The Bedrock Geology of 

Massachusetts

E. Stratigraphy of the Milford-Dedharn Zone, Eastern 

Massachusetts: An Avalonian Terrane

F. Stratigraphy of the Nashoba Zone, Eastern Massachusetts: An 

Enigmatic Terrane

G. Stratigraphy of the Merrimack Belt, Central Massachusetts 

H. Structural and Metamorphic History of Eastern Massachusetts

I. Intrusive Rocks of Eastern Massachusetts

J. Radiometric Ages of Rocks in Massachusetts

L.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-E-J
The Bedrock Geology of 

Massachusetts

NORMAN L. HATCH, JR., Editor

E. Stratigraphy of the Milford-Dedham Zone, Eastern

Massachusetts: An Avalonian Terrane 

By RICHARD GOLDSMITH

F. Stratigraphy of the Nashoba Zone, Eastern Massachusetts: An

Enigmatic Terrane

By RICHARD GOLDSMITH

G. Stratigraphy of the Merrimack Belt, Central Massachusetts 

By PETER ROBINSON and RICHARD GOLDSMITH

H. Structural and Metamorphic History of Eastern Massachusetts 

By RICHARD GOLDSMITH

I. Intrusive Rocks of Eastern Massachusetts

By DAVID R. WONES and RICHARD GOLDSMITH

J. Radiometric Ages of Rocks in Massachusetts

By ROBERT E. ZARTMAN and RICHARD F. MARVIN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-E-J

Chapters E J are issued as a single volume 

and are not available separately

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1991
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for 

and does not imj" 

.S. Government

descriptive purposes only and_does not imply endorsement by the

Library of Congress Cataloging in Publication Data

The bedrock geology of Massachusetts.

(U.S. Geological Survey professional paper ; 1366-E-J)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1366E-J

1. Geology Massachusetts. I. Hatch, Norman L., Jr. II. Series: Geological Survey professional paper ; 1366-E-J.

QE123.B44 1988b 557.44 87-600472

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
Editor's Preface to Chapters E through J

This Professional Paper was planned as a companion to 

the bedrock geologic map of Massachusetts (Zen and 

others, 1983; hereafter referred to as the State bedrock 

map). It is being published as lettered chapters of 

Professional Paper 1366, six of which are included in this 

volume, and four of which were published in a single 

volume as chapters A-D (Hatch, 1988). Compilation of 

the geology for the State bedrock map was completed in 

1980. Some of the chapters in this Professional Paper 

reflect field or laboratory data that were gleaned as much 

as 6 years later. Each chapter was prepared, however, 

with the objective of explaining and further describing 

the geology as portrayed on the State bedrock map. In 

some instances, information and interpretations developed since 1980 have caused chapter authors to suggest 

revisions that they would make to the map if they were 

able to redraw it, but in each case these suggested 

revisions are discussed in the context of the map as it was 

published.

The previous State bedrock map (which also showed 

the geology of Rhode Island) was published in 1917 by 

Benjamin K. Emerson as U.S. Geological Survey Bulletin 597. (The publication date of Bulletin 597 is 1917. 

Some confusion arises from the fact that the bedrock map 

of the two States, which is included in the pocket of the 

Bulletin, bears the date of 1916.) All who were involved 

in the preparation of the new bedrock map, particularly 

those responsible for the parts of the State in which 

Emerson himself had done the original field work, feel a 

great deal of respect for Professor Emerson and his 

remarkably perceptive and thorough understanding and 

portrayal of the geology. Although the new map is very 

different from Emerson's in many aspects, particularly 

with regard to the interpretation of the geologic history, 

the basic distribution of map units is remarkably similar.

The State bedrock map and this report are direct 

outgrowths of a cooperative geologic mapping program 

between the U.S. Geological Survey and the Commonwealth of Massachusetts, which was begun in 1938. They 

also include the results of more than 25 years of mapping 

and topical studies by faculty and students at the University of Massachusetts at Amherst and at many other 

colleges and universities.

The subdivision of the material in this Professional 

Paper into the constituent chapters is based on the

grouping of the 343 individual lithic units on the State 

bedrock map into the 8 lithotectonic packages discussed 

by Hatch and others (1984). The temporal and geographic distributions of these eight packages are indicated on figures 1 and 2. Also indicated on the figures are 

the geographic and geologic coverages of the chapters 

included in this volume. In this packaging scheme, the 

older, primarily pre-Silurian, rocks of the State are 

grouped into five "zones" whose exposed and buried 

parts completely cover the State. From west to east, 

these zones are the Taconic-Berkshire, the RoweHawley, the Bronson Hill, the Nashoba, and the MilfordDedham. Their mutual boundaries are, or could reasonably be interpreted to be, faults. Overlying and 

overlapping the zones in the central part of the State are 

the Connecticut Valley and Merrimack "belts" of primarily Silurian and Devonian strata. Their mutual boundary 

is somewhat arbitrarily taken to be the east contact of 

the easternmost exposed Silurian Clough Quartzite. 

Finally, the Mesozoic "basins" unconformably overlie the 

Connecticut Valley belt.

For some packages, all aspects of the geology are 

treated in the same chapter. For others, aspects such as 

the structure, metamorphism, and tectonics are discussed separately from stratigraphy and lithology. These 

differences in treatment resulted from peculiarities of 

the geology and the preferences of the individual 

authors. Many of the plutonic rocks of the State are 

described and discussed in chapter I.

Many of the lithologic subdivisions of formal units on 

the State bedrock map have not been given formal 

names. In order to avoid potentially cumbersome discussions of such things as "the thick-bedded micaceous 

quartzite and mica schist unit of the XYZ Formation," 

many chapter authors have chosen to refer to such units 

simply by their map symbols. Thus the micaceous quartzite, quartz-mica-garnet schist, and calc-silicate unit of 

the Devonian Goshen Formation may be referred to 

simply by its map symbol "Dgq," but in a context where 

the reader will be easily guided to the correct unit.

The terms "granulite" and "granofels" have been used 

rather arbitrarily and interchangeably throughout this 

Professional Paper, although on the State bedrock map 

the term "granofels" was used exclusively. Both terms 

are used to describe a metamorphic rock composed

in
IV PREFACE

predominantly of even-sized, interlocking granular minerals; no implication as to the grade of metamorphism is 

intended by either term. The choice of words merely 

reflects individual author preference, and we hope that 

no confusion to the reader will result from the unrestrained use of two words for the same kind of rock.

This volume contains six chapters that deal primarily 

with terranes of eastern Massachusetts, terranes that 

were amalgamated and accreted to North America during the Paleozoic. Chapter E, on the stratigraphy of the 

Milford-Dedham zone, deals with the stratified rocks of 

the easternmost part of the State, a terrane considered 

to represent part of Avalonia. Chapter F deals with the 

stratified rocks of the Nashoba zone, a "suspect terrane" 

west of the Milford-Dedham zone, that is bounded by the 

Bloody Bluff fault on the east and the Clinton-Newbury

fault on the west. Chapter G describes the stratigraphy 

of the Silurian and Devonian rocks of the Merrimack belt, 

which adjoins and overlies the Nashoba zone on the west. 

Chapter H discusses the structure and metamorphism of 

the rocks of the Milford-Dedham and Nashoba zones and 

of the easternmost part of the Merrimack belt. Chapter 

I describes and discusses the critically distinctive intrusive rocks of the eastern part of the State. Finally, 

Chapter J tabulates all of the isotopic ages on the rocks 

of the whole State available as of 1986. Most of these ages 

are from the abundant plutonic rocks of the eastern part 

of the State.

We would herein like to acknowledge the invaluable 

contributions to this Professional Paper of two key 

people. Jewel Dickson did the cartographic work on the 

majority of the illustrations, the principal exceptions

-Chapters H and I -

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MISSISSIPPI AN 

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

CAMBRIAN TO 

LATE PROTEROZOIC

LATE PROTEROZOIC

MIDDLE 

PROTEROZOIC

1

-1

1 Mesozoic basins ~S| g

V.

«- 

0)

_c .*-

J2 E

1 1

i «"

8 E

1 Ga | 

North >

basement

^^--^-^.~~^ ^ - -

i

Merrimack belt ^ 3 

Connecticut Volley belt chapter G -f *

^^ ^ o

0

!ley Rowe Haw zone - S

thrust Bistol 

No siolic 

basement 

known

s

X

S

0

co

w?

i

ID 

C w_

S -5

O CO

o

-C ^

1 8

CO

750 Mo gneiss

8

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1

il

"o

'c 

_o 

o

igneous rocks

rhantor F

Chapter F 1

T

K

J

*

P

IH

M

D

S

O

Z

Z

Y

FIGURE 1. Diagram simplified from the "Correlation of Map Units" on the State bedrock map 

showing the eight lithotectonic packages into which the rock units have been divided. Also 

indicated are the letter designation(s) of the chapter(s) in this volume covering various aspects of 

the geology. Chapter J deals with rocks from the entire State and thus is not shown on the figure. 

Modified from Hatch and others (1984, fig. 1).
PREFACE

| CONNECTICUT . . J^ 1. .

1 VALLEY BELT

Conn. Valley belt 

| above R.-H. zone

Massabesic 

Gneiss Comdex 1 -<3r

TACONICBERKSHIRE ZONE

ChapterF

Chapter E' \ ^

\

Martha's 

Vineyard Nantucket

FIGURE 2. Map of Massachusetts showing the geographic distribution of the eight lithotectonic packages into which the rock units of the State 

have been grouped and the letter designation(s) of the chapter(s) in this volume in which aspects of the geology are discussed. Chapter J deals 

with rocks from the entire State and thus is not shown on the figure. Modified from Hatch and and others (1984, fig. 2).

being those prepared by author Peter Robinson. Kathleen Krafft Gohn suffered bravely over the years with 

the editor and authors of this Professional Paper as its 

technical editor.

REFERENCES CITED

Emerson, B.K., 1917, Geology of Massachusetts and Rhode Island: 

U.S. Geological Survey Bulletin 597, 289 p.

Hatch, N.L., Jr., editor, 1988, The bedrock geology of Massachusetts: 

U.S. Geological Survey Professional Paper 1366-A-D, variously

Hatch, N.L., Jr., Zen, E-an, Goldsmith, Richard, Ratcliffe, N.M., 

Robinson, Peter, and Wones, D.R., 1984, Lithotectonic assemblages as portrayed on the new bedrock geologic map of Massachusetts: American Journal of Science, v. 284, p. 1026-1034.

Zen, E-an, editor, Goldsmith, Richard, Ratcliffe, N.M., Robinson, 

Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 

of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 

scale 1:250,000.

VOLUME CONTENTS

[Letters designate chapters]

Editor's Preface to Chapters E through J, by Norman L. Hatch, Jr.

(E) Stratigraphy of the Milford-Dedham zone, eastern Massachusetts: An Avalonian terrane, by 

Richard Goldsmith

(F) Stratigraphy of the Nashoba zone, eastern Massachusetts: An enigmatic terrane, by Richard 

Goldsmith

(G) Stratigraphy of the Merrimack belt, central Massachusetts, by Peter Robinson and Richard 

Goldsmith

(H) Structural and metamorphic history of eastern Massachusetts, by Richard Goldsmith 

(I) Intrusive rocks of eastern Massachusetts, by David R. Wones and Richard Goldsmith 

(J) Radiometric ages of rocks in Massachusetts, by Robert E. Zartman and Richard F. Marvin

The following chapters of Professional Paper 1366 were published in a single volume in 1988:

(A) The pre-Silurian geology of the Rowe-Hawley zone, by Rolfe S. Stanley and Norman L. 

Hatch, Jr.

(B) Stratigraphy of the Connecticut Valley belt, by Norman L. Hatch, Jr., Peter Robinson, and 

Rolfe S. Stanley

(C) Post-Taconian structural geology of the Rowe-Hawley zone and the Connecticut Valley belt 

west of the Mesozoic basins, by Norman L. Hatch, Jr., and Rolfe S. Stanley

(D) The Whately thrust: A structural solution to the stratigraphic dilemma of the Erving Formation, by Peter Robinson, Norman L. Hatch, Jr., and Rolfe S. Stanley

Stratigraphy of the 

Milford-Dedham Zone, 

Eastern Massachusetts: 

An Avalonian Terrane

By RICHARD GOLDSMITH

With a section on MESOZOIC AND TERTIARY STRATIGRAPHY OF 

CAPE COD AND THE NEARBY ISLANDS 

By E.G.A. WEED

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-E

CONTENTS

Page

Abstract............................................................................. El

Introduction........................................................................ 2

Metasedimentary and metavolcanic rocks older than the

Proterozoic Z Rhode Island and Dedham batholiths.......... 2

Rocks possibly older than the quartzitic assemblage ............ 4

Quartzitic assemblage..................................................... 4

Westboro Formation (Zw)......................................... 4

Plainfield Formation (Zp).......................................... 5

Blackstone Group: Mica schist and phyllite (Zbs) and

Quinnville Quartzite (Zbq)................................ 5

Paleoenvironment of deposition of the quartzitic

assemblage.................................................... 13

Metavolcanic assemblage................................................. 13

Metamorphosed mafic to felsic flow, volcaniclastic,

and hypabyssal intrusive rocks (Zv)................... 17

Blackstone Group: Greenstone and amphibolite

(Hunting Hill Greenstone) (Zbv)........................ 20

Metamorphosed felsic volcanic rock (Zvf)..................... 20

Correlation of the mafic metavolcanic rocks (Zv) and

the Hunting Hill Greenstone (Zbv).................... 20

Metamorphic rocks of southeastern Massachusetts .............. 21

Gneiss and schist near New Bedford (Zgs)................... 21

Biotite gneiss near New Bedford (Zgn) ....................... 21

Correlation ............................................................ 22

Age of the prebatholithic rocks ....................................... 23

Proterozoic Z rocks younger than or equivalent to the

southeastern Massachusetts batholith............................. 23

Mattapan and Lynn Volcanic Complexes (Zm, DZ1)............. 24

Felsic and mafic rocks southwest of the Boston basin (Zfm).. 26 

Age of the Mattapan and Lynn Volcanic Complexes ............ 26

Boston Bay Group ......................................................... 28

Paleoenvironment.................................................... 29

Age....................................................................... 29

Cambrian strata.................................................................. 30

Hoppin Formation (Ch).................................................. 30

Weymouth Formation and Braintree Argillite (Cbw)........... 32

Green Lodge Formation of Rhodes and Graves (1931) (Cg)... 32 

Paleoenvironment.......................................................... 33

Newbury Volcanic Complex (Silurian and Devonian).................. 33

Pennsylvanian strata.......................................................... E33

Bellingham Conglomerate (PZb).................................... 34

Pondville Conglomerate (Pp), Wamsutta Formation 

(Pw, Pwv), Rhode Island Formation (Pr, Prc), and

Dighton Conglomerate (Pd)..................................... 34

Paleogeography and age ............................................... 37

Triassic and Jurassic rocks .................................................. 37

Red arkosic conglomerate, sandstone, and siltstone ("Re).... 37

Stratigraphic problems....................................................... 37

Felsic volcanic rocks..................................................... 38

Lynn Volcanic Complex ................................................ 39

Boston Bay Group ....................................................... 39

Bellingham Conglomerate ............................................. 39

Brighton Melaphyre..................................................... 39

Sequence in the Burlington area..................................... 40

Mafic metavolcanic rocks and the Marlboro Formation....... 40

Newbury Volcanic Complex........................................... 40

The stratigraphic record in the Milford-Dedham zone .............. 40

Regional relations in southeastern New England .................... 42

Quartzitic assemblage................................................... 44

Volcanic-plutonic complex in eastern Massachusetts and

correlative rocks ................................................... 44

Rhode Island and southeastern Massachusetts batholiths

and related granitoids ............................................ 45

Mattapan Volcanic Complex and equivalent rocks ............. 45

Boston Bay Group and equivalent rocks .......................... 45

Younger units ............................................................ 45

The Milford-Dedham zone in the Caledonides Correlation....... 45

Mesozoic and Tertiary stratigraphy of Cape Cod and the

nearby islands, by E.G.A. Weed ................................. 46

Introduction................................................................ 46

Geologic setting........................................................... 47

Description of post-Paleozoic units.................................. 47

Triassic and Jurassic basalt (J"fcb) ............................ 47

Triassic and Jurassic sediments and volcanic rocks (J"fc) 50

Cretaceous sediments (K)........................................ 50

Tertiary sediments (T)............................................ 55

Stratigraphic history.................................................... 57

References cited................................................................ 57

ILLUSTRATIONS

FIGURE 1. Sketch map of the major features of the geology in the Milford-Dedham zone in eastern Massachusetts ............................... E3

2. Correlation diagram of stratigraphic units in the Milford-Dedham zone........................................................................... 6

in
IV CONTENTS

FIGURES 3-5. Maps of:

3. Stratigraphic units north and northwest of the Boston basin....................................................................... E9

4. Stratigraphic units west and southwest of the Boston basin........................................................................ 10

5. Stratigraphic units in southeastern Massachusetts and adjacent Rhode Island ............................................... 11

6. Correlation chart of stratified rocks older than Proterozoic Z granitoids in eastern Massachusetts and northern

Rhode Island............................................................................................................................................. 12

7. Correlation chart of stratified rocks older than Proterozoic Z granitoids in southeastern Massachusetts and southern

Rhode Island............................................................................................................................................. 13

8. Map of Stratigraphic units in and around the Boston basin...................................................................................... 18

9. Stratigraphic section and lithologic description of the Boston Bay Group................................................................... 25

10. Map of Stratigraphic units in and around the Narragansett basin............................................................................. 27

11. Geologic map and measured sections on the east side of Hoppin Hill, Attleboro, Mass................................................. 31

12. Measured section of Lower and Middle Cambrian strata of the Hoppin Formation at the north end of Hoppin Hill

Reservoir................................................................................................................................................. 32

13. Map showing distribution of major groups of rocks in the Milford-Dedham zone, Massachusetts, Rhode Island, and

Connecticut............................................................................................................................................... 42

14. Correlation diagram of some Proterozoic Z units in the Milford-Dedham zone in southeastern Connecticut, eastern

Massachusetts, and Rhode Island ................................................................................................................. 43

15. Map showing locations of outcrop and auger- and core-drilling sites that provide information on the pre-Mesozoic

basement and Mesozoic and Tertiary deposits in the area of Cape Cod and the nearby islands, Massachusetts .......... 48

16. Diagram showing correlation of the Cretaceous section in bore hole USGS 6001, Nantucket, with the exposed Cretaceous

section in New Jersey................................................................................................................................. 56

TABLES

TABLE 1. Sedimentary basins in the Milford-Dedham zone, Massachusetts..................................................................................... E4

2. Nomenclature of stratified metamorphic rocks older than the Proterozoic Z Rhode Island and southeastern Massachusetts

batholiths ........................................................................................................................................................ 8

3. Descriptions from previously published works of Stratigraphic units in eastern Massachusetts and northern Rhode Island

older than the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths................................................. 14

4. Stratigraphic units of the Boston basin and vicinity ..................................................................................................... 23

5. Description of Stratigraphic units in the Narragansett and Norfolk basins........................................................................ 35

6. Stratigraphic record in the Milford-Dedham zone......................................................................................................... 41

7. Location and identification of outcrops and auger- and core-drilling sites in the area of Cape Cod and the nearby islands,

Massachusetts.................................................................................................................................................. 49

8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands,

Massachusetts.................................................................................................................................................. 51
THE BEDROCK GEOLOGY OF MASSACHUSETTS

STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, 

EASTERN MASSACHUSETTS: AN AVALONIAN TERRANE

By RICHARD GOLDSMITH

ABSTRACT

The sedimentary and volcanic rocks and their metamorphic equivalents in the Milford-Dedham zone include rocks of Proterozoic Y? and 

Proterozoic Z, Proterozoic Z, Cambrian, Silurian and Devonian, Pennsylvanian, Triassic-Jurassic, and Cretaceous and Tertiary ages. Rocks 

older than Proterozoic Z batholithic rocks are divided into two major 

sequences. A sequence of quartzitic rocks includes the Westboro 

Formation, the Plainfield Formation, and mica schist and phyllite and 

Quinnville Quartzite of the lower part of the Blackstone Group. These 

formations represent shelf-edge sedimentation associated with minor 

volcanism. A group of largely mafic metavolcanic rocks younger than 

the quartzitic group, and probably arc related, includes metamorphosed mafic to felsic flow, volcaniclastic, and hypabyssal intrusive 

rocks, which in the literature of northeastern Massachusetts have been 

mapped as Middlesex Fells Volcanic Complex, Cherry Hill Formation 

and associated units, and Marlboro Formation. Included within this 

group in northern Rhode Island and adjacent Massachusetts are 

greenstone and amphibolite equivalent to the Hunting Hill Greenstone 

of the upper part of the Blackstone Group. In southeastern Massachusetts, near New Bedford, rocks older than Proterozoic Z batholithic 

rocks include gneiss and schist and biotite gneiss. These rocks were 

derived from metamorphosed clastic, volcaniclastic, and felsic to intermediate volcanic material. The correlation of these rocks to the 

quartzitic or mafic volcanic assemblages is uncertain.

Proterozoic Z rocks equivalent to or younger than the batholithic 

rocks are located primarily in and around the Boston basin. The 

Mattapan and Lynn Volcanic Complexes, primarily felsic volcanic 

rocks, may be in part coeval with younger phases of the batholithic 

rocks. Above the Mattapan and Lynn is the Boston Bay Group, a 

largely sedimentary, probably turbidite sequence of conglomerate, 

sandstone, and argillite consisting of the Roxbury Conglomerate and 

the Cambridge Argillite. Within the Roxbury Conglomerate, in the 

lower part of the group, are horizons of mafic volcanic rock, the 

Brighton Melaphyre. The Cambridge Argillite contains Proterozoic Z 

to Cambrian(?) acritarchs.

Lower and Middle Cambrian strata containing fossil assemblages 

typical of the Acado-Baltic province overlie the Proterozoic Z rocks 

around the Boston basin (Weymouth Formation, Braintree Argillite) 

and in the northwest corner of the Narragansett basin, near North 

Attleboro and West Wrentham (Hoppin Formation). The Lower Cambrian consists of basal quartzite, overlain by slaty phyllite, limey 

phyllite, and limestone; the Middle Cambrian consists primarily of

Manuscript approved for publication November 16, 1987.

argillite and slate. The Upper Cambrian is represented by only one 

locality near Dedham, where the Green Lodge Formation of Rhodes 

and Graves consists of quartzite and phyllite.

Silurian and Devonian strata are confined to two narrow faultbounded basins in northeastern Massachusetts, near Newbury and 

near Middleton. These strata, the Newbury Volcanic Complex, consist 

in the lower part primarily of volcanic rocks, including felsic and mafic 

flows and volcaniclastic material, and in the upper part largely of 

mudstone and siltstone and subordinate volcanic detritus. Fossil fauna, 

also of Acado-Baltic affinity, range from latest Silurian (Pridolian) to 

earliest Devonian (Gedinnian).

Pennsylvanian strata occupy one large basin (Narragansett basin) 

and two smaller basins (Norfolk and Bellingham (Woonsocket) basins). 

The Pennsylvanian strata are primarily continental sandstone, shale, 

and conglomerate (Pondville Conglomerate, Wamsutta Formation, 

Rhode Island Formation, and Dighton Conglomerate). The Wamsutta 

and Rhode Island Formations contain plant fossils, and the Rhode 

Island Formation is coal bearing. The strata in the Bellingham basin 

are nonfossiliferous, and their age is in question. The Wamsutta 

Formation contains basalt and felsic volcanic rocks in the northwestern 

part of the Narragansett basin and the southwestern part of the 

Norfolk basin. The Dighton represents channel deposits high in the 

sequence.

Red arkosic conglomerate, sandstone, and siltstone of Late Triassic 

to Early Jurassic age occupy a small fault-bounded basin, the Middleton 

basin, in northeastern Massachusetts. A Triassic-Jurassic basin lies in 

the subsurface beneath Coastal Plain sediments on Nantucket Island 

and Martha's Vineyard and beneath Nantucket Sound. The basin 

contains Triassic-Jurassic basalt, which was identified in a deep drill 

hole on Nantucket, and overlying sedimentary strata known only 

through seismic profiles.

Cretaceous strata of the Atlantic Coastal Plain crop out and are 

identified in drill holes on Nantucket and Martha's Vineyard. Tertiary 

strata are identified in drill holes and sparse outcrops on the islands, on 

Cape Cod, and in the Marshfield area on the mainland. Most material 

containing Tertiary spores or pollen, however, seems to have been 

disturbed and is in places physically mixed with, or reworked into, 

Pleistocene materials.

The stratigraphic record indicates four major episodes. A Proterozoic episode involves several phases: (1) arc margin and volcanic-arc 

accumulation in a compressional phase (Marlboro Formation; metamorphosed mafic to felsic flow, volcaniclastic, and hypabyssal intrusive 

rocks), (2) felsic volcanism and plutonism (Mattapan and Lynn Volcanic 

Complexes; Dedham Granite), (3) uplift and erosion, and (4) flysch to

El
E2 THE BEDROCK GEOLOGY OF MASSACHUSETTS

molasse deposition and mafic volcanism in an extensional regime 

(Boston Bay Group). A second episode began with encroachment of 

Cambrian seas and shelf-deposition of Cambrian sediments (Hoppin 

Formation, Weymouth Formation, and Braintree Argillite) on a passive continental margin and lasted with little deposition into the middle 

Paleozoic. Tectonic conditions were primarily extensional and were 

characterized by static plutonism, largely alkalic, from Ordovician into 

Devonian time, but mafic and felsic arc volcanism and marine sedimentation occurred locally(?) in the Late Silurian and Early Devonian 

(Newbury Volcanic Complex). In the Pennsylvanian, continental sedimentary deposits and minor volcanic material (Pondville Conglomerate, Wamsutta Formation, volcanic rocks in the Wamsutta Formation, 

Rhode Island Formation, Dighton Conglomerate) accumulated in a 

shallow basin or basins on the Proterozoic and the older Paleozoic 

rocks. An episode of sedimentation and volcanism associated with 

rifting and the opening of the present Atlantic Ocean occurred during 

Triassic and Jurassic time. A new episode has begun with onlap of 

Cretaceous and Tertiary coastal plain sediments.

The stratigraphy within the Milford-Dedham terrane is in many 

respects similar to that in the Avalon terrane of southeastern Newfoundland and can be correlated with that of similar terranes in the 

Piedmont of the southern Appalachians, in the Maritime Provinces of 

Canada, in Wales, in England, and in northwest Africa.

INTRODUCTION

The Milford-Dedham zone is defined as encompassing 

those rocks and sediments lying east and southeast of the 

Bloody Bluff-Lake Char fault system. The zone includes 

Cape Cod and the outer islands and extends eastward an 

unknown distance beneath the Atlantic Ocean. The zone 

is further characterized in that it contains intrusive 

granites of Proterozoic Z age, in contrast to the adjacent 

Nashoba zone, which lacks such granites. Rocks of the 

zone continue southward into Rhode Island and adjacent 

eastern Connecticut.

The Milford-Dedham zone contains two extensive 

batholithic masses of granitoid rock of Proterozoic Z age, 

one in western Rhode Island and in the Milford antiform 

of adjacent Massachusetts (hereinafter called the Rhode 

Island batholith) and the other in eastern and southeastern Massachusetts comprising the Dedham and equivalent granites (hereinafter called the southeastern Massachusetts batholith) (fig. 1). Older metasedimentary, 

metavolcanic, and metaplutonic rocks are preserved in 

discontinuous belts on the flanks and in septa or roof 

pendants. Plutons of gabbro to granite, the latter primarily alkalic, of Ordovician to Devonian age intrude the 

older rocks. Superimposed on the Proterozoic rocks are 

structural and stratigraphic basins containing unmetamorphosed to variably metamorphosed sedimentary and 

volcanic rocks ranging in age from latest Proterozoic to 

Triassic and Jurassic (table 1). Coastal plain deposits of 

Cretaceous and Tertiary age overlap the bedrock on the 

southeast. These and their substrata are described later 

in this chapter. The deposits of Pleistocene age are not 

shown on the State bedrock map (Zen and others, 1983;

hereinafter referred to as the State bedrock map) and are 

not discussed in this chapter.

The State bedrock map contains 36 named units of 

sedimentary and volcanic rocks and their metamorphic 

equivalents in the Milford-Dedham zone (fig. 2). This 

number is condensed from a larger number of units that 

have been described and named by many different people 

working in different areas and at different times in 

eastern Massachusetts. In addition, the terrane is broken up by many faults so that many units are exposed 

only in one or two fault blocks. The result is that the 

geology as presented in this chapter tends to be fragmented. To remedy this, an attempt has been made to 

group units systematically within the limits of available 

fossil control and isotopic data.

This chapter summarizes descriptions of stratigraphic 

units partly described elsewhere, presents more detail in 

areas where new observations have been made, and 

concentrates discussion on controversial points of interpretation and on observations that are important in 

developing a regional synthesis. The plutonic rocks 

within the zone are described in another chapter (Wones 

and Goldsmith, this vol., chap. I). The structure is 

described in chapter H (Goldsmith, this vol.). The 

descriptions and discussion to follow are arranged primarily by relative age and only secondarily by structural 

block or basin. The data in this chapter are drawn from 

a number of sources used in the preparation of the State 

bedrock map and listed thereon. Nomenclature used 

consists of existing names and names modified for use on 

the State bedrock map (table 2; fig. 2; Goldsmith and 

others, 1982c). Significant exposures of rock units in the 

Milford-Dedham zone are described in guidebooks of the 

New England Intercollegiate Geological Conference published over a long span of years. Some of the stratigraphic units in the Greater Boston area can be seen by 

following a field guide prepared by Skehan (1979).

METASEDIMENTARY AND METAVOLCANIC

ROCKS OLDER THAN THE PROTEROZOIC Z

RHODE ISLAND AND DEDHAM BATHOLITHS

Metasedimentary and metavolcanic rocks older than 

the Proterozoic Z granitoids of the Rhode Island and 

Dedham batholiths are found (1) in a metavolcanicmetaplutonic complex primarily in northeastern Massachusetts (fig. 3), (2) on the flanks of and as inclusions in 

the Rhode Island batholith (fig. 4), and (3) as septa in the 

southeastern Massachusetts batholith in the New Bedford area (fig. 5). Most of the lower part of the sequence 

of stratified rocks in and around the batholiths in Massachusetts consists of an assemblage of metasedimentary 

formations characterized by significant amounts of
43°

72°

STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS

71°

E3

70°

HAMPSHIRE 

MASSACHUSETTS

42°

FIGURE 1. Major features of the geology in the Milford-Dedham zone in eastern Massachusetts.

quartzite the Westboro Formation (Zw), the Plainfield 

Formation (Zp), and the lower part of the Blackstone 

Group (Zbs, Zbq, Zb). Most of the upper part is an 

assemblage of formations characterized by significant 

amounts of mafic metavolcanic rocks metamorphosed 

mafic to felsic flow, volcaniclastic, and hypabyssal intrusive rocks (Zv); metamorphosed felsic volcanic rocks 

(Zvf); and greenstone and amphibolite (Zbv) of the 

Blackstone Group. The two assemblages are gradational 

through interlayering. Gneiss and schist near New Bedford (Zgs) and biotite gneiss near New Bedford (Zgn) are 

Proterozoic Z in age, but their correlation with the 

assemblages mentioned above is uncertain. In a few 

places, metasedimentary strata apparently lie below the 

quartzitic assemblage, and, at others, metasedimentary 

strata apparently lie above the mafic metavolcanic 

assemblage. These strata, however, are thin and hence 

are included in one or another of the larger units on the 

State bedrock map. The correlation of these older stratified rocks is shown in figures 6 and 7, and descriptions
E4 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE I. Sedimentary basins in the Milford-Dedham zone, Massachusetts

Basin Lithotype Fossil type Metamorphism Age of strata

Middleton Fluviatile conglomerate, arkose, Plant 

and shale.

Norfolk Fluviatile conglomerate, sandstone, Plant 

and siltstone; minor volcanic component.

Narragansett Fluviatile and deltaic conglomerate, Plant 

sandstone, and coal. Minor mafic 

and felsic volcanic rocks in northwest part.

Newbury Mafic and felsic volcanic flows and Shelly fauna 

tuffs; marine siltstone and mudstone.

Bellingham Fluviatile(?) and marine(?) conglom- None 

erate, sandstone, and siltstone; 

minor felsic volcanic rocks.

Boston Marine sandstone, siltstone, shale, Shelly fauna

calcareous shale, and limestone. 

Fluviatile and marine conglomerate, Acritarchs 

sandstone, siltstone, shale, tillite; 

mafic and felsic volcanic rocks.

None

Diagenetic to zeolite facies(?) 

(well-developed cleavage in 

slaty beds).

Diagenetic to greenschist 

facies (mainly polydeformed 

poor to well-developed cleavage).

None

Greenschist facies (foliated)

Diagenetic

Late Triassic and Early Jurassic.

Late Mississippian(?) to Pennsylvanian.

Late Mississippian to PennsylvaLate Silurian and Early Devonian.

Proterozoic Z or Pennsylvania^

Early and Middle Cambrian.

Diagenetic (incipient cleavage) Proterozoic Z to Early

Cambrian.

from previously published works are summarized in 

table 3.

ROCKS POSSIBLY OLDER THAN THE QUARTZITIC 

ASSEMBLAGE

Bell and Alvord (1976) described "unnamed stratified 

remnants" consisting of fine-grained gneisses and 

quartzite from a hill in the western part of Saugus (fig. 3, 

table 3). These remnants form a band of poorly exposed 

slablike xenoliths, lying apparently conformably below 

thick-bedded quartzite assigned to the Westboro Formation cropping out 0.5 km to the north, and a zone of 

scattered inclusions in the Dedham Granite near Lynn. 

These rocks occupy a limited area and have been included 

in the Westboro Formation on the State bedrock map.

QUARTZITIC ASSEMBLAGE

WESTBORO FORMATION (Zw)

The Westboro Formation (Zw), the Westboro Quartzite of Perry and Emerson (1903, p. 155), is primarily a 

quartzitic unit consisting of thick- to thin-bedded orthoquartzite and subordinate mica schist, calc-silicate rock, 

amphibolite, and biotite gneiss and schist, the latter 

commonly quartzitic. Although the rock sequences of 

Nelson (1974) and of Bell and Alvord (1976) (table 3) 

cannot be correlated precisely, their descriptions are 

similar and typify the formation in eastern Massachusetts. Hepburn and DiNitto's (1978) description of the 

Westboro is of rocks closest to Emerson's type area near

Westboro. They proposed that the type area of Westboro 

be exposures in 200 m of cuts on 1-495 north of Mt. Nebo, 

south of the Southboro-Westboro town line.

West and north of Boston, disconnected masses of 

quartzite and associated rock in Proterozoic Z mafic 

plutonic and volcanic rocks are shown on the State 

bedrock map as Westboro, although they are not in a 

continuous belt with the Westboro of Nelson (1974) or of 

Hepburn and DiNitto (1978). Other isolated masses of 

quartzite, such as those mapped by Castle (1964) in the 

Reading area, are also assigned to the Westboro. Bell 

and Alvord (1976) mapped a thin-bedded quartzite primarily in their Burlington Formation (table 3), in a group 

of formations near Burlington that lie between the 

Bloody Bluff fault and their Middlesex Fells Volcanic 

Complex. Although this unit lies 2,300 m above the 

Westboro on their section (table 3), and they cited 

indirect evidence that the group lies unconformably on 

the Middlesex Fells, the Burlington has been shown 

arbitrarily as Westboro on the State bedrock map to 

indicate its quartzitic composition and to decrease the 

number of small units on the map. The remainder of this 

group (Bell and Alvord's Greenleaf Mountain Formation 

and unnamed gneiss and quartzite; see table 3) has been 

included in the metamorphosed mafic to volcanic rocks 

(Zv) of Proterozoic Z age because these units contain 

metavolcanic rock. I now believe that the three units 

should be assigned to a separate metamorphic suite 

above the main sequence of metavolcanic rocks (Zv, Zvf).

Some small masses of quartzite reported in the literature have not been shown on the State bedrock map. 

Emerson's (1917) map of Massachusetts shows small 

masses of quartzite in the Essex and Ipswich areas,
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E5

which later geologists mapping in these areas have not 

recognized. Emerson included many small areas of 

quartzite, in addition to the larger ones shown as Westboro on the State bedrock map, in his Marlboro Formation (a term now confined to the Nashoba zone for a 

sequence characterized by amphibolites) because of their 

intercalation in the Milford-Dedham zone with metavolcanic rocks that he mapped as Marlboro but that are now 

assigned to the mafic and felsic metavolcanic rocks (Zv, 

Zvf). Sears (1905, p. 110) mentioned that a well in 

quartzite at Lynnfield penetrated white limestone interstratified with a light-blue slate and quartzite. A description of the quartzite given by Sears indicates that the 

rock had been crushed and recrystallized. It may have 

been a silicified zone.

The Westboro in the Framingham area (figs. 4, 8) and 

to the northeast is overlain by the assemblage of metamorphosed mafic and felsic volcanic rocks (Zv, Zvf) 

(tables 2, 3). South of Westborough, the Westboro is 

truncated by the Bloody Bluff-Lake Char fault system. 

The Westboro is intruded by Proterozoic Z batholithic 

rocks.

Nelson (1974) placed a metasedimentary-metavolcanic 

unit of gneiss, schist, and quartzite he named the Rice 

Gneiss (table 3) below thick-bedded quartzite of the 

Westboro in the Natick quadrangle. The Rice Gneiss 

could be the equivalent of the unnamed stratified remnants of Bell and Alvord on the basis of their descriptions. The gneiss and calc-silicate rocks described as Rice 

Gneiss by Volckmann (1977) in the Holliston area (fig. 4; 

table 3), however, do not resemble the description of 

Rice Gneiss by Nelson (1974). Rocks similar to the Rice 

Gneiss have not been identified elsewhere in the region, 

although they may form part of the Blackstone Group in 

northern Rhode Island. The Rice Gneiss may be equivalent to gneiss and schist lying below the Plainfield 

Formation in the Lyme Dome of southeastern Connecticut (Lundgren, 1967, p. 14). However, that is a longrange correlation for such a small unit; furthermore, the 

complicated fold pattern and lack of topping information 

in southeastern Connecticut render questionable the 

proper stratigraphic assignment of the inner rocks of the 

Lyme Dome. Because the Rice Gneiss is not characteristically quartzitic but predominantly feldspathic, and 

because it lies in a different fault block from the Westboro, it was included in the comprehensive unit of 

metavolcanic rocks (Zv) of Proterozoic Z age on the State 

bedrock map. I now believe that assignment to the 

Westboro or as a separate unit below the Westboro 

would be preferable.

The Westboro is equivalent to the Plainfield Formation of eastern Connecticut because it lies in the same 

strike belt, and the Westboro is believed to be equivalent 

to the Quinnville Quartzite (Zbq) and mica schist and

phyllite (Zbs) of the Blackstone Group. Both have similar 

stratigraphic positions below mafic metavolcanic rocks 

greenstone and amphibolite (Zbv) in northern Rhode 

Island and metamorphosed mafic and felsic volcanic 

rocks (Zv, Zvf) in eastern Massachusetts. The boundary 

between the Westboro and the Blackstone is drawn 

arbitrarily on the State bedrock map on the basis of 

proximity of the isolated exposures of the two to their 

respective type areas. No area of continuous exposure 

exists between the two units.

PLAINFIELD FORMATION (Zp)

The Plainfield Formation (Zp) extends into Massachusetts in the Webster-Oxford area (fig. 4) from eastern 

Connecticut and western Rhode Island along the west 

flank of the Rhode Island batholith. Elongate lenses of 

quartzite mapped as Westboro on the State bedrock 

map, trending northeast through Grafton towards Westborough, are on strike with the Plainfield in the WebsterOxford area.

The lithology of the Plainfield, as described by Goldsmith (1966,1976) in the New London area, southeastern 

Connecticut, and by Harwood and Goldsmith (1971) and 

Dixon (1974) in eastern Connecticut, and its structural 

and stratigraphic position indicate that the Plainfield and 

Westboro are equivalent formations. For example, Nelson's sequence (1974; as given in table 3) approximates 

the three-part division of the Plainfield Formation suggested by Goldsmith (1976) in the New London area. 

However, this division of the Plainfield does not seem to 

carry northward into the Thompson quadrangle in northeastern Connecticut and northwestern Rhode Island 

(Dixon, 1974).

The Plainfield is truncated along most of the 

Connecticut-Rhode Island border by the Lake Char 

fault, but, in the New London area, the Plainfield is 

overlain by a suite of largely mafic metavolcanic rocks 

called the Waterford Group (Goldsmith, 1980). These 

relations are similar to those of the Westboro Formation 

(Zw) and overlying metavolcanic rocks (Zv) in Massachusetts. The base of the Plainfield is not known, but 

possibly gneiss and schist in the center of the Lyme 

Dome, referred to earlier, lie below the Plainfield.

The Plainfield is believed to be equivalent to the 

quartzite (Zbq) and schist (Zbs) in the Blackstone Group 

because of its similar lithology and structural relations 

with the plutonic rocks of the Rhode Island batholith.

BLACKSTONE GROUP: MICA SCHIST AND PHYLLITE (Zbs) 

AND QUINNVILLE QUARTZITE (Zbq)

The Blackstone Group, the Blackstone Series of Woodworth (in Shaler and others, 1899, p. 106; Quinn and 

others, 1948, 1949), was named for exposures along the
E6 THE BEDROCK GEOLOGY OF MASSACHUSETTS

FIGURE 2. Stratigraphic units in the Milford-Dedham zone.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E7

EXPLANATION

Tertiary sediments

Cretaceous sediments

Sedimentary and volcanic rocks

Altered amygdaloidal basalt

Red arkosic conglomerate, sandstone, and siltstone

Dighton Conglomerate

Rhode Island Formation

Conglomerate, sandstone, and graywacke in the Rhode Island Formation

Wamsutta Formation

Rhyolite and mafic volcanic rocks in the Wamsutta Formation

Pondville Conglomerate

Bellingham Conglomerate

Cambridge Argillite

Roxbury Conglomerate

Melaphyre in the Roxbury Conglomerate

Newbury Volcanic Complex

Micrographic rhyolite

Undivided sedimentary and volcanic rocks

Upper member

Porphyritic andesite

Lower member 

Lynn Volcanic Complex

Green Lodge Formation of Rhodes and Graves (1931) 

Braintree Argillite and Weymouth Formation 

Hoppin Formation 

Mattapan Volcanic Complex 

Felsic and mafic volcanic rocks 

Plainfield Formation 

Westboro Formation 

Blackstone Group

Greenstone and amphibolite Includes Hunting Hill Greenstone

Quinnville Quartzite

Undivided

Mica schist and phyllite Includes Mussey Brook Schist and Sneech Pond Schist 

Metamorphosed mafic to felsic flow, volcaniclastic, and hypabyssal intrusive rocks 

Metamorphosed felsic volcanic rocks 

Gneiss and schist near New Bedford 

Biotite gneiss near New Bedford 

Marks a fossil-bearing unit 

Intrusive rocks 

Diabase dikes and sills 

Massive quartz and silicified rock

Alkalic granitic rocks

Includes Peabody Granite (Dpgr); Wenham Monzonite (Dwm); Cherry Hill 

Granite (Dcygr); granite of Rattlesnake Hill pluton (Drgr)

Alkalic granitic and gabbroic rocks

Includes Blue Hills Granite Porphyry (SObgr); Cape Ann Complex (SOcgr, 

SOcsm, SOcb); Quincy Granite (SOqgr); alkalic granite in Franklin (DOgr); 

Nahant Gabbro (Ongb)

Serpentinite

Batholithic rocks

Includes biotite granite (Zgr), Milford Granite (Zmgr, Zmgd); Hope Valley Alaskite 

Gneiss (Zhg); alaskite (Zagr); Scituate Granite Gneiss (Zsg); Esmond Granite 

(Zegr); Topsfield Granodiorite (Ztgd); Grant Mills Granodiorite (Zgmgd); 

Dedham Granite (Zdgr, Zdngr); Westwood Granite (Zwgr); fine-grained granite 

and granite porphyry (fgr); granite of the Fall River pluton (Zfgr); Ponaganset 

Gneiss (Zpg); porphyritic granite (Zpgr); granite, gneiss, and schist, undivided (Zgg)

Mafic rocks

Includes diorite at Rowley (Zrdi); diorite (Zdi); diorite and gabbro (Zdigb); 

gabbro (Zgb); Sharon Syenite (Zssy)

FIGURE 2. Continued.

Blackstone River in northern Rhode Island. The Blackstone as defined by Quinn and others (1948; table 3) 

consists of three lower, largely metasedimentary units, 

the Mussey Brook Schist (Zbs), Quinnville Quartzite

(Zbq), and Sneech Pond Schist (Zbs), and an upper, 

largely metavolcanic unit, the Hunting Hill Greenstone 

(Zbv). Only the lower metasedimentary part of the 

Blackstone Group is correlated with the Westboro and
E8 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 2. Nomenclature of stratified metamorphic rocks older than the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths

(stratigraphic order only approximate)

Used on State bedrock map Previously published nomenclature

Metamorphosed mafic to felsic flow, volcaniclastic, and hypabyssal 

intrusive rocks (Zv). Metamorphosed felsic volcanic rocks (Zvf).

Blackstone Group (Zb): 

Greenstone and amphibolite (Zbv) 

Mica schist and phyllite (Zbs) 

Quinnville Quartzite (Zbq) 

Mica schist and phyllite (Zbs)

Westboro Formation (Zw)

Plainfield Formation (Zp)

Westboro Formation (Zw)

Gneiss and schist near New Bedford (Zgs)

Biotite gneiss near New Bedford (Zgn)

Burlington Formation

Greenleaf Mountain Formation

Unnamed gneiss and quartzite

Claypit Hill Formation

Cherry Brook Formation

Kendall Green Formation

Middlesex Fells Volcanic Complex

Amphibolite, mixed rocks, banded volcanic rocks, biotite-hornblende

schist (Volckmann, 1977) 

Rice Gneiss

Blackstone Group: 

Hunting Hill Greenstone 

Sneech Pond Schist 

Quinnville Quartzite 

Mussey Brook Schist

Westboro Formation

Plainfield Formation*

Unnamed stratified remnants (Bell and Alvord, 1976)

(new unit)

(new unit)

"The Plainfield extends into the Webster-Oxford area of Massachusetts from eastern Connecticut and western Rhode Island and is equivalent to the Westboro in Massachusetts.

Plainfield Formations (fig. 6). The base of the Blackstone 

Group is not known.

The Quinnville Quartzite (Zbq) (table 3) was earlier 

called Westboro Quartzite by Emerson (1917) and 

included an Albion Schist Member (of the Quinnville). 

Other schist in the area distant from the quartzite beds 

and an interbedded marble were included by him in the 

Marlboro Formation, as was the greenstone. Quinn 

(1971) thought the quartzite was not equivalent to the 

Westboro in the type area; he named the quartzite 

Quinnville and schists above and below it Sneech Pond 

and Mussey Brook, respectively. The schists (table 3) 

contain similar assemblages of rock, and recent mapping 

by Drier and Mosher (1981) in the Blackstone River area 

northwest of Pawtucket (fig. 4) indicates no basis for 

distinguishing the two schist units. Possibly the Quinnville is a quartzitic lens in the sequence, or possibly the 

schists are repeated by folding. The schist units, including the marble beds, are combined into a unit of mica 

schist and phyllite (Zbs) on the State bedrock map. This 

unit and the Quinnville Quartzite (Zbq) are shown only in 

their type areas. Outside those areas, the Blackstone 

Group consists of rock believed to be equivalent to the 

Quinnville Quartzite and the mica schist and phyllite but 

which cannot be distinguished separately, and the Blackstone is therefore shown as undivided (Zb) on the State 

bedrock map. The Blackstone also includes, in places, 

layers, dikes, and sills of greenstone or amphibolite 

believed to be equivalent to the greenstone and amphibolite of the Hunting Hill Greenstone (Zbv), which is also 

shown only in its type area. The amphibolite layers, 

where present outside the type area, are usually interlayered with quartzite or schist and are not volumetrically large enough to show at the scale of the map.

The undivided Blackstone Group (Zb) extends along 

the Blackstone River into Massachusetts in the towns of 

Blackstone, Milford, and Uxbridge (figs. 4, 8). It is also 

present to the north in the Franklin and Medway areas, 

where it appears as disconnected segments of phyllite 

(see below) and locally quartzite in the plutonic rocks. 

West of Blackstone, the Blackstone Group is difficult to 

divide because outcrops are poor and quartzite and 

amphibolite are extensively interlayered. A large area of 

poorly exposed amphibolite, hornblende gneiss, quartzite, and minor rusty-weathering feldspathic mica-quartz 

schist occupies a belt from the town of Blackstone north 

through Hopedale and Milford, mainly west of the Mill 

River. A similar belt of Blackstone (Zb) extends south 

from Nipmuck Pond, southwest of Mendon. The Blackstone in both these areas lies largely in valleys. In places 

sheets of Milford Granite have intruded the Blackstone, 

producing layered migmatite as on Bear Hill, 1 km 

southeast of Milford. Southeast of East Douglas, quartzite is exposed in a large area of undivided Blackstone, 

which has the form of a large fold nose. Rocks included in 

the undivided Blackstone Group (Zb) include amphibolite 

interlayered with quartzite mapped by Shaw (1967) in 

the Milford area and by McKniff (1964) in the Blackstone
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS

42°45' -

42°30' -

EXPLANATION

Contact Dotted where concealed 

Fault Dotted where concealed

FIGURE 3. Stratigraphic units in the Milford-Dedham zone north and northwest of the Boston basin. Unit designations as on figure 2.

area, small areas of amphibolite containing epidote pods 

and stringers, and epidote-biotite-hornblende gneiss in 

the Franklin and Wrentham areas. The rocks mapped by 

Shaw and McKniff were considered by them to be 

correlative with the Hunting Hill Greenstone.

East of the Bellingham basin (also called the Woonsocket basin), the Blackstone Group (Zb) mostly lacks

quartzite and consists mainly of phyllite and schist. 

Gray-green laminated phyllite and schist, locally 

ankeritic, are conspicuous. Also present are epidotechlorite-biotite schist and small masses of metadiorite. 

Some of the schists are feldspathic and are subporphyritic metavolcanic rocks. A conglomerate containing lenticular quartzite pebbles and cut by thin sills and dikes of
E10 THE BEDROCK GEOLOGY OF MASSACHUSETTS

71°45'

42°15' -

42°00' -

^Y/M-w a£ > ///f-,V>v,-Tzb.v/:y^

MASSACHUSETTS? 

RHODE^ ISLAND

.-- . Pawtuckel >'-;-^^:;- ;^-:: v-'

FIGURE 4. Stratigraphic units in the Milford-Dedham zone west and southwest of the Boston basin. Unit designations as on figure 2.

greenstone crops out on Bound Road, just south of the 

Massachusetts-Rhode Island State line, west of Woonsocket. A similar conglomerate and clean quartzite crop 

out sporadically in a zone extending from this area north 

to Framingham and Natick.

The boundary between rocks mapped as Blackstone 

Group and rocks mapped as Westboro is arbitrarily 

drawn as mentioned above, on the basis of proximity to 

their respective type areas. For example, small areas of 

quartzite near Walpole, Mass. (figs. 4, 8), have been
42°00

STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS

71°00' 70°45'

Ell

41°45' 2.

41°30

* .. '|..'*j;.';.';' !'. .' _'.'. 'Pr- '. ; -';:;';.'.' : '-.*s^". \ /:- f'.i\.Assauiornps& 

:' /.' ;:'. ' '.;','; . -." "..' : '. ." ." ' '' .' .' ' .'. /.'yC^J/'.-;' /'ifond. .''-. 

. - .' '. ' :;'. ' ;V':V. Great. 

 ' »-"'-Quitacas

^UM^ , ^ ^ r- ^N^r -7

, ' r-/ A V -7 -7 ^ v . 7 >\

FIGURE 5. Stratigraphic units in the Milford-Dedham zone in the New Bedford area, southeastern Massachusetts and

adjacent Rhode Island. Unit designations as on figure 2.
H i ' 

to

Emerson 

(1917)

Marlboro 

Formation 

and gneiss 

and schist of 

undetermined 

age

Westboro 

Quartzite

La Forge 

(1932)

Marlboro 

Formation

Woburn 

Formation

Westboro 

Quartzite

Quinn 

(1971)

 )

Blackstone Series

Hunting 

Hill 

Greenstone

Sneech Pond 

Schist

Quinnville 

Quartzite

Mussey 

Brook 

Schist

Nelson 

(1974)

(Marlboro 

Formation)

fa u It 

Claypit 

Hill 

Formation

Cherry 

Brook 

Formation

Kendall 

Green 

Formation

Westboro 

Quartzite

Rice Gneiss

Bell and Alvord 

(1976)

(Marlboro 

Formation)

fault 

Burlington Formation

Greenleaf Mountain 

Formation

Unnamed gneiss 

and quartzite

Middlesex 

Fells 

Volcanic 

Complex

Protoclastic 

rocks

Westboro 

Formation

Unnamed 

1 stratified 

remnants

Volckmann 

(1977)

Cherry Brook 

Formation 

amphibolite; 

mixed rocks; 

banded volcanic 

rocks; biotitehornblende 

schist

Westboro 

Quartzite

1 Rice Gneiss

Hepburn and 

DiNitto (1978)

(Marlboro 

Formation)

fault 

fault 

Westboro 

Formation

Biotite 

schist 

member

Geologic map of Massachusetts 

(Zen and others, 1983)

(Marlboro Formation (OZm)) 

fa u It 

Metamorphosed mafic to felsic 

flow, volcaniclastic, and hypabyssal 

intrusive rocks (Zv); includes 

diorite and gabbro (Zdigb) 

north and northwest of Boston; 

metamorphosed felsic volcanic 

rocks (Zvf); greenstone and 

amphibolite (Hunting Hill 

Greenstone) (Zbv) of the Blackstone 

Group, and undivided Blackstone 

Group (Zb), in part

Westboro Formation (Zw); 

Plainfield Formation (Zp); 

Quinnville Quartzite (Zbq); 

mica schist and phyllite of 

the Blackstone Group (Zbs); 

undivided Blackstone Group 

(Zb), in part

Rice Gneiss included in Zv; 

unnamed remnants not shown 

on map

This 

chapter

Metavolcanic assemblage

Quartzitic assemblage

O OO 

H 

OO O

co CG

fe

FIGURE 6. Stratified rocks older than Proterozoic Z granitoids in eastern Massachusetts and northern Rhode Island.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E13

Newport area, Rhode Island

Past and Skehan (1981)

Price Neck 

Formation

Newport 

Formation

Quinn(1971)

Volcanic tuff, 

conglomerate, 

and quartzite r 

of Newport 

vicinity

r

r*

rs

Slate and quartzite 

of Newport vicinity

Sakonnet River area, 

Rhode Island

Quinn (1971)

Mica-chlorite schist 

of Sakonnet; 

chlorite-biotite schist 

of Tiverton; mica 

schist of Bristol

r

r 

r

rNew Bedford area, Massachusetts

Geologic map of Massachusetts 

(Zen and others, 1983)

Gneiss and schist 

near New Bedford 

(Zgs)

? ? ? ?

Biotite gneiss 

near New Bedford 

(possibly a facies 

of above) (Zgn)

FIGURE 7. Stratified rocks older than Proterozoic Z granitoids in southeastern Massachusetts and southern Rhode Island.

mapped as Westboro Quartzite by Volckmarm (1977) but 

are shown as undivided Blackstone Group (Zb) on the 

State bedrock map because they are near the area of 

Blackstone to the south. Outcrops of quartzite near 

Hopkinton, Mass. (fig. 4), adjacent to large areas of 

Westboro Formation in the Framingham area, are 

shown as Westboro Formation.

Small areas consisting largely of phyllite near Franklin 

and Medway have been assigned to the Blackstone 

Group. Tan, crinkled phyllite and phyllitic metawacke 

crop out west of a sliver of Dedham Granite on Pond 

Street, Norfolk, 1.5 km west of Pondville in the middle of 

the Norfolk basin. The Wamsutta Formation of Lower to 

Middle Pennsylvanian age to the west is unmetamorphosed. Greenish-gray, calcite-bearing, locally pyritic 

phyllite and metasandstone are exposed in roadcuts on 

1^195 at the Maple Street overpass, 2 km northeast of 

Bellingham. Gray felsite exposed in these cuts is probably a continuation of an extensive area of felsite to the 

south on the flanks of the Bellingham basin. Phyllite 

crops out on the banks of the Charles River south of the 

Lincoln Street Bridge at Medway.

The correlation of the phyllite in the Franklin and 

Medway areas is uncertain. On the one hand, its composition and grade of metamorphism are similar to those of 

pelite in the type area of the Blackstone Group in Rhode 

Island and its degree of metamorphism contrasts with 

the lack of metamorphism in the nearby Wamsutta 

Formation. Thus I have shown it as Blackstone Group 

(Zbs) on the State bedrock map. On the other hand, its 

association with felsic volcanic rocks (Zfm; fig. 2), which

may be equivalent to the younger Mattapan Volcanic 

Complex (Zm), suggests that it might be a sedimentary 

part of that complex in a more metamorphosed regime 

than the Boston area.

PALEOENVIRONMENT OF DEPOSITION OF THE 

QUARTZITIC ASSEMBLAGE

The paleoenvironment of deposition of the quartzitic 

assemblage was probably the flank of a volcanic arc. 

Although the Plainfield and Westboro contain calcsilicate layers, and the Blackstone contains marble 

locally in northern Rhode Island, no carbonates of the 

shelf or platform type exist in the sequence. On the 

contrary, the Plainfield, Westboro, and lower part of the 

Blackstone are in sequence below, and are interlayered 

with, rocks of primarily mafic volcanic derivation (table 

3). This association suggests that the formations of the 

quartzitic assemblage were deposited on the margins of a 

probably mature volcanic arc, possibly of the continentalmargin type. Zircons from the Westboro indicate a much 

older source than the rocks now present in the DedhamMilford zone, and the zircons were probably derived from 

continental crust (Olszewski, 1980).

MET A VOLCANIC ASSEMBLAGE

In eastern Massachusetts, the assemblage of primarily 

mafic metamorphosed volcanic rocks (Zv, Zvf, Zbv) is 

associated temporally and spatially with gabbro, diorite, 

and syenite (Zgb, Zdigb, Zssy); together, these units are
E14 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 3. Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths 

[Areas described roughly follow a southwest-trending line, from around Boston, Mass., to northern Rhode Island]

Symbol 

on

State 

bedrock

map

Stratigraphic unit Description Protolith Thickness Metamorphic 

facies

Northeastern Massachusetts (Bell and Alvord, 1976)

Zw Burlington Formation

Zv

Zw

Zv

Greenleaf Mountain 

Formation.

Unnamed gneiss and 

quartzite.

Middlesex Fells 

Volcanic Complex.

Zw

Zw

Westboro Formation, 

upper part.

Westboro Formation, 

lower part.

Zw Unnamed stratified 

remnants.

Fine-grained, randomly interlayered impure 

quartzite, quartz-feldspar gneiss, micaquartz feldspar gneiss, amphibolite, 

metawacke. Metaconglomerate at top. Layers a few to several centimeters thick. [Age 

uncertain, Proterozoic Z to Ordovician.].

Fine-grained, thinly laminated, dark-green 

amphibolite and minor pale-green calcsilicate rock. Layers 1 cm thick.

Light-colored, fine-grained quartzite, 

feldspar-quartz gneiss, biotite gneiss and 

hornblende gneiss as xenoliths in Cape Ann 

Complex. Bedding obscure.

Fine-grained, dark-gray to black, foliated, 

thickly layered to massive amphibolite, and 

hornblende gneiss, pods and lentils of epidote common, locally pillowed; laminated 

amphibolite, rare quartzite and calc-silicate 

rock and light-gray metadacite. Tuff predominates in upper part, flows in lower 

part. An apparently conformable contact 

with the Westboro Formation is exposed in 

two places north of Boston.

Fine-grained, white to pale-gray quartzite 

and pale-green hornblende gneiss. Layers a 

few centimeters to 15 m thick.

At top, interlayered fine-grained quartzite, 

argillite, slate, and quartz-bearing calcsilicate rock in beds a few centimeters to 3 

m thick; in middle, massive white to palegray fine-grained quartzite in lenticular 

masses 5-60 m long; at bottom, massive 

quartzite, interbedded quartzite and micaceous quartzite. 3-5 m of reddish-brown 

biotite quartzite near basal contact.

Very fine grained, gray biotite gneiss in slablike xenoliths. Fine-grained quartzite, feldspathic quartzite, laminated light-gray 

gneiss containing minute biotite flakes. 

Forms inclusions in Dedham Granite.

Shallow marine to littoral 

sands, fine-grained mafic 

tuff, and volcaniclastic 

detritus. Metaconglomerate is a channel deposit, 

possibly unconformable 

on rest of unit.

Fine-grained marine tuff; 

some layers contain carbonate.

Epiclastic sediment and 

tuff.

Mafic flows and tuffs in 

marine environment. 

Amygdular and pillow 

structures evident.

Shallow marine and littoral 

sand, silt, and mud, 

partly calcareous, and 

airborne tuff.

Shallow marine and littoral 

sands, silt, and mud, 

partly calcareous.

Near-shore marine sand 

and volcaniclastic detritus.

300m Amphibolite.

225m Amphibolite.

<250 m Amphibolite.

1,500 m Greenschist.

680m

420m

Amphibolite.

Amphibolite.

150 m for bio- Obscured by 

tite gneiss; intrusion, 

thickness of 

quartzite 

not known.

Framingham and Natick area, Massachusetts (Nelson, 1974)

Zv Claypit Hill Formation Mostly dark-gray and greenish-gray, finegrained hornblende gneiss interlayered with 

two-mica gneiss locally containing garnet 

and sillimanite; biotite-hornblende gneiss, 

biotite gneiss, and thin beds of amphibolite. 

Layers 8 cm to 0.5 m thick. Unconformable(?) on Cherry Brook Formation.

Not given 460-610 m Amphibolite 

(sillimanitemuscovite).
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E15

TABLE 3. Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths Continued

Symbol 

on

State 

bedrock

map

Stratigraphic unit Description Protolith Thickness Metamorphic 

fades

Framingham and Natick area, Massachusetts (Nelson, 1974) Continued

Zv Cherry Brook Formation, upper part.

Zv Cherry Brook Formation, lower part.

Zv Kendall Green Formation.

Zw Westboro Quartzite

Felsic tuff

Zv Rice Gneiss

Predominantly amphibolite and minor intercalated biotite schist and quartzite. Amphibolite is fine to coarse grained, equigranular, 

thin to thick layered, locally massive, 

amygdaloidal, and pillowed.

Light- to pinkish-gray, massive felsic crystal Crystal tuff 

tuff and a few thin beds of medium-grained 

schist.

Very fine grained, light-tan, laminated felsic 

tuff interlayered with dark-greenish-gray, 

fine-grained tuff and discontinuous layers of 

quartzite. [A zone of strong shearing. Mylonite according to Castle and others (1976). 

Woburn Formation of LaForge (1932).].

Upper part, light-gray, thick to massively 

bedded quartzite, a few thin layers of biotite gneiss near base. Middle part, dark- to 

medium-gray, fine-grained biotite and hornblende gneiss and schist; relatively pure 

quartzite, feldspathic quartzite, and 

amphibolite; beds 3 cm to 1.5 m thick. 

Lower part, light-gray, thick-bedded 

quartzite in beds as much as 5 m thick and 

minor interbedded biotite gneiss.

Medium- to dark-gray, fine- to mediumgrained, equigranular to inequigranular biotite gneiss and schist, two-mica schist, and 

thin beds of quartzite and feldspathic 

quartzite.

Basaltic and minor andesitic 

tuffs and flows.

900-> 1,200 m Amphibolite.

290m

215m

Amphibolite. 

Not given.

Volcaniclastic detritus and 

beach sand.

980m Amphibolite.

Beach sand and tuffaceous 

detritus.

>760m Amphibolite.

Marlboro area, Massachusetts (Hepburn and DiNitto, 1978)

Zw Westboro Formation

Zw Westboro Formation, 

(lower) biotite schist 

member.

Light- to dark-gray, tan, and pinkish-gray Not given 

feldspathic quartzite, orthoquartzite, and 

micaceous quartzite; massive with thin 

micaceous partings to well bedded in beds a 

few centimeters to 0.5 m thick. Gray 

quartz-rich mica schist. Calc-silicate quartzite and granofels in layers and lenses as 

much as 0.5 m thick. Light-gray muscovitequartz schist and dark-gray rustyweathering biotite-quartz-feldspar schist. 

Amphibolite and biotite amphibolite in beds 

as much as 1 m thick.

Dark-gray, fine- to medium-grained biotite Not given 

schist and interlayered minor, thin, impure 

quartzite. Subordinate biotite amphibolite, 

amphibolite, quartz-rich biotite schist, and 

biotite-plagioclase gneiss.

Not given Amphibolite.

Not given Amphibolite.
E16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 3. Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths Continued 

[Areas described roughly follow a southwest-trending line, from around Boston, Mass., to northern Rhode Island]

Symbol

on 

State Stratigraphic unit Description 

bedrock

map

 ,,.,, ml . , Metamorphic Protolith Thickness , . * facies

Holliston and Medfield areas, Massachusetts (Volckmann, 1977)

Zv Cherry Brook Formation.

Zv Amphibolite

None Mixed rocks

Interlayered blastoporphyritic to equigranular 

amphibolite, hornblende-biotite gneiss and 

hornblende-biotite-quartz gneiss; contains 

relic clasts and bombs as much as 2.5 m in 

diameter; layers a few centimeters to 10 m 

thick. These rocks overlie the Westboro 

Quartzite. [Appears to be equivalent to 

only the upper part of the Cherry Brook 

Formation of Nelson (1974).].

Amphibolite and hornblende gneiss containing Not given 

thin layers of alternating felsic and mafic 

constituents; epidote porphyroblasts common; local intercalations of microcline-rich 

quartz-feldspar rock. [Probably equivalent 

to part of Cherry Brook Formation of Nelson (1974).].

Interlayered metamorphosed blastoporphyritic andesitic tuff, biotite-epidoteplagioclase schist, chlorite schist, and massive amphibolite.

Chaotically bedded volcanic 

flows, breccias, and tuffs.

480m Amphibolite.

Not given Amphibolite.

Assorted volcanic and volcaniclastic rocks.

Zv Banded volcanic rocks

None Biotite-hornblende 

schist.

Zw Westboro Quartzite

Zv Rice Gneiss

Biotite-hornblende gneiss in alternate thin (1 Volcanic rocks 

mm to 2 cm) layers of mafic and felsic minerals; interleaved with light-colored biotite 

gneiss, feldspathic quartzite, and massive 

black porphyritic rock containing andesine 

laths as much as 2 cm in length. [Possibly 

equivalent to part of Cherry Brook Formation of Nelson (1974).].

Biotite-hornblende (-epidote) schist and Not given 

gneiss, biotite gneiss; indistinctly layered. 

[Stratigraphic correlation uncertain.].

Massive to thick-bedded orthoquartzite, minor Not given 

interbedded quartz-muscovite schist, and 

biotite-hornblende gneiss. Intruded by 

hornblende gabbro and Milford Granite.

Interlayered light- to medium-gray and Not given 

greenish-gray biotite gneiss and schist 

showing a swirled foliation, quartz-feldspar 

gneiss, epidote-biotite schist, and calcsilicate gneiss. Intruded by diorite and Milford Granite.

2,700 m

900m

Amphibolite 

partly retrograded to 

greenschist 

facies.

Amphibolite.

1,200 m

750m

1,600 m

Amphibolite. 

Amphibolite.

Amphibolite.

Northern Rhode Island (Quinn, 1971)

Zb Blackstone Series 

Zbv Hunting Hill Greenstone.

Dark-green, fine-grained greenstone, locally 

pillowed; contains knots and veins of epidote; local pyroclastic texture. Rare serpentine and steatite. [Drier and Mosher (1981) 

recognized thin quartzite (chert?) and volcaniclastic layers as well as flows.].

Basaltic and minor andesitic 

flows and tuffs. Intrusive 

dikes and sills.

800-1,200(7) m Greenschist 

near Pawtucket, 

R.I.;

amphibolite 

in the Milford, 

Mass., 

area.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E17

TABLE 3. Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths Continued

Symbol 

on

State 

bedrock

map

Stratigraphic unit Description Protolith Thickness Metamorphic 

facies

Northern Rhode Island (Quinn, 1971) Continued

Zbs Sneech Pond Schist

Zbq Quinnville Quartzite

Zbs Mussey Brook Schist

Greenish-gray, fine-grained, thin-bedded 

chlorite-quartz schist, thin-bedded quartzite, marble, greenstone, amphibolite schist, 

feldspathic mica schist, and serpentine.

Light-gray, clean, massive, medium-grained 

quartzite and minor thin beds of light-gray 

to greenish-gray quartz-mica schist.

Same as Sneech Pond Schist. [Drier and 

Mosher (1981) recognized only one schist 

unit in the Blackstone. The Mussey Brook 

is, then, equivalent to the Sneech Pond 

Schist.].

Marine sand and silt, and 

mafic and intermediate 

volcanic and volcaniclastic 

material.

Mature quartz sand and 

interbedded silt and clay.

Same as Sneech Pond 

Schist.

2,600 m

1,100 m

Same as 

Sneech 

Pond Schist.

Amphibolite 

and greenschist.

Greenschist.

Same as 

Sneech 

Pond 

Schist.

here called the mafic volcanic-plutonic complex in eastern 

Massachusetts. This complex was subsequently intruded 

by the Dedham Granite (Zdgr) and related Proterozoic Z 

granitoids. The metavolcanic rocks have been given 

different names in different areas (table 3), but they 

appear in general to belong to one episode of Proterozoic 

Z volcanism and plutonism.

The stratigraphic positions of many of the assemblages 

of metavolcanic rocks in eastern Massachusetts are difficult to determine because of the complex fault pattern, 

which has broken the terrane into numerous wedges and 

blocks; the lack of topping evidence; and extensive 

igneous intrusion. On the State bedrock map, the named 

and unnamed metavolcanic rocks in eastern Massachusetts (table 2) are combined into a single unit of metamorphosed mafic and felsic flow, volcaniclastic, and 

hypabyssal intrusive rocks (Zv). Mappable areas of metamorphosed felsic rock (Zvf) that appear to be older than 

the Dedham and Milford Granites are identified by a 

separate symbol. Greenstone and amphibolite (Hunting 

Hill Greenstone) (Zbv) of the Blackstone Group in Rhode 

Island are considered to be the equivalent of the mafic 

metavolcanic unit in Massachusetts.

Some of the rocks in the metavolcanic assemblage 

were included in the Marlboro Formation by Emerson 

(1917). The name Marlboro is not now applicable east of 

the Bloody Bluff fault. The mafic volcanic-plutonic complex of eastern Massachusetts is intruded by Proterozoic 

Z granitoids and lies east of the Lake Char-Bloody Bluff 

fault zone, whereas the Marlboro Formation is intruded 

only by Paleozoic granitoids and lies west of the Lake 

Char-Bloody Bluff fault zone.

METAMORPHOSED MAFIC TO FELSIC FLOW, 

VOLCANICLASTIC, AND HYPABYSSAL INTRUSIVE ROCKS

(Zv)

The metamorphosed mafic to felsic flow, volcaniclastic, 

and hypabyssal intrusive rocks (Zv) (referred to hereafter as mafic metavolcanic rocks) as shown on the State 

bedrock map include primarily the Middlesex Fells Volcanic Complex (Bell and Alvord, 1976) and the Cherry 

Brook Formation (Nelson, 1974) (table 3, fig. 6). The 

mafic metavolcanic rocks include also the Claypit Hill 

Formation and unnamed metavolcanic rocks mapped by 

Nelson (1975a,b) in the Natick and Framingham areas 

and by Volckmann (1977) in the Medfield and Holliston 

areas; biotite-hornblende schist southwest of Holliston 

(Volckmann, 1977); and the unnamed gneiss and quartzite and Greenleaf Mountain Formation mapped by Bell 

and Alvord (1976) in the Burlington area. Widely scattered small masses of metavolcanic rock north and east of 

the Narragansett basin, including the greenstone at 

North Plympton, are also included in the mafic metavolcanic unit.

North and northeast of Boston in the Salem and 

Danvers area (fig. 3), small masses of the mafic metavolcanic rocks (Zv) amphibolite, chlorite schist, and augen 

gneiss mapped by Toulmin (1964) as Marlboro Formation are engulfed in gabbro and diorite (Zdigb), primarily 

Salem Gabbro-Diorite (Toulmin, 1964; Wones and Goldsmith, this vol., chap. I). Near the Bloody Bluff fault in 

the Reading area and northeast in Georgetown, Ipswich, 

and Rowley are more extensive areas of mafic metavolcanic rock (Zv), primarily amphibolite, and less mafic 

plutonic rock. Those in the Reading area were mapped as
E18 THE BEDROCK GEOLOGY OF MASSACHUSETTS

42°30'

71°30' 71°00 r 70°45'

42°00'

Zbs

FIGURE 8. Stratigraphic units in and around the Boston basin. M, Milton. Unit designations as on figure 2. Jd shown on islands east of Boston

intrudes RiZc.

Marlboro Formation (Emerson, 1917; Castle, 1964). 

Those in the Georgetown area were included in the 

Middlesex Fells Volcanic Complex in the regional synthesis by Bell and Alvord (1976). Parts of this general 

area, particularly the Danvers area, where plutonic and 

volcanic rocks are mixed could have been more appropriately shown on the State bedrock map as an undifferentiated volcanic-plutonic complex (volcanic-plutonic complex in eastern Massachusetts).

The Kendall Green Formation as used by Nelson (1974; 

table 3") is included in the mafic metavolcanic rocks (Zv) 

on the State bedrock map. The protolith of the Kendall 

Green Formation (Kendall Green slate of Hobbs, 1899; 

Woburn Formation of LaForge, 1932; fig. 6) has been the 

subject of several interpretations in recent years. Nelson

(1974) considered that the Kendall Green represented 

thinly layered metamorphosed felsic and mafic tuffs. Bell 

and Alvord (1976) considered the rock to be a protoclastic 

phase of the Ordovician and Silurian Cape Ann Complex. 

Castle and others (1976) considered the rock to be a 

mylonite in the Bloody Bluff fault zone. I think all three 

interpretations are partly correct. The rock was originally either layered felsic and mafic tuffs or sheets of 

felsic rock intruded into the mafic pile, which have been 

smeared out into thinner layers and laminae within the 

Bloody Bluff and other fault zones in the WestonWaltham area west of Boston. Corresponding rock in a 

similar Stratigraphic position can be seen in southeastern 

Connecticut, where thinly interlayered fine-grained 

Hope Valley Alaskite Gneiss and (hornblende)-biotite-
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E19

quartz-plagioclase rock have been milled down in a splay 

of the Honey Hill fault zone to a fine-grained blastomylonite consisting of alternating thin dark and light layers, 

lenses, and discontinuous laminae. This blastomylonite 

lies near the base of a predominantly amphibolitic part of 

the metavolcanic-plutonic Waterford Group (Goldsmith, 

1980). I consider the Kendall Green Formation of Nelson 

part of the mafic metavolcanic assemblage (Zv) rather 

than a valid formation; it is merely a zone rendered 

distinctive through sheetlike intrusion and cataclasis. In 

eastern Connecticut and adjacent Rhode Island and 

Massachusetts, felsic layers may be intrusions of Hope 

Valley or may be felsic volcanic rock that is part of the 

carapace for the Sterling Plutonic Group and Milford 

Granite of the Rhode Island batholith. In the Framingham area, mappable zones of felsic volcanic rock (Zvf) 

interpreted in the latter way are shown on the State 

bedrock map.

The Claypit Hill Formation and the group of three 

units of Bell and Alvord (1976) above their Middlesex 

Fells Volcanic Complex (fig. 6) in the Burlington area are 

less certainly part of the mafic metavolcanic assemblage 

(Zv) but have been included in it on the State bedrock 

map. Nelson (1974) suggested that the Claypit Hill is 

unconformable on the Cherry Brook Formation, on the 

basis that attitudes of layering in the Claypit Hill were 

discordant to those in the Cherry Brook, but he did not 

discount the possibility that the contact is a fault. The 

Claypit Hill resembles the Marlboro Formation in the 

Nashoba zone to the northwest (Goldsmith, this vol., 

chap. F), and it may be a slice of Marlboro caught in the 

Bloody Bluff fault zone. The Claypit Hill, according to 

Nelson's map (1975b), is intruded by Proterozoic Z 

granite, however, placing it clearly within the MilfordDedham zone.

Two of the three formations exposed in the Greenleaf 

Mountain area, Burlington (fig. 3; Bell and Alvord, 1976; 

table 3), are included in the mafic metavolcanic assemblage (Zv). These are an unnamed gneiss and quartzite 

and the Greenleaf Mountain Formation. The third formation, the Burlington Formation, has been assigned to 

the quartzitic assemblage on the State bedrock map. 

These formations form a lens between the Bloody Bluff 

fault on the west and outcrops of the Middlesex Fells 

Volcanic Complex on the east. These formations rest 

unconformably on the eroded surface of the Middlesex 

Fells Volcanic Complex although no actual contact has 

been seen (Bell and Alvord, 1976, p. 202-203). They are 

assigned to the mafic metavolcanic assemblage because 

they are near the metavolcanic rocks of the Middlesex 

Fells Volcanic Complex and because they contain biotite 

gneiss and amphibolite as major components. However, 

as discussed in the section on the quartzitic assemblage, 

the three units in the Burlington area should be assigned

to a separate suite of their own. They may actually lie in 

a fault sliver of the Bloody Bluff fault system. In contrast 

to the Claypit Hill Formation and Middlesex Fells Volcanic Complex, these formations are intruded not by the 

Proterozoic Z Dedham Granite but only by Paleozoic 

alkalic granite, and therefore they could be younger than 

Proterozoic Z. I did not attempt to correlate the Claypit 

Hill with these units (fig. 6), and their assignment to the 

mafic metavolcanic assemblage (Zv) on the State bedrock 

map is arbitrary.

Massive greenstone is exposed on the edge of the 

Narragansett basin west of Plymouth, at North Plympton (fig. 5). The rock is greenish gray and contains a 

few scattered, small, greenish-white phenocrysts. It 

has a weak but measurable cleavage. In thin section, the 

rock has a relict trachytic or felty texture in a flow 

pattern and contains scattered larger saussuritized plagioclase grains. Mineral constituents are light-green to 

yellowish-green amphibole, locally in aggregates as 

much as 2 mm in diameter, clinozoisite, olive-green 

chlorite, yellow epidote, and fine-grained accessory 

albite, oriented tabular flakes of leucoxene, and white 

mica. Quartz is dispersed but also forms a few aggregates that contain rutile. The greenschist-facies metamorphism of this rock is in contrast to the lack of 

metamorphism and cleavage in the adjacent Rhode 

Island Formation. The relationship can be seen in exposures on a farm 0.8 km northeast of North Plympton 

(Lyons, 1977, Plympton quadrangle).

Woodworth (in Shaler and others, 1899, p. 116) considered the greenstone near North Plympton to be of 

Pennsylvanian age on the basis of comparison with a 

similar rock cutting fine-grained "granitite" south of the 

main exposure of the felsite north of Plympton, and thus 

he considered it to be younger than the southeastern 

Massachusetts batholith. He correlated the greenstone 

with the felsite at Attleboro in the Wamsutta Formation 

(Pwv). However, the greenstone at North Plympton is 

altered, as are the plutonic rocks in the area, and has a 

cleavage and is thus unlike the adjacent Middle to Upper 

Pennsylvanian Rhode Island Formation. The rock is 

either pre-Dedham (and its equivalents), and part of the 

volcanic-plutonic complex in eastern Massachusetts, or it 

is para- and post-Dedham and correctable with the 

Mattapan Volcanic Complex. On the State bedrock map 

it is mapped as metamorphosed mafic metavolcanic rock 

(Zv).

The greenstone at North Plympton is somewhat similar to a flow-banded light-greenish-gray porphyry in 

ledges 1.3 km south of Rock. This porphyry contains 

saussuritized plagioclase phenocrysts as much as 1 cm in 

longest dimension and twinned green hornblende, locally 

clustered, in a matrix of chlorite, quartz, potassium 

feldspar, epidote, and rare biotite and sphene. Cleavage
E20 THE BEDROCK GEOLOGY OF MASSACHUSETTS

is not apparent in this rock. Its contact with adjacent 

granite is not exposed. This porphyry, not shown on the 

State bedrock map because of its small size, is considered 

to be the same age as the greenstone at North Plympton 

because it has the same greenschist-facies metamorphism.

BLACKSTONE GROUP: GREENSTONE AND AMPHIBOLITE 

(HUNTING HILL GREENSTONE) (Zbv)

The Hunting Hill Greenstone as described by Quinn 

(Quinn and others, 1948; summarized in Quinn, 1971; 

table 3) is quite similar to the Cherry Brook Formation 

(Nelson, 1974) and Middlesex Fells Volcanic Complex 

(Bell and Alvord, 1976) and is in a similar stratigraphic 

position. In the type area in northern Rhode Island, 

northwest of Pawtucket, the Hunting Hill forms massive 

ledges, in some of which pillow structure can be discerned. The Hunting Hill Greenstone also forms sills and 

dikes in the mica schist and phyllite unit (Zbs) (Sneech 

Pond and Mussey Brook Schists) in northeastern Rhode 

Island (Rutherford and Carroll, 1981).

The term "greenstone and amphibolite" (Zbv) used on 

the State bedrock map was originally meant to include 

not only the Hunting Hill Greenstone in the type area but 

also amphibolite and other metavolcanic or metavolcaniclastic rocks of middle to high metamorphic grade in 

areas west and northwest of the Blackstone type area. 

However, the amphibolites in these areas could not be 

readily separated on the scale of the map from the rest of 

the Blackstone Group and so were included in the 

undivided Blackstone Group (Zb) rather than being 

mapped separately. Some of these rocks have been 

mentioned above in the description of the lower part of 

the Blackstone Group.

METAMORPHOSED FELSIC VOLCANIC ROCK (Zvf)

Metamorphosed felsic volcanic rock (Zvf) forms a 

subordinate part of the metamorphosed mafic and felsic 

volcanic rock assemblage (Zv). Mappable units of felsic 

metavolcanic rock are shown only in the Framingham 

area (figs. 4, 8) on the State bedrock map. The Cherry 

Brook Formation, as described by Nelson (1974; table 3), 

has a lower felsic part consisting of a metamorphosed 

massive crystal tuff containing a few layers of biotite 

schist. This felsic part was not recognized by Bell and 

Alvord to the northeast in the Medford area, where 

pillow lavas of the Middlesex Fells Volcanic Complex 

rest directly on quartzite (Bell and Alvord, 1976, p. 198), 

nor was an equivalent zone of felsic rock or interlayered 

felsic and mafic rock described by Quinn (1971) below the 

Hunting Hill Greenstone in the Blackstone Group. However, the felsic part of the Cherry Brook may be correlative with felsic layers described by Volckmann (1977) in

the units lying below the Cherry Brook Formation (table 

3). The Cherry Brook studied by Volckmann is equivalent to only the upper part of the Cherry Brook studied 

by Nelson. However, Nelson's crystal tuff is similar in 

composition and texture to, although finer grained than, 

mafic-mineral-poor felsic rocks north and east of the 

Milford Granite that Volckmann (1977) mapped as Milford Granite. I have shown the felsic rocks of Volckmann 

as Hope Valley Alaskite Gneiss on the State bedrock 

map. I could not separate out the felsic rocks of the 

Cherry Brook from the rest of the Cherry Brook for 

compilation as metamorphosed felsic volcanic rocks on 

the State bedrock map, but my reconnaissance in the 

Framingham area in preparation for compiling the State 

bedrock map convinced me that the northern belts of 

rock that Nelson (1975b) mapped as Milford Granite 

would be better shown as metamorphosed felsic volcanic 

rock (Zvf) because of their fine-grained texture and 

mafic-mineral-poor composition.

CORRELATION OF THE MAFIC METAVOLCANIC ROCKS 

(Zv) AND THE HUNTING HILL GREENSTONE (Zbv)

The Middlesex Fells Volcanic Complex, Cherry Brook 

Formation, Claypit Hill Formation, and Rice Gneiss 

have been included in the metamorphosed mafic and 

felsic volcanic rocks (Zv) on the State bedrock map. Bell 

and Alvord's (1976, p. 199-200) Middlesex Fells Volcanic 

Complex encompassed the rocks mapped by Nelson as 

Cherry Brook and Claypit Hill Formations as well as the 

mafic rocks in the upper part of the Blackstone Group 

(Bell and Alvord, 1976, p. 199-200) shown on the State 

bedrock map as greenstone and amphibolite (Hunting 

Hill Greenstone (Zbv)). The descriptions of the Middlesex Fells Volcanic Complex in the type area by Bell and 

Alvord (1976) and of the Hunting Hill Greenstone by 

Quinn and others (1948) are similar (table 3), and the 

units occupy a similar stratigraphic position. I agree with 

the correlation of these units.

I used the term "metamorphosed mafic to felsic flow, 

and volcaniclastic and hypabyssal intrusive rocks" (Zv) 

on the State bedrock map instead of Middlesex Fells 

Volcanic Complex because the type area of the Middlesex 

Fells is in a fault-bounded block unique in containing the 

Lynn Volcanic Complex (DZ1), and I was wary of extending the usage to the amphibolites and gneisses outside 

the block to the northeast and north, as well as to units 

which Nelson had already mapped in the Framingham 

area. Furthermore, the Middlesex Fells contains no 

felsic rocks like those in the Cherry Brook Formation. 

Bell and Alvord's (1976) usage of Middlesex Fells Volcanic Complex in a broad sense may be correct, but I 

thought it better to use an informal lithologic name on 

the State bedrock map pending further work.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E21

METAMORPHIC ROCKS OF SOUTHEASTERN 

MASSACHUSETTS

Layered schist and gneiss form arcuate and linear 

septa in the plutonic rocks of the southeastern Massachusetts batholith southeast of the Narragansett basin 

(fig. 5). Two types of rock are present, thinly layered 

gneiss and schist (gneiss and schist near New Bedford, 

Zgs, table 2) and thickly layered biotite gneiss (biotite 

gneiss near New Bedford, Zgn, table 2). They are 

exposed primarily in the New Bedford-Fall River area, 

but the poorly exposed terrane to the east in Plymouth 

and Barnstable Counties probably contains similar rocks. 

A deep drill hole near Harwich on Cape Cod (see fig. 15) 

encountered thinly layered phyllitic schist and subordinate limestone (Koteff and Cotton, 1962). The gneiss and 

schist near New Bedford (Zgs) and the biotite gneiss 

near New Bedford (Zgn) are described below in more 

detail than are rocks in other areas because no previous 

descriptions of them exist.

GNEISS AND SCHIST NEAR NEW BEDFORD (Zgs)

The most extensive exposures of the gneiss and schist 

near New Bedford (Zgs) are in cuts on 1-195 near New 

Bedford (fig. 5). Elsewhere only a few exposures were 

seen in road reconnaissance. The belt of gneiss and schist 

shown on the State bedrock map extending from Hixville 

to Brayleys is based on the trend of a valley, the 

magnetic pattern (U.S. Geological Survey, 1971a), and 

the strike of schist in one outcrop. Similarly, the belt 

trending southwest from the south part of Dartmouth 

across the East Branch of the Westport River is based on 

the strike of one outcrop and the magnetic pattern (U.S. 

Geological Survey, 1971b).

The gneiss and schist are thin-bedded to laminated, 

medium-gray to dark-greenish-gray rocks differing in 

proportions of feldspar, quartz, hornblende, biotite, and 

epidote; they locally contain muscovite. Amphibolite is 

rare. Rocks exposed on 1-195 west of New Bedford 

consist of garnet-biotite-quartz schist containing small 

calc-silicate pods and randomly oriented elongate black 

poikiloblastic hornblende, muscovite-green-biotiteplagioclase-quartz schist, locally containing pyrite, and 

layers of laminated epidote-biotite-two-feldspar schist 

or gneiss containing green biotite, clusters of green 

biotite and epidote, quartz, calcic (An20_29) oligoclase, 

minor hornblende, sphene, allanite, and microcline. Similar rocks are exposed in other places. Microcline-bearing 

phases tend to be more abundant near granitic contacts. 

The microcline in some of these layers is poikiloblastic 

and aggregated with plagioclase and quartz forming 

light-colored knots. Some layers are of coarse twofeldspar-quartz augen gneiss containing green biotite, 

accessory epidote, opaque minerals, and sphene. Others

are a splintery-weathering, gray, sugary-textured 

quartz-biotite-feldspar gneiss. A laminated, greenishgray schist near Center Village, Westport, contains 

plagioclase, biotite, calcite, epidote, allanite, opaque 

minerals, and rare scattered aggregates of quartz. 

Flanking the diorite at Acushnet is somewhat granitoid, 

medium-gray to dark-gray gneiss consisting of clusters of 

epidote, blue-green hornblende, and green biotite in a 

matrix of plagioclase and rare quartz. A lighter colored 

phase lacks hornblende and contains a minor amount of 

potassium feldspar. In a nearby quarry, a streaked and 

spotted rock flanking the diorite contains elongate palegreen hornblende prisms in a matrix of greenish-brown 

biotite locally clustered with fine-grained hornblende and 

epidote, saussuritized plagioclase, and quartz. In most of 

the rocks described above, the plagioclase is poorly 

twinned.

A septum of epidote-biotite schist containing quartz, 

calcic oligoclase, and accessory apatite and sphene is 

exposed near the Route 24 1-195 interchange at Fall 

River. Inclusions of similar rock are abundant in the 

band of porphyritic granite in that area. Similar schist 

forms inclusions in the porphyritic granite along Long 

Pond, north of Deans Point (fig. 5).

The textures and composition of the rocks suggest that 

their protolith was intermediate to felsic volcaniclastic 

rocks, primarily tuffaceous sediments and tuffs, deposited in an aqueous environment.

BIOTITE GNEISS NEAR NEW BEDFORD (Zgn)

Layered granitoid biotite gneiss (Zgn) forms a tightly 

arcuate band south of New Bedford along the north shore 

of Buzzards Bay (fig. 5). It is flanked on either side by 

gneissic alaskite. The biotite gneiss is best exposed at 

Fort Phoenix Park, Fairhaven, east of New Bedford (fig. 

5). Other exposures are at Wilbur Point and on U.S. 

Route 6, Mattapoisett, 200 m west of its junction with 

Mattapoisett Neck Road, Marion quadrangle.

The gneiss is thickly layered, gray, and relatively 

homogeneous in texture. Layers differ in proportions of 

feldspars and biotite; some layers are quite poor in mafic 

minerals. Typical layers are flecked with biotite and 

magnetite, giving the rock a salt-and-pepper appearance. 

Foliation is pervasive and approximately parallel to 

layering. It is marked by uniform orientation of the 

disseminated biotite flakes. Some dark-gray layers, however, contain elongate clots of biotite, sphene, and allanite. Some small pegmatitic streaks and patches contain 

only magnetite. Mineralogy is simple. The gneiss consists of plagioclase, quartz, biotite, microcline, and accessory sphene and allanite. The exposure at Mattapoisett 

contains about 20 percent biotite and augen of fleshcolored potassium feldspar as much as 1 cm long. The
E22 THE BEDROCK GEOLOGY OF MASSACHUSETTS

biotite gneiss is not well exposed in the hinge of the arc 

in Dartmouth. Exposures at Bayview, Dartmouth, consist of lineated and foliated inequigranular biotite gneiss 

containing flesh-colored feldspar 2.5-5 mm in diameter. 

Pegmatite masses in these outcrops contain muscovite as 

well as biotite, quartz, and two feldspars. Exposures in a 

playground off Front Street in southern New Bedford on 

strike with the Fort Phoenix exposures are of relatively 

massive granitoid biotite gneiss in which pegmatite 

masses have large books of biotite. Granitoid phases at 

Fort Phoenix approach in composition and aspect 

gneissic phases of the granite of the Fall River pluton. 

The biotite gneiss, however, is distinguished by its 

layered character and greater amount of biotite and 

magnetite.

The protolith of the biotite gneiss was probably a 

sequence of layered intermediate to felsic volcanic rocks, 

more likely flows and massive tuffs rather than waterlain 

volcaniclastic materials. Alternatively, the gneiss could 

represent metamorphosed sheeted intrusions.

Contact relations with the granite of the Fall River 

pluton are not clear. The biotite gneiss appears to be 

conformable with the adjacent gneissic alaskite and is in 

fact interlayered with it. On the one hand, both may be 

part of a plutonic or volcanic assemblage older than the 

granite of the Fall River pluton. On the other hand, both 

could be phases of the southeastern Massachusetts batholith but metamorphosed and deformed under higher 

pressure-temperature conditions than the batholith to 

the north and west.

CORRELATION

The gneiss and schist (Zgs) and the biotite gneiss (Zgn) 

near New Bedford are not well enough exposed to 

provide a good basis for correlation with rock units of 

apparently similar age in adjacent Rhode Island (fig. 7). 

However, on the basis of similar composition and bedding characteristics, they are probably equivalent to 

mica-chlorite schist and chlorite-biotite schist of Pollock 

(1964, p. D2) exposed at several places along the east 

side of the Sakonnet River in the Tiverton area. The 

mineralogy of the schist as described by Pollock (1964, p. 

D2-D3) is similar to that in layers in the gneiss and schist 

in the New Bedford area: "Chief constituents are quartz, 

biotite, and chlorite; muscovite, epidote, calcite, and 

hornblende are locally abundant. Minor and accessory 

constituents are albite, microcline, microperthite, zircon, 

apatite, sphene, magnetite, ilmenite, and garnet." Quinn 

(1971), following Pollock (1964), described the micachlorite schist on the Sakonnet River south of Tiverton 

near Browns Point (fig. 5) as light-gray to greenish-gray, 

thin-bedded mica-chlorite schist containing thin beds of 

marble and quartzite. The mica-chlorite schist is

intruded by the Bulgarmarsh Granite, which is here 

considered a phase of the Fall River pluton. Quinn (1971) 

described the chlorite-biotite schist mapped by Pollock 

(1964) near Tiverton Four Corners (fig. 5) as green to 

gray, fine-grained, poorly foliated chlorite-biotite schist 

containing subordinate layers of amphibolite, epidotehornblende schist, and quartzite. Quinn (1971) equated a 

pink- to light-gray, thin-bedded, lineated muscovitequartz-biotite schist at Bristol Neck, Rhode Island, 

northwest of Tiverton, with the rocks in the Sakonnet 

River area. The rocks in the Sakonnet River and Bristol 

areas described above are in the greenschist facies to 

epidote-amphibolite facies of metamorphism.

Not seen in the New Bedford area were the thin beds 

of limestone in the mica-chlorite schist near Browns 

Point, nor the quartzite at Tiverton Four Corners, nor a 

volcanic breccia that Pollock mapped on Gould Island in 

the Sakonnet River (fig. 5). Pollock tentatively correlated these rocks with the Blackstone Group, but Quinn 

(1971, p. 54) did not venture such a correlation. The mica 

schist at Bristol Neck, R.I. (Quinn, 1971, p. 23), resembles some of the gneiss and schist near New Bedford. It 

is reasonable to correlate the rocks mostly east of the 

Sakonnet River in Rhode Island, mentioned above, with 

Proterozoic Z volcanic and sedimentary rocks near Newport on Aquidneck and Conanicut Islands described by 

Kay and Chappie (1976), and more recently by Rast and 

Skehan (1981), because they are both intruded by Proterozoic Z granites. Rast and Skehan (1981) assigned the 

volcanic and sedimentary rocks to two formations. The 

lower, the Newport Formation, consists of graded graywacke, siltstone and pelite, and minor conglomerate, 

felsic volcanic rock, quartzite, calc-silicate rock, dolomite, and diamictite containing fragments of quartzite, 

calc-silicate rock, dolomite, and serpentine. They interpreted the formation as turbidite deposits that included 

minor felsic volcanic material. The upper formation, the 

Price Neck Formation, consists primarily of volcanogenie materials: agglomerate breccias (in part lahars), 

conglomerate, coarse tuff, thinly bedded tuff and laminated sediments; graded-bedded tuff, siltstone and slate; 

and possible rhyolite flows. Both formations are in the 

lower greenschist facies of metamorphism.

Correlation of the metavolcanic and metasedimentary 

rocks in the New Bedford area (Zgs, Zgn) with either the 

mafic metavolcanic rocks (Zv) or the quartzitic assemblage (Zp, Zw, Zbs, and Zbq) north and west of the 

Narragansett basin is uncertain. Mafic metavolcanic 

rocks are generally lacking in the New Bedford area of 

Massachusetts and the Sakonnet River area and Aquidneck and Conanicut Islands in Rhode Island. Volcanic 

rocks, where they do exist, tend to be felsic. A more 

likely correlation would be with the mica schist and 

phyllite (Zbs) of the lower part of the Blackstone Group
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E23

TABLE 4. Stratigraphic units of the Boston basin and vicinity 

[Descriptions and thicknesses adapted from Rehmer and Roy (1976)]

Unit on State bedrock map

Braintree Argillite and Weymouth Formation (Cbw).

Formation

Braintree Argillite 

Weymouth Formation

Preferred age

Middle Cambrian 

Early Cambrian

Description

Argillite and slate 

Argillite, slate, and limestone

Thickness 

(meters)

Not given. 

Not given.

Boston Bay Group 

Cambridge Argillite (ftZc)

Roxbury Conglomerate 

(ftZr).

Melaphyre in the Roxbury 

Conglomerate (ftZrb).

Mattapan Volcanic Complex (Zm); Lynn Volcanic 

Complex (DZ1).

Dedham Granite (Zdgr,

Zdngr). 

Metamorphosed mafic and

felsic volcanic rocks (Zv);

diorite and gabbro

(Zdigb).

Boston Bay Group 

Cambridge Argillite

Roxbury Conglomerate 

Squantum Member 

Dorchester Member 

Brookline Member

Mattapan Volcanic Complex (and Lynn Volcanic Complex).

Dedham Granite

Volcanic-plutonic complex 

in eastern Massachusetts.

Unconformity^?)

Proterozoic Z and

Early Cambrian(?). 

Proterozoic Z

Unconformity

Proterozoic Z

Disconformity(?)

Proterozoic Z 

Proterozoic Z

Argillite, slate, sandstone, 

and quartzite.

Diamictite

Argillite and sandstone 

Conglomerate, argillite, sandstone, and melaphyre 

(Brighton Melaphyre).

Felsite, tuff, melaphyre, and 

argillite.

Granite and granodiorite

Mafic plutonic and volcanic 

rocks.

>2,300 (from tunnel data).

20-180.

360.

150-1,300 (thins

abruptly to

south).

0-1,000?

Not applicable. 

Not applicable.

in northern Rhode Island or possibly the lower part of 

the Cherry Brook Formation (included in Zv) in the 

Framingham area. On the other hand, the gneiss and 

schist (Zgs) and the biotite gneiss (Zgn) near New 

Bedford and the rocks in the Sakonnet River area, Rhode 

Island, are not correctable directly with either of the 

major groups of Proterozoic Z, prebatholithic rocks north 

of the Narragansett basin.

AGE OF THE PREBATHOLITHIC ROCKS

A minimum age for the metamorphic rocks older than 

the Rhode Island and southeastern Massachusetts batholiths is established by the Proterozoic Z age (595-630 

Ma) of the plutonic rocks (Dedham Granite, Milford 

Granite) intrusive into them (Goldsmith, 1980; Zartman 

and Naylor, 1984; Zartman and Marvin, this vol., chap. 

J, table 1). A maximum age is indicated by a 1,500-Ma 

U-Pb age on detrital zircon from the Westboro Formation (Olszewski, 1980). It may be, however, that some 

quartzites now mapped as Westboro or Plainfield and 

some felsic volcanic rocks now included in the mafic 

metavolcanic rocks are actually younger than or equivalent to the batholithic rocks. Some of these particular 

problems will be addressed in a later section of this 

chapter, "Stratigraphic problems."

PROTEROZOIC Z ROCKS YOUNGER THAN OR

EQUIVALENT TO THE SOUTHEASTERN

MASSACHUSETTS BATHOLITH

Unmetamorphosed to weakly metamorphosed volcanic 

and sedimentary rocks of known or of possible Proterozoic Z to early Paleozoic age occupy and flank structural 

and stratigraphic basins in the Proterozoic Z crystalline 

complex of eastern Massachusetts and Rhode Island. 

Proterozoic Z volcanic rocks (Zm, ftZrb) and Proterozoic 

Z to Cambrian sedimentary rocks (ftZr, ftZc) have been 

identified in and around the Boston basin (fig. 8). Similar 

rocks flanking the Norfolk basin (Pwv), and in the 

Bellingham basin (PZb, Zm), may be similar in age to 

those in the Boston basin. In the Boston basin (table 4), 

the basal Mattapan Volcanic Complex has been shown to 

be of Proterozoic Z age (602±3 Ma; Kaye and Zartman, 

1980) and in places to intrude and to lie unconformably 

above the Dedham Granite (LaForge, 1932, p. 31).
E24 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Metasedimentary and metavolcanic rocks of the Boston 

Bay Group, formerly thought to be of Silurian to Devonian or Pennsylvanian age (Billings, 1979), overlie the 

Mattapan and are now considered to be of Proterozoic Z 

to Early Cambrian(?) age (Skehan, 1979; Kaye and 

Zartman, 1980; Skehan and Murray, 1980a; Billings, 

1982; Lenk and others, 1982). The volcanic rocks will be 

described first.

MATTAPAN AND LYNN VOLCANIC COMPLEXES (Zm, DZ1)

The volcanic rocks in the Boston basin area younger 

than the Dedham Granite include an earlier, primarily 

felsic suite (Mattapan (Zm) and Lynn (DZ1) Volcanic 

Complexes) and a later mafic suite (Brighton Melaphyre 

(RzZrb) of the Boston Bay Group) (table 4). The Mattapan 

is distributed in the west and southwest part of the 

Boston basin and beyond, and to the south in the Blue 

Hills (Chute, 1969; fig. 8); the Lynn is confined to a block 

north of the Boston basin between the northern border 

fault and the Walden Pond Fault of Bell and Alvord 

(1976, fig. 1). The Mattapan and Lynn Volcanic Complexes have similar lithologies. According to Kaye (1980), 

they consist largely of rhyolite and rhyodacite flows, in 

part porphyritic; welded ash flows; vitric, lithic, and 

lapilli tuffs; flow breccias; breccia pipes; and extrusion 

domes. LaForge (1932, p. 30-33) mentioned the presence 

of andesitic and basaltic rocks, some of them amygdaloidal. However, he may have been referring to the older 

Middlesex Fells Volcanic Complex cropping out in the 

area of the Lynn Volcanic Complex or to the younger 

Brighton Melaphyre. Rhyolites and rhyodacites in the 

Mattapan are thinner and less varied in composition and 

texture than those in the Lynn, and volcanic breccias are 

largely absent in the Lynn. In composition, the volcanic 

rocks tend to be sodic rather than potassic (Chute, 1966). 

The Mattapan contains thick zones of interbedded metasedimentary rocks ranging from laminated argillite with 

fine-scale graded bedding (Lyons and Goldsmith, 1983) 

to volcanic conglomerate. Some of the latter have been 

confused with the Roxbury Conglomerate, as noted by 

LaForge (1932, p. 34-35). A pinkish-red to red-maroon 

conglomerate, formerly considered to be Roxbury Conglomerate, containing mainly granite and rhyolite fragments interfingers with welded ash-flow tuff in the 

northwestern part of the Medfield quadrangle and is part 

of the Mattapan (Volckmann, 1977). Other kinds of 

volcanic rock have been noted elsewhere. Nelson (1974) 

described a laharic unit and andesitic rocks in the Mattapan in the Natick quadrangle. Volckmann (1977) 

described a quartz-latite crystal-vitric tuff in the Medfield quadrangle. The volcanic rocks in the Blue Hills are

masses of red, pink, purple, brown, and gray altered and 

devitrified rhyolitic flows, ash flows, and breccias (aporhyolite of Emerson, 1917).

The relative stratigraphic position of the Mattapan and 

Lynn is fairly clear. The Mattapan and Lynn are 

reported to lie nonconformably on the Dedham Granite 

and the volcanic-plutonic complex in eastern Massachusetts. LaForge (1932, p. 31) cited exposures near Medford and in the Saugus area where the Lynn overlies 

weathered plutonic rocks; it consists in one place of a 

basal arkose and in another of an agglomerate containing 

disintegrated granite. Bell (1976, p. 289) cited a location 

in the Saugus area where the Lynn overlies both the 

Middlesex Fells Volcanic Complex and diorite intruded 

into it. Mattapan dikes and stocks cut the Dedham 

Granite and other basement rocks (see Billings, 1976a, p. 

8; Kaye and Zartman, 1980, p. 258; Chute, 1966, p. B27). 

On the other hand, Zarrow (1978, cited by Naylor, 1981) 

described a mass of Dedham Granite at Pine Hill, Medford, that contains inclusions of Lynn. Kaye (Kaye and 

Zartman, 1980, p. 258) described two places where the 

Dedham grades upward through finer grained phases 

including granophyre into rhyolite and aphanitic rock 

resembling rhyolite. Chute (1966, p. B15) noted several 

exposures southwest of Dedham where a fine-grained 

phase of the Westwood Granite (Zwgr), which intrudes 

the Dedham Granite (Zdgr), resembles phases of the 

Mattapan. Thus it would seem that some of the Mattapan 

is or could be penecontemporaneous with younger phases 

of the Proterozoic Z Dedham batholith. In support of this 

contention, no cobbles of Westwood Granite have been 

observed in the Roxbury Conglomerate that overlies the 

Mattapan, so that it is possible that the Westwood is an 

intrusive equivalent of the extrusive Mattapan Volcanic 

Complex and was not exposed to erosion at the time of 

deposition of the Roxbury. Radiometric ages, discussed 

below in the section on ages of the Mattapan and Lynn 

Volcanic Complexes, permit the possibility of equivalent 

ages for the Mattapan and the Westwood.

The Mattapan is overlain by the Roxbury Conglomerate of the Boston Bay Group in what was described by 

LaForge (1932, p. 34) as a fairly continuous horizon. 

Billings (1929, p. 104) noted an angular relation between 

the Mattapan and the overlying Roxbury north of the 

Neponset River in Hyde Park and Mattapan, although 

elsewhere he observed that the contact seems to be 

conformable. The Roxbury Conglomerate contains pebbles and cobbles of the volcanic rocks. LaForge (1932, p. 

34-35) cautioned against possible confusion in identifying 

Roxbury Conglomerate because of the similarity of some 

of its layers containing the volcanic material to some of 

the sedimentary layers interbedded in the volcanic rocks 

of the Mattapan. However, the Boston Bay Group in
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E25

NORTH

THICKNESS

(IN METERS)

8000

7000

6000

5000

4000

3000

2000

1000

SOUTH

data

Cambridge Argillite

Squantum Member Milton quartzite unitof Billings (1976a)

Brookline Brighton Melaphyre 

Member .___ /

Mattapan Volcanic Complex

Dedham Granite

-i Roxbury 

^Conglomerate

Roxbury 

Conglomerate

Modified from Billings (1976a, figure 4)

0 5000 10,000 15,000 20,000 FEET

i i i I I

0 1000 2000 3000 4000 5000 METERS

EXPLANATION

(Descriptions modified from Rehmer and Roy (1976))

CAMBRIDGE ARGILLITE Fine-grained argillite. mostly argillaceous, some siltstone and tuff; typically 90 percent argillite. 

10 percent slightly calcareous feldspathic sandstone and quartzite. fossil acritarchs. Common pinch-and-swell bedding and 

small-scale crossbedding. oscillation and interference ripple marks, scour marks, graded bedding, slump structures and 

contorted zones, and load casts. Rhythmic bedding in one-half of the formation, beds 1.2-7.6 cm thick; pinch-and-swell in 

beds 7.5 mm thick. Color mostly gray to the north. 60 percent reddish to purplish gray and 40 percent gray or greenish gray 

to the south. Minimum thickness probably 2.300 m

ROXBURY CONGLOMERATE-Squantum Member: Diamictite. 50 to 63 percent matrix of silt and clay, locally sandy, 

some thin sandstone and argillite layers Pebble to boulder clasts: subrounded to angular, rarely striated, some faceted; 

mostly quartzite and granite, some felsite and argillite; average size 7 to 15 cm, common large size 1 m, rarely 6 m; some 

large argillite clasts bent and deformed Bedding obscure or unstratified Dropstones. slump structures, and contorted zones 

in argillite layers. Thickness 20 to 180 m Dorchester Member: Approximately 60 percent argillite. 25 percent sandstone, 

15 percent conglomerate, 1 percent tuff. Sand fine- to medium-grained, feldspathic; quartz grains rounded. Pebbles 

mostly quartzite. some granite, no argillite clasts; average maximum pebble size 14 cm. Crossbedding and ripple marks 

common. Bedding absent or indistinct in sandstone beds, distinct in argillite. Thickness 180-500 m, mostly about 350 m. 

Brookline Member: 43 to 60 percent conglomerate. 20 to 55 percent argillite. 2 to 20 percent sandstone. Interbedded 

mafic volcanic rocks (Brighton Melaphyre) in lower part. Clasts well rounded, generally 2.5 to 7.6 cm, average large size 

10 cm, a few 30 cm; mostly quartzite, granite, felsite. lesser melaphyre (Brighton Melaphyre) and argillite; clasts in basal 

beds larger and typically of underlying formation. Oscillation ripple marks, current lamination and crossbedding, graded 

bedding. Bedding mostly lacking or obscure, rarely well stratified; some sand and shale partings and lenses. Thickness 

150-1.300 m; thins rapidly to south

Approximate location of fossiliferous horizon (Billings. 1982, p 912)

FIGURE 9. Stratigraphic section and lithologic description of the Boston Bay Group.

general overlies the Mattapan as shown by Billings 

(1976a,b). The Lynn has no known overlying strata.

The thickness of the Mattapan is indicated on table 4 

and figure 9. It is thickest on the central anticline, 

Boston (1,000 m on projection from surface exposure), 

600 m in Hyde Park, near Milton (M, fig. 8), and 760 m in 

Natick (Billings, 1979, p. A17), but it is missing to the 

southeast in Hingham and Nantasket where the Roxbury lies directly on the Dedham Granite (K.G. Bell, 

written commun., 1976; Billings, 1979). Its extent and 

thickness to the north under the Charles River syncline 

between Boston and Medford are not known.

Metavolcanic rocks that resemble the Mattapan in the 

Blue Hills, south of Boston, are assigned on the State 

bedrock map to the Mattapan. These rocks were considered for many years to be either Silurian and Devonian 

(LaForge, 1932; Naylor and Sayer, 1976) or Carboniferous (Emerson, 1917; Billings, 1929). These metavolcanic 

rocks are chemically and mineralogically distinct from 

the nearby Blue Hills Granite Porphyry (SObgr), which 

is comagmatic with the Quincy Granite (SOqgr) (Sayer, 

1974). Most geologists agree that the volcanic rocks in 

the Blue Hills are older than the Quincy Granite and the 

Blue Hills Granite Porphyry (Naylor, 1981), which Billings (1982) postulated to be an Ordovician caldera complex. Chute (1966) could find no difference between the 

volcanic rocks (Zm) in the Blue Hills and the Mattapan 

Volcanic Complex in the type area. We have followed 

Chute in assigning these rocks to the Mattapan Volcanic 

Complex.
E26 THE BEDROCK GEOLOGY OF MASSACHUSETTS

FELSIC AND MAFIC ROCKS SOUTHWEST OF THE BOSTON 

BASIN (Zfm)

Felsic and minor mafic volcanic rocks flanking other 

basins southwest of the Boston basin are correlated or 

could be correlated with the Mattapan Volcanic Complex. These rocks lie in a structural belt that also 

encompasses the Bellingham and Norfolk basins (Emerson, 1917; LaForge, 1932). Modern mapping has carried 

the Mattapan as far south as Medfield (Volckmann, 

1977). Similar felsic volcanic rocks not previously 

mapped as Mattapan lie on the flanks of the Bellingham 

and Norfolk basins.

In or on the flanks of the Bellingham basin, a felsite 

porphyry (Zfm, mislabeled Zm on the State bedrock 

map) crops out in a series of exposures east of Maple 

Street, about 1 km east of Bellingham. The felsite 

porphyry contains quartz "eyes" and has a measurable 

cleavage. Igneous texture is still recognizable in thin 

section. The porphyry consists of perthite, quartz, both 

interstitial and in phenocrysts, tabular zoned plagioclase, 

and interstitial white mica, quartz, and an opaque mineral. The felsite is also exposed on 1-495 to the north.

Felsic and minor mafic volcanic rocks exposed west of 

Lake Pearl, Wrentham, on the west flank of the Norfolk 

basin are possibly also equivalent to the Mattapan Volcanic Complex but are shown on the State bedrock map 

as volcanic rocks in the Wamsutta Formation (Pwv). At 

one stage in the compilation of the State bedrock map, 

these rocks were shown as being equivalent to the 

Mattapan, as is the porphyry near Bellingham (Zfm). 

However, the proximity of these rocks to similar felsite 

at Diamond Hill, R.I., to the south, mapped by Quinn 

(1971) as Pennsylvanian, made an assignment to a Pennsylvanian age preferable. These rocks will be described 

in the section on the Pennsylvanian rocks, under the 

heading "Rhyolite and mafic volcanic rocks in the Wamsutta Formation."

Two small exposures of rhyolite were mapped by 

Chute (1965) north of Plymouth in the Duxbury quadrangle at Green Harbor, Duxbury (GH, fig. 10), and at 

Cripple Rocks (CR) in Kingstown Bay. Their age is not 

known, and they are not shown on the State bedrock map 

but are included in the unit granite, gneiss, and schist, 

undivided (Zgg) underlying the poorly exposed southeast 

corner of the State. They are possibly correlative with 

the Mattapan. The granite, gneiss, and schist unit (Zgg) 

is discussed in more detail in Wones and Goldsmith (this 

vol., chap. I).

AGE OF THE MATTAPAN AND LYNN VOLCANIC 

COMPLEXES

The Mattapan Volcanic Complex was for many years 

considered to be Pennsylvanian in age, through correlation with the volcanic rocks in the Narragansett basin, or 

Silurian and Devonian, through correlation with the 

Newbury Volcanic Complex. Recently, however, Zartman (Kaye and Zartman, 1980) reported a U-Th-Pb 

isotopic age on zircons from the Mattapan of 602 ±3 Ma. 

Although Billings (1979) questioned the reliability of 

zircon ages from volcanic rocks, particularly those from 

vents, the discovery of Proterozoic Z acritarchs in the 

overlying Cambridge Argillite (Lenk and others, 1982) 

indicates that the zircon age is appropriate.

The age of the Lynn Volcanic Complex, like that of the 

Mattapan, has been equivocal. Emerson (1917) considered the Lynn, as well as the Mattapan, to be Carboniferous. LaForge (1932) correlated the Mattapan and 

Lynn with the Newbury Volcanic Complex; Naylor and 

Sayer (1976) suggested a similar correlation. Billings 

(1979, 1982) accepted a Proterozoic Z age for the Mattapan but preferred to correlate the Lynn with the Newbury because of its proximity and lithologic similarity to 

the Newbury. The argument based on the proximity of 

the Lynn to the Newbury is not strong, because the 

Lynn is geographically and structurally much closer to 

the Mattapan. The closest Newbury lies in a tectonic 

wedge 13 km to the north, whereas projection of the 

Mattapan northward beneath the Boston Bay Group in 

the Charles River syncline to the northern border fault of 

the basin places the Mattapan at most a few kilometers 

from the Lynn. The throw on the northern border fault 

probably is not great (Billings, 1976b, p. 41); my reconstruction suggests a throw of 1-1.5 km on the border 

fault using top of basement (Dedham Granite) as an 

approximate horizon. This amount of offset would place 

the two volcanic complexes within a short distance of 

each other. Zarrow (1978, cited in Naylor, 1981) has 

shown that the rare-earth-element pattern in one of the 

units in the Lynn is identical to that in part of the 

Mattapan. I think it reasonable to assume that the Lynn 

and the Mattapan are parts of the same volcanic complex 

and that the Lynn is also Proterozoic Z in age. However, 

in view of the uncertainties in the correlation at the time 

the State bedrock map was prepared, the Lynn is shown 

on the map as Lower Devonian, Silurian, or Proterozoic 

Z.

Although much of the Mattapan and Lynn is younger 

than the Dedham Granite, some evidence suggests a 

genetic relation between the Dedham and the felsic 

volcanic rocks. Kaye (Kaye and Zartman, 1980, p. 258) 

described the gradation from granite to rhyolite seen in 

places and pointed out that Crosby (1880,1893,1900) saw 

field evidence that convinced him that the rhyolitic 

volcanic rocks were genetically related to the granites. 

Zarrow (1978, cited in Naylor, 1981) found rock considered to be Dedham containing inclusions of Lynn and 

apparently truncating mappable members of the Lynn.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E27

42°00'

"y.-tuv I "-- TV '' " "« - r- "-" - ,| 

^Paw/tucket;":; -"-'."\- ;

Contact

Fault Dotted where concealed

Geographic features 

Green Harbor 

Cripple Rocks

10 MILES

J

10 KILOMETERS

FIGURE 10. Stratigraphic units in and around the Narragansett basin. WW, West Wrentham. Unit designations as on figure 2.

The identity of granitic rocks intrusive into the Lynn 

might be questioned, as Bell (1976) mentioned granite 

dikes of the Paleozoic Cape Ann Complex in a block of 

rock containing the Lynn. Agreement exists, however, 

that the intrusive rock is Dedham (G.R. Robinson, Jr., 

oral commun., 1980). If so, it is more likely to be a 

younger phase of the Dedham batholith. A similarity in 

trace-element content between the Lynn and rock 

mapped by Zarrow as Dedham in this area suggests 

consanguinity (Zarrow, 1978, cited in Nay lor, 1981). The 

observation that the Mattapan and Lynn are both older 

and younger than the Dedham is resolved if a genetic 

affinity does exist between the felsic volcanic rocks and 

the Dedham and if we recognize that the Dedham

consists of several phases. More than one kind of granite 

is observed in the Dedham batholith of southeastern 

Massachusetts, where type Dedham Granite is found. 

The Westwood Granite (Zwgr), also found in the Dedham 

batholith, is younger than the Dedham (Chute, 1966). A 

slightly more mafic granite north of Boston mapped as 

Dedham (Zdngr) is another phase. Kaye (Kaye and 

Zartman, 1980, p. 259; Kaye, 1980) recognized at least 

four pulses of intrusion in the block containing the Lynn 

Volcanic Complex.

The isotopic age for Dedham Granite as a whole is 

630±15 Ma (Zartman and Marvin, this vol., chap. J, table 

1). This is probably a maximum age; because multiple 

intrusions exist, the zircon age of 602 Ma for the Matta-
E28 THE BEDROCK GEOLOGY OF MASSACHUSETTS

pan given by Kaye and Zartman (1980) is quite within 

reason. It is well below the approximate date of the base 

of the Cambrian at about 570 Ma. Ample time is available 

for granite plutonism, accumulation of felsic volcanic 

rocks, and deposition of the Boston Bay Group before 

commencement of Cambrian deposition. The Mattapan 

and Lynn quite probably represent a period of felsic 

volcanism that started during at least the late stages 

of emplacement of the southeastern Massachusetts 

batholith.

BOSTON BAY GROUP

The primarily sedimentary rocks of the Boston Bay 

Group (table 4) comprise the Roxbury Conglomerate 

(FfeZr), which consists of the Brookline, Dorchester, and 

Squantum Members and a volcanic unit in the Brookline, 

the Brighton Melaphyre (FfeZrb), and the Cambridge 

Argillite (FfeZc). The Boston Bay Group overlies the 

Mattapan Volcanic Complex disconformably (LaForge, 

1932; Billings, 1929). The most detailed and comprehensive studies of the Boston Bay Group have been made by 

M.P. Billings, by his associates and students over many 

years, and by LaForge (1932). Early studies of the 

Boston Bay Group were made by W.O. Crosby in the late 

1800's. More recent work has been done by Kaye (Kaye 

and Zartman, 1980; Kaye, 1980). Bell (written commun., 

1976) has mapped marginal areas of the basin. On the 

State bedrock map, the three members of the Roxbury 

Conglomerate (FfeZr) are not shown separately. Only the 

Brighton Melaphyre unit (FfeZrb) is shown within the 

Roxbury Conglomerate. The stratigraphy is summarized 

in table 4. Bedding characteristics and other distinguishing features of the units of the Boston Bay Group are 

shown in figure 9.

The Roxbury Conglomerate (FfeZr) forms the base of 

the Boston Bay Group. Both LaForge (1932) and Billings 

(1929; 1976a,b; 1982) considered the base of the Roxbury 

to be an unconformity or disconformity. The Roxbury 

clearly lies nonconformably on the Dedham Granite near 

Hull, and the base of the Roxbury can be traced above 

the Mattapan Volcanic Complex as a continuous horizon, 

according to LaForge (1932). Emerson (1917) and Billings (1929, 1976a) separated the Roxbury into three 

clearly defined members (table 4). LaForge (1932), however, claimed that the threefold division did not persist 

throughout the area and that the middle Dorchester 

Member had no clear base. Since then, Billings and 

Tierney (1964) found evidence that the Roxbury interfingers with the lower part of the overlying Cambridge 

Argillite in the northern part of the Boston basin, 

indicating that the Squantum Member pinches out and 

that the Dorchester has no clearly defined top in this part 

of the basin. Conglomerate in the Brookline Member

contains clasts of Dedham Granite (Zdgr), quartzite, and 

volcanic rock from the underlying Mattapan Volcanic 

Complex (Zm). The source of the quartzite clasts in the 

Roxbury Conglomerate is most likely the Westboro 

Formation. If the Dedham Granite was exposed to 

erosion to provide the clasts of granite, then the Westboro was most likely exposed also. The quartzite clasts in 

the Roxbury do not contain traces of fossils as do the 

quartzite pebbles in the Pennsylvanian Purgatory Conglomerate in Rhode Island (Shaler and others, 1899). The 

Dorchester Member consists of interbedded argillite and 

sandstone forming an intermediate unit between the 

primarily conglomeratic Brookline Member and the overlying Cambridge Argillite. The uppermost Squantum 

Member of the Roxbury is a distinctive diamictite, best 

exposed north of Quincy; its origin has been subject 

to differing interpretations, most being that the Squantum Member is a tillite (Cameron, 1979). It is not 

everywhere a diamictite, however. In Brighton (Newton 

area) and Hingham, the Squantum Member is a very 

coarse conglomerate (Billings, 1976a, p. 10). It may not 

be as continuous as indicated by Billings (Kaye, 1980) 

and, as mentioned above, appears to pinch out in the 

northern part of the basin. Most recently, Caldwell 

(1981) presented arguments indicating that the Boston 

Bay Group as a whole, including the Squantum, is unlike 

Pleistocene glacial deposits in that deltaic deposits are 

lacking, sand-size fractions are rare, the varvelike Cambridge Argillite is too thick to be a glacial-lake deposit, 

and evidence of multiple episodes of glaciation is lacking.

Within the lowermost Brookline Member of the Roxbury Conglomerate are mafic volcanic rocks, the Brighton Melaphyre (FfeZrb). The Brighton Melaphyre consists 

primarily of quartz keratophyre, keratophyre, and spilite (Kaye, 1980). These are dark-gray to dark-greenishgray and reddish-gray aphanitic rocks. The spilites form 

flows, pillow lavas, feeder pipes and vents, and pyroclastic rocks. The keratophyres form massive flows, breccias, pillow lavas, and laminated devitrified palagonite 

tuff. Altered mafic dikes in the Mattapan are considered 

to be part of the Brighton (LaForge, 1932, p. 42). Nelson 

(1975a) described the Brighton Melaphyre in the Natick 

quadrangle as consisting of bluish- to dark-greenish-gray 

basaltic and andesitic flows and tuffs and minor interbedded, very fine grained ash and slate. Bouchard (1979) 

described the Brighton in the Newton area as a complex 

composed primarily of varicolored mafic basaltic extrusive rocks and subordinate andesitic to rhyolitic lavas 

and tuffs; lahars, mudflows, breccias, and agglomerate 

are also present. Some of these rocks are aphanitic, 

others are amygdaloidal, others are lapilli rich.

The Brighton is younger than the Mattapan and 

intrudes it. Possibly some of the Brighton is contemporaneous with part of the Mattapan. The Brighton appar-
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E29

ently rises no higher in the section than the Brookline 

Member of the Roxbury, although tuffaceous beds are 

noted in the Dorchester Member and in the Cambridge 

Argillite (fig. 9).

The sandy horizons in the Cambridge Argillite (ftZc) 

are in places quartzite. The most prominent of these are 

the Milton quartzite unit of Billings (1976a), in the 

southern part of the basin, and the Tufts Quartzite 

Member, described by Billings (1929) and LaForge 

(1932), in the northern part of the basin. Billings (1976a, 

p. 12) projected the Milton quartzite unit to lie about 820 

m above the Squantum Member of the Roxbury Conglomerate in east Milton. He projected the Tufts Quartzite Member to lie 2,280 m below the Squantum Member 

(Billings, 1976b, p. 35), which places it quite low in the 

section (fig. 9). Red sandstone and sandy argillite shown 

by Kaye (1980) in the Chelsea and Revere area and in the 

Milton-Quincy area intertongue with green argillite of 

the Cambridge. Kaye (1980) claimed that the red beds lie 

above the cleaner quartzites such as the Tufts Member 

and Milton unit. The possible significance of these 

quartzites in the otherwise turbidite-type bedding of the 

Cambridge is discussed in the following section.

PALEOENVIRONMENT

The descriptions of the formations of the Boston Bay 

Group given by Rehmer and Roy (1976; fig. 9) indicate 

that the group was deposited in a fairly low-energy 

environment in a marine or lacustrine basin. The Roxbury Conglomerate represents deposits proximal to a 

volcanic source area in an alluvial fan-delta complex. 

Laminar graded beds of fine-grained sand and silt are 

interstratified with the coarser clastic beds and the 

volcanic material. The Cambridge Argillite represents 

primarily distal deposits of mud and silt characterized by 

rhythmic bedding but containing wedges of sand from 

near-shore sources intertonguing with the silt and mud 

as water levels shifted. It is not certain whether there is 

more than one quartzite horizon and whether the quartzite represents shoal areas during Cambridge deposition 

or marine regression followed by transgression in the 

Cambrian. The presence of acritarchs (Lenk and others, 

1982) suggests that the basin was marine, possibly 

protected in some way. The bedding style in the Cambridge suggests turbidite deposition. The Squantum 

Member "tillite" could be more aptly considered in this 

environment to be a submarine-landslide deposit, perhaps derived from glacial deposits, as suggested by 

Rehmer and Hepburn (1974), or from other sediments, in 

accordance with the doubts of Caldwell (1981) concerning 

the glacial origin of the Boston Bay Group. Laminar 

graded beds like those in the Cambridge are present in

the Mattapan Volcanic Complex (Lyons and Goldsmith, 

1983), indicating a long-standing basin in which the 

environment of deposition changed from primarily volcanic (Mattapan) to primarily proximal turbidite (Roxbury) 

to primarily distal turbidite (Cambridge).

The total thickness of the Boston Bay Group increases 

from south to north (Billings, 1976a; fig. 9). The Brookline Member is thickest over the central anticline and 

thins to the south. These relations indicate that the 

material was derived from a southern and southwestern 

source and that the basin of deposition deepened to the 

north and northeast. The center of the basin subsided 

more rapidly than the margins (Billings, 1976a). The 

Squantum Member, however, appears to maintain a 

fairly uniform thickness (fig. 9), suggesting a cessation of 

subsidence toward the end of Roxbury deposition.

AGE

The age of the Boston Bay Group has long been a 

matter of controversy. Suggested ages have ranged from 

Primordial (Cambrian) (Crosby, 1880) to Pennsylvanian 

(Crosby, 1900; Emerson, 1917), on the basis of lithologic 

correlation with the strata in the Narragansett basin and 

now discounted plant fossils; the most recent suggestion 

is Proterozoic Z (Kaye and Zartman, 1980; Lenk and 

others, 1982). Kaye and Zartman concluded that the 

Boston Bay Group lay below the fossiliferous Cambrian 

strata located on the margin of the basin, on the basis of 

an interpretation of field relations between the Cambrian 

strata and the strata within the basin. This interpretation has been recently supported by the identification of 

acritarchs in the Cambridge Argillite, including a diagnostic species that ranges in age from Proterozoic Z to 

Early Cambrian but that is most abundant in Proterozoic 

Z time (Lenk and others, 1982). Billings (1982, p. 912) 

estimated that the horizon containing the acritarchs lies 

about 3,300 m below the highest beds of the Cambridge, 

as deduced from his tunnel investigations in the Boston 

area, and about 2,500 m above the lowest beds (fig. 9). 

No acritarchs or other microfossils were found by Lenk 

and others in the known Cambrian rocks adjacent to the 

basin (P.K. Strother, oral commun., 1982). No reliable 

diagnostic fossils other than these have been found in the 

rocks of the Boston Bay Group (Lyons and Goldsmith, 

1983). The Proterozoic Z age indicated by the acritarchs 

is supported on several other counts: none of the plant 

fossils so numerous in the strata of the Narragansett 

basin have been found; Kaye (1980) showed that the 

Quincy Granite of Late Ordovician to Early Silurian age 

contains argillite inclusions that are on strike with Cambridge Argillite; and the primarily marine stratigraphy is 

not similar to the terrestrial stratigraphy in the Nar-
E30 THE BEDROCK GEOLOGY OF MASSACHUSETTS

ragansett basin (Mutch, 1968). The basal Cambrian 

strata in eastern Massachusetts are quite unlike the 

Roxbury Conglomerate.

Existing evidence thus indicates a certainly Proterozoic Z to possibly Early Cambrian age for the Boston Bay 

Group. How much of the Boston Bay Group may be 

Cambrian is uncertain. David D. Ashenden (written 

commun., 1980) of the Metropolitan District Commission, who has studied in detail cores of both the Cambridge Argillite in the Roslindale syncline and the Braintree Argillite (Cbw on the State bedrock map) at the Old 

Quincy Reservoir, Braintree, concluded that the two 

rocks are identical. C.A. Kaye (oral commun., 1979) 

believed that quartzites like the Tufts Quartzite and the 

Milton quartzite unit as used by Billings (1976a) and 

overlying red beds may be basal Cambrian and that 

overlying red sandstone and sandy argillite are Cambrian strata. These red beds interfinger with green and 

red argillite mapped as Cambridge in the northern and 

southern parts of the basin. However, to date, Cambrian 

strata have been found only on or beyond the margins of 

the Boston basin.

CAMBRIAN STRATA

Fossiliferous rocks of Cambrian age in eastern Massachusetts are found in and around the Boston basin, at 

Hoppin Hill in North Attleboro, and north of Diamond 

Hill in West Wrentham. The largest area of fossiliferous 

Cambrian is south of the Boston basin in the Braintree 

and Weymouth areas (fig. 8) where the Weymouth 

Formation and Braintree Argillite contain Lower and 

Middle Cambrian fossils, respectively. The many studies 

of fossils from these formations have been summarized 

by Theokritoff (1968). Mr. G. Stinson Lord of Quincy has 

made extensive fossil collections in these formations. 

Lower Cambrian fossils have been found on the north 

side of the Boston basin at Nahant (Foerste, 1889) and at 

Revere Beach by C.A. Kaye (oral commun., 1981). The 

Lower and Middle Cambrian Hoppin Formation at Attleboro and West Wrentham (WW, fig. 10) has been 

described by Foerste (in Shaler and others, 1899), Shaw 

(1950, 1961), and, more recently, Anstey (1979) and 

Landing and Brett (1982). Fossils reported as Upper 

Cambrian by Rhodes and Graves (1931) from the Green 

Lodge Formation near Dedham, Mass., cannot be reliably assigned to that period according to Shaw (1961, p. 

436). The Braintree Argillite and the Weymouth Formation have been combined into a single unit (Cbw) on the 

State bedrock map because of their small areal distribution.

Rocks of Cambrian age may be more widely distributed than has been mapped. C.A. Kaye (oral commun.,

1979) believed that some of the red sandstones in the 

Cambridge Argillite at the northern part of the Boston 

basin could be Cambrian rather than Proterozoic Z in 

age. Chute (1964) described beds resembling Cambrian 

strata in the Wamsutta Formation in the Norfolk basin, 

and J.P. Schafer (oral commun., 1982) has observed 

limestone-bearing beds in the Wamsutta near outcrops of 

Dedham Granite in the Attleboro area southeast of 

Hoppin Hill that might be Cambrian strata. Cambrian 

strata have been found to be much more extensive than 

previously thought in southern Rhode Island (Skehan 

and others, 1981), although, as Skehan and others 

pointed out, Dale (1885a,b) early differentiated what are 

known to be Cambrian strata from the Pennsylvanian 

strata in Rhode Island.

The Cambrian fossil assemblages in eastern Massachusetts and Rhode Island are all of the Acado-Baltic 

province (Theokritoff, 1968) and compare favorably with 

assemblages from southeastern Newfoundland, England, and Morocco (Landing and Brett, 1982; Skehan and 

others, 1978).

HOPPIN FORMATION ( h)

A sequence of strata containing Lower Cambrian 

fauna at Hoppin Hill, North Attleboro (fig. 10), was 

named the Hoppin Slate by Foerste (in Shaler and 

others, 1899) and more recently named the Hoppin 

Formation (Goldsmith and others, 1982a) because the 

sequence contains rocks other than slate. The Hoppin 

Formation (Ch) consists primarily of green and red slaty 

shale, locally containing calcareous nodules, and lenses 

and layers of red argillaceous limestone. At the base it 

contains arkosic quartzite, which is locally conglomeratic, and sandstone. The nonconformity at the base of the 

Hoppin Formation is clearly exposed at Hoppin Hill 

(Dowse, 1950). Anstey (1979) studied the Hoppin Formation in detail and measured several sections in and 

around the Hoppin Hill Reservoir (fig. 11). The section 

shown in figure 12, measured by me before I was aware 

of Anste/s work, duplicates section A of Anstey. On the 

east side of Hoppin Hill, a basal quartzite and grit unit 

containing quartz and feldspar grains from the underlying Dedham Granite rests nonconformably on the Dedham. Obscure crossbedding indicates the beds top away 

from the granite. The quartzite (10-15 m thick), which is 

conglomeratic in places, is interbedded with and passes 

upward into arenaceous slate, argillaceous siltstone, and 

dark-green slate containing thin sandy horizons above 

scour-fill channels (18-30 m). This unit is succeeded by 

red argillaceous limestone, containing fossil fragments 

(biomicrite, biomicrudite) dominated by Volborthellids, 

hyolithids, and trilobites, interbedded with red slate 

(34-44 m). This limestone and slate unit is overlain by a
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E31

305500 E.

4649000 N -

4648000 N. -

1000-meter Universal Transverse Mercator 

grid ticks, zone 19, shown for reference

EXPLANATION

C/3 

C/3 

LLJ

z ^

175

150

125

100

75

50

25

OL

c °

 si

Limestone ( hl) 

Slate ( hs) 

Quartette ( hq) 

Covered

Line of measured section 

---- Contact Dotted where projected 

Fault 

Bedrock exposures

C'

Pp

Pondville 

Conglomerate

 h

Hoppin 

Formation

Zdgr 

Dedham 

Granite

B

FIGURE 11. Geologic map and measured sections on the east side of Hoppin Hill, Attleboro, Mass.

thick nonfossiliferous section of red and green slate 

(more than 186 m). The top of the sequence is overlain in 

angular unconformity by the Pondville Conglomerate of 

Early Pennsylvanian age. Measured sections indicate the 

exposed maximum thickness of the Hoppin Formation is 

approximately 244 m. Skehan (1969, p. 798) believed the 

Hoppin Hill section is similar in lithology to the Manuels 

Brook section in the Avalon terrane of southeastern

Newfoundland described by Walcott (1890; cited by 

Hutchinson, 1962).

The Hoppin Formation at West Wrentham is poorly 

exposed in a hillock beneath a powerline east of Cumberland Street, just north of the Rhode Island State line. 

Here, red and green slate and red argillaceous limestone 

containing fossil fragments similar to those at Hoppin 

Hill can be found. A small outcrop of quartz on the
E32 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Thicknes 

(meters

60-

55

50- 

45 

40-

35 ~ 

30- 

25- 

20-

10 

5- 

0-

s

 £=> ~__ 

E-^EE-bzE-EZ

W^S^lj^A

 .:: :. ' ; . rr-'.-.v.^-:

.i.v{ :;, >. .- ' ; jo. '.;.£v: * 

.> ' : AV?;-' ' "'.'.' o3S&&2S

B£^c5£ '&$&&&

Hoppin Hill Reservoir

Covered

^ Red slaty shale

(Float of red slaty shale and limestone)

^ Red limestone underlain by green slaty shale

(Float of red slaty shale)

Red slaty shale containing calcareous concretions

Covered

Red slaty shale and a few green slaty lenses; scattered 

calcareous concretions

Covered

C^ Red shaly limestone

(Float of red siliceous shale)

Micaceous quartzite and quartz wacke, fissile, brown-weathering

Covered

/ Quartz wacke and quartzite

Covered

^ Quartz wacke and quartzite. fissile, brown to gray

x> Pebble conglomerate and pebbly quartzite

Quartzite

(Float of pebbly quartzite and pebble conglomerate)

Crossbedded quartzite, pebbly quartzite, and pebble conglomerate

(Float of quartzite and pebbly quartzite) 

(Float of granite in 71-m interval)

Dedham Granite (Zdgr)

FIGURE 12. Measured section of Lower and Middle Cambrian strata 

of the Hoppin Formation at the north end of Hoppin Hill Reservoir.

southwest side of the hillock may be vein quartz, related 

to the massive vein quartz at Diamond Hill immediately 

to the south, rather than the basal quartzite. An inclusion of "slate" in ledges of Dedham Granite adjacent to 

the west mentioned by Foerste (in Shaler and others, 

1899, p. 393) is not the Hoppin Formation strata but is a 

chloritoid-bearing phyllite (E-an Zen, oral commun., 

1979) belonging to the Blackstone Group, which crops out 

0.7 km to the southwest. The contact with the Dedham to 

the west is probably a fault. To the north and east of the 

hillock is the Wamsutta Formation, and to the south are 

the felsic volcanic rocks of Diamond Hill.

WEYMOUTH FORMATION AND BRAINTREE ARGILLITE

( bw)

The Weymouth Formation (Cbw) at Weymouth consists primarily of greenish-gray and dark-red slate containing calcareous nodules, and subordinate beds and 

lenses of argillaceous limestone. It was locally converted 

to hornfels by the intrusion of Quincy Granite. At Nahant

(fig. 8), the Weymouth includes a layer of greenish-white 

limestone. Kaye (oral commun., 1981; Kaye, 1980) found 

exposures of Weymouth Formation at Revere Beach 

following a storm that scoured sand from the beach, 

which resembled rock in the exposures at Nahant. Clark 

(1923, p. 475) reported a concentration of fossiliferous 

pebbles within a mile of the south end of the beach. 

Fossils in the Weymouth are largely conoidal forms but 

include some olenellids. The lithology of the Weymouth is 

similar to that of the Hoppin Formation at Hoppin Hill, 

except that the lower quartzitic part of the section has 

not been recognized. A complete section of the Weymouth Formation has been nowhere described. The base 

of the Weymouth is not exposed in the Boston area, and 

its thickness can only be inferred to be at least 100 m 

(LaForge, 1932, p. 20).

The Braintree Argillite (Cbw) is present only south of 

the Boston basin in and around the Quincy Granite. The 

formation consists of noncalcareous green to dark-gray 

or black massive slate or argillite. Adjacent to the Quincy 

Granite it is a greenish-gray hornfels with indistinct 

bedding. Chute (1969) described the Braintree in the 

Blue Hills quadrangle as a dark-gray slate containing 

thin beds of light- and medium-gray siltstone. The slate 

is cyclically thinly bedded. LaForge (1932) estimated the 

thickness to be at least 1,000 ft (300 m). Its contact with 

the underlying Weymouth Formation is nowhere 

exposed, nor is the top of the formation known. No 

complete section has been described. The faunal assemblage in the Braintree is characterized by Middle Cambrian representatives of Paradoxides. At Conanicut 

Island in southern Rhode Island, Skehan and others 

(1978, 1981) described a Middle Cambrian sequence 350 

m thick of siltstone and phyllite in which the beds in the 

lower part coarsen upward, indicating deposition in 

shallow water, and the beds in the upper part are cyclic 

and fine grained, indicating deposition in quieter water. 

The lower part of the sequence contains a trilobite fauna 

characterized by Badelusia and Paradoxides. The upper 

part is nonfossiliferous. The lithologies described there 

appear to be similar to those of the Braintree.

GREEN LODGE FORMATION OF RHODES AND GRAVES

(1931) ( g)

Poorly preserved brachiopod impressions in a quartzite (Cg) in the Dedham area were reported by Rhodes 

and Graves (1931). The fossiliferous quartzite is in a 

sequence of light-gray quartzite overlain by dark-gray 

phyllite containing siltstone laminae and thin layers of 

siltstone containing limonite-bearing pits. These rocks 

were exposed in a hill south of Route 128 near the 

Westwood-Dedham town line. Rhodes and Graves (1931) 

named the sequence the Green Lodge Formation and
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E33

assigned it to the Upper Cambrian on the basis of the 

fossil impressions. Shaw (1961, p. 436), however, examined the fossils from the Green Lodge and said that they 

were too poorly preserved for reliable assignment. The 

Green Lodge therefore is shown as questionable Upper 

Cambrian on the State bedrock map. The outcrops, also 

seen by Loughlin (1911; Loughlin and Hechinger, 1914) 

and by Chute (1964), are no longer accessible because of 

construction. Chute estimated that about 150 m of phyllite is present overlying an undeterminable thickness of 

quartzite. Rhodes and Graves estimated the thickness of 

the Green Lodge to be not less than 300 m. Chute (1964) 

mapped a locality of similar phyllite on the main Amtrak 

railroad line in southern Norwood but did not correlate 

this phyllite with the Green Lodge.

The Green Lodge is most likely the source of the 

fossiliferous quartzite pebbles found in the Pennsylvanian Purgatory Conglomerate in the Narragansett basin 

in Rhode Island and also in the glacial drift on Cape Cod 

and the offshore islands (Kaye, 1983b). Samples of 

quartzite containing impressions of fossils gathered from 

outcrops of the Green Lodge shown to me by C.A. Kaye 

are similar to pebbles he has collected from the drift. The 

Green Lodge then may be more extensive beneath the 

glacial drift than it appears; it is shown on the State 

bedrock map as covering a larger area than originally 

shown by Rhodes and Graves (1931) and Chute (1964).

PALEOENVIRONMENT

The Cambrian strata in eastern Massachusetts represent a marine littoral to shelf sequence of sediments. 

Anstey (1979) described the Lower to Middle Cambrian 

Hoppin Formation as a deepening-upward transgressive 

marine sequence. The basal quartzite in the Hoppin Hill 

area is a littoral deposit lying nonconformably on a 

Proterozoic Z granitic terrane. The calcareous fossiliferous beds represent a near-shore shoal deposit, and the 

overlying nonfossiliferous slates in the upper part of the 

sequence represent detrital muds washed further offshore. The lithologically similar Weymouth Formation 

around the Boston basin can be interpreted as representing the same sedimentary sequence. The cyclically bedded fossiliferous silt and shale of the Middle Cambrian 

Braintree Argillite were deposited in relatively shallow 

water but in a low-energy environment. The Middle 

Cambrian at Conanicut Island, R.L, described by Skehan and others (1978, 1981) represents a transgressive 

sequence. A minor regression may have occurred 

between Early and Middle Cambrian time. If the quartzite of the Green Lodge is indeed Upper Cambrian, then 

a return to littoral conditions occurred in Late Cambrian 

time. The paleoenvironment of the Cambrian thus seems 

to be one of low tectonic activity in which crustal

movements were epeirogenic. The volcanic activity of 

Proterozoic Z time had ceased. The Proterozoic Z batholiths were exposed and were being eroded on a surface 

of low relief flanked by a transgressing shallow sea.

NEWBURY VOLCANIC COMPLEX (SILURIAN 

AND DEVONIAN)

Sedimentary and volcanic rocks of known Silurian and 

Devonian age in eastern Massachusetts are confined to 

two wedge-shaped and lenticular fault-bounded basins 

aligned between Newburyport and Middleton-Topsfield 

(fig. 3). The Newbury Volcanic Complex (DSn) was 

carefully described by Shride (1976) in the Newburyport 

and Rowley area and by Toulmin (1964) in the MiddletonTopsfield area. The information below is condensed from 

Shride's description.

The lower part of the Newbury Volcanic Complex 

(DSnl, DSna), 2,900 m thick, is composed largely of 

volcanic materials consisting of basalt flows; flow-banded 

rhyolite, vitrophyre, and ash-flow tuff; and porphyritic 

andesite flows, breccias, and tuff, partly waterlain. The 

upper part of the complex (DSnu), 1,500 m thick, consists 

of shallow marine siliceous siltstone, red sandy mudstone 

containing some volcanic detritus, and calcareous mudstone. The porphyritic andesite (DSna) contains a shelly 

fauna indicating a latest Silurian age (Pridolian). The 

calcareous mudstone of the upper part (DSnu) contains 

ostracodes having a greater range than the andesite 

fauna, and it could be as young as earliest Devonian 

(Gedinnian). Lenticular masses of micrographic rhyolite 

(DSnr) 100-600 m thick intrude the sequence subparallel 

to the strata at various horizons. Neither the top nor the 

bottom of the complex is known. The rocks have undergone no more than diagenesis and propylitization, as 

indicated by the descriptions by Shride (1976). Primary 

textures are well preserved.

The Newbury Volcanic Complex can be correlated 

with rocks along the Appalachian trend to the northeast, 

but no Upper Silurian and Lower Devonian rocks of this 

sort are known to the southwest. The Newbury is 

equivalent to the Leighton Formation of the Pembroke 

Group in the Eastport area, Maine, on the basis of 

correlation of the shelly fauna. Faunal assemblages of the 

Leighton are of Acado-Baltic affinity. The Ames Knob 

Formation and the Thorofare Andesite of the Penobscot 

region, Maine, are also temporal equivalents. On the 

basis of similar lithologies, the Newbury has been 

equated in the past with the Mattapan and Lynn Volcanic 

Complexes in the Boston area, but the Mattapan and the 

Lynn now are believed to be Proterozoic Z in age.

PENNSYLVANIAN STRATA

Fossiliferous rocks of Pennsylvanian age in the 

Milford-Dedham zone in Massachusetts occupy the
E34 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Norfolk and Narragansett basins (figs. 1, 8, 10). These 

rocks consist of the Pondville Conglomerate (Pp), Wamsutta Formation (Pw, Pwv), Rhode Island Formation 

(Pr, Prc), and Dighton Conglomerate (Pd). The nonfossiliferous Bellingham Conglomerate (PZb) occupies 

the Bellingham basin. The Bellingham is usually correlated with the Pennsylvanian strata in the Narragansett 

and Norfolk basins, but it may instead be of Proterozoic 

Z age. The only other known fossiliferous rocks of 

Pennsylvanian age in Massachusetts are the Coal Mine 

Brook Formation (Pcm) at Worcester on the east edge of 

the Merrimack synclinorium (Goldsmith and others, 

1982a). The Coal Mine Brook Formation is discussed in 

the chapter on the Merrimack belt (Robinson and Goldsmith, this vol., chap. G).

BELLINGHAM CONGLOMERATE (PZb)

The Bellingham Conglomerate (PZb) (Mansfield, 1906; 

Hall, 1963; Quinn, 1971) is primarily a conglomerate and 

lithic graywacke and is confined to the Bellingham basin. 

The dominant lithology, conglomerate, consists of 

quartzite and granite pebbles and cobbles, flattened in 

varying degrees, set in a green and greenish-gray micaceous matrix. Interbedded with the conglomerate and 

lithic graywacke is chlorite phyllite composed of muscovite, quartz, chlorite, zoisite, magnetite, and locally 

chloritoid (Warren and Powers, 1914). An outcrop of 

Bellingham Conglomerate near the intersection of Blackstone Street and River Street in Woonsocket (fig. 4) 

consists of green conglomerate containing pebbles of 

granite in a sandy matrix, green sandstone, and darkgreen phyllite containing a lens of gray limestone. The 

Bellingham Conglomerate contains pebbles of quartzite 

from the adjacent Blackstone Group and of Milford 

Granite containing its typical blue quartz; it is therefore 

locally derived. An instructive exposure lies on the east 

side of Woonsocket Hill, southeast of Woonsocket. Here 

cliffs of steeply dipping, thin-bedded, white to gray 

quartzite of the Blackstone Group stand above green 

schistose conglomerate, containing many flattened white 

to gray quartzite pebbles, and interbedded green calcareous quartz schist. The contact is probably a fault here, 

but the source of the quartz pebbles is obvious. It is 

difficult to distinguish schists of the Blackstone Group 

from those of the Bellingham in this area because of the 

low-grade metamorphism of the Blackstone. Warren and 

Powers (1914) claimed the distinction could be made in 

that schists of the Blackstone contain knots of epidote, 

whereas schists of the Bellingham do not. This criterion 

is not always reliable, and it is possible, if not probable, 

that some of the low-grade Blackstone Group mapped in 

the Blackstone River valley northwest of Woonsocket is 

part of the Bellingham.

Volcanic rocks are present in a few places in the 

Bellingham. Near the edge of the basin, in a roadcut on 

new Route 146 at Premisy Hill, west of Woonsocket, a 

rhyolite porphyry sill 30 cm thick cuts actinolitic greenstone, schist, thin gray quartzite, and gray, streaked 

biotite schist containing epidote pods, rocks probably 

belonging to the Blackstone Group. East of Bellingham 

in the Franklin area are exposures of felsite porphyry 

described in an earlier section that may be the same age 

as the Bellingham Conglomerate. Warren and Powers 

(1914, p. 448) mentioned the presence of amygdaloids in 

the Bellingham area, but I could not locate these in my 

reconnaissance. Exposures are poor in the Bellingham 

basin north of Woonsocket, and the distribution of the 

Bellingham is mapped largely from float.

The age of the Bellingham is conjectural. No fossils 

have been found. Customarily the rocks have been 

correlated with rocks of the Narragansett basin and thus 

labeled as Pennsylvanian in age. Rocks in some exposures, such as the one at River Street and Blackstone 

Street in Woonsocket, resemble outcrops of Roxbury 

Conglomerate seen in the Boston basin, and it is not 

unreasonable to assume that the age of the Bellingham 

conglomerate is similar to that of those rocks in the 

Boston basin as suggested by Skehan and Murray (in 

Skehan and others, 1979, p. 14-15). Some support for 

this conclusion lies in the observation that the Bellingham basin is a structural trough (Goldsmith, this vol., 

chap. H) that extends southwest from the Boston basin 

and that separates primarily gneissic Proterozoic Z granitoids from altered but nongneissic Proterozoic Z granitoids (Wones and Goldsmith, this vol., chap. I). In deference to tradition, however, and because a Proterozoic Z 

age is not proven but only suspected, the Bellingham 

Conglomerate in the Bellingham basin is shown as Pennsylvanian to Proterozoic Z in age on the State bedrock 

map.

PONDVILLE CONGLOMERATE (Pp), WAMSUTTA

FORMATION (Pw, Pwv), RHODE ISLAND FORMATION

(Pr, Prc), AND DIGHTON CONGLOMERATE (Pd)

The Pondville Conglomerate, Wamsutta Formation, 

Rhode Island Formation, and Dighton Conglomerate are 

the stratigraphic units of known Pennsylvanian age in 

the Norfolk and Narragansett basins in Massachusetts. 

These formations and the Purgatory Conglomerate in 

Rhode Island were referred to collectively as the Narragansett Bay Group by Skehan and Murray (in Skehan 

and others, 1979, p. A4). The Pennsylvanian strata in the 

Narragansett and Norfolk basins have been described by 

many authors from Shaler and others (1899) through 

Quinn and Oliver (1962), Mutch (1968), and Skehan and 

Murray (1978). This work was summarized by Skehan
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E35

Table 5. Description of stratigraphic units in the Narragansett and Norfolk basins 

[Modified slightly from Skehan and others (1979, table 1)]

Name

Purgatory

Conglomerate.

Dighton

Conglomerate.

Rhode Island

Formation.

Wamsutta

Formation.

Pondville

Description

Coarse-grained to very coarse grained

conglomerate containing thin lenses of

sandstone and magnetite-rich sandstone; clasts in conglomerate consist of

several varieties of quartzite.

Gray conglomerate consisting primarily

of rounded quartzite cobbles to boulders and containing subordinate

rounded granite cobbles and slate pebbles; very little sand matrix; lenses of

medium-grained sandstone form less

than 20 percent of the unit.

Gray sandstone and siltstone and lesser

amounts of gray to black shale, gray

conglomerate, and coal beds 10 m

thick. Sandstone and conglomerate are

quartz rich.

Interbedded red coarse-grained conglomerate, lithic graywacke, sandstone,

and shale; conglomerate layers less

than 1.2 m thick contain felsite clasts;

a few lenses of limestone, one rhyolite

flow, and several sheets of basalt are

present.

At type locality (Pondville Station,

Sedimentary Approxiand other mate 

distinguishing thickness 

features (meters)

Not given 30

Sandstone lenses are <300-450

faintly crossbedded

and coarsen both

upward and downward into adjacent

conglomerate.

Contains both fining- <3,000

and coarseningupward sequences;

paleocurrents have

been defined only

locally; conglomerate

is relatively less abundant than in Dighton

Conglomerate.

Crossbedding and inter- 300

fingering of layers are

characteristic.

First-deposited beds are 0-150

Age Additional references 1

No Pennsylvanian Mosher and Wood

flora yet known; (1976).

distinctive early

Paleozoic faunas

are present in

quartzite clasts.

Late Pennsylvanian; None.

small isolated

amounts of allochthonous nondiagnostic plant debris

are present.

Late and Middle Skehan and Murray

Pennsylvanian. (1978), Lyons and

Chase (1976).

Middle and Early Lidback (1977).

Pennsylvanian;

partly equivalent

to Rhode Island

Formation as the

red layers interfinger with gray and

black; contains a

few plant fossils.

Early Pennsylvanian None.

Conglom- Mass.), interbedded red and green 

erate. slate, siltstone, arkose, and quartzitepebble conglomerate; elsewhere 

includes gray to greenish-gray coarse 

conglomerate containing clasts 15-60 

cm in diameter and abundant sandy 

matrix (clasts mostly quartzite, but 

some are granite or schist) and darkgray granule conglomerate containing 

pebbles of smoky quartz 5 mm in 

diameter irregularly bedded with 

sandstone and lithic graywacke.

siltstone or arkosic 

sandstone, rarely conglomerate; however, 

sandstone and shale of 

the Wamsutta Formation or Rhode Island 

Formation may lie 

directly on older 

rocks.

lrThese references are in addition to Quinn and Oliver (1962), Mutch (1968), and Quinn (1971), which contain information on all these stratigraphic units.

and Murray and Murray and Skehan (in Skehan and 

others, 1979). Detailed maps have been made by Chute 

(1950,1966,1969), Hartshorn (1960,1967), Koteff (1964), 

and Lyons (1969). Lyons (1977) mapped the Massachusetts part of the Narragansett basin in reconnaissance 

fashion and reviewed existing detailed work. Drilling by 

the U.S. Geological Survey (USGS) in 1977 and 1978 

showed that the Narragansett basin and its deposits

extend to Massachusetts Bay a few kilometers south of 

Scituate.

The units in the Narragansett and Norfolk basins are 

described in table 5. Briefly, the strata are primarily 

fluviatile and consist of sandstone, siltstone, conglomerate, shale, and coal. Volcanic rocks are present in the 

northwest part of the basin (fig. 10). Chute (1966, p. B32) 

described lenses of carbonate rock in red and green
E36 THE BEDROCK GEOLOGY OF MASSACHUSETTS

shales in the Wamsutta Formation in the Norwood 

quadrangle. Limestone has been observed in rock 

mapped as Wamsutta adjacent to exposed Dedham Granite at the Manchester Pond Reservoir, Attleboro (Schafer, oral commun., 1982). Possibly these red and green 

shales and calcareous beds are actually Cambrian in age. 

Total thickness of the strata is estimated to be 3,700 m 

(Skehan and Murray, 1978). The Rhode Island Formation is the thickest and most extensive of the formations 

although it does not extend into the Norfolk basin. Coal 

beds are found only in the Rhode Island Formation. Only 

the Pondville Conglomerate and the Wamsutta Formation are present in the Norfolk basin. Chute (1964, 1966, 

1969) recognized a lower boulder conglomerate member 

and an upper sandstone to pebble conglomerate member 

in the Pondville in the northeast part of the Norfolk 

basin, but such a division is not readily made to the 

southwest because of facies changes. The upper member 

grades into and interfingers with the Wamsutta Formation; in turn, the Wamsutta interfingers with the Rhode 

Island Formation in the northwest part of the Narragansett basin. In the northern part of the Narragansett 

basin, basal beds of the Pondville, the Wamsutta, and in 

places the Rhode Island Formation rest nonconformably 

on weathered Dedham Granite. Basal beds are usually 

sandstone or arkose rather than conglomerate; little 

indication exists that the material has been transported 

far. A drill hole near Assonet at the edge of the basin 

revealed that the basal beds of the Rhode Island Formation were a reworked regolith (J.A. Sinnott, oral commun., 1979). At the northeast end of the Norfolk basin 

(fig. 8), the Pondville overlies weathered and partly 

transported Blue Hills Granite Porphyry without clear 

definition (see Naylor and Sayer, 1976; Naylor, 1981).

Daniels Street, between Medway and Franklin, 

crosses the top of a hill covered with float of polymict 

conglomerate containing red-stained gray and white 

quartzite cobbles and pebbles and fragments of reddishpurple slate. The matrix appears to be reddish-colored 

sand. No rock was seen in place. The material does not 

seem to be metamorphic. The bedrock surrounding the 

hill is granite. This material is shown on the State 

bedrock map as Wamsutta Formation (Pw). It could be 

Pondville Conglomerate or possibly Bellingham Conglomerate, as it is somewhat on line with the Bellingham 

basin to the southwest; however, the rock matenal does 

not seem to be sufficiently metamorphosed to be the 

Bellingham.

Rhyolite and mafic volcanic rocks in the Wamsutta 

Formation (Pwv). A rhyolite flow and two sheets of 

basalt flanked by fossiliferous horizons lie within the 

Wamsutta Formation near Attleboro (Lyons, 1977; fig. 

10). Here and in the Norfolk basin,, conglomerate in the 

Wamsutta contains many clasts of volcanic rock. Bottino

(1963) attempted to date the rhyolite by using the 

whole-rock Rb-Sr method, but the material did not yield 

a reliable date. Differences in initial ratios of strontium 

isotopes, however, indicated that the rhyolite and the 

basalts were probably not related (Bottino, 1963).

Northwest of Attleboro, near Grants Mill, R.I., the 

Diamond Hill Felsite as used by Skehan and Murray (in 

Skehan and others, 1979, p. A5) is overlain by the 

Wamsutta Formation at the south end and underlain by 

the Wamsutta at the north end of the Diamond Hill 

Reservoir (Quinn and others, 1948, p. 18). The Diamond 

Hill Felsite is primarily dacite, much altered and cut 

through by vein quartz; it is gray, greenish gray, and 

reddish purple, fine grained, and porphyritic (Quinn, 

1971, p. 41). Phenocrysts are quartz and altered plagioclase. The rock locally shows flow structure, but much of 

it is massive. At the north end of the reservoir, I 

observed greenish-gray, gray, and reddish-purple felsite 

and agglomerate that pass downward through interbedded tuff, sandstone, slate showing cleavage, and conglomerate containing white quartzite and granite cobbles 

into red and green conglomerate typical of the Wamsutta 

Formation. Some faulting has occurred near the contact 

of the Diamond Hill with the Wamsutta, but displacement appears to have been minor. The amount of volcanic detritus in the Wamsutta indicates that volcanoes 

were active close to the time of its deposition.

Volcanic rocks somewhat similar to the Diamond Hill 

Felsite crop out west of Lake Pearl, between Franklin 

and Wrentham, on the west flank of the Norfolk basin. 

These are shown as volcanic rocks in the Wamsutta 

Formation (Pwv) on the State bedrock map because of 

their proximity to Diamond Hill. They also resemble the 

Mattapan Volcanic Complex, as mentioned in that section of this chapter. The volcanic rocks at Lake Pearl 

consist of dark- and light-colored aphanitic felsite, 

agglomerate^), and breccia. One type is a greenish-gray 

aphanitic rock containing saussuritized lath-shaped plagioclase and clots of chlorite and iron-oxide in a finegrained matrix of alteration products (white mica, epidote, chlorite). The plagioclase is locally glomerophyric. 

Another type is a white-weathering, aphanitic rock containing fine-grained angular quartz, feldspar, and rock 

fragments in a fine-grained sericitic matrix. A lightcolored felsite contains quartz "eyes" like those in the 

felsite in Franklin. A grayish-green agglomerate or 

conglomerate containing fragments of felsite porphyry 

and smaller clasts of quartz and pink feldspar in a 

greenish-gray sandy matrix exposed on the east side of a 

large quarry west of Lake Pearl may be part of the 

volcanic assemblage or it may be basal Wamsutta Formation. Further west, a felsite containing pale-tan phenocrysts in a pale-red aphanitic matrix crops out west of 

Uncas Brook and east of Summer Street, 2 km south of
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E37

Franklin. On Summer Street, due west of Uncas Pond, a 

dike of spherulitic rhyolite containing scattered quartz 

phenocrysts cuts Dedham Granite. The rocks near and 

along Summer Street are separated from the volcanic 

rocks near Lake Pearl by the Wamsutta Formation and 

a sliver of Dedham Granite but are considered to be part 

of the volcanic assemblage. Outcrops of fine-grained 

granite (fgr) and of porphyry exposed in roadcuts north 

of Lake Pearl in the town of Norfolk and in a few cuts 

northeast of Lake Pearl near the Wrentham State School 

may also be part of the volcanic assemblage.

PALEOGEOGRAPHY AND AGE

Mutch (1968, p. 201-203) discussed the paleogeography of the Narragansett basin; he concluded that a 

highland supplying most of the detritus for the Narragansett Bay Group of Skehan and Murray (in Skehan 

and others, 1979) existed to the west and northwest of 

the basin and that a stable source to the east supplied 

less material. Current directions measured in the Rhode 

Island Formation in the northern part of the basin 

indicate a flow from northeast to southwest. In support 

of this observation, conglomerate is more abundant on 

the west and northwest side of the basin than to the east 

and southeast. In the southern part of the basin in Rhode 

Island, on the contrary, coarse cobble to pebble conglomerate in the Purgatory Conglomerate is interpreted as 

being derived from the northeast and east. The center of 

volcanism was to the northwest. The basin fill is primarily alluvial; conglomerates represent stream channel 

deposits, and coal beds indicate interchannel swamps.

The age of the deposits ranges from Early to Late 

Pennsylvanian (table 5), although Skehan and Murray 

(1980b, p. 69) assigned the lower part of the Pondville to 

the Upper Mississippian. It is possible that lower parts of 

the sequence in the Narragansett Bay Group of Skehan 

and others (1979), below the fossiliferous horizons, may 

contain Paleozoic strata older than Mississippian or 

Pennsylvanian. However, little, if any, stratigraphic 

thickness is present in the northern part of the basin 

between the fossiliferous horizons and the Cambrian and 

older rocks, so that such strata must be thin, if they 

exist. Possibly some nonfossiliferous Cambrian beds 

have been mistaken for Pennsylvanian beds. The northwest part of the Narragansett basin and the southern 

part, at least, of the Norfolk basin need critical study in 

this regard.

TRIASSIC AND JURASSIC ROCKS

Triassic and Jurassic rocks occupy a small basin, the 

Middleton basin in Essex County ("Re), and lie in the

subsurface beneath the Coastal Plain cover in Nantucket 

Sound and Nantucket Island. The Triassic and Jurassic 

sedimentary (Jfj) and volcanic (JTJb) rocks in the Nantucket area are described in a later section of this 

chapter. An extensive basin of Triassic and Jurassic 

rocks lies northeast of Essex County in the Gulf of Maine 

(Uchupi, 1966) and in the Bay of Fundy but is outside the 

area of the State bedrock map. In this section, only the 

rocks in the Middleton basin are described.

RED ARKOSIC CONGLOMERATE, SANDSTONE, AND 

SILTSTONE ("Re)

A narrow wedge of red to reddish-gray conglomerate, 

arkosic sandstone, and siltstone ("Re) lying near the 

southernmost of the Newbury basins in the towns of 

Peabody, Danvers, Middleton, and Topsfield northwest 

of Salem, Essex County (fig. 3), contains a few shale beds 

bearing plant fossils of Late Triassic or possibly Early 

Jurassic age (Kaye, 1983a). Fragments of rock of possible 

Triassic age were first recognized in the glacial drift in 

this area (Oldale, 1962). The conglomerate was later 

exposed on one edge of a quarry in northeast Peabody 

and called to the attention of C.A. Kaye by A.E. Shride. 

Plant fossils found later by Peter Robinson from a red 

shale layer in the conglomerate were examined by E.S. 

Barghoorn and found to correspond to fossils from the 

Newark Group of Late Triassic and Early Jurassic age 

(Kaye, 1983a). The true size and shape of the basin called 

the Middleton basin by Kaye (1983a) is not known. It is 

fault bounded on its southeast side. Its northwest side is 

not exposed; the strata are only exposed in the quarry 

face. Its size is inferred, from its topography and aeromagnetic signature, to be about 5.7 km by 0.5 km.

The strata are poorly to well bedded. Conglomerates 

are poorly sorted and have abundant sandy matrix. The 

conglomerate contains rounded to subrounded cobbles of 

nearby granite and granodiorite and poorly rounded to 

angular fragments of red shale and sandstone.

STRATIGRAPHIC PROBLEMS

The descriptions of the various units in the MilfordDedham zone and the discussions of their relationships 

presented in the preceding sections have revealed a 

number of problems and uncertainties, which need further investigation. All of these problems require field 

work, but some require an emphasis on petrologic, 

petrochemical, and isotopic methods; others an attention 

to structural features and degree and nature of metamorphism; and others a more thorough stratigraphic 

analysis.
E38 THE BEDROCK GEOLOGY OF MASSACHUSETTS

FELSIC VOLCANIC ROCKS

Immediately obvious from the descriptions of the rock 

units presented earlier in this chapter is the confusion 

between two similar-appearing volcanic assemblages, 

one dated as Proterozoic Z (the Mattapan and Lynn 

Volcanic Complexes) and the other dated as Early and 

Middle Pennsylvanian (the volcanic rocks in the Wamsutta Formation at Attleboro and at Diamond Hill). 

Emerson (1917) equated the Diamond Hill Felsite with 

the Mattapan, both of which were then considered to be 

Carboniferous in age. Since then, the Mattapan has been 

revealed to be Proterozoic Z in age, whereas the Diamond Hill Felsite is still considered to be Carboniferous. 

LaForge (1932), who considered the Mattapan to be 

Silurian or Devonian, recognized the problem of having 

dissimilar ages for similar rocks. He said (LaForge, 1932, 

p. 29):

So far there is no difficulty in correlating the volcanic rocks (Mattapan 

and Lynn), but an argument of the same sort fails when applied to the 

volcanic rocks of the Attleboro district, in the Narragansett basin. 

Those rocks, which are of the same lithologic types as part of those of 

the Mattapan complex and hence might be supposed to be of the same 

age, are interstratified with sedimentary rocks that are undoubtedly 

Pennsylvanian. At what point in the chain of reasoning there is a flaw 

has not been determined***.

The felsic volcanic rocks flanking the Norfolk basin near 

Lake Pearl and flanking the northern part of the Bellingham basin lie between the two areas, and their age 

assignment is moot. Petrologic geochemical studies such 

as Zarrow's (1978) studies on the Lynn and Mattapan 

Volcanic Complexes might be of considerable aid in 

comparing the suites, as would radiometric analyses. 

Detailed mapping in the different areas might reveal 

significant differences in the associated sedimentary 

strata and in the structural position of the rocks. At the 

present time, I am not convinced that all of the strata 

mapped as Wamsutta Formation in the northwest Narragansett basin and southern Norfolk basin are of Pennsylvanian age. Just as agglomerate of the Mattapan has 

been mistaken for Roxbury Conglomerate in the Boston 

basin, so may fragmental rocks to the south in the 

Norfolk basin and northwest part of the Narragansett 

basin have been wrongly assigned to the Carboniferous. 

In this regard, the volcanic rocks at Attleboro and at 

Diamond Hill appear only in the part of the basin where 

basement is shallow and projects in two places through 

the Pennsylvanian strata; one at Hoppin Hill and another 

to the south-southwest at Manchester Pond Reservoir, 

Attleboro (J.P. Schafer, oral commun., 1982), where a 

sliver of granite like that at Hoppin Hill is brought up 

along a reverse fault. The granite is overlain by carbonate rocks and red beds (J.P. Schafer, oral commun., 

1982) that resemble the Cambrian strata at Hoppin Hill. 

Perhaps even older strata such as those in the Boston 

basin may be faulted up in this area. However, in neither

area are there volcanic rocks between the Pennsylvanian 

strata and the basement. The Cambrian, where known, 

rests directly on granitic basement. Possibly Precambrian rocks are preserved in down-dropped blocks such 

as the Bellingham basin. The Pennsylvanian age for the 

volcanic rocks at Attleboro seems fairly firm. The most 

likely possibility is that the felsic volcanic rocks near 

Lake Pearl flanking the Norfolk basin are Pennsylvanian 

but the felsic rocks flanking the Bellingham basin, Zfm 

(Zm on map), are equivalent to the Mattapan. This is the 

interpretation used on the State bedrock map. The 

Boston basin and the Bellingham basin lie along the same 

structural grain (figs. 1, 8), whereas the Norfolk basin is 

offset south of the Boston basin.

A less obvious problem pertaining to the felsic volcanic 

rocks in the Milford-Dedham zone is the correlation of 

the layered felsic rocks in the Natick-Framingham area 

shown as metamorphosed felsic volcanic rocks (Zvf) on 

the State bedrock map (lower part of the Cherry Brook 

Formation of Nelson) and the Mattapan Volcanic Complex in the Boston area, which lies above the Middlesex 

Fells Volcanic Complex. The felsic volcanic rocks at 

Framingham have been suggested as being the volcanic 

cover into which the Milford-Dedham granite plutons 

intruded. Most of the Mattapan, however, lies nonconformably above the batholithic rocks and is intrusive into 

them, or in a few places is paracontemporaneous with 

them. Because the Proterozoic Z batholiths of eastern 

Massachusetts comprise several plutons differing in age 

and composition, the associated felsic volcanic rocks in 

different areas could be pre-, post-, or syngranite. The 

Mattapan and the felsic rocks in the Framingham area 

thus could belong to a suite of felsic volcanic rocks having 

a range of age similar to that in the granitic plutonic 

rocks. Similar relations in rocks of the same time interval 

have been described by Wood (1974) in Anglesey, Wales, 

and by Hughes and Bruckner (1971) in Newfoundland. 

Bell and Alvord (1976) did not have a zone of felsic 

metavolcanic rocks below the Middlesex Fells, and Drier 

and Mosher (1981) pointed out the lack of felsic volcanic 

rocks below the Hunting Hill Greenstone in northern 

Rhode Island. I suggest that the felsic layers in the 

Framingham area are not volcanic rocks but thin intrusive sheets into the mafic volcanic suite. They are here in 

a more highly metamorphosed regime than in the Boston 

basin and in addition are close to and in a zone of ductile 

shear associated with the Bloody Bluff fault zone (Nelson, 1976). In this interpretation the felsic layers are 

younger than the mafic volcanic rocks; they are more or 

less synchronous with emplacement of the Proterozoic Z 

granites and thus more or less equivalent in age to the 

Mattapan. A similar situation exists in southeastern 

Connecticut, where thin sheets of metamorphosed finegrained alaskite are interlayered with metamorphosed
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E39

mafic volcanic rocks above thick intrusive sheets of Hope 

Valley Alaskite Gneiss. These thin sheets have been 

ductilely deformed, producing the thinly layered to laminated blastomylonites in the Honey Hill fault zone, 

referred to earlier, and alternating layers and laminae of 

felsic and mafic material. Another explanation for the 

apparent interlayering of mafic and felsic rock is that the 

mafic layers are dikes, for example dikes of Brighton 

Melaphyre, cutting the intrusive granite sheets, which 

have been subsequently brought into parallelism through 

pervasive ductile shear. One sees a suggestion of this 

along the Massachusetts Turnpike (1-90) from Natick to 

west of Framingham. Mafic dikes to the east in lightcolored granite become increasingly sheared, recrystallized, and less discordant to foliation and eventually 

become black amphibolitic layers parallel to the foliation 

in gneissic granite. Clearly, the problem of the felsic 

volcanic rocks of eastern Massachusetts has not yet been 

resolved.

LYNN VOLCANIC COMPLEX

The case for a Proterozoic Z age for the Lynn Volcanic 

Complex is not entirely closed, although evidence presented in previous pages tends to support that assignment. Additional petrologic work and detailed field studies in the Mattapan, Lynn, and Newbury Volcanic 

Complexes might provide additional evidence for such an 

age assignment.

BOSTON BAY GROUP

A Proterozoic Z to Early Cambrian(?) age for the 

Boston Bay Group is confirmed by the find of acritarchs 

of that age range in the Cambridge Argillite. A Pennsylvanian age for the Boston basin even before this find was 

at any rate quite convincingly ruled out by the difference 

in paleoenvironment and depositional style between the 

deposits in the Boston basin and those in the Narragansett basin (fig. 9, table 5) and the lack of plant fossils in 

the Boston basin as compared with the abundance of 

plant fossils in the Narragansett basin. However, the 

question remains as to how much of the Boston basin fill 

is Cambrian, equivalent to the Weymouth Formation 

and Braintree Argillite. The proposal by C.A. Kaye 

(1980) that the quartzites and red beds in the Cambridge 

Argillite are Cambrian needs to be explored. Are the 

Tufts Quartzite Member and Milton quartzite unit equivalent to the quartzite and feldspathic quartzite at the 

base of the Hoppin Formation, or are they merely 

deposits on shoals formed during Cambridge deposition 

and before Cambrian encroachment as Billings (1976a) 

indicated? The Cambridge and other units of the Boston

Bay Group contain many thin to laminar graded beds, 

which should be useful in detecting an unconformity or 

disconformity.

BELLINGHAM CONGLOMERATE

The age of the Bellingham Conglomerate in the Bellingham basin is highly uncertain. Like the rocks in the 

Boston basin, the Bellingham lacks the plant fossils so 

abundant in the rocks of the Narragansett basin, and 

other similarities to the rocks of the Boston basin exist. 

Skehan and Murray (in Skehan and others, 1979) suggested that the Bellingham Conglomerate is correlative 

with conglomerates in the Boston basin. The structural 

alignment of the Boston basin with the Bellingham basin 

and the North Scituate basin in Rhode Island, their 

similar lithologies, and their lack of fossils certainly 

suggest that the three might be of similar age. The rocks 

in the Bellingham basin are, however, more metamorphosed than are the rocks of the Boston basin.

An ancillary problem that requires careful mapping is 

the similarity of the Bellingham Conglomerate in the 

Bellingham basin to the Blackstone Group in the Woonsocket area adjacent to the basin. Both are in the 

greenschist facies of metamorphism. The criterion of 

Warren and Powers (1914) involving the presence or 

absence of epidote to distinguish rocks of the Blackstone 

Group from the Bellingham Conglomerate needs to be 

reexamined, although in field reconnaissance the criterion seemed to be useful. The area of particular interest 

is that occupied by the schist of the Blackstone Group 

east of the Bellingham basin and north of the belt of 

Quinnville Quartzite. The State bedrock map probably 

shows the correct distribution of the two sets of rocks, 

but in the field it is not easy to distinguish the two nor to 

draw a line between them with confidence.

BRIGHTON MELAPHYRE

The Brighton Melaphyre represents an interval of 

volcanism, predominantly mafic, which is younger than 

the Mattapan and Lynn (LaForge, 1932, p. 42). As 

described, it appears to have phases similar to some in 

the Mattapan Volcanic Complex; these phases may represent the transition from sialic Mattapan volcanism to 

the more simatic Brighton volcanism. This change in 

volcanism probably reflects a change to extensional 

tectonics associated with late Proterozoic Z rifting. We 

do not know where similar rocks of this age exist in 

eastern Massachusetts other than in the Boston area. 

The areal extent of the Brighton is very small. Possibly 

the amphibolitized dikes to the west on 1-90 described
E40 THE BEDROCK GEOLOGY OF MASSACHUSETTS

above are Brighton. The dikes described as cutting 

Precambrian Z rocks in the Jamestown area in Rhode 

Island might also be of Brighton age. The temporal and 

areal extent of the Brighton needs study.

SEQUENCE IN THE BURLINGTON AREA

The units in the Greenleaf Mountain area, Burlington, 

described by Bell and Alvord (1976; table 3) are not 

clearly related to the other units in the Milford-Dedham 

zone. One can only speculate as to where they fit into the 

stratigraphy. As they are close to the Bloody Bluff fault 

zone, they might be a fault-bounded slice of allochthonous rock or merely a mylonitized part of the mafic 

metavolcanic assemblage.

MAFIC METAVOLCANIC ROCKS AND THE MARLBORO 

FORMATION

The relationship between the juxtaposed mafic volcanic complex in eastern Massachusetts (Zv), east of the 

Bloody Bluff fault, and the Marlboro Formation to the 

west of it is an intriguing question. The Marlboro as 

described in chapter F (Goldsmith, this vol.) consists of 

amphibolite, hornblende gneiss, biotite-quartz-feldspar 

gneiss and granofels, calc-silicate rock and rare marble, 

rusty-weathering garnet-sillimanite schist, and garnetmuscovite-biotite schist. Some layers contain coticule. 

The metamorphosed mafic and felsic volcanic rocks of the 

Milford-Dedham zone (Zv; table 3) contain amphibolite, 

hornblende gneiss, and minor intercalated schist and 

quartzite, as well as felsic layers, but lack the calcareous 

rocks, the rusty schists, and the distinctive coticulebearing layers of the Marlboro. The two contain some 

rocks that are similar in lithology, primarily the amphibolite and hornblende-bearing rocks; this similarity 

has led previous workers to consider them to be one 

suite of rocks (Emerson, 1917). Current studies indicate that they are two different suites, but where to 

place the boundary between them in the MarlboroughFramingham-Concord area is in places unclear. The 

Marlboro Formation, on the whole, is a somewhat more 

varied unit. The primary distinction, as pointed out 

above in this chapter, is that the Middlesex Fells is 

intruded by Proterozoic Z granitoids, whereas the Marlboro is not. However, we do not know with certainty the 

age of the formations in the Nashoba zone. If they are 

Proterozoic as they seem to be, they are perhaps a part 

of the Proterozoic volcanic-arc complex, but which accumulated in a different place than where they are at 

present and which represent a somewhat different 

fades. Stratigraphic analysis of the formations in the 

Nashoba zone (Goldsmith, this vol., chap. F) suggested

that the center of volcanism was to the east. Possibly, 

the volcanic-plutonic complex in eastern Massachusetts 

represented the core of an arc, and the formations of the 

Nashoba zone were deposited on its flanks. At some later 

time the original continuity was disrupted and the rocks 

of the Nashoba zone were transported from their original 

position relative to the Milford-Dedham zone, to become 

affixed, probably in telescoped fashion, in their present 

position against the Milford-Dedham zone. Naylor (1976, 

p. 422) has noted the juxtaposition of two lithologically 

similar but probably temporally different terranes along 

a continuation of the Bloody Bluff fault in southern 

Connecticut. The rocks of the two zones could, however, 

have been originally quite unrelated.

NEWBURY VOLCANIC COMPLEX

Deposition of the Newbury Volcanic Complex does not 

readily fit into the proposed history of the MilfordDedham zone. The fossil assemblages in the Newbury, 

however, indicate Acado-Baltic affinities, indicating that 

the Newbury accumulated on the same side of the 

lapetus Ocean as did the Cambrian of the MilfordDedham zone. One can only draw the conclusion that the 

Newbury did not accumulate at its present site. However, it could possibly have accumulated on the flanks of 

the zone as a volcanic belt developed along the leading 

edge of the zone during its westward movement in the 

middle Paleozoic (Robinson and Hall, 1980). The actual 

site of the superposition of the Newbury on the crystalline basement has since been obliterated by faulting.

THE STRATIGRAPHIC RECORD IN THE 

MILFORD-DEDHAM ZONE

The stratigraphic record in the Milford-Dedham zone 

(table 6) comprises two dissimilar cycles. A Proterozoic 

Y or Z cycle of arc-margin accumulation, volcanic arc 

accumulation, and orogenesis followed by Proterozoic Z 

plutonism, volcanism, and subsequent flysch to molasse 

deposition of turbidites during an extensional phase was 

completed by the Cambrian Period. The Cambrian initiated a new cycle, spanning the Paleozoic, beginning with 

shelf deposition and closing with deposition during rifting 

in the Mesozoic. In the earlier cycle, the felsic volcanism 

(and plutonism) represented by the Mattapan Volcanic 

Complex (and Dedham Granite) is considered to be a 

precursor to the opening of the proto-Atlantic Ocean 

(lapetus) (Rankin, 1975), and the subsequent basin filling 

and mafic volcanism (Brighton Melaphyre) to be associated with the actual rifting. The length of the hiatus 

between the end of development of the volcanic-plutonic 

complex in eastern Massachusetts and the beginning of 

the phase of felsic volcanism and plutonism is not known.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E41

TABLE 6. Stratigraphic record in the Milford-Dedham zone

Event Paleotectonic environment Age Representative units

Deposition of arenite, calc-arenite, 

and pelite. A volcanic component 

increasing towards top.

Volcanism and plutonism, predominantly mafic to intermediate calcalkalic; volcaniclastic and epiclastic sedimentation.

Intrusion, calc-alkalic to subalkalic 

granite and associated volcanism 

and later felsic volcanism.

Deposition of turbidite sequence 

accompanied in early stages by 

felsic to mafic volcanism.

Deposition of arenite, pelite, and 

carbonate.

Gabbroic to granite intrusion, typically alkalic; associated volcanism.

Mafic to felsic volcanism, and volcaniclastic and epiclastic sedimentation.

Deposition of conglomerate, sand, 

silt, and coal. Mafic to felsic volcanism in early stage(?).

Plutonism (in southern Rhode

Island). Intrusion of mafic dikes

in Boston area. 

Deposition of conglomerate, arkose,

and mud. Extrusion of basalt as

flows, dikes, and sills. 

Deposition of sand, silt, and clay

Littoral to shelf deposition on unknown 

basement. Source of material a 1,500- 

Ma terrane, probably to the east.

Island arc-continental margin(?) association in compressional tectonic 

regime.

Pre-rifting intrusion and volcanism in a 

compressional to extensional tectonic 

regime.

Post-rifting continental to marine deposition in deepening marginal basin or 

aulacogen formed in extensional tectonic regime. Highlands and volcanic 

centers to the south, southwest, and 

west(?).

Transgressive littoral and shallow-shelf 

sequence in epeirogenic regime.

Anorogenic intrusion in epeirogenicextensional tectonic regime.

Island-arc(?) volcanism and sedimentation in compressional tectonic 

regime(?).

Alluvial-fan fill in shallow basin; highlands and volcanic center(?) to northwest.

Pre-rifting plutonism and volcanism in 

extensional tectonic regime.

Alluvial filling of rift basin, extrusion

of simatic material in extensional

environment. 

Marine transgressive and regressive

sequences on a continental shelf in an

epeirogenic regime.

Middle or Late Proterozoic; 

older than 630 Ma, 

younger than 1,500 Ma.

Middle or Late Proterozoic;

older than 630 Ma,

younger than 1,500 Ma.

Possibly about 750 Ma. 

Late Proterozoic; 630 Ma

and younger (about 600

Ma?). 

Late Proterozoic, probably

younger than 600 Ma.

Early and Middle 

Cambrian.

Ordovician to Devonian

Late Silurian and Early 

Devonian.

Late Mississippian and 

Pennsylvanian.

Permian

Triassic and Jurassic

Cretaceous to Tertiary

Westboro and Plainfield 

Formations; Quinnville 

Quartzite and Sneech 

Pond Schist.

Middlesex Fells Volcanic 

Complex, Hunting Hill 

Greenstone, Salem 

Gabbro-Diorite.

Dedham, Milford, and 

Westwood Granites, Mattapan Volcanic Complex.

Boston Bay Group including 

Brighton Melaphyre.

Weymouth and Hoppin 

Formations and Braintree Argillite.

Quincy Granite, Cape Ann 

Granite, Peabody Granite, Blue Hill Granite 

Porphyry, Nahant Gabbro.

Newbury Volcanic Complex.

Narragansett Bay Group of 

Skehan and others (1979), 

volcanic rocks in Wamsutta Formation.

Narragansett Pier Granite.

Middleton-basin fill; Medford dike.

Coastal plain deposits.

A period of uplift and erosion must have occurred before 

deposition of the Roxbury Conglomerate. A basin must 

have been available during accumulation of the Mattapan, because the volcanic rocks contain intercalated 

terrestrial and waterlain beds. This suggests a marine or 

partly marine basin-and-range environment. The sandstone in the upper part of the Cambridge Argillite 

indicates shoaling at that time, which may have been a 

precursor for uplift and erosion before the Cambrian 

transgression. In the later, Paleozoic cycle, however, the 

depositional record within the zone does not indicate a 

period of orogenesis of the sort recorded in the Proterozoic. The tectonic processes were primarily extensional, 

as indicated by the static Paleozoic plutonism. The

western part of the Milford-Dedham zone was involved, 

however, if only passively, in Paleozoic compressive 

tectonics (Goldsmith, this vol., chap. H). A new cycle has 

begun with the Cretaceous and Tertiary marine onlap 

onto the passive continental margin of accreted North 

America. The scheme of tectonic evolution derived is 

similar to that proposed by Williams and King (1979) for 

the Trepassy area of the Avalon Peninsula, Newfoundland. The alternation of compressional and extensional 

tectonics evident from the depositional history shown in 

table 6 can be translated into terms of plate-tectonic 

theory better when Massachusetts and the New England 

region are considered as a whole in another chapter of 

this volume (Goldsmith, this vol., chap. H).
E42 THE BEDROCK GEOLOGY OF MASSACHUSETTS

43°00r

72°30' 72°00' 71°00' 70°30'

EXPLANATION

Milford-Dedham Zone

Paleozoic granite (Permian to Ordovician)

Pennsylvanian strata 

Devonian and Silurian strata 

Cambrian strata

Boston Bay Group and equivalent (?) strata 

(Early Cambrian to Proterozoic Z)

Mattapan Volcanic Complex and equivalent (?) 

rocks (Proterozoic Z)

Rhode Island and southeastern Massachusetts 

batholiths and equivalent rocks in 

eastern Connecticut (Proterozoic Z) ATLANTIC 

OCEAN

Mafic volcanic-plutonic complex in eastern Massachusetts 

_?y_L ?e_! ancj equivalent rocks in eastern Connecticut (Proterozoic Z) 

(Ze): volcanic and volcaniclastic rocks (Zev), plutonic rocks 

(Zep), Claypit Hill Formation and sequence near Burlington, 

Mass. (Zee)

Quartzitic assemblage (Proterozoic Z)

Nashoba Zone and Putnam Terrane

Paleozoic and Proterozoic rocks

Note' Units and their symbols do not match 

those on the State bedrock map.

Willimantic domeX -

Block [stand Sound

42W -

41°30' -

FIGURE 13. Distribution of major groups of rocks in the Milford-Dedham zone, Massachusetts, Rhode Island, and Connecticut.

REGIONAL RELATIONS IN SOUTHEASTERN 

NEW ENGLAND

The relations of the Milford-Dedham zone in Massachusetts to the rest of southeastern New England are 

summarized in figures 13 and 14. Primary sources for the 

compilation were the State bedrock map, Quinn's (1971)

paper on the bedrock geology of Rhode Island, and the 

preliminary bedrock geologic map of Connecticut (Rodgers, 1982). Structural data have been omitted except for 

the names of the bounding faults on the west. A certain 

amount of interpretation has been introduced in the 

compilation, and some of the debatable units discussed in 

this chapter have been assigned to age brackets that
Hunts Brook

syncline, Preston Gabbro Eastern

southeastern area, southeastern Natick area, Massachusetts Northern

Connecticut Connecticut eastern (Bell and Rhode Island

(Goldsmith, (Goldsmith, Massachusetts Alvord, (Quinn,

111

cj oL

N

o

o

°~

1980) 1980) (Nelson, 1974) 1976) 1971)

.. ^

a.

0

0

 o

1

ro

Brook

Gneiss

New

London 

Gneiss

Mamacoke 

Formation

Plainfield

Formation

\"K^^

\^^^

^^Honey Hill fault

Approximate location 

of Kendall Green' 

lithology

Quinebaug

Formation

Waterford

Group

^

Plainfield

Formation

k^^^J

Lake Char fault

~> ^

^^^

Marlboro

Formation

Claypit Hill

Formation

~^^^*_x^^^

Brook 

Formation

Kendall 

Green Fm.

Westboro

Quartzite

Rice 

Gneiss

Bloody Bluff

?'~~~-~--^^

-V-Y^^ ?

<//'CoV^?-^J^~^

nforr^7)r~

~~->

^~~~~~~~--~ -__^

^^^?^-^

Marlboro

Formation

Burlington

Formation

Greenleaf

Formation

-^s^^'-s-^s*

Middlesex 

Fells 

Volcanic 

Complex

Westboro

Quartzite

Unnamed 

stratified

rocks

fault

/ r1

/s _T

\unnamed unit

?

'^^^^

^ 

Hunting 

Hill 

Greenstone

Sneech

Pond 

Schist

Quinnville

Mussey 

Brook

Schist

<n

V)

2

u

CD

FIGURE 14. Some Proterozoic Z units in the Milford-Dedham zone in southeastern Connecticut, eastern Massachusetts, and R
E44 THE BEDROCK GEOLOGY OF MASSACHUSETTS

differ from those on the State bedrock map. Such assignments can stimulate future investigation and discussion. 

These changes are discussed below.

QUARTZITIC ASSEMBLAGE

The quartzitic assemblages (Zq on fig. 13) include the 

Westboro Formation (Zw), the Plainfield Formation 

(Zp), the lower part of the Blackstone Group (Zbq, Zbs), 

part of the undivided Blackstone Group (Zb) in Massachusetts and northern Rhode Island, and rocks mapped 

as Blackstone Group in southern Rhode Island. The 

undivided Blackstone in Massachusetts and northern 

Rhode Island includes some amphibolite and other metavolcanic rocks, but, where quartzite is the most conspicuous lithology, the undivided Blackstone has been 

included in the quartzitic assemblage, as for example in 

the town of Uxbridge, Mass. These formations do not 

contain a particular quartzite layer but represent formations in which quartzite layers are conspicuous if not the 

predominant lithology.

The Rice Gneiss, which has a volcanic component, has 

been included in the quartzitic assemblage in figure 13 

rather than in the overlying metavolcanic assemblage 

because the Rice lies stratigraphically below the Westboro (Nelson, 1974) and is in part quartzitic (table 3). 

Also, as no clear base has been observed for the quartzitic units in the quartzitic assemblage elsewhere, and as 

the Rice Gneiss occupies only a small area, it is most 

conveniently placed in the quartzitic assemblage rather 

than in an isolated unit lying below the quartzitic formations. As described by Nelson (1974), the Rice Gneiss 

contains lithologies resembling some of those in the 

Plainfield Formation.

VOLCANIC-PLUTONIC COMPLEX IN EASTERN 

MASSACHUSETTS AND CORRELATIVE ROCKS

The metavolcanic assemblage that lies primarily above 

the quartzitic assemblage is part of the volcanic-plutonic 

complex in eastern Massachusetts. In Connecticut it is 

called the Waterford Group (fig. 14); in Rhode Island it is 

not recognized as a unit but consists of separately 

mapped metavolcanic rocks and plutons of gabbro and 

diorite (Zgb, Zdi). Shown on figure 13 are both the 

metavolcanic component, Zev, and the plutonic component, Zep. Where these components are too mixed or too 

thin to be shown separately, they are shown as a single 

unit, Ze.

In northeastern Massachusetts, Bell and Alvord (1976) 

used the term "Middlesex Fells Volcanic Complex" for 

the largely mafic metavolcanic rocks older than the 

Dedham Granite. Their term, as used, includes the 

Cherry Brook Formation, the Middlesex Fells Volcanic

Complex in its type area, stray pieces of stratified 

metavolcanic rocks such as the unnamed units of Volckmann (1977) in the Holliston area, amphibolite and augen 

gneiss mapped by Toulmin (1964) in the Salem quadrangle, and mafic metavolcanic rocks mapped by Bell and 

others (1977) and Dennen (1975) in the Georgetown and 

Ipswich quadrangles, respectively. This collective use of 

the term makes it more useful than the more areally 

limited term Cherry Brook Formation, in spite of the fact 

that the term Cherry Brook Formation has priority. Bell 

and Alvord considered the Middlesex Fells Volcanic 

Complex to be the equivalent of the Hunting Hill Greenstone of the Blackstone Group. The metamorphosed 

mafic and felsic volcanic rock unit (Zv) used on the State 

bedrock map is equivalent to the Middlesex Fells Volcanic Complex as used by Bell and Alvord (1976). On figure 

13, however, I have used volcanic and volcaniclastic 

rocks (Zev) as a collective name for the units in the 

metavolcanic assemblage that overlies the quartzitic 

assemblage in eastern Massachusetts and northern 

Rhode Island.

Included in the metavolcanic assemblage are those 

parts of the undivided Blackstone Group (Zb) that contain prominent amphibolite, such as near Hopedale and 

Milford and near Blackstone, Mass. Also included in the 

metavolcanic assemblage are the three metamorphic 

units in the Georgiaville quadrangle in northern Rhode 

Island that Richmond (1952) named the Absalona and 

Woonasquatucket Formations and the Nipsachuck 

Gneiss. His description of these rocks suggests that they 

are largely metamorphosed felsic or intermediate volcanic rocks, in part at least tuffaceous. Their exact stratigraphic position is uncertain (Quinn, 1971, p. 14-15), for 

they may lie above or below the Blackstone Group. 

Because they seem to lie above the quartzitic formations 

on the east flank of the Rhode Island anticlinorium 

(Goldsmith, this vol., chap. H), I have assigned these 

rocks to the metavolcanic assemblage.

The plutonic component of the volcanic-plutonic complex in eastern Massachusetts includes all the mafic 

plutonic rocks older than the granites and granodiorites 

of the Rhode Island and southeastern Massachusetts 

batholiths and related plutons; for example, the Dedham 

Granite (Zdgr, Zdngr), Milford Granite (Zmgr), and 

Topsfield Granodiorite (Ztgd). These mafic-plutonic 

rocks consist mostly of gabbro, diorite, and syenite, such 

as the Salem Gabbro-Diorite (Zdigb), unnamed diorite 

and gabbro (Zdi, Zgb), and the Sharon Syenite (Zssy). 

These plutonic rocks are discussed in Wones and Goldsmith (this vol., chap. I). North of Salem and southeast of 

the Middleton basin, the plutonic and volcanic components are not distinguished separately on figure 13 

because of their intermixing.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E45

The Claypit Hill Formation and the formations of Bell 

and Alvord (1976) near Burlington (see table 3), which 

were grouped with the metavolcanic assemblage on the 

State bedrock map, are shown separately on figure 13 as 

a possibly younger group of metasedimentary and metavolcanic rocks (Zee) lying above the metavolcanic assemblage (Zev).

In southeastern Connecticut, the Waterford Group 

(Goldsmith, 1980) is the stratigraphic equivalent of the 

volcanic-plutonic complex in eastern Massachusetts (fig. 

14). It contains both stratified metavolcanic and metasedimentary rocks and metaplutonic rocks; for example, 

the New London Gneiss, the Rope Ferry Gneiss, and 

possibly the Stonington Gneiss phase (Rodgers and others, 1959) of the Mamacoke Formation. The volcanic and 

plutonic rocks in this complex are not distinguished 

separately on figure 13 because of the thinness of the 

units.

The older metamorphic rocks of southeastern Massachusetts and the rocks of similar age in the Newport and 

Sakonnet areas (fig. 7) are included in the metavolcanic 

assemblage because of their apparently large original 

volcaniclastic component, although the rocks are not 

necessarily mafic. They could, however, be all or in part 

equivalent to part of the mica schist and phyllite (Zbs) of 

the Blackstone Group in northeastern Rhode Island. 

These units in southeastern Massachusetts and southern 

Rhode Island are the gneiss and schist near New Bedford 

(Zgs), the biotite gneiss near New Bedford (Zgn), the 

Proterozoic Z strata at different places along the Sakonnet River (Quinn, 1971), the Jamestown Formation of 

Skehan and Murray (1980b), and the Price Neck Formation of Rast and Skehan (1981) in the Newport area 

following the age assignments and the descriptions of 

Rast and Skehan (1981).

RHODE ISLAND AND SOUTHEASTERN MASSACHUSETTS 

BATHOLITHS AND RELATED GRANITOIDS

The Rhode Island and southeastern Massachusetts 

batholiths (Zgr on fig. 13) occupy much of the area of the 

Milford-Dedham zone. Units included in this assemblage 

are the Dedham Granite (Ddgr, Ddngr), Topsfield Granodiorite (Dtgd), Westwood Granite (Zwgr), Milford 

Granite (Zmgr, Zmgd), Esmond Granite (Zegr), granite 

of the Fall River pluton (Zfgr), porphyritic granite 

(Zpgr), alaskite (Zagr), the Ponagansett Gneiss (Zpg), 

Hope Valley Alaskite Gneiss (Zhg), Scituate Granite 

Gneiss of the Sterling Plutonic Group (Zsg), and biotite 

granite (Zgr). In southeastern Connecticut and southern 

Rhode Island, the units belonging to the Sterling Plutonic Group are present including the Potter Hill Granite 

Gneiss. Hermes and others (1981) have found that a large 

area formerly mapped as Scituate Granite Gneiss in

central Rhode Island is of middle Paleozoic rather than 

Proterozoic Z age. (This area is shown on fig. 13 as Ffeg.) 

Also included in Zgr are the Newport Granite of Rast 

and Skehan (1981) and the Willimantic Gneiss in eastcentral Connecticut. The Bulgarmarsh Granite is a phase 

of the granite of the Fall River pluton.

MATTAPAN VOLCANIC COMPLEX AND EQUIVALENT 

ROCKS

The Mattapan and Lynn Volcanic Complexes (Zm, 

DZ1), and the felsite porphyry near Bellingham (Zfm (Zm 

on the State geologic map)) are shown as Zm on figure 

13. The volcanic rocks near Lake Pearl (Pwv) are shown 

as equivalent to the Mattapan because of their lithologic 

similarity and because they lie in a structural trough 

more or less aligned with the Boston basin. The felsite at 

Diamond Hill remains assigned to the Pennsylvanian, 

although detailed mapping in this area might reveal that 

all or part of the rocks mapped as Wamsutta in this area 

are actually equivalent to part of the Roxbury Conglomerate.

BOSTON BAY GROUP AND EQUIVALENT ROCKS

The Bellingham Conglomerate (PZb) in the Bellingham and North Scituate (Rhode Island) basins is shown 

on figure 13 as equivalent in age to the Boston Bay 

Group. The Bellingham resembles the Roxbury Conglomerate, and no fossils have been found in the Bellingham in contrast to the highly fossiliferous Pennsylvanian 

strata in the Narragansett basin to the east. As mentioned in an earlier part of this chapter, the Bellingham 

and North Scituate basins lie in a structural trough 

aligned with the Boston basin.

YOUNGER UNITS

The distribution of the Cambrian and younger rocks 

(C, DS, Ps, and ftg on fig. 13) has not been changed from 

the State bedrock map except that the Pennsylvanian 

volcanic rocks near Lake Pearl have been correlated with 

the Mattapan Volcanic Complex (Zm on fig. 13). The 

distribution of Cambrian strata on Conanicut Island, 

R.I., is taken from Skehan and others (1981).

THE MILFORD-DEDHAM ZONE IN THE 

CALEDONIDES - CORRELATION

The stratigraphic sequence in the Milford-Dedham 

zone has been correlated by numerous authors with 

sequences in the Maritime Provinces, Newfoundland, 

England, Wales, and Morocco. The term "Avalonian"
E46 THE BEDROCK GEOLOGY OF MASSACHUSETTS

(Kay and Colbert, 1965) has been applied to terranes 

containing these sequences in North America. The Avalonian terranes form a southeastern platform of the 

Appalachian orogenic belt, as first pointed out by 

Williams (1964). Summary sections and their correlations 

in the different terranes, most of which cover eastern 

Massachusetts, have been presented by Weeks (1957), 

Schenk (1971), Rast and others (1976), and Strong (1979). 

The most recent summary descriptions of rocks in the 

Avalon Peninsula of Newfoundland are by King (1980; 

O'Brien and King, 1982) and Anderson (1981). Equivalent sequences have been described for example in 

Anglesey, Wales, by Wood (1974), in the English Midlands by Rast and others (1976), and in Morocco by 

Affaton and others (1980). The relationship of eastern 

Massachusetts and Rhode Island to the above terranes is 

discussed by Skehan (1969, 1973), Skehan and others 

(1978), Rast (1980), and Skehan and Murray (1980a).

Gross correlation of rocks of eastern Massachusetts 

with those of the Avalon Peninsula is fairly easily made. 

The Mattapan Volcanic Complex and Boston Bay Group 

are similar to the Proterozoic Z sequence of the Avalon, 

the Conception Group and Harbour Main Volcanics, as 

summarized by King (1980; O'Brien and King, 1982) 

except that there the fossil record is more abundant, the 

sequence is apparently thicker, and no older basement is 

exposed. In Nova Scotia and New Brunswick, however, 

as in Massachusetts, a basement (Basement Complex, 

Greenhead Group, Kelly Mountain and George River 

Group, and Brookville gneiss) is present beneath the 

Proterozoic Z (Hadrynian) rocks (Georgeville Group, 

Fourchu Group, Coldbrook Group). The basement rocks 

cannot be readily correlated with the pre-Dedham Granite strata, however. According to Rast (1980, p. 63), 

these rocks, apparently older than 770 Ma, consist of 

quartzites, argillites, and carbonates in part converted to 

schists, paragneisses and marbles. J.W. Skehan (written 

commun., 1983) suggested that these rocks are equivalent to the Westboro Formation and Blackstone Group. 

None of these rocks clearly resembles the volcanicplutonic complex in eastern Massachusetts. Possibly the 

Hadrynian(?) Bass River Complex and George River 

Group ( = Greenhead Group) of Nova Scotia (Keppie, 

1979; Keppie and Schenk, 1982) are equivalents of the 

complex.

The eastern Massachusetts terrane has also been 

correlated with rocks of the slate belt in Virginia, North 

Carolina, and South Carolina (Glover and Sinha, 1973; 

Rast and others, 1976; Williams, 1978; Nelson, 1981). 

The strata in the Carolina slate belt are equivalent to 

some of the rocks of the Boston basin area in lithology 

and fossil fauna, but it is not generally realized that the 

adjacent Charlotte belt bears a remarkable resemblance 

to the volcanic-plutonic complex in eastern Massachusetts. In North Carolina, a probably Proterozoic Z 

volcanic-plutonic complex, consisting largely of plutonic 

rocks ranging from gabbro to granodiorite and enclosing 

masses of mafic to felsic volcanic rocks and metasediments, forms the core of the Charlotte belt (Goldsmith 

and others, 1982b). This complex is flanked by Proterozoic Z to Cambrian volcanic rocks and sediments of the 

Carolina slate belt and is intruded by early to midPaleozoic plutons of gabbro (and syenite) to granite and 

by late Paleozoic plutons of granite. This terrane thus 

described is quite similar to that in eastern Massachusetts, particularly west and north of the Boston basin. 

Although no late Paleozoic granites are present in the 

Boston area, they are present in southern Rhode Island, 

and late Paleozoic volcanic rocks are present in northern 

Rhode Island.

MESOZOIC AND TERTIARY STRATIGRAPHY OF 

CAPE COD AND THE NEARBY ISLANDS

By E.G.A. WEED

INTRODUCTION

Cape Cod and the nearby islands, the Elizabeth 

Islands, Martha's Vineyard, and Nantucket, are almost 

entirely covered by Quaternary surficial deposits. No 

pre-Pleistocene rocks are exposed southeast of the Cape 

Cod Canal, except the distorted beds in the cliffs at the 

western end of Martha's Vineyard. However, pebbles, 

fragments, and blocks of older rocks are found in the 

glacial drift and may be encountered in sand and gravel 

pits.

Data on the pre-Pleistocene rocks are available only 

from drill holes and other manmade excavations and the 

natural exposures on Martha's Vineyard noted above. 

These few data sources are clearly inadequate to accurately portray the distribution of rocks in the subsurface, 

even at 1:250,000 scale. Their very paucity, however, 

makes the information obtained from them all the more 

critical.

Early studies of the geology of Cape Cod and the 

nearby islands mostly addressed materials found in outcrop (Emerson, 1917). Cretaceous and Tertiary fossils in 

this area were found by Lyell (1843), Desor (1849), 

Shaler (1888, 1889, 1890), White (1890), Ball (1894), 

Woodworth (1897), Rollick (1906), and Berry (1915), and 

later by Woodworth and Wigglesworth (1934) and Kaye 

(1964a,b,c; 1983b). Offshore studies of the U.S. Atlantic 

margin (Emery and Uchupi, 1972; Weed and others, 

1974; Austin and others, 1980) and hydrologic studies on 

land (Maevsky and Drake, 1963; Kohout and others, 

1977; Delaney, 1980; Walker, 1980; Guswa and LeBlanc,
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E47

1985) have contributed to our understanding of the 

Coastal Plain geology. Geophysical investigations in the 

Cape Cod area have been made by Hoskins and Knott 

(1961), Oldale and Tuttle (1964), Oldale (1969), Oldale 

and others (1973), Ballard and Uchupi (1975), Grow and 

Schlee (1976), Bothner (1977), and Klitgord and 

Behrendt (1979, 1980). The mineralogy of the Coastal 

Plain materials, in particular the clays, has been studied 

by L.J. Poppe (written commun., 1976) and S.A. WoodNeedell (written commun., 1976).

As part of the investigations made for the State 

bedrock map (Zen and others, 1983), 15 shallow bore 

holes (sites 13-17, 23-24 and 26-33, fig. 15; tables 7, 8) 

were drilled in Martha's Vineyard, Cape Cod, Marshfield, and Scituate from 1977 through 1982. Unconsolidated Tertiary material was recovered in two bore holes 

(sites 26 and 29). Hard material (granodiorite, sandstone, shale, hardpan (till?)) was reached in eight holes 

but recovered in only six holes (sites 27-32).

The locations of the drill and auger holes and the 

natural exposures that serve as sources for the data for 

this report are shown on figure 15. Lithology, depth, 

thickness, age, and principal references are presented in 

table 8. Because the pre-Mesozoic rocks of the subsurface 

are described by Wones and Goldsmith in chapter I of 

this volume under the heading "Proterozoic Z batholithic 

rocks," only the Mesozoic and Tertiary strata are 

described in this section. Data on the pre-Mesozoic 

basement encountered in the holes are included, however, in table 8.

GEOLOGIC SETTING

The rocks beneath the Quaternary cover on Cape Cod 

and the islands lie southeast of the exposed rocks of the 

Milford-Dedham zone (Zen and others, 1983; Wones and 

Goldsmith, this vol., chap. I), which is interpreted as 

extending to the east of Cape Cod (Klitgord, 1984). The 

rocks known to be present in the subsurface of Cape Cod 

and the islands include, from oldest to youngest, granite, 

gneiss, and schist of probable Proterozoic Z age (Zgg of 

the State bedrock map), Triassic and Jurassic basalt 

(J~Eb), Cretaceous sediments (K), and Tertiary sediments (T). K-Ar radiometric analyses of biotite and 

hornblende obtained from samples of the core of the 

Tubman hole (site 3) gave ages as follows: biotite from 

granodiorite (depth unknown) 348±12 Ma, hornblende 

518±30 Ma; biotite from a mafic xenolith at 190.2 m (624 

ft) depth 400 ±14 Ma, hornblende 566 ±22 Ma; biotite 

from granodiorite at 294.5 m (966 ft) depth 380±19 Ma 

(Weston Geophysical Research, Inc., 1977; Zartman and 

Marvin, this vol., chap. J, table 1). As these are minimum ages, a Proterozoic Z age for at least part of the 

basement under Cape Cod is confirmed. Probable Proterozoic rock was encountered in a few other drill holes on 

Cape Cod, at the holes at Harwich, Brewster, and Woods 

Hole (sites 1-5, fig. 15).

The northeastern continuation of the Atlantic Coastal 

Plain, which is well exposed on the surface as far 

northeast as northern New Jersey (Weed and others, 

1974), and is present in the subsurface in Long Island (de 

Laguna, 1963; Perlmutter and Todd, 1965; Soren, 1971, 

1977; Minard and others, 1974; Weed and others, 1974), 

lies beneath Cape Cod and the islands. Because the sites 

of pre-Pleistocene materials known in southeastern Massachusetts are few, each has considerable importance for 

interpreting the geology of the northern Coastal Plain 

and the Appalachian region.

DESCRIPTION OF POST-PALEOZOIC UNITS

Triassic and Jurassic strata consisting of basalt (J~Eb) 

and sedimentary and volcanic rock (J~E) have been identified only beneath Nantucket and Nantucket Sound. 

Cretaceous clay, sand, and silt (K) have been reported 

from wells and outcrops at many places in southeastern 

Massachusetts and the islands (table 8), but no rocks of 

Mesozoic age have been identified in Cape Cod proper. 

The northwestern limit of Cretaceous materials in the 

subsurface is shown on the State bedrock map southeast 

of the main part of the Cape (fig. 15). The boundary 

passes through Monomoy Point to the northeast and 

between the Elizabeth Islands and Martha's Vineyard to 

the southwest. Tertiary deposits are scattered and fragmentary.

TRIASSIC AND JURASSIC BASALT (J"6b)

A 514-m hole drilled on Nantucket (site 7, USGS 6001) 

encountered Triassic and Jurassic basalt (J~Eb) from 470 

m depth to the bottom of the hole. Interpretations of 

seismic profiles in the area, including USGS multichannel 

seismic line 5 (Grow and others, 1979; Austin and others, 

1980), indicate that a northeast-trending Triassic and 

Jurassic basin, here called the Nantucket basin, extends 

from the middle of Nantucket Sound southeast about 25 

km (fig. 15). It is estimated to be about 100 km long, and 

the seismic profile of line 5 indicates that it is about 8 km 

deep (Folger and others, 1978, fig. 2). The basalt encountered on Nantucket underlies a sedimentary and volcanic 

section (J~E) interpreted from seismic profiles to be 

present under Nantucket Sound. The profiles indicate 

that the beds dip northeastward toward a normal fault 

south of Cape Cod (cross section E-E' on the State 

bedrock map).

The basalt, although highly altered, is similar in 

appearance to basalts of the Newark basin in New Jersey 

(Faust, 1975). The alteration is apparently the result of
E48 THE BEDROCK GEOLOGY OF MASSACHUSETTS

70°30' 70°00'

42W

41°30'

Scituate

31,

.29

27. *28

26*0,

Marshfield .21 20. ,19 -18

Plymouth

y

CAPE COD BAY

/^Approximate limit of 

I bedrock outcrop

Canal

\

Brewster. .4 2r 20

\ Harwich

Marioi CAPE COD

Monomoy 

Point

24

Woods Hole =

or

BUZZARDS BAY 

Elizabeth Islands

.10

NANTUCKET

10 20 MILES

20 KILOMETERS

FIGURE 15. Outcrop and auger- and core-drilling sites that provide information on the pre-Mesozoic basement and Mesozoic and 

Tertiary deposits in the area of Cape Cod and the nearby islands, Massachusetts.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E49

TABLE 7 . Location and identification of outcrops and auger- and core-drilling sites in the area of Cape Cod and the nearby islands,

Massachusetts

[Locations of sites shown on figure 15; total depth measured from Kelly bushing (K.B.); WHOI, Woods Hole Oceanographic Institution; , not given. See table 8 for

principal data sources]

Site 

no.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Site designation

HJW-106 (Hole A)

BMW-23 (Hole C)

BMW-24 (Hole D)

"Tubman."

BMW-25 (Hole B)

WB-1 (WHOI dock)

Assorted outcrops in

bay west of Marion.

USGS 6001

ENW-50

Gay Head Cliffs

Zack's Cliff

Nonamesset

Coskata

MV-1

MV-2

OBW 33-35 (MV-3)

MV^l

ENW 70-73

Holden's Pond

Stark's Well

Jim's II

Race Point

DGW-193-195

YAW

Location

Harwich, Cape Cod

Brewster, Cape Cod

Brewster, Cape Cod

Brewster, Cape Cod

Woods Hole, Cape

Cod.

Buzzards Bay coast

in Plymouth and

Bristol Counties.

Nantucket

Martha's Vineyard

Martha's Vineyard

Martha's Vineyard

Lackey's Bay, Nonamesset Island.

Nantucket

West Tisbury, Martha's Vineyard.

Katama Beach,

Martha's Vineyard.

Sengekontacket

Pond, Martha's

Vineyard.

Katama Beach,

Martha's Vineyard.

Trapps Pond, Martha's Vineyard.

Cape Cod

Cape Cod

Cape Cod

Cape Cod

Corporation Beach,

Dennis, Cape

Cod.

Windmill Park, Yarmouth, Cape Cod.

Latitude, 

longitude

41°41'06"N.,

70°06'55"W.

41°44'35"N.,

70°06'22"W.

41°44'50"N.,

70°05'11"W.

41°45'03"N.,

70°05'01"W.

40°31'30"N.,

70°40'20"W.

41°30' N. to

41°40'N.,

70°50'W. to

71°02'W.

41°15'55"N.,

70°02'17"W.

41°23'50"N.,

70°35'30"W.

41°21'00"N.,

70°50'10"W.

41°19'30"N.,

70°48'30"W.

41°30'35"N.,

70°41'25"W.

41°22'00"N.,

70°01'30"W.

42°03'30"N.,

70°06'25"W.

42°03'50"N.,

70°09'00"W.

42°03'25"N.,

70°11'00"W.

42°02'40"N.,

70°14'40"W.

41°45'04"N.,

70°11'21"W.

41°39'16"N.,

70°11'48"W.

Site altitude, 

meters 

(feet)

8.2 (26.8)

21.3 (70)

22.9 (75)

30.5 (100)

-2.4 (-8)

11 (36)

10 (33)

0(0)

0(0)

0(0)

0(0)

3.1 (10)

4.3 (14)

3.1 (10)

6.2 (20)

1.6 (5)

1.6 (5)

Total depth, 

meters 

(feet)

304.8 (1,000)

304.8 (1,000)

304.8 (1,000)

132.6 (435)

85.7 (281)

0(0)

514 (1,686)

262 (860)

0(0)

0(0)

0(0)

102.2 (335)

80.5 (264)

61.9 (203)

66.5 (217)

69.8 (229)

94.2 (309)

91.5 (300)

Recovery method

Drilled, core

Drilled, core

Drilled, core

Redrilled, core

Drilled, core

Outcrops

Drilled, rotary,

split-spoon, and

punch or driven

barrel coring.

Drilled, split-spoon

coring.

Outcrop

Outcrop, sea cliffs

Outcrop, not

exposed at time

of inspection

(1977).

Drilled, cable tool

Mobile 50 rig auger

Mobile 50 rig auger

Mobile 50 rig auger

Mobile 50 rig auger

Mobile 50 rig auger

Pipe driven as casing, samples.

Pipe driven as casing, samples.

Pipe driven as casing, samples.

Pipe driven as casing, samples.

Drilled, auger and

split spoon.

Drilled, auger and

split spoon.

Year

1961.

1962.

1962.

1961.

1965.

1978.

1975.

1976.

1888-1984.

1977.

1934.

1933.

1977.

1977.

1977.

1977.

1977.

1960.

1960.

1960.

1960.

1977.

1977.
E50 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 7 . Location and identification of outcrops and auger- and core-drilling sites in the area of Cape Cod and the nearby islands,

Massachusetts Continued

Site 

no.

Site designation Location Latitude, 

longitude

Site altitude, 

meters 

(feet)

Total depth, 

meters 

(feet)

Recovery method Year

24 FA-8-82-1

25 PH-77-1

Maravista,

Falmouth, Cape

Cod. 

Pine Hill, Manomet,

Plymouth County.

41°32'45"N., 

70°35'10"W.

1.6 (5) 83.3 (273) Drilled, wire line, 

core barrel.

33.6 (110) Drilled, auger

1982.

1977.

26

27

28

29

30

31

32

33

MB-77-1

MB-77-2

MK-79-1

MH-77-1

MH-78-1, Pine Street

NR-78-1, Bridge

Street.

SC-79-1

TC-77-1

Marshfield

Marshfield

Kent Park

Marshfield Hills

Marshfield Hills

North River, Norwell.

Scituate filtration

plant.

Third Cliff, Scituate

 

 

42°06'00"N., -

70°41'00"W.

 

 

 

42°10'00"N., -

70°43'00"W.

- -

5.5 (18)

11.6 (38)

16.2 (53)

39.3 (129)

30.2 (99)

22.6 (74)

Drilled, auger

Drilled, auger

Drilled, auger

Drilled, 11 cores

Drilled, auger

Drilled, auger

Drilled, auger

Drilled, auger, 10

cores.

1977.

1977.

1977.

1977.

1978.

1978.

1979.

1977.

hydrothermal activity not long after emplacement of the 

basalt (Folger and others, 1978). The maroon clay above 

the basalt (table 8) mimics the structure of the basalt and 

appears to be a saprolite. This clay is directly overlain by 

Cretaceous sediments. K-Ar whole-rock age determinations of the basalt of 183±8 Ma and 164±3 Ma (Zartman 

and Marvin, this vol., chap. J, table 1) suggest that the 

basalt is Early Jurassic or older. The chemistry of the 

basalt is similar to that of basalts from various Eastern 

U.S. Mesozoic basins (Gottfried and others, 1977) except 

that it contains anomalous amounts of titanium and 

phosphorus (Folger and others, 1978, table 1). The basalt 

is at least 44 m thick; its lateral extent is unknown.

TRIASSIC AND JURASSIC SEDIMENTS AND VOLCANIC 

ROCKS (JT?)

The presence of the Triassic and Jurassic sediments 

and volcanic rocks (Jl) is inferred from seismic data 

through comparison of the seismic record with records 

from down-faulted rift basins in the Gulf of Maine 

(Ballard and Uchupi, 1975) and the New England continental margin (Grow and others, 1979). The sediments 

and volcanic rocks in the Nantucket basin are probably 

similar to Triassic sediments that crop out along the Bay 

of Fundy and the Gulf of Maine, which are continental 

red beds that interfinger with tholeiitic basalts. The red 

beds consist of graywacke, arkose, orthoquartzite, and, 

at the base in places, sharpstone conglomerate (Powers, 

1916; Klein, 1962).

CRETACEOUS SEDIMENTS (K)

Cretaceous sediments (K) are known from drill holes in 

Nantucket and Martha's Vineyard and from surface 

exposures on Martha's Vineyard and Nonamesset (sites 

7-11, table 8). Bore hole USGS 6001 in Nantucket (site 7) 

contains the most complete Cretaceous section in Massachusetts. This section, 329 m thick, and that at site 8,140 

m thick, are probably representative of the Cretaceous 

section in the area. The Cretaceous section of clays, silts, 

and sands in USGS 6001 (site 7) was divided by Folger 

and others (1978) into a lower unit, 108 m thick, of 

unconsolidated clayey sand and an upper unit, 221 m 

thick, of silty clay containing a few beds of silt and sand. 

The unfossiliferous lower unit that rests on the saprolite 

is considered to be part of the Albian Stage, by correlation on the basis of lithology with the Cretaceous Potomac Group in the Coastal Plain of New Jersey (fig. 16). 

The lowest 40 m of the lower unit consists of gray to 

white, clayey sand composed of quartz, plagioclase, 

kaolinite, illite, and minor amounts of smectite or mixedlayer illite-smectite. The middle 40 m of the lower unit 

consists of dark-red, gray, olive, and white mottled silty 

sandy clay composed of quartz, plagioclase, orthoclase, 

kaolinite, illite, and sparse smectite and hematite. The 

upper 25 m of the lower unit consists of coarse, light-gray 

silty sand composed of quartz, plagioclase, orthoclase, 

kaolinite, illite, and sparse smectite. The basal contact 

appears to be a disconformity representing Jurassic and 

Early Cretaceous time during which clastic and carbonate sediments were being deposited in similar basins to
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E51

TABLE 8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, Massachusetts 

[Locations of sites shown on figure 15; datum for unit depths is mean sea level. USGS, U.S. Geological Survey; , not given]

Unit 

Site Site depth, 

no. designation meters 

(feet)

1 HJW-106 +7.3 (+24)

(Hole A).

-41.3 (-136)

-87.9 (-289)

-123.4 (-405)

-125.3 (-411)

Unit 

thickness, 

meters 

(feet)

48.6 (160)

46.6 (153)

35.4 (116)

1.8 (6)

18.3 (60)

Description of units

Fine sand and scattered layers

of coarse sand.

Bluish-gray, coarse to clayey

silt.

Boulder till boulders, mainly

granite, in silt matrix.

Iron-stained till

Fine-grained, bluish-gray phylAge

Quaternary

Quaternary

Quaternary

Quaternary

Proterozoic(?)

Principal data source

Maevsky and Drake, 1963;

Koteff and Cotton, 1962;

unpublished well data,

USGS, Boston, Mass.;

Oldale, 1976.

BMW-23

(Hole C).

BMW-24 

(Hole D) 

"Tubman.

4 BMW-25 

(Hole B).

-143.8 (-471)

-299.3 (-981) 

+21.3 (+70)

-15.2 (-50)

-36.3 (-119)

-62.5 (-205)

-105.2 (-345)

-110.7 (-363)

-283.7 (-930) 

+22.9 (+75)

-97.9 (-321)

-117.4 (-385)

+30.5 (+100)

-100.0 (-328)

155.5 (510)

36.5 (120) 

21.0 (69)

26.2 (86) 

42.7 (140)

5.5 (18) 

172.9 (567)

120.8 (396)

19.5 (64)

160.1 (525)

130.5 (428) 

2.1 (7)

litic schist and, in upper 20 m, 

30 percent greenish-gray to 

gray crystalline limestone in 

beds 1 cm thick.

Phyllitic schist as above lacking 

carbonate but containing abundant quartz veins and chlorite 

alteration; thinly bedded, well 

foliated. 38 cm (15 in) recovered at -181 m(-173.5m 

datum) shows good foliation 

dipping about 80°.

Fine to very coarse yellowishbrown sand containing scattered layers of gravel.

Fine to very coarse gray sand 

containing scattered layers of 

gravel.

Greenish-gray silty clay containing sandy layers.

Very fine to very coarse gray 

sand containing layers of 

gravel.

Till

Quartz-biotite gneiss (three samples from core examined by 

Gore appeared to be granulated granodiorite).

Unconsolidated material

Granodiorite

Proterozoic(?)

Quaternary 

Quaternary

Quaternary 

Quaternary

Quaternary 

Proterozoic Z(?)

Quaternary

Proterozoic Z

Quartz monzonite; contains xeno- Proterozoic Z 

liths at 164.7 m (540 ft) and 

190.3 m (624 ft).

Unconsolidated cobble gravel,

sand, and silt. 

White, pink, and rusty granite

(boulder?).

Quaternary 

Proterozoic Z(?)

Maevsky and Drake, 1963; 

unpublished well data, 

USGS, Boston, Mass.; 

R.Z. Gore, written commun., 1978.

Maevsky and Drake, 1963; 

unpub. well data, USGS, 

Boston, Mass.; Zartman 

and Naylor, 1984; Zartman and Marvin, this 

vol., chap. J, table 1; 

R.Z. Gore, written commun., 1964, 1978.

Unpub. well data, USGS, 

Boston, Mass.
E52 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands,

Massachusetts Continued

Site 

no.

5

6

7

Site 

designation

WB-1

(WHOI

dock).

Assorted

outcrops

near

Marion.

USGS 6001

Unit 

depth, 

meters 

(feet)

-102.2 (-335)

-2.4 (-8)

-85.4 (-280)

-

+ 11.0 (+36)

Unit 

thickness, 

meters 

(feet)

28.4 (90.9)

83.0 (272)

3.1 (10)

-

128.1 (420)

Description of units

Unconsolidated material as

above. Refusal at 130.5 m (428

ft) in till or granitic rock.

Light-brown fine to coarse sand

Gray to pink granodiorite

Fine- to coarse-grained granitic

rock and local layers and

lenses of gneiss.

Tan, gray, olive, green, mediumAge

Proterozoic Z(?)

Quaternary

Proterozoic Z(?)

Proterozoic Z

Pleistocene

Principal data source

Unpub. well data, American Drilling and Boring

Company, Inc., East

Providence, R.I.

Williams and Tasker, 1978.

Folger and others, 1978;

8 ENW-50

to coarse-grained quartz and 

plagioclase sand; dominant 

clay mineral is illite, minor 

kaolinite. Some glauconite 

below -82 m. Material contains sparse Tertiary sporomorphs and dinoflagellates and 

Cretaceous palynomorphs.

-117.1 (-384) 221.1 (725) Variegated red, gray, and yel- Late Cretaceous

low clay and silt interbedded 

with light-gray sand; lignite in 

darker layers, subbituminous 

coal at -317 m; dominant minerals are kaolinite and quartz.

108.0 (354) Light-gray, fine- to coarse- Late Cretaceous 

grained quartz sand in kaolinite matrix; subordinate interbedded clay.

13.1 (43) Massive maroon clay containing Jurassic and 

white blebs and veins. Triassic.

43.9 (144) Altered maroon to gray amygda- Jurassic and 

loidal basalt containing veins Triassic. 

and amygdaloid fillings of calcite, zeolite, and saprolite.

80.2 (262) Medium- to coarse-grained white Pleistocene 

sand, lignite, and several layers of clay. Material contains 

Tertiary sporomorphs and 

microfossils.

-70.2 (-230) 47.9 (157) Medium to fine sand and layers Cretaceous

of silty clay.

-118.0 (-387) 92.1 (302) Mottled micaceous silty clay; Cretaceous

white sand at -119 m and lignite between -164 and -170 

m; layers partly indurated.

Kohout and others, 1977; 

unpub. data, USGS, 

Woods Hole; R.A. 

Christopher, written 

commun., 1982; Walter 

Barrett, written commun., 1936; E.G.A. 

Weed, unpub. data; L.J. 

Poppe, written 

commun., 1976; S.A. 

Wood-Needell, written 

commun., 1976.

-338.2 (-1,109)

-446.2 (-1,463)

-459.3 (-1,506)

-503.3 (-1,650) 

+ 10.1 (+33) Hall and others, 1980; 

Kaye, 1964a, 1983b; 

Delaney, 1980.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E53

TABLE 8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands,

Massachusetts Continued

Site 

no.

Site 

designation

Unit

depth, 

meters

(feet)

Unit

thickness, _ . ,. Description of units meters

(feet)

Age Principal data source

9 Gay Head

Cliffs.

10 Zack's Cliff

11 Nonamesset

-210.1 (-689) 42.1(138) Sand and sandstone; clay at

-230 m, and red arkosic sand-

-252.2 (-827) stone at bottom.

 Shelly, light-yellow to light-gray

sand, thin beds of fine sand 

and silt; massive ferruginous 

clay and silt.

Greensand, largely glauconite, 

minor quartz and feldspar; 

contains apatite nodules and 

chert.

Kaolinite, red, white, and lignitic clay containing siderite 

concretions. Clays largely kaolinite, minor quartz and feldspar.

 Greensand

Greensand

 Greensand

Lignitic clay, gray clay, quartz 

gravel, and yellow sand. Sideritic nodules on nearby beach 

characteristic of known Cretaceous beds at Gay Head.

12 Coskata

13 MV-1

14 MV-2

15 OBW 33-35

(MV-3).

16

0(0)

-44.2 (-145)

-55.2 (-181)

-75.0 (-246)

-102.2 (-335)

-

-

44.2 (145) Sand and gn

11.0 (36) Silt and clay

19.8 (65) Silty clay

27.1(89) Silty clay

 Brown sand

Gray sand

 No recovery

 Gray sand

Brown sand

Olive sand

Olive clay

-134 (-439)

17 ENW 70-73

Brown sand 

Gray sand 

White sand 

Clayey sand 

Orange sand 

Gray sand 

Brown sand 

Gray sand 

White sand 

Olive-gray sand

Cretaceous

Pliocene and 

Miocene.

Miocene

Cretaceous

Tertiary 

Cretaceous 

Miocene 

Cretaceous

Pleistocene 

Pliocene and

Miocene. 

Eocene 

Paleocene

Pleistocene

Pleistocene(?) 

Pleistocene 

Pleistocene 

Pleistocene 

early Pleistocene. 

Pleistocene

Pleistocene

Kaye, 1964a, 1983b; 

Shaler, 1888, 1890; 

Woodworth and Wigglesworth, 1934; Fredericksen, 1984; L.J. 

Poppe, written 

commun., 1978.

R.A. Christopher, written

commun., 1977. 

Woodworth and

Wigglesworth, 1934.

Folger and others, 1978.

Delaney, 1980.

Delaney, 1980. 

Delaney, 1980.

Delaney, 1980.

Delaney, 1980.
E54 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands,

Massachusetts Continued

Site 

no.

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

Unit 

Site depth, 

designation meters 

(feet)

Holden'sPond 3.1(10)

Stark's Well 4.3 (14)

Jim's II 3.1 (10)

Race Point 6.2 (20)

DGW-193-195 -

YAW

FA-8-82-1 -

PH-77-1 -

MB-77-1 -

MB-77-2 -

11.6 (38)

MK-79-1 -

MH-77-1 -

MH-78-1 -

33.2 (109)

NR-78-1 -

18.3 (60)

SC-79-1

29.6 (97)

TC-77-1 -

Unit 

thickness, 

meters 

(feet)

80.5 (264)

61.9 (203)

66.5 (218)

69.8 (229)

-

97 (300)

24.4 (80)

6.1 (20)

51.9 (170)

-

2.7 (9)

1.5 (5)

0.9 (3)

7.6 (25)

3.1 (10)

1.2 (4)

15.6 (51)

4.6 (15)

12.2 (40)

25.9 (85)

37.8 (124)

3.1 (10)

4.6 (15)

1.5 (5)

1.5 (5)

Description of units

Clean sand

Silty sand containing Eocene

pollen, possibly reworked.

Clean sand

Silty sand

Clean sand

Silty sand

Clean sand

Silty sand

Brown sand

Gray sand and silt

Brown sand

Gray silt and sand

Gravel

Gray clay

Basal till

Brown medium sand

Fine yellow sand

Gray silty clay

Hard layer (ate bit)

Dry yellow sand, cobbles at

35, 50, 60, and 75 ft.

Gray clayey silty sand

Yellow sand

Greensand

Rottenstone (disaggregated

granite).

Olive clay, varved

Yellow sand

Granodiorite

Sand

Granite

Silt and sand

Gray clay

Yellow sand

Granodiorite

Sand

Black shale

Sand

Red sandstone

Sand

Granite

Dark-gray till

Clayey sand

Gray clay

Age

Pleistocene

Eocene(?)

Pleistocene

Eocene(?)

Eocene(?)

Eocene(?)

Pleistocene

Pleistocene

Pleistocene

Pleistocene

Pleistocene

Pleistocene

Miocene(?)

Proterozoic Z(?)

Pleistocene

Pleistocene

Proterozoic Z(?)

Pleistocene

Proterozoic Z(?)

Tertiary

Tertiary

Pleistocene

Proterozoic Z(?)

Pleistocene

Carboniferous

Pleistocene

Carboniferous

Pleistocene

Proterozoic Z(?)

Pleistocene

Principal data source

Zeigler and others, 1960.

Zeigler and others, 1960.

Zeigler and others, 1960.

Zeigler and others, 1960.

Guswa and LeBlanc, 1985.

Guswa and LeBlanc, 1985.

E.G. A. Weed, unpub.

data, 1982.

E.G. A. Weed, unpub.

data, 1977.

E.G.A. Weed, unpub.

data, 1977.

E.G.A. Weed, unpub.

data, 1977.

E.G.A. Weed, unpub.

data, 1977.

E.G.A. Weed, unpub.

data, 1977.

E.G.A. Weed, unpub.

data, 1978.

P.C. Lyons, written commun., 1979.

E.G.A. Weed, unpub.

data, 1978.

P.C. Lyons, written commun., 1979.

E.G.A. Weed, unpub.

data, 1979.

E.G.A. Weed, unpub.

data, 1977.
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E55

TABLE 8. Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands,

Massachusetts Continued

Unit 

Site Site depth, 

no. designation meters 

(feet)

22.6 (74)

Unit 

thickness, 

meters 

(feet)

1.5 (5)

3.1 (10)

3.1 (10)

3.1 (10)

1.5 (5)

4.9 (16)

Description of units Age Principal data source

Olive sand

Clay layers containing sand

Silty clay

Brown sand

Till

Yellow sand containing

cobbles.

Till(?), refusal

the east (Cousminer and Manspeizer, written commun., 

1984). The middle unit of Folger and others (1978) is 

considered by them to extend from the Cenomanian into 

the Campanian (middle Late Cretaceous). The upper 

(221-m) unit consists of variegated red, gray, and yellow 

clay interbedded with light-gray sand. Lignite and subbituminous coal about 20 m above the top of the lower 

unit indicate a period of subaerial deposition. Of 30 

samples from the Cretaceous part of the core, 13 contained spore and pollen assemblages. These assemblages 

are correlated with assemblages from the Cretaceous 

section of the New Jersey Coastal Plain (R.A. Christopher, written commun., 1976). Correlation of the strata 

in the bore hole with the New Jersey section is shown on 

figure 16. No formal names have been assigned to the 

Cretaceous strata in Massachusetts.

Hall and others (1980) divided the Cretaceous section 

in hole ENW-50 (site 8) into 11 parts, correlated them 

with USGS 6001 (site 7), and noted the presence of 

greensand, sphalerite, and lignite, as well as the clays 

and sand. X-ray diffraction analyses by Hall and others 

(1980, table 1, p. 11) showed three distinct mineral 

assemblages with increasing depth: (1) quartz-feldspar to 

-50 m; (2) quartz-mica-chlorite-smectite, -50 to -70 m; 

(3) quartz-kaolinite, -70 m to bottom. These three 

assemblages have been used to subdivide the Cretaceous 

section into three units in table 8.

Kaye (1983b) divided the Cretaceous section in the 

ENW-50 hole (site 8, table 8) into five informal zones: A, 

white beds, 71-137 m depth, consisting primarily of 

white kaolinitic clays and sands; B, gray and black beds, 

138-173 m, consisting of interbedded gray clay and silt 

and some light-gray silt and sandy silt; lignite is present 

at 161-164 m, 166-168 m, and 172-173 m depth; C, 

marine horizon, 174-189 m, consisting of greenish-gray 

to medium-gray, micaceous, glauconitic, silty clay and 

silty sand that contains foraminifers; D, white beds, 

190-224 m, containing white kaolinitic quartz sand and

minor sandy silt, and beds of white clay containing thin 

zones of red and yellow mottling from 202-209 m; and E, 

dense white beds, 225-245 m, consisting of white clay 

and coarse sand, more compact and having a higher ratio 

of clay to sand than the beds just above. Kaye assigned 

the entire Cretaceous section in this hole to the Upper 

Cretaceous Raritan Formation. He correlated the 

marine horizon (C) with marginal marine intervals containing foraminifers and plant fossils in USGS 6001, on 

Nantucket (site 7) at 310, 320, and 321 m identified by 

Folger and others (1978, p. 18).

The Cretaceous at Gay Head, Martha's Vineyard (site 

9), and at Nonamesset (site 11) is characterized by 

kaolinite; red, gray and white clays, in part containing 

siderite concretions; and lignitic clay. Pollen analyses 

(R.A. Christopher, written commun., 1977) confirm the 

Late Cretaceous age for the clays at Gay Head, and 

Christopher correlated them with the bottom part of the 

Upper Cretaceous section of New Jersey, either the 

South Amboy Fire Clay Member of the Raritan Formation or the basal part of the Old Bridge Sand Member of 

the Magothy Formation. Greensand at Zack's Cliff, 

Martha's Vineyard (site 10), was identified as Cretaceous 

(R.A. Christopher, written commun., 1977), but most of 

the greensand in the region is of Tertiary age (Kaye, 

1983b).

TERTIARY SEDIMENTS (T)

Tertiary sediments (T) are scattered and fragmentary 

and no longer form a continuous stratum. They all appear 

to be of nearshore marine origin. Sediments containing 

Tertiary macrofossils and microfossils are incorporated 

as blocks or disaggregated material within Pleistocene 

deposits (sites 7, 8, 29, 33). The most complete section of 

Tertiary deposits is a 58-m sequence of silt and clay on 

Nantucket (site 12, table 8) that ranges from Paleocene 

to Miocene and Pliocene in age. Greensand of Tertiary 

age, identified in places by pollen analyses as Miocene or
E56 THE BEDROCK GEOLOGY OF MASSACHUSETTS

System 

or 

Series

(B

C

(B

O

O

CO

(B

Q_

CO

(B

03

(B

0

Q.

Q.

D

CO

Jr O

o 2

O

 .y

-3 £

European 

Stage

Upper

Lower

Campanian

Santonian

K

Goniacian? \,

Turonian? /

Cenomanian

Albian

New Jersey 

outcrop 

equivalent 

formation

Basal Navesink Formation

Wenonah Formation

 .Englishtown Formation, 

? \ and Woodbury Clay / ?

Magothy 

Formation

Raritan 

Formation

Units of 

Folger 

and others 

(1978)

upper

middle

lower

Sample location

- 0 -

- O -

-Q -

~ w

- 8 

- 0 -

-a-

- o -

- 0 -

 

H:

- O -

- o -

- o -

- o -

Depth 

Feet Meters

-100

-200

- 300

100 -

- 400

- 500

- 600

200-

- 700

- 800

- 900

300 - 

- 1000

- 1100

- 1200

- 1300 4QO

- 1400

- 1500 

- 1600

500 -

FIGURE 16. Correlation of the Cretaceous section in bore hole USGS 6001, Nantucket, with the 

exposed Cretaceous section in New Jersey. Modified from R.A. Christopher (written commun., 

1976). Solid circles indicate samples containing microfossils.

Miocene and Pliocene, is exposed on Martha's Vineyard 

at Gay Head and Zack's Cliff (sites 9 and 10) and on 

Nonamesset Island (site 11). Greensand has also been 

reported from Wequobsque Cliff about 9.5 km east of 

Gay Head (Kaye, 1983b).

Tertiary sediments of possible Eocene age are 

reported from the outer tip of Cape Cod (sites 18, 19, 20,

21). Eocene pollen was identified in the silty sand at 

Holden's Pond (site 18), but this material may be 

reworked, and there is some doubt that the sediments 

themselves, and possibly those at sites 19, 20, and 21 as 

well, are Eocene. Reworking of Tertiary material into 

Pleistocene sediments is evident at the top of the section 

at hole ENW-50 (site 8) on Martha's Vineyard. Here the
STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E57

inversion of normal stratigraphic position of fossils suggests a disordered structure like that visible at Gay Head 

as described by Kaye (1964c, 1983b).

A similar disruption of stratigraphic order is evident 

on the mainland in the Marshfield Hills area. At Marshfield Hills (site 29), the 17 m of Tertiary silt, sand, and 

clay overlying Pleistocene glacial deposits is allochthonous. Greensand, probably Miocene, at site 26 appears to 

be autochthonous, however. Kaye (1983b) described two 

places in Marshfield near sites 26 and 27 where presumably autochthonous greensand overlies bedrock.

STRATIGRAPHIC HISTORY

The Triassic and Jurassic basalt and sediments were 

deposited in the Nantucket basin during rifting of the 

Proterozoic and Paleozoic basement in the early Mesozoic (Ballard and Uchupi, 1975; Klitgord and Behrendt, 

1979; Goldsmith, this vol., chap. H). The sediments were 

presumably nonmarine, similar to those in the onshore 

Mesozoic basins of the East Coast. Following an interval 

of subaerial weathering (see site 7, -446 m, table 8) and 

erosion, littoral and shallow offshore-marine strata were 

deposited as onlap facies in the Cretaceous as eastern 

North America became a passive trailing edge of the 

westward-moving North American plate. Shallowmarine deposition continued into and probably through 

the Tertiary until disrupted by the glaciation of the 

Pleistocene. The northwest marine limit of the Tertiary 

marine deposits may be marked by fragments found in 

glacial drift, as at Marshfield Hills (site 29, fig. 15).

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STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS E61

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E62 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 

scale 1:250,000.
Stratigraphy of the Nashoba 

Zone, Eastern Massachusetts: 

An Enigmatic Terrane

By RICHARD GOLDSMITH

With a section on MASSABESIC GNEISS COMPLEX (OZma) 

By PETER ROBINSON

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-F

CONTENTS

Abstract............................................................................ Fl

Introduction....................................................................... 1

Description of units............................................................. 3

Marlboro Formation (OZm, OZmg).................................. 3

Shawsheen Gneiss (OZsh) .............................................. 5

Fish Brook Gneiss (OZf)................................................ 6

Nashoba Formation (OZn, OZnb).................................... 6

Tadmuck Brook Schist (SZtb)......................................... 7

Stratigraphic considerations ................................................. F7

Orientation.................................................................. 7

Thickness and sequence ................................................. 8

Age.................................................................................. 11

Regional correlations........................................................... 12

Summary and conclusions..................................................... 17

Massabesic Gneiss Complex (OZma), by Peter Robinson ........... 18

References cited................................................................. 20

ILLUSTRATIONS

FIGURE 1. Map showing maj or rock units and structural features in the Nashoba zone, eastern Massachusetts.................................... F2

2. Correlation diagram of stratified and major intrusive rock units in the Nashoba zone and some rock units in the

Merrimack belt............................................................................................................................................... 3

3. Lithostratigraphic columns of the formations in the Putnam terrane and Nashoba zone, Connecticut and eastern

Massachusetts................................................................................................................................................ 4

4. Diagram showing correlation of units in the Nashoba zone using repetition by faulting: A, nomenclature as published;

B, alternative nomenclature suggested in this paper............................................................................................. 10

5. Map showing terranes in New England possibly correlative with the Nashoba zone......................................................... 13

6. Correlation diagram and relative thicknesses of formations in the Nashoba zone and of some possibly correlative

formations elsewhere in Massachusetts, Connecticut, New Hampshire, and Maine .................................................... 14

7. Lithostratigraphic columns of units in southern and east-central Maine and southeastern New Hampshire that are

probably correlative with units of the Nashoba zone............................................................................................. 15

8. Correlation diagram of stratified lower Paleozoic rocks in eastern Connecticut, as shown by Rodgers (1982)........................ 16

in

THE BEDROCK GEOLOGY OF MASSACHUSETTS

STRATIGRAPHY OF THE NASHOBA ZONE,

EASTERN MASSACHUSETTS:

AN ENIGMATIC TERRANE

By RICHARD GOLDSMITH

ABSTRACT

The Nashoba zone is a fault-bounded block of high-grade, steeply 

dipping metamorphic rocks, largely metavolcanic to the east (Marlboro 

Formation) and largely metasedimentary to the west (Shawsheen 

Gneiss, Fish Brook Gneiss, Nashoba Formation, and Tadmuck Brook 

Schist). The Marlboro Formation, presumably stratigraphically the 

lowest formation, consists of amphibolite, feldspathic gneiss, and 

subordinate pelitic schist and calc-silicate rock. The Shawsheen Gneiss, 

above the Marlboro, is predominantly a pelitic schist and gneiss, 

commonly rusty weathering. The Fish Brook Gneiss, above, is a 

conspicuous mottled-appearing feldspathic gneiss believed to be of 

felsic igneous origin, probably volcanic. The Nashoba Formation, which 

occupies a large part of the zone, consists of pelitic and semipelitic 

(metawacke) gneiss and schist, commonly rusty weathering, and subordinate calc-silicate rock, marble, and amphibolite. The Tadmuck 

Brook Schist, consisting of pelitic schist and phyllite and subordinate 

quartzite, may lie unconformably above the Nashoba. Protoliths of the 

Marlboro were volcanic, volcaniclastic, and intercalated epiclastic 

materials deposited in a marine environment close to a volcanic source, 

probably to the east. The volcanic materials were primarily basaltic but 

also included andesitic, dacitic, and rhyodacitic materials. The protoliths of the Shawsheen, Fish Brook, and Nashoba were marine volcaniclastic, epiclastic, and minor volcanic materials and carbonate sediments deposited in a slope or basin off a volcanic center (volcanic arc).

The source of the material seems to be from the east, although two 

source terranes are indicated a deeply weathered terrane and a 

volcanic source. The age of the material is bracketed between a 750-Ma 

U-Pb age on zircons from the Fish Brook Gneiss and a 1,500-Ma U-Pb 

age on detrital zircon from the Shawsheen Gneiss.

Rocks of the Nashoba zone correlate fairly well with the Tatnic Hill 

and Quinebaug Formations in Connecticut and with the Gushing 

Formation of southeastern Maine and the Rye Formation of southeastern New Hampshire. The Passagassawakeag Gneiss of eastern Maine 

and the rocks of the Gander Group in southeastern Newfoundland bear 

similarities to the Nashoba Formation. Correlation across strike with 

rocks of the Brimfield Group or rocks on the flanks of the Bronson Hill 

anticlinorium is less certain and involves complex structural interpretations. The Massabesic Gneiss Complex of south-central New Hampshire contains rocks resembling those of the Nashoba zone. The

Manuscript approved for publication November 16, 1987.

Nashoba zone is distinct from the Merrimack belt, adjacent to the west, 

and from the Milford-Dedham zone to the east. It is considered to be an 

accretionary terrane that was part of a volcanic-arc complex lying 

southeast of the North American plate and containing material probably derived from an African source.

INTRODUCTION

The Nashoba zone of eastern Massachusetts is defined 

as that mass of rock lying between the Merrimack belt on 

the west and the Milford-Dedham zone on the east. The 

Clinton-Newbury fault system bounds the block on the 

west and the Bloody Bluff-Lake Char fault system 

bounds the zone on the east (fig. 1). The zone consists of 

high-grade metamorphic rocks and intrusive plutonic 

rocks that together form a distinctive metamorphicplutonic terrane extending northeast across eastern 

Massachusetts from Oxford, Mass., to the Gulf of Maine, 

south of Newburyport. Near Newburyport, the zone 

narrows appreciably between the Clinton-Newbury fault 

and the faults bounding the Newbury basins. These 

basins are small tectonic wedges between the Nashoba 

and Milford-Dedham zones and are discussed in the 

chapter on the Milford-Dedham zone (Goldsmith, this 

vol., chap. E). To the southwest, the Nashoba zone 

narrows at the salient in the Milford-Dedham zone near 

Oxford, where it is in tenuous continuity with the terrane 

containing the Putnam Group (Putnam block) in eastern 

Connecticut.

The rock units within the Nashoba zone are considered 

to be part of a single lithotectonic entity because common 

lithologies are interlayered and because the zone is 

flanked by terranes of different lithologic and structural 

character. Internally, however, the structure is complex. Early synmetamorphic folds are largely obscured 

by later folds and faults. Differential vertical and lateral

Fl
F2 THE BEDROCK GEOLOGY OF MASSACHUSETTS

71°00' 70°30'

xT-J 

__NEW HAMPSHIRE J \ Lawrence - _'-y- . rayiroHIKt, / \ 

MASSACHUSETTS 1

Pepperell

 Fault Dashed where inferred

42°00' -

0 20 MILES

0 10 20 KILOMETERS *

FIGURE 1. Major rock units and structural features in the Nashoba zone, eastern Massachusetts. Correlation diagram and list of units are shown

on figure 2.

translation along faults has cut the zone into lenses of 

rock, the sequence of which is not now clear.

The five formations of stratified rock within the Nashoba zone in Massachusetts shown on the State bedrock 

map (Zen and others, 1983; fig. 2) the Marlboro Formation (OZm, OZmg), Shawsheen Gneiss (OZsh), Fish 

Brook Gneiss (OZf), Nashoba Formation (OZn, OZnb), 

and Tadmuck Brook Schist (SZtb) represent an assemblage of sedimentary, volcaniclastic, and volcanic strata 

metamorphosed to nonsulfidic and sulfidic, pelitic and
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F3

MERRIMACK BELT

Intrusive 

rocks

[W|

k "Dfgr// J

Dl -: iv s - i

 ^- ' I bswj

^^ _^^ 

~" Massabesic x 

Gneiss ,

Complex ~-QZi-f

K&rill /// ^

-DZq' ^ m

NASHOBA 

ZONE

Sharpners Pond 

D orite and related 

rocks

/ 1 >Ss^Kl

t I/A,,^ - I Andover 

NSOfo/l Granite

I

isztb; '<& 

5&

OZfx

OZs'hl

-;g=fl

aP

1

 Includes units Sgr, Ssagd, and igd 

of Zen and others (1983)

Pennsylvanian

Devonian

Silurian

Ordovician

Cambrian

Proterozoic Z

* Includes units OZty and OZtf of Zen 

and others (1983)

NASHOBA ZONE

SZtb Tadmuck Brook Schist

OZn Nashoba Formation

OZnb Boxford Member

OZf Fish Brook Gneiss

OZsh Shawsheen Gneiss

OZm Marlboro Formation

OZmg Homogeneous light-gray feldspathic gneiss

OZt Tatnic Hill Formation

OZq Quinebaug Formation

OZma Massabesic Gneiss Complex

	Intrusive Rocks

Ssqd Sharpners Pond Diorite

SOagr Andover Granite

MERRIMACK BELT

Dl Littleton Formation

DSw Worcester Formation

Sp Paxton Formation

Intrusive Rocks

Pgr Fine- to medium-grained biotite granite 

Dfgr Fitchburg Complex

FIGURE 2. Stratified and major intrusive rock units in the Nashoba 

zone and some stratified and intrusive rock units in the Merrimack 

belt.

semipelitic schist and gneiss, calc-silicate rock, amphibolite, felsic gneiss, and subordinate marble and quartzite 

(fig. 3). In adjacent Connecticut, along the strike of the

units, the Quinebaug Formation (OZq) is equivalent to 

the Marlboro, and the Tatnic Hill Formation (OZt) is 

equivalent to the Nashoba. In Massachusetts, metavolcanic rocks increase in abundance eastward and presumably downsection, from the Tadmuck Brook Schist to the 

Marlboro Formation. The Tadmuck Brook Schist and 

Nashoba Formation are primarily metasedimentary, but 

the latter has a significant volcaniclastic component 

(Abu-Moustafa and Skehan, 1976). The Fish Brook 

Gneiss has a volcanic or volcaniclastic protolith. The 

Shawsheen Gneiss consists primarily of pelitic and semipelitic metasediments. The Marlboro Formation is primarily metavolcanic but has a significant metasedimentary component. Sulfidic sillimanitic schist and gneiss, 

calc-silicate rock, amphibolite, and biotite-quartzfeldspar gneiss with and without garnet are present in 

varying proportions in all the formations except the Fish 

Brook.

The stratified rocks are intruded by the syn- or 

late-tectonic Silurian and Ordovician(?) Andover Granite 

(SOagr), dated by Zartman and Marvin (this vol., chap. 

J, table I), and associated but possibly in part younger 

aplite and pegmatite (Zartman and Naylor, 1984), which 

was shown on the State bedrock map as part of the 

Andover map unit, and by a post-tectonic suite of dioritic 

to granitic plutons that includes the Silurian Sharpners 

Pond Diorite (Ssqd) and related rocks. The plutonic rocks 

are particularly abundant near Lowell and Billerica and 

decrease in abundance to the southwest (fig. I). The 

intrusive rocks in the Nashoba zone are described in 

Wones and Goldsmith (this vol., chap. I).

DESCRIPTION OF UNITS

MARLBORO FORMATION (OZm, OZmg)

The Marlboro Formation consists primarily of interlayered metavolcanic and metavolcaniclastic rocks and 

marine metasedimentary rocks (fig. 3). It was named by 

Emerson (1917) for ledges of biotite-hornblende schist in 

the town of Marlboro (in the Township of Marlborough). 

The Marlboro is bounded on the east by the Bloody Bluff 

fault zone. North of Concord the Marlboro is cut out by 

the Bloody Bluff and by plutonic rocks, so that no 

Marlboro has been identified with certainty north of 

the Concord area. The Marlboro is in contact east of the 

fault with different units of the Milford-Dedham zone: 

Proterozoic Z granitoids, the Proterozoic Z Westboro 

(Zw) and Plainfield (Zp) Formations, and the Proterozoic 

volcanic-plutonic complex exemplified by the metamorphosed mafic and felsic volcanic rocks (Zv) and intrusive 

diorite and gabbro (Zdigb, Zdi, Zgb). The upper contact 

of the Marlboro is apparently conformable with the 

Nashoba Formation in the Shrewsbury-Marlborough
F4 THE BEDROCK GEOLOGY OF MASSACHUSETTS

area, Middlesex and Worcester Counties (fig. 1) (Skehan 

and Abu-Moustafa, 1976), but Hepburn and DiNitto 

(1978) showed the contact as a fault. Thickness considerations imply that Hepburn and DiNitto were probably 

correct in this area. To the north, in northern Middlesex 

County, the contact of the Marlboro with the overlying 

Shawsheen Gneiss is mostly cut out by the Andover 

Granite, but Alvord and others (1976, p. 319) believed 

that the top of the Sandy Pond Member (fig. 3) of the 

Marlboro was conformable with the Shawsheen Gneiss. 

In eastern Connecticut, amphibolite increases in abundance downward in the basal member of the Tatnic Hill 

Formation (OZt) (Dixon, 1974, for example), suggesting 

conformity with the underlying Quinebaug Formation 

(OZq), which is equivalent to the Marlboro (fig. 3).

Bell and Alvord (1976) divided the Marlboro into two 

members (fig. 3), mainly on the basis of mapping in

Essex and northern Middlesex Counties (fig. 1). The 

lower, unnamed member consists of mica schist, calcsilicate rock, marble, and amphibolite. The upper member, the Sandy Pond Member, is predominantly amphibolitic. Skehan and Abu-Moustafa (1976), mapping in the 

Wachusett-Marlborough tunnel in southern Middlesex 

County, divided the Marlboro into 31 members (shown as 

10 units on fig. 3). In contrast to Bell and Alvord's 

section, most of the amphibolitic rocks are at the east or 

bottom part of Skehan and Abu-Moustafa's section. 

Hepburn and DiNitto (1978) and Hepburn (1978), mapping the surface rocks above the tunnel in the Marlborough and the Shrewsbury areas respectively, recognized 

four divisions of the Marlboro, one of which, their Sandy 

Pond amphibolite member, coincides with the Sandy 

Pond Member of Bell and Alvord and with the amphibolitic lower part of Skehan and Abu-Moustafa's section.

Dixon (1965a) 

Eastern Connecticut

Hebron Formation

OL

D

O

CD

^

z

1-

D

0_

E

c w

._ rco

£ a) 

° 5

"- !y

I E

C £

w 'x

sl

*J CM

E »

0 » 

U. .£

D3 

 Q E

11

O <o

^

Yantic

Member

Fly Pond

Member

Lower

member

- --

Upper

Black Hill

Member

Lower

member

(Muscovite)-biotite-oligoclase-quartz schist

containing sillimanite, kyanite, or staurolite at

y Calc-silicate gneiss, local marble; minor mica

\ schist and amphibolite

(Sillimanite)-garnet-(muscovite)-biotite gneiss;

local calc-silicate gneiss

/ Garnet-biotite gneiss, rusty-weathering sillimanite

mica schist, calc-silicate gneiss

Rusty-weathering graphitic garnet-sillimanitek mica schist and minor amphibolite

\ /

\

(Hornblende)-epidote-biotite-quartz-plagioclase

gneiss, locally porphyroblastic; amphibolite, calcsilicate rock, rare marble

(Garnet)-muscovite-biotite-quartz schist.

\ oligoclase gneiss, calcite-muscovite-plagioclase-

\ quartz rock and calc-silicate rock

Garnet-biotite-epidote-quartz-plagioclase gneiss.

(biotite)-epidote-quartz-hornblende-plagioclase

gneiss, amphibolite, biotite-quartz-plagioclase

gneiss, and calc-silicate gneiss

1 nkp Thrir fault

Note: Possible correlations 

referred to in text

(1)

(2)-

«

Granulite 

member

Rusty

schist

member

Schist and 

granulite 

member

Sandy 

Pond

amphibolite

member

Hepburn and DiNitto (1978) 1 

Marlborough, Mass., area

 Assabet River fault (?) -

Biotite-muscovite-plagioclase-quartz granulite, 

locally containing small plagioclase megacrysts; 

minor sillimanite-mica schist and amphibolite

Rusty-weathering (garnet)-(sillimanite)-biotitemuscovite schist and gneiss; minor amphibolite

Calc-silicate rock, coticule, amphibolite

Amphibolite, locally containing knots and boudins 

rich in epidote; minor garnet-(muscovite)-biotite 

schist and gneiss, hornblende gneiss, and calcsilicate rock

Biotite-rich gneiss and schist; thickness 

uncertain

-Bloody Bluff faultThickness computed from average width of unit 

on map, corrected for dip

(3)

FIGURE 3. Lithostratigraphic columns of the formations in the Putnam terrane and Nashoba zone, Connecticut and eastern 

Massachusetts. Column of Bell and Alvord (1976) is at 2.5 times smaller scale than the other columns.
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F5

On the basis of the sections shown on figure 3, it seems 

likely that Hepburn and DiNitto's Sandy Pond is equivalent to Skehan and Abu-Moustafa's units M5-M16.

A lenticular mass of biotite granodioritic gneiss 

(OZmg) near Grafton, considered by Emerson (1917) to 

be a pluton of Milford Granite (Zmgr), does not resemble 

the Milford and was named informally the Grafton gneiss 

by Dixon (written commun., 1977). The Grafton is not 

layered, is fairly uniform in composition, and has a 

foliation formed by oriented flakes of biotite. The Grafton

does, however, contain some inclusions of amphibolite 

and biotite schist of the Marlboro. Hepburn (1978) considered the Grafton to be a plutonic rock, but it could be 

a metamorphosed felsic volcanic rock.

SHAWSHEEN GNEISS (OZsh)

The Shawsheen Gneiss (Bell and Alvord, 1976) consists primarily of sillimanitic muscovite-biotite schist and 

gneiss, sulfidic near the base, and it contains a few lenses

Bell and Alvord (1976) 

Middlesex County, Mass.

Zen and others (1983) 

Massachusetts State 

bedrock map

Skehan and Abu-Moustafa (1976) 

Wachusett-Marlborough tunnel, Mass.

U1

3740 Nashoba Formation Thickness m u

IMarlboro Formation 2702 Thickness m 

N29

N26-28

N22-25

N21

N 19-20

N16-18

N15

N12-14

N11

N7-10

N6

N1-5

M25-31

M 23-24

M20-22

M 17-1 9

M15-16

M12-14

M10-11

M6-9

M5

M1-4

SCALE,

x Muscovite-biotite schist and gne ss; minor

\ Calc-silicate gneiss and marble

(Sillimanite)-garnet-(muscovite)-biotite gneiss and

v Amphibolite; minor mica schist and gneiss; 

\ coticule

(Sillimanite)-garnet-(muscovite)-biotite gneiss and ^^- 

schist ^3.

Calc-silicate rock and marble; minor mica gneiss 

\ and schist

(Sillimanite)-garnet-(muscovite)-biotite schist and 

gneiss; calc-silicate rock; amph bolite at base / 

\ Fault within interval n/

\ Calc-silicate rock; marble; quartzite; coticule /;

\ Mica schist and gneiss; amph bolite at base /

(Sillimanite)-garnet-(muscovite)-biotite gneiss and / 

\ schist /

(Sillimanite)-garnet-(muscovite)-biotite schist and J / 

gneiss; subordinate calc-silicate rock, / / 

amphibolite, and quartzite; coticule near top /? ^

Amphibolite and mica gneiss and schist """ ~~

^ (Sillimanite)-garnet-(muscovite)-biotite gneiss and 

schist; minor amphibolite V 

Fault within unit \ ^_______-

Amphibolite, quartzite, and micaceous quartzite, \^\ 

coticule; muscovite-(chlorite) schist and gneiss \\\

/ Amphibolite, calc-silicate rock; coticule and \\ N^^ 

/ epidote-quartz rock \ \

/ Calc-silicate rock and biotite-rich gneiss and \ \ 

/ schist; minor amphibolite \ \

/ Calc-silicate rock; amphibolite; quartzite; mica \ \ 

/ schist; coticule \

/ Amphibolite; quartzite; coticule; epidote quartz \ \

rock , \ \

/ Amphibolite; calc-silicate rock; quartzite ' v \

/ Garnet-(muscovite)-biotite schist and gneiss; calc- \ \ \ 

silicate rock and amphibolite . \\

quartzite . .)

/ Amphibolite and hornblende gneiss; minor biotite ,.--'" 

gneiss ,-- ''

(Muscovite)-biotite gneiss and schist; calc-silicate """-- ^^^ 

rock; amphibolite; quartzite 

' Bloody Bluff fault v

IN METERS SCALE. IN METERS \ 

Bell and Alvord (1 976) only \

0 - -0 \ 

200- -500 \

r*>- * M- - *- iTadmuck 

Brook Schist

Biiierica Nashoba Formation 10,890 Thickness mMember. 

Beaver 

Brook 

Member

Long Pond 

Gneiss 

Member

Fort Pond 

Member

Nagog 

Pond 

Gneiss 

Member

Nashoba 

Brook 

Member

Tophet 

Swamp 

Gneiss Mbr.

Spencer 

Brook Mbr.

Hill 

Member

Boxford 

Member

Fish Brook 

Gneiss 

Thickness 

1250 m

Shawsheen 

Gneiss

Thickness 

2600 m

Formation Marlboro 

Thickness 2140m

Sandy Pond 

Member 

(Thickness 

640 m)

undivided 

(Thickness 

1500m)

Rusty-weathering graphitic (sillimanite)- 

(andalusite)-(staurolite) schist and phyllite; thin- 

\ bedded micaceous quartzite mainly at base /

Sillimanite-muscovite-biotite gneiss and schist; 

minor amphibolite, calc-silicate rock, and marble

Sillimanite-muscovite-biotite gneiss; rare 

amphibolite

Upper part Calc-silicate rock; marble; sillimanitemica gneiss and amphibolite. Lower part 

amphibole- biotite gneiss and amphibolite; calcsilicate rock; sillimanite gneiss and schist

Sillimanite-muscovite-biotite gneiss; subordinate 

amphibole and amphibole gneiss

Calc-silicate rock; sillimanite-muscovite-biotite 

schist and amphibole-bearing gneiss

Sillimanite-(muscovite)-biotite gneiss; rare 

amphibolite

Amphibolite and calc-silicate rock

Sillimanite-biotite-muscovite schist

Upper part Amphibolite and subordinate 

sillimanite-(muscovite)-biotite schist. Lower part 

\ (sillimanitej-biotite-(muscovite) schist and gneiss; / 

\ minor amphibolite /

Amphibolite and amphibole-bearing gneiss and 

schist; minor sillimanite gneiss, calc-silicate , 

\ gneiss; rare marble /

Mafic-poor biotite-quartz-sodic plagioclase gneiss; 

very minor biotite-rich gneiss and amphibolite

Muscovite-biotite-oligoclase-quartz gneiss, locally 

sillimanitic; rare amphibolite; rusty-weathering 

sillimanite-muscovite-biotite schist near base

Biotite-(actinolite) amphibolite; minor andalusitemica schist and muscovite-biotite gneiss

Plagioclase-quartz-mica schist; hornblendebiotite-plagioclase gneiss; calc-silicate gneiss and 

rare lenses of marble; minor amphibolite. Unit 

too poorly exposed to determine proportions of 

lithologies

Tadmuck 

Brook Schist 

(SZtb)

N

g

c 

0

Format Nashoba 

Boxford 

Member 

(OZnb)

Fish Brook 

Gneiss (OZf)

Shawsheen 

Gneiss 

(OZsh)

Marlboro 

Formation 

(OZm)

600- -1500 

800 _2QOO

FIGURE 3. Continued.
F6 THE BEDROCK GEOLOGY OF MASSACHUSETTS

of layered and massive amphibolite (fig. 3). The Shawsheen is lithologically similar to much of the Nashoba 

Formation and was considered by Hansen (1956) to be 

part of the Nashoba. Bell and Alvord (1976) separated 

the Shawsheen from the Nashoba because the Fish 

Brook Gneiss intervened between the Shawsheen and 

the rest of the Nashoba Formation. On the other hand, 

Barosh and others (1977) included both the Shawsheen 

and the Fish Brook in the Nashoba. Hansen (1956) did 

not recognize the rock later called the Fish Brook Gneiss 

(Castle, 1965) as a mappable unit and similarly included 

all the rocks in the Nashoba Formation.

FISH BROOK GNEISS (OZf)

The Fish Brook Gneiss is a fine- to medium-grained, 

"pearly white to very light gray, distinctly foliated but 

generally unlayered biotite-quartz-plagioclase rock" 

(Castle, 1965, p. C81). The foliation is marked by oriented biotite flakes, which are more abundant to the 

north than to the south. According to Castle, the Fish 

Brook contains inclusions ranging in size and shape from 

schlieren to rectangular zones. These inclusions consist 

of amphibolite, thinly layered biotite gneiss, and other 

rock types; the foliation in the gneiss generally passes 

through these inclusions, although it wraps around some 

zones. The Fish Brook, like other units in the Nashoba 

zone, is intruded by the Andover and related granites. 

Castle (1965) considered the Fish Brook to be either a 

premetamorphic intrusive rock or a core gneiss of 

intrusive or sedimentary ancestry. Bell and Alvord 

(1976), however, claimed a volcanic and volcaniclastic 

origin for it. Zircons from the Fish Brook are almost 

certainly of volcanic origin (Olszewski, 1980, p. 1411). 

Alvord and others (1976, p. 320) described an exposure of 

the contact of the Fish Brook Gneiss and the Nashoba 

Formation near Billerica, where very thin beds of finegrained amphibolite of the Boxford Member of the Nashoba are interlayered with very thin beds of light-gray 

Fish Brook Gneiss.

NASHOBA FORMATION (OZn, OZnb)

The Nashoba Formation (Hansen, 1956) consists of 

interlayered sillimanite-bearing, partly sulfidic schist 

and gneiss, (hornblende)-biotite-quartz-feldspar gneiss, 

calc-silicate gneiss, and subordinate quartzite and marble (fig. 3). Protoliths are primarily volcanogenic sediments, interlayered with limy marine sediments and 

volcanic rocks (Abu-Moustafa and Skehan, 1976; Bell and 

Alvord, 1976). Bell and Alvord divided the Nashoba into

10 members on the basis of lithology. Sillimanite-bearing 

pelitic and semipelitic schist and gneiss are interlayered 

with other rock types throughout their section. Calcsilicate rocks and marble characterize some members. 

Amphibolite is most abundant near the presumed base of 

the Nashoba, primarily in the Boxford Member (OZnb) 

(Boxford Formation of Castle, 1965). Alvord and others 

(1976) identified localities where some of their subdivisions of the Nashoba can be seen. Skehan and AbuMoustafa (1976) subdivided the Nashoba as seen in the 

Wachusett-Marlborough tunnel into 30 members, which 

I have condensed for economy to 13 on figure 3. Lithologies are similar to those described by Bell and Alvord, 

and in a general way their sections are similar, although 

Bell and Alvord's section is much thicker (note scale 

change for their section on fig. 3). The Boxford Member 

is not readily identifiable in the tunnel section. Possibly 

it thins and pinches out or is faulted out before it reaches 

the tunnel section. The Boxford is, however, extensive 

in the Lowell-Billerica area, where Castle (1965) divided 

it into an upper and a lower member. His upper member consists mostly of amphibolite and hornblendeplagioclase gneisses. His lower member consists chiefly 

of mica schist and quartzofeldspathic gneisses and subordinate amounts of amphibolite and calc-silicate gneiss. 

This lower member is to a certain extent lithologically 

like the undivided lower part of Bell and Alvord's Marlboro Formation.

Subdivision of the Nashoba is conjectural south of 

Marlborough and Shrewsbury where only reconnaissance mapping has been done. Hepburn (1978) and H.R. 

Dixon (written commun., 1978) identified a rustyweathering sulfidic schist at the base of the Nashoba, 

which extends from south of Shrewsbury to the Webster 

area and which is probably the same sulfidic schist that 

forms the basal member of the Tatnic Hill Formation in 

Connecticut. Dixon (written commun., 1978) recognized 

rock in the Nashoba in the Oxford quadrangle that was 

similar to subdivisions of the Tatnic Hill that she has 

mapped in Connecticut.

The Boxford Member (OZnb) has been separated out 

from the rest of the Nashoba on the State bedrock map 

because this unit is the only one that can be recognized 

clearly in several areas. I doubt that a definite sequence 

of units exists throughout the Nashoba because of the 

lenticularity of assemblages and repetition of rock types, 

both of which could be accounted for by either sedimentary or tectonic processes. Evidence for tectonism has 

been found by practically everyone who has mapped in 

the Nashoba zone (Hansen, 1956; Castle, 1964; Alvord, 

1975; Bell and Alvord, 1976; Skehan and Abu-Moustafa, 

1976; Barosh, 1978; Hepburn, 1978; Hepburn and 

DiNitto, 1978).
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F7

TADMUCK BROOK SCHIST (SZtb)

Bell and Alvord (1976) gave the name Tadmuck Brook 

Schist to a sequence of largely pelitic rocks lying west of 

and presumably above the Nashoba (figs. 1, 3). The 

Tadmuck Brook consists primarily of sillimanite schist, 

graphitic staurolite-andalusite phyllite, and chloritebiotite-muscovite phyllite in decreasing metamorphic 

grade from east to west. Near its base to the east, the 

unit contains subordinate layers and lenses of amphibolite and discontinuous beds of quartzite. Where exposed 

on Route 2 near Littleton (fig. 1), the Tadmuck Brook is 

rusty weathering. According to Alvord and others 

(1976), the Tadmuck Brook intertongues locally with 

quartzofeldspathic layers of the Nashoba Formation, 

but, from the Littleton area to the vicinity of Lawrence, 

the Tadmuck Brook appears to truncate units in the 

Nashoba; those authors, therefore, suggested the possibility of an unconformity or disconformity. However, as 

they pointed out, this contact is a fault in most places 

north of Route 2 (Littleton area) and has been so 

interpreted by Castle and others (1976) on the basis of a 

truncation of aeromagnetic pattern along this contact. 

Skehan and Murray (1980, p. 295) considered the Tadmuck Brook to be unconformable on the Nashoba 

because of a small angular discordance coupled with 

absence of fault-related features at the contact and the 

sharp contrast in deformation and metamorphism 

between the two units.

The Tadmuck Brook is probably represented in Skehan and Abu-Moustafa's (1976) tunnel section by their 

unnamed units U1-U9. Ul consists largely of quartzite, 

and in their section it is shown as lying possibly unconformably on the Nashoba. Hepburn (1978) was unable to 

identify the Tadmuck Brook south of Shrewsbury where 

the Boylston Schist (SObo) of the Merrimack belt lies 

against the Nashoba. To the north, the Tadmuck Brook 

is truncated at the top, sliced throughout, and locally 

phyllonitized by the Clinton-Newbury fault system 

(Alvord and others, 1976).

STRATIGRAPHIC CONSIDERATIONS

ORIENTATION

The formations in the Nashoba zone have customarily 

been considered to be a sequence topping to the west, 

presumably because they lie on the east flank of the 

Merrimack synclinorium. This assumption may not be 

valid, however, because the bounding faults of the zone 

appear to be major dislocations. Even so, Bell and 

Alvord (1976) considered the Marlboro-to-Nashoba section to be homoclinal and topping to the west on the basis

of many features seen in outcrop that they considered to 

be relict primary sedimentary features. On the other 

hand, Castle (1965, p. C84) observed no unambiguous 

primary structures, at least in the Boxford Member 

(compare Bell and Alvord, 1976, fig. 4, p. 186). Skehan 

and Abu-Moustafa (1976) made no mention of primary 

structures except bedding and presented no topping 

evidence in their description of their WachusettMarlborough tunnel section. They followed convention, 

however, and described the section as homoclinal and 

presumably topping to the west. They cited no evidence 

in their section that units are repeated by folding or 

faulting except on a small scale.

I do not believe we can say with certainty which way 

the units face, nor can we say that the sequence is 

complete, if it is homoclinal, because of the flanking 

faults. What evidence there is indicates that the 

sequence is west facing. We do not see a depositional 

bottom and probably not a depositional top. We do not 

know for sure whether the Tadmuck Brook Schist is part 

of a Nashoba-Marlboro package or if it is the base of an 

overlying unconformable sequence. However, we can see 

that a difference exists in sedimentary facies within the 

terrane. The facies on the east indicate deposition in a 

marine basin next to a volcanic terrane, as evidenced by 

the greater amount of amphibolite interpreted to be of 

volcanic origin. Facies to the west indicate deposition in 

a somewhat more distal part of the basin, as evidenced 

by the greater amount of aluminous pelitic schist and 

gneiss and thinly layered calc-silicate rocks, although 

these western facies still contain a noticeable volcanic 

and volcaniclastic component.

Abu-Moustafa and Skehan (1976), who have studied 

the rocks intensively, described the sediments of the 

Nashoba and Marlboro as having been derived from two 

major sources: one a deeply saprolitized, even lateritic, 

terrane and the other a terrane containing unweathered 

volcanic and plutonic rocks, whose average composition 

is dacite, and volcanogenic sediments. The hornblenderich rocks of the Marlboro and Nashoba were volcanogenic sediments and flows and pyroclastic rocks of 

predominantly basaltic composition and subordinate calcalkalic andesitic dacitic and rhyodacitic compositions 

(Nockolds, 1954). Abu-Moustafa and Skehan (1976, p. 32) 

inferred that the paleotectonic environment of deposition 

of the Nashoba (and presumably also the Marlboro) was 

a "relatively shallow marine basin that received deeply 

weathered soils, unaltered volcanogenic sediments, and 

some volcanic rocks, occasionally interbedded with thin 

limey beds..., thin quartz and quartz-feldspar sand..., 

and distinctive manganiferous-iron chert...." This basin 

presumably flanked an island arc lying to the east. 

However, the original Nashoba protolith need not have 

formed where it is now. The Nashoba zone could have
F8 THE BEDROCK GEOLOGY OF MASSACHUSETTS

been moved into place with or without rotation and could 

have moved a considerable distance.

THICKNESS AND SEQUENCE

The true thicknesses of the formations are difficult to 

determine because of the differences in thicknesses given 

by different authors or estimated by me from their maps. 

Differences in these thicknesses need to be considered 

with regard to folding and faulting. Bell and Alvord's 

(1976) section of the Nashoba Formation is almost five 

times thicker than Skehan and Abu-Moustafa's (1976) 

tunnel section. This difference may be a function of 

differences in allowances for internal folding and faulting 

in preparing the two sections, although it would appear 

from their descriptions that the thicknesses in both 

sections were derived from the width of the lithologic 

unit or subunit corrected for dip. On the other hand, the 

units may thin stratigraphically or tectonically southward. In contrast to the discrepancy in thickness of the 

Nashoba Formation between sections, thicknesses of the 

Marlboro in Massachusetts given in the two reports are 

in fairly good agreement and agree also with thicknesses 

of the equivalent Quinebaug Formation in Connecticut 

(fig. 3). In fact, the Marlboro is thicker in Skehan and 

Abu-Moustafa's section than in Bell and Alvord's section, 

although this might be accounted for by the much greater 

amount of plutonic intrusive rock in Bell and Alvord's 

area.

Let us first consider folding. Some repetition of lithologic assemblages is evident in the sections of the Nashoba, as for example the alternations between calcsilicate-bearing assemblages and pelitic schist and gneiss 

assemblages (fig. 3). These repetitions could be 

explained by cyclic sedimentation or, alternatively, by 

folding. Skehan and Abu-Moustafa's cross section (1976, 

fig. 1, p. 220) shows two major synforms and a central 

antiform, although all are broken by faults. These large 

folds were apparently not taken into account in their 

compilation of aggregate thicknesses because they could 

recognize no repetitions of units except where layers are 

repeated by small-scale folding and by drag associated 

with minor faults. Hansen (1956) did not show a complete 

section across the Nashoba but showed clearly at least 

one synform within the Nashoba, an antiform beyond the 

west flank, and a broad antiform or dome in the Marlboro 

on the east flank. These relations indicate a synformal 

structure for the Marlboro-to-Nashoba sequence but 

with apparent truncation on the west side where there is 

no Marlboro, unless the Reubens Hill Formation (SOrh) 

(Skehan, 1967) in the Merrimack belt is the stratigraphic 

equivalent of the Marlboro. Foliation and layering symbols in the Nashoba on the maps of the Billerica and 

Westford quadrangles (Alvord, 1975) are steeply dipping, but these symbols on their maps form zones of 

alternate east dip and west dip, suggesting tight folding 

of layering and foliation and thus probable repetition of 

units and an exaggerated thickness of section.

Faulting probably has had an appreciable effect on the 

thicknesses of sections. Skehan and Abu-Moustafa (1976) 

described numerous faults of different kinds and ages 

and of large and small magnitude in their tunnel section. 

Many of Bell and Alvord's (1976) units are totally or in 

part bounded by faults. Such faults not only make 

estimates of thickness of each individual unit a minimum, 

disregarding folding, but also raise the possibility that 

total thickness of the formations in the zone is exaggerated because of unrecognized repetition of units. The 

latter would be particularly important if appreciable 

imbrication has occurred as a result of movement along 

larger faults such as the Spencer Brook and Assabet 

River faults (fig. 1). The Assabet River fault separates 

the Shawsheen from the Fish Brook Gneiss in most 

places, but this contact is viewed by Bell and Alvord as 

stratigraphically conformable. The Assabet River fault is 

not identified in the tunnel section but is either a fault 

zone in unit N6 (near Station 266+35 of Skehan and 

Abu-Moustafa, 1976) west of the Nashoba-Marlboro contact or more likely, a thrust fault in N4 (at Station 

285+10-15 of Skehan and Abu-Moustafa, 1976). Hepburn and DiNitto (1978) mapped a fault along the 

Nashoba-Marlboro contact in the Marlborough quadrangle that may be the Assabet River fault and may be a 

surface representation of the appreciable faulting near 

the base of unit Nl described by Skehan and AbuMoustafa. To the north, the Assabet River fault cuts off 

the top of the Shawsheen (Bell and Alvord, 1976, Appendix 1). The Spencer Brook fault separates the Billerica 

Schist from the Bellows Hill Member of the Nashoba. 

Alvord and others (1976, p. 322) presumed that if the 

contact were not obscured by faulting, the two units 

would intertongue and be conformable. The Spencer 

Brook may be a fault zone described by Skehan and 

Abu-Moustafa (1976, table 6) between units N17 and N18 

(at Station 177+53), or it could be one of the faults nearer 

the Marlboro contact. Possibly two faults in the tunnel 

section, one in N1-N5 and the other at the top of N6, 

may be the Assabet River and Spencer Brook faults, 

respectively. The Shawsheen and Marlboro are in normal 

stratigraphic contact, according to Bell and Alvord, and 

the Nashoba and Marlboro are in normal stratigraphic 

contact, according to Skehan and Abu-Moustafa, despite 

evidence for appreciable faulting in the vicinity of the 

contact. From Bell and Alvord's map (1976, fig. 1), 

though, it is not clear that the Shawsheen is ever in 

contact with the Marlboro without intervening plutonic 

rocks. The Shawsheen-Fish Brook contact was considered by Alvord and others (1976, p. 320) to be conform-
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F9

able, as were the Fish Brook-Boxford and the Billerica 

Schist-Bellows Hill contacts. In their view, the numerous faults shown by Bell and Alvord (1976, fig. 4) have 

done little to disrupt the sections. Yet, looking at the 

sections as presented in figure 3, we see that a thick 

wedge of rock comprising the Shawsheen Gneiss, the 

Fish Brook Gneiss, and possibly the lower part of Bell 

and Alvord's Nashoba section, including the Boxford 

Member, is missing in the tunnel section. One explanation is that the Shawsheen and Fish Brook are stratigraphic lenses, although this seems unlikely because the 

truncation is extremely angular. Another explanation, 

despite the statement of conformity by Bell and Alvord 

(1976), is that these units constitute tectonic slices (Goldsmith, this vol., chap. H). Possibly the larger faults, 

although they do not everywhere determine the formational boundaries, have sliced once-continuous sections of 

rock into disjointed lenses, so that although masses of 

like rock may be in contact with each other, they have 

been transported there from some other part of the 

strike belt.

A closer look at possible correlations between the 

sections may resolve the problem of thickness and apparent loss of section. A number of correlation schemes are 

possible. The most obvious are shown on figure 3 as solid 

lines; tielines of the more questionable correlations are 

marked by queries. A reasonable correlation can be made 

for the upper part of the Nashoba sections of Bell and 

Alvord (1976) and of Skehan and Abu-Moustafa (1976). 

However, the Nashoba Brook, Tophet Swamp Gneiss, 

Spencer Brook, and Billerica Schist Members are not 

recognizable in the tunnel section. These units may be 

cut out by the Spencer Brook fault. Possibly the Nashoba 

Brook is a lens or fold nose, and the Tophet Swamp is 

equivalent to part of N7-N10. The Bellows Hill Member 

is correlated with units N1-N6 in the lower part of the 

Nashoba in the tunnel section. The Boxford Member, the 

Fish Brook Gneiss, and the Shawsheen Gneiss lens out or 

are cut out. Possibly the Shawsheen Gneiss is equivalent 

to Hepburn and DiNitto's rusty schist member of the 

Marlboro. Their granulite member may be a facies 

equivalent of the Fish Brook Gneiss. Neither of these 

units is clearly recognizable in the tunnel section. Bell 

and Alvord's Sandy Pond Member of the Marlboro 

Formation can be correlated (possibility 2 of fig. 3) with 

units M5-M16 (?) of the tunnel section, a correlation 

supported by lithologic similarity and by the mapping of 

Hepburn and DiNitto (1978) and Hepburn (1978); this 

correlation, however, requires accounting for lack of 

equivalents in Bell and Alvord's section to the interval 

M17-M31 of Skehan and Abu-Moustafa's section. The 

missing upper part of the Marlboro in Bell and Alvord's 

section may be replaced by granite (SOagr) as suggested 

by the State bedrock map. A third possibility (possibility

3, fig. 3), is that Bell and Alvord's Marlboro sequence is 

inverted by folding, in which case a fault is required 

between the Shawsheen and Marlboro. As Hepburn and 

DiNitto's section approximately corresponds on the surface to the tunnel section below, the discrepancy in 

detailed description of the units in the two sections, 

particularly between the interval M25-M31 of the tunnel 

section and the granulite member, can be construed to 

mean that the kinds of rock seen in scattered exposures 

on the surface in the zone of weathering do not necessarily represent the relative abundances of lithologies that 

can be seen in a corresponding section of rocks underground. Both Hepburn and DiNitto's section and Bell 

and Alvord's section are based on surface exposures, so 

it is not surprising that these two correspond somewhat 

better than either does to the tunnel section.

Other possible correlations that require complicated 

structure equate the Shawsheen Gneiss and Boxford 

Member with tunnel units N1-N5 and N6, or alternatively the Shawsheen and Sandy Pond with units 

N7-N10 and N6 in reverse order. The Fish Brook again 

would have to terminate in some way, but this is not 

difficult if it is intrusive. None of the possible correlations is wholly satisfactory, and a fault solution seems the 

most promising.

A correlation suggested by the lithostratigraphic columns of figure 3 involves repetition by faulting (fig. 4A). 

In this scheme, the Boxford is equated with the Sandy 

Pond Member, the Shawsheen of slice A is repeated 

above the Assabet River fault in slice B as the Bellows 

Hill member and above the Spencer Brook fault in slice C 

as the Billerica Schist and Spencer Brook members. 

Fault slice C could consist of other fault slices as the 

numerous faults in the sections on figure 3 suggest.

This scheme depends on the validity of the correlation 

of the Boxford with the Sandy Pond. I have pointed out 

above that the Boxford section of Castle (1965) resembles 

Bell and Alvord's (1976) Marlboro section. Castle (1965) 

pointed out the similarity of the Boxford to isolated 

amphibolitic rocks to the southwest that he mapped as 

Marlboro. These isolated rocks of Castle are apparently 

on strike with the type belt of Marlboro extending from 

Maryborough to Concord. Castle's upper and lower Boxford Formation are similar to Bell and Alvord's upper 

and lower Marlboro, although seemingly in reverse 

sequence to the Marlboro in the tunnel section. The 

Boxford Member of the Nashoba of Bell and Alvord thus 

could be part of the Marlboro repeated by faulting but 

representing the opposite limb of an earlier fold. The 

Fish Brook becomes part of the Marlboro Formation, in 

a stratigraphic position somewhat similar to that of the 

Grafton gneiss to the south. Barosh (1976, p. 309) noted 

rock resembling Fish Brook Gneiss near the top of the 

Marlboro near the intersection of 1-495 and 1-290 west of
FIG

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NASHOBA MARLBORO 

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NASHOBA MARLBORO 

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STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS Fll

Maryborough. Possibly Hepburn and DiNitto's granulite 

member of the Marlboro is a fades of the Fish Brook 

Gneiss and their rusty schist member is equivalent to the 

Shawsheen Gneiss. If so, the Assabet River fault must 

cut below the granulite member, as suggested on figure 

4A Hepburn and DiNitto's Marlboro section correlates 

fairly well with the Quinebaug section of Dixon (1965a; 

fig. 3). If this correlation is carried across to the sections 

of Skehan and Abu-Moustafa and Bell and Alvord (fig. 

44.), the Shawsheen Gneiss, units N1-N6, and the Bellows Hill, Spencer Brook, and Billerica Schist Members 

of the Nashoba would be assigned to the Marlboro rather 

than to the Nashoba.

The correlation scheme presented in figure 4A is, then, 

probably only one of several possible. The simplest 

possible scheme would be based on the interpretation 

that the faults have done little to disrupt the MarlboroNashoba sequence, as was implied by Alvord and others 

(1976) and that the sequence is homoclinal. The map 

pattern of Bell and Alvord (1976) can then be interpreted 

as showing a cross-sectional view of a metamorphosed 

stack of lenticular and intertonguing sedimentary, volcaniclastic, and volcanic rocks. Certainly intertonguing 

facies would be expected in the depositional environment 

of the protoliths described earlier; however, the indications of complex structure mentioned above in the section on thickness and sequence seem to me to make this 

view oversimplified.

It seems clear from figure 3 that no consistently 

applied boundary exists between the Marlboro and the 

Nashoba. Possibly Bell and Alvord (1976) are correct in 

implying that the Boxford is a tongue of amphibolite in 

the Nashoba lying well above the Sandy Pond of the 

Marlboro and that the Shawsheen is a tongue of 

Nashoba-like schist below the Sandy Pond. The boundary between the Marlboro and the Nashoba can only be 

placed, then, as it has been in the past, where sequences 

containing abundant amphibolite give way to sequences 

in which pelitic schist and gneiss predominate. My suggested assignment of the boundary, based on the interpretation of figure 45, is that the top of the Sandy Pond 

and the Boxford (of Bell and Alvord, 1976) are more 

clearly defined boundaries than any of those lying above, 

such as the top of the Spencer Brook Member or the top 

of N6 of the tunnel section, and thus should mark the top 

of the Marlboro Formation.

The inconsistent thicknesses of major units within the 

Nashoba zone, the lenticular map patterns of units, the 

repetition of rock types, and the difficulties in correlation 

of map units along strike demonstrated in the above 

discussion suggest that, rather than subdividing the 

Nashoba Formation into many members or dividing the 

zone into many formations, it would be preferable to 

keep only units that are distinct or readily recognizable

throughout the zone. This approach has been taken on 

the State bedrock map, where the column of Bell and 

Alvord has been used (fig. 3). On this map, the Nashoba 

and Marlboro are obvious units of formational rank, and 

the Fish Brook Gneiss and Shawsheen Gneiss are 

retained as discrete units also of formational rank. The 

Boxford Member of the Nashoba is the only one of Bell 

and Alvord's members considered distinct enough to be 

shown on the map. Actually, the Fish Brook and Shawsheen are limited in area and should not have formational 

rank. The Fish Brook and Shawsheen were given formational rank by Bell and Alvord (1976) because the Fish 

Brook has a distinctive lithology and separates the 

Shawsheen from the Nashoba to the west. However, 

they saw no reason why the Shawsheen could not otherwise be considered part of the Nashoba. Barosh and 

others (1977), in their compilation of the Boston I°x2° 

sheet, placed the Shawsheen in the Nashoba, as did 

Hansen (1956) originally. Accordingly, in figure 45, I 

have placed the Shawsheen in the Nashoba. Until unified 

detailed mapping is done in the Nashoba zone, and the 

scheme of figure 45 tested, the order of units probably 

should be kept as on the State bedrock map.

The weight of evidence seems to show that the present 

distribution of units within the zone is in large part 

controlled by structure. Folding may be partly responsible, but I would suggest that the pattern can best be 

explained by faults, along some of which appreciable 

differential lateral movement has occurred. One can 

easily visualize that formerly continuous masses of rock 

have been sliced and transported into disjointed lenses so 

that masses of rock from different parts of the section are 

now juxtaposed. These masses have subsequently been 

given separate formational or member names by people 

mapping in different parts of the strike belt. A uniform 

effort of detailed mapping in the Nashoba zone may 

clarify some of the uncertainties.

AGE

The stratified rocks of the Nashoba zone are unfossiliferous; their age can only be approximated within limits 

determined by radiometric data on rocks that intrude 

them. Upper limits on their age are established by ages 

on the Sharpners Pond Diorite (Ssqd) and phases of the 

Andover Granite (SOagr). The Sharpners Pond is nonfoliated and gives a concordant U-Pb age of 430 ±5 Ma 

(Zartman and Marvin, this vol., chap. J, table 1), or early 

Silurian. The Andover Granite is in part foliated and, 

from field relations, is believed to be older than the 

Sharpners Pond. The most reliable age from what 

appears to be the oldest part of the Andover places it in 

the Ordovician or possibly early Silurian, on the basis of
F12 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Rb-Sr whole-rock ages of 450 ±23 Ma (Zartman and 

Marvin, this vol., chap. J, table 1; Handford, 1965). Thus 

the youngest possible age for the rocks of the Nashoba 

zone is Ordovician. A minimum age of Ordovician for the 

Tatnic Hill Formation (equivalent to Nashoba on strike 

in southeastern Connecticut) was determined by Zartman and others (1965, p. D6).

The stratified rocks of the Nashoba zone are probably 

as old as Proterozoic. Olszewski (1980) obtained a 

730±26-Ma age on a concordia plot of apparently igneous 

zircons from the volcanic (or plutonic) Fish Brook Gneiss. 

If the Fish Brook Gneiss is a core gneiss, as Castle (1965) 

suggested, then the zircons date only that unit and not 

necessarily the surrounding strata. However, field relations seen by Bell and Alvord (cited above in the section 

on descriptions of units) indicate that the Fish Brook is 

part of the Nashoba-Marlboro sequence. A concordia plot 

of probably detrital zircons in the Shawsheen Gneiss 

gave an age of more than 1,500 Ma, thus dating a source 

terrane, proto-Africa(?), not now present, to the east. 

Olszewski (1980, p. 1414) pointed out that the pattern of 

ages in the Nashoba and Milford-Dedham zones is similar 

to the pattern in northwest Africa as described by 

Schenk (1971) and Hurley and others (1974). He also 

pointed out that the detrital zircons could be derived 

from the Grenville terrane to the west as well as from a 

proto-African source to the east.

Skehan and Murray (1980, fig. 3) suggested that the 

Tadmuck Brook Schist, which overlies the Nashoba 

Formation, is Ordovician(?) to Cambrian in age, on the 

basis of the unconformity at its base and an Ordovician to 

Silurian age on the Ayer Granite that intrudes the units 

above the Nashoba in and west of the Clinton-Newbury 

fault zone. This suggestion is discussed in the chapter on 

the Merrimack belt (Robinson and Goldsmith, this vol., 

chap. G).

The rocks of the Nashoba zone are shown on the State 

bedrock map as Ordovician or Proterozoic Z. This dual 

age was given because at the time the map was prepared, 

I was uncertain about the actual rocks sampled by 

Olszewski (1978), and because I held a strong belief in a 

correlation of rocks of the Nashoba zone with similar 

rocks to the west considered to be Ordovician in age (see 

below). Now, because of the radiometric ages, I believe 

that the rocks of the Nashoba zone, except possibly for 

the Tadmuck Brook Schist, could be Proterozoic. If so, 

however, they are unlike the Proterozoic rocks in the 

Milford-Dedham zone to the east (Goldsmith, this vol., 

chap. E).

REGIONAL CORRELATIONS

The rocks of the Nashoba zone can with some confidence be correlated with units along strike in Connecticut and in southeastern Maine (figs. 5, 6). They can be 

correlated with less confidence with rocks across strike 

in the Merrimack belt, with those on the east flank of the 

Bronson Hill anticlinorium in east-central Massachusetts, Connecticut, and southwestern New Hampshire, 

and with the Massabesic Gneiss Complex of northernmost central Massachusetts and adjacent New Hampshire.

The Nashoba and Marlboro Formations are equivalent 

to the Tatnic Hill and Quinebaug Formations of eastern 

Connecticut, respectively (figs. 3, 5, 6). The Nashoba 

thins near Oxford but is continuous with the Tatnic Hill 

to the south, although the passage near Webster is along 

a number of closely spaced fault slices (Barosh, 1974; 

H.R. Dixon, written commun., 1978). The Marlboro and 

Quinebaug are not continuous across the salient between 

Webster and Oxford, but the two have similar lithologies 

and occupy the same position with respect to the Nashoba and Tatnic Hill. Emerson (1917) and Dixon (1965a) 

initially correlated the two sequences, and the correlation is now generally accepted (Barosh, 1977; Barosh and 

others, 1977; H.R. Dixon, written commun., 1978). 

Detailed lithologic correlation of members within the 

formations cannot be made between the two terranes 

(Nashoba and Putnam). However, similar groups of 

lithologies occupy similar structural positions (fig. 3). 

Dixon (written commun., 1978) recognized units of the 

Tatnic Hill in the Nashoba in the Oxford and Grafton 

areas during reconnaissance mapping for the State bedrock map. The Fly Pond Member of the Tatnic Hill might 

correspond to either unit N29 or units N12-20 of the 

tunnel section (Skehan and Abu-Moustafa, 1976). The 

Quinebaug, on the whole, however, appears to contain 

less amphibolite than the Marlboro. In Connecticut, the 

Tatnic fault (Wintsch and Hudson, 1978) separates the 

Tatnic Hill and the Quinebaug Formation in many places 

(Dixon, 1965b, 1968, 1974). This fault appears to be a 

bedding-plane fault that lies close to the normal stratigraphic contact between the two formations. Interlayering of rock types that does not appear to be tectonic is 

common in the contact zone, and a rusty schist that 

Dixon considered to be the basal part of the Tatnic Hill 

(Dixon, 1965a) can be recognized at many places along 

the contact zone in Connecticut and at the base of the 

Nashoba in Massachusetts from Webster to Shrewsbury. 

The Tatnic Hill is much thinner than the Nashoba 

Formation. This thinning could be stratigraphic but more 

likely is tectonic, attributable to faulting along the Lake 

Char, Honey Hill, and Tatnic fault systems in Connecticut (Goldsmith, this vol., chap. H).

Correlation of rocks of the Nashoba zone with rocks to 

the northeast in southern Maine and New Hampshire 

(figs. 5-7) must be made across a gap. The Nashoba 

Formation in the Lowell area is truncated against the
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F13

46° 11

UNITED STATES^ 

 (iV/I Middle fig.J £ >P

W^/f S

42°

FIGURE 5. Terranes in New England possibly correlative with the Nashoba zone. Terrane boundaries dashed where uncertain.
Brimfield area, 

ConnecticutMassachusetts 

Peper and Pease 

(1976)

VERTICAL 

EXAGGERATION X0.25

Bigelow Brook 

Formation 

( = part of

Paxton 

Formation)

fault

Eastern Connecticut 

Dixon(1965a)

WachusettMarl borough

tunnel. 

Massachusetts

Skehan and

Abu-Moustafa

(1976)

X0.5

Middlesex County. 

Massachusetts 

Bell and Alvord

(1976) 

Zen and others

(1983)

X0.25

Southern Maine

and southeastern

New Hampshire

Hussey

(1968)

X1.5

fault

Upper 

member

Lower 

member

East-central

Maine 

Osberg (1979)

X1.5

J1.

a1

(3

&

CD

5

Jewell

Formation

Spurwlnk Lmst.

Scarboro Formation

Diamond 

Island Fm.

Spring Point

Cape Elizabeth

Formation

Gushing

Formation _ ^X^_^^ ^~~~~~?~^^^ ^^~-~~<^ / // s/ ,s s/ /

Vassalboro

Formation'

^^ ^x-x-SwX-^^-^^* ^^

Mine Hill

Formation

Lenticular marble

Mine Hill Formation

Gushing

Formation

FIGURE 6. Correlation and relative thicknesses of formations in the Nashoba zone and of some possibly correlative formations elsewhere in Massachusetts, Connecticut, New 

Hampshire, and Maine. Shawsheen Gneiss and Fish Brook Gneiss are presently ranked as formations by the U.S. Geological Survey. Wavy line indicates unconformity. Lines 

queried where conjectured. Alternative tielines are dashed or dotted.

wO 

O 

W

O 

H 

O 

f

O

02g O

w
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F15

Hussey (1968, 1971) 

Southern Maine

Thickness 

(meters)

Q.

D 

O

O 

>

CD

CD 

O

O 

V) 

CD

O

Jewell 

Formation ^^

Spurwink / 

Limestone /

Scarboro / 

Formation //

Diamond // 

Island // 

Formation / j

Spring / I 

Point / / 

Greenstone / /

Cape / 

Elizabeth / 

Formation /

Gushing 

Formation

~^>&r~^

150 ^30

0-45

0-120

300- 

1500

>600

Sulfidic and nonsulfidic two-mica schist 

containing thin beds of mica schist and 

quartzite___________________

Ribbon limestone containing thin beds of 

biotite, phyllite, and schist________

Same as Jewell Formation

Laminated black, sulfidic quartz-schist 

and phyllite_________________ 

Chlorite schist, actinolite schist (or 

amphibolite); feldspathic quartzite 

and metafelsite at top__________

Muscovite-biotite-plagioclase-quartz 

schist, micaceous quartzite and lenses 

of calc-silicate rock____________ 

Biotite-quartz-feldspar gneiss, amphibolite, 

biotite-feldspar-quartz schist and 

gneiss, calc-silicate gneiss; minor marble 

and sulfidic two-mica quartz schist

Hussey (1968) 

Southeastern New Hampshire

Thickness 

(meters)

O 

LL.

0) 

>

OC

Upper 

member

Lower 

member

300- 

600

>300

-faultPorphyroblastic biotite-quartz-feldspar 

gneiss containing thin beds of feldspathic 

metagraywacke; felsic metatuff in upper 

part; includes amphibolite, calcareous 

metagraywacke, marble, and graphitic 

phyllite

(Staurolite)-(sillimanite)-muscovitebiotite-feldspar-quartz schist; feldspar 

content varies

Osberg (1979) 

East-central Maine

Thickness 

(meters)

Mine Hill 

Formation

Gushing 

Formation

200

Rusty-weathering quartz mica schist and 

phyllite, and sparse quartz-feldspar 

layers; contains garnet and sillimanite

Lenticular marble and calc-silicate rock 

Slightly rusty-weathering garnet - 

(sillimanite)-quartz-mica schist

Upper massive biotite-quartz-plagioclase 

gneiss; medial interlayered biotite-quartzplagioclase gneiss and biotite amphibolite

Novotny (1969) 

Southeastern New Hampshire

Thickness 

(meters)

SCALE, IN METERS 

Hussey (1968,1971), Osberg (1979)

-- 0

- 100

200

300

400

-- 500

-- 600

SCALE, IN METERS 

Novotny (1969)

 0

 - 200

 400

 600

 - 800

 1000

 1200

c 

g ' *->

CD

E

0

u_

> 

cc

'co

CD 

_O

o IT

CD -Q

« E

£ E

a

^

CD

C

1 -

CD -D

ca ez

4-^ CD IE

1

16001

>2700

 IdUll 

Quartz-biotite-plagioclase gneiss

containing laminae of quartz-biotite

schist and quartz-actinolite schist; 

biotite and hornblendic injection gneiss;

amphibolite and hornblende schist; minor

quartzite

(Garnet)-(sillimanite)-mica schist and

quartz-feldspar-mica schist; feldspathic

and garnetiferous quartzite; (diopside)-

(garnet) amphibolite

Thickness calculated from cross sections

on map of Novotny (1969)

FIGURE 7. Lithostratigraphic columns of units in southern and east-central Maine and southeastern New Hampshire that are probably

correlative with units of the Nashoba zone.

Clinton-Newbury fault and is largely engulfed in granite 

of the Andover pluton, so that only the Boxford Member 

of the Nashoba and the Fish Brook Gneiss extend to the 

Newburyport area and the Gulf of Maine. The Marlboro 

is similarly truncated or cut out to the northeast. However, a rock sequence similar to the Nashoba is present 

in southern Maine from south of Augusta at least as far 

as the Brunswick area (figs. 5-7). These rocks have been

mapped by Osberg (1979) as the Gushing Formation, 

part of the Casco Bay Group of Katz (1918) redefined by 

Hussey (1962, 1968, 1971). The age of the Gushing is 

uncertain, but the unit is considered to lie disconformably below the Silurian turbidite section of eastern and 

central Maine (Osberg, 1979). The Mine Hill Formation 

of Osberg (1979) and a similar sulfidic schist mapped by 

Hussey (1971) in Casco Bay intervene between the
F16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Gushing and the turbidite section. These sulfidic schists 

are similar in lithology and occupy the same stratigraphic 

position relative to the Gushing as the Tadmuck Brook 

Schist does to the Nashoba (fig. 6), as pointed out by 

Osberg (1979, p. 42). The Gushing terrane in Maine is, 

however, appreciably offset to the west from the trend of 

the Nashoba zone in Massachusetts (fig. 5). Farther 

north, in the Belfast area, Maine, the Passagassawakeag 

Formation as described by Bickel (1976) appears to 

resemble the Nashoba Formation. The Passagassawakeag is considered to be probably Precambrian in 

age. Far to the north, Kennedy's (1976) description of 

rocks in the Gander zone of Newfoundland is remarkably 

similar to the descriptions of the rocks in the Nashoba 

zone. A specific correlation might be made between the 

Nashoba Formation and the Little Passage Gneisses of 

the Gander zone (Colman-Sadd, 1980).

Similarities in structural position and in lithology 

indicate that parts of the Marlboro Formation are quite 

likely equivalent to the Rye Formation of southeastern 

New Hampshire and Maine (figs. 3, 6, 7). Castle (1965) 

suggested correlation of his Boxford Formation with the 

Rye Formation. Novotny's (1969) Rye section (fig. 7) is 

similar to, if somewhat thicker than, that of Hussey 

(1968), although the thickness may be an artifact of my 

calculations of thickness from Novotny's map and sections. Both Rye sections resemble in lithology the part of 

Hepburn and DiNitto's Marlboro section above their 

Sandy Pond amphibolite member (fig. 3). The upper 

member of the Rye resembles Hepburn and DiNitto's 

granulite member of the Marlboro and the upper member 

of the Quinebaug Formation of Dixon (1965a) and resembles the undivided part of the Marlboro Formation of Bell 

and Alvord (1976) that lies below their Sandy Pond 

Member. The lower member of the Rye resembles to 

some extent Hepburn and DiNitto's rusty schist member 

of the Marlboro and the Shawsheen Gneiss as described 

by Bell and Alvord (1976). Correlation of the Marlboro 

Formation with the Rye Formation is not clear cut.

Correlation of the rock units of the Nashoba zone with 

units west of the Nashoba strike belt, such as those of 

Peper and Pease (1976) (fig. 6) and the units on the east 

flank of the Bronson Hill anticlinorium (Robinson, 1979), 

is less certain. Prior to the isotopic work of Olszewski 

(1978, 1980), the Nashoba was considered to be of 

Ordovician age, on the basis of a more or less continuous 

mapping of rusty-schist lithologies from the Ordovician 

Partridge Formation, on the west flank of the Bronson 

Hill anticlinorium in New Hampshire (fig. 5), southward 

through the Brimfield Schist terrane in Massachusetts, 

where the rusty schists on the east flank of the anticlinorium and in the Merrimack synclinorium have been 

called Brimfield Schist (Emerson's (1917) term; Brimfield Group of Peper and Pease (1976)) and, in the Middle

BRONSON HILL 

ANTICLINORIUM

MERRIMACK SYNCLINORIUM 

Hebron and equivalent formations

(including equivalents of Paxton

and Oakdale Formations of

Massachusetts)

Bigelow Brook 

Formation

SOh: Hebron Formation

SOs: Southbridge

Formation

Brimfield Schist and equivalent formations

I Och I I Obr I I Ota

Collins Hill Formation

( = Partridge Formation

of New Hampshire)

Brimfield Schist Tatnic Hill 

(including Hamilton Formation 

Reservoir Formation)

Quinebaug 

Formation

I Omo [ 

Monson Gneiss

wcc

-faultsAVALONIAN TERRANE 

(Proterozoic Z)

FIGURE 8. Stratified lower Paleozoic rocks in eastern Connecticut, as 

shown by Rodgers (1982).

Haddam area, the Collins Hill Schist (Snyder, 1970; 

Eaton and Rosenfeld, 1972; Collins Hill Formation of 

Rodgers, 1982, on fig. 8) around the Chester syncline 

(Dixon and Lundgren, 1968) above the Honey Hill fault 

into the Tatnic Hill Formation of eastern Connecticut 

and thence north into the Nashoba Formation. Emerson 

(1917) mapped rusty schists in the Nashoba as Brimfield. 

Peter Robinson (oral commun., 1979; Hall and Robinson, 

1982, p. 27) pointed out that the Tatnic Hill and Nashoba, 

although consisting of lithologies similar to those in the 

Partridge Formation on the Bronson Hill, contain appreciable magnetite and represent a more oxidized metamorphic facies than does the Partridge. The Quinebaug 

(Marlboro) underlying the Tatnic Hill (Nashoba) was 

equated with the Middletown Gneiss on the east flank of 

the Bronson Hill in Connecticut (Dixon and Lundgren, 

1968), which in turn was equated by them with the 

Ammonoosuc Volcanics underlying the Partridge of central Massachusetts and western New Hampshire. This 

interpretation of the relations was followed by Rodgers 

(1982) in the preliminary bedrock geologic map of Connecticut from which figure 8 has been adapted.

The continuity on which the above correlation is based 

has been questioned recently. Wintsch (1979a; Wintsch 

and Kodidek, 1981) presented evidence that the Tatnic
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F17

Hill is not continuous with the rusty schists in the 

Chester syncline and with the Brimfield and Collins Hill 

Schists to the west on the Bronson Hill. Thrust faults 

beneath the Brimfield Group (Peper and others, 1975) 

project into a high-angle fault, the Bone Mill Brook fault 

(Peper and others, 1975; Pease and Fahey, 1978; Pease, 

1982) along the east side of the Bronson Hill anticlinorium in Connecticut, which creates a fault wedge of 

Emerson's type area of Brimfield Schist that is separate 

from the Bronson Hill sequence. However, Robinson and 

Tucker (1982) saw no evidence farther north in Massachusetts for a major fault along the east flank of the 

Bronson Hill; they equated the rusty schists of the 

Brimfield Group, Peper and Pease's (1976) Hamilton 

Reservoir Formation, with the Partridge, but in a thickened, basinward section. The Hamilton Reservoir, from 

detailed descriptions by Peper and Pease (1976) and 

Peper and others (1975), does not correlate particularly 

well with the Tatnic Hill Formation, but in general it 

does somewhat resemble Bell and Alvord's (1976) 

description of the Nashoba. Regional considerations suggest to some (Snyder, 1970, for example) that the 

Brimfield Group is inverted, although this does not affect 

the correlation. In support of this inversion, the Bigelow 

Brook Formation of Peper and Pease (1976) passes along 

strike into the Paxton Formation in Massachusetts 

where it overlies the Partridge-type rocks on the east 

flank of the Bronson Hill anticlinorium (Peter Robinson, 

oral commun., 1978). A U-Pb radiometric age of 440 Ma 

on zircon from a granite gneiss intruding the Hamilton 

Reservoir Formation (Pease and Barosh, 1981, p. 23) 

indicates an Ordovician or older age for this part, at 

least, of the Brimfield Group. G.R. Robinson, Jr. (written commun., 1982), has suggested that possibly only the 

Tadmuck Brook Schist is correlative with the Partridge 

and is Ordovician in age. This suggestion is in accord 

with Skehan and Murray's (1980) interpretation of a 

Cambrian-0rdovician(?) age for the Tadmuck Brook.

Rocks similar to parts of the Nashoba have been 

observed in the Massabesic Gneiss Complex of southcentral New Hampshire and adjacent Massachusetts 

(Massabesic terrane of fig. 5) within what has previously 

been mapped as the Fitchburg pluton (Billings, 1956). 

The Massabesic Gneiss Complex also contains rocks 

resembling the Ordovician Monson Gneiss of the Bronson 

Hill anticlinorium and the Quinebaug Formation in the 

Willimantic Dome in east-central Connecticut. U-Pb 

isotopic data on zircons obtained from the Massabesic, 

however, indicate both Proterozoic Z (645-Ma) and Ordovician (480-Ma) ages, as well as Permian (275-Ma) ages in 

late granite (Besancon and others, 1977; Aleinikoff and 

others, 1979). The Massabesic lies in an anticlinal area 

within the Merrimack synclinorium, northwest of the 

Nashoba zone. Its structural position relative to the

Nashoba zone has yet to be determined. The Massabesic 

Gneiss Complex is described by Peter Robinson in a 

separate section at the end of this chapter.

SUMMARY AND CONCLUSIONS

The Nashoba zone is a fault-bounded wedge consisting 

of high-grade, steeply west-dipping metamorphic rocks, 

largely metasedimentary to the west (Tadmuck Brook 

Schist, SZtb; Nashoba Formation, OZn; Fish Brook 

Gneiss, OZf; and Shawsheen Gneiss, OZsh) and largely 

metavolcanic to the east (Marlboro Formation, OZm, 

OZmg). These rocks occupy a terrane distinct from the 

Merrimack belt to the west and the Milford-Dedham zone 

to the east. Complex faulting and folding and much 

granite intrusion hinder determination of a stratigraphic 

sequence throughout the zone. However, two major 

formations, presumably topping to the west, are recognized: the Nashoba Formation, consisting of pelitic and 

semipelitic (metawacke) gneiss and schist and subordinate calc-silicate rock, marble, and amphibolite, some of 

which was formed from impure carbonates; and the 

Marlboro Formation, consisting of amphibolite, feldspathic gneiss, and subordinate pelitic schist and calcsilicate rock. The Tadmuck Brook Schist, consisting of 

pelitic schist and phyllite and subordinate quartzite, lies 

above the Nashoba, probably unconformably. The Shawsheen Gneiss, consisting of pelitic and semipelitic schist 

and gneiss similar to parts of the Nashoba, lies above the 

Marlboro. Above the Shawsheen and below the Nashoba 

is the Fish Brook Gneiss, a lenticular, felsic metavolcanic 

or intrusive rock. Possible assignment of stratigraphic 

units that differs from that shown on the State bedrock 

map is based on the similarity in lithology of the Shawsheen Gneiss to units in the Nashoba and on the observation that the Fish Brook Gneiss appears to be lenticular. According to this scheme, the Shawsheen and the 

Fish Brook would become members of the Nashoba 

Formation. A third scheme that can be deduced from the 

fault pattern within the zone (figs. 1, 4) may be worth 

consideration. Under this scheme, the Boxford Member 

of the Nashoba correlates with the Sandy Pond Amphibolite Member of the Marlboro. If the Boxford is reassigned to the Marlboro, the Fish Brook Gneiss, lying 

below the Boxford Member and above the Assabet River 

fault, becomes the lowest unit in the Marlboro. The 

Shawsheen, lying in a different fault block below the 

Assabet River fault, could be assigned to the Nashoba.

The protoliths of the Marlboro Formation were volcanic, volcaniclastic, and epiclastic materials deposited in 

a marine environment close to a volcanic source to the 

east. The protoliths of the Nashoba Formation were 

volcaniclastic, epiclastic, and minor volcanic materials
F18 THE BEDROCK GEOLOGY OF MASSACHUSETTS

and carbonate rocks deposited in the basin farther from 

the volcanic center. The minor volcanic component was 

primarily basaltic but also included andesitic, dacitic, and 

rhyodacitic materials. The Fish Brook Gneiss is a prominent felsic, probably volcanic unit. The material in the 

zone seems to have been derived from both a deeply 

weathered terrane and a volcanic source, according to 

Abu-Moustafa and Skehan (1976). A 1,500-Ma age for 

detrital zircon in the Shawsheen Gneiss (Olszewski, 1980) 

indicates an old source area not now recognized. A source 

of zircons of this age can be found in northwest Africa. 

No rocks of this age are known in the Grenville terrane 

to the west. Nearer at hand is the allochthonous Chain 

Lakes massif, western Maine (fig. 5), from which a 1.5- to 

1.6-Ga age is reported on zircon (Naylor and others, 

1973; Boudette and Boone, 1982). The rocks of the 

Nashoba zone may have been derived from two sources, 

but if they came from one, the African source seems most 

likely, on the basis of Olszewski's work.

Igneous zircons from the Fish Brook Gneiss indicate a 

Proterozoic Z age (at least 750 Ma (Olszewski, 1980)), 

older than those so far recorded in the batholithic rocks 

of the Milford-Dedham zone to the east. Unless the Fish 

Brook is intrusive, this is a possible age for the Nashoba 

Formation. The pattern of volcanic and detrital ages is 

similar to that in northwest Africa in both time of 

volcanism and source of detritus according to Olszewski. 

The age of the rocks of the Nashoba zone therefore 

appears to be Proterozoic Z. The rocks can be no younger 

than Ordovician because they are intruded by the 

Andover Granite, parts of which could be as old as 

Ordovician (Zartman and Marvin, this vol., chap. J, table 

1). A strongly argued correlation with some of the rocks 

of the Brimfield Group to the west on the flanks of the 

Bronson Hill anticlinorium suggests an Ordovician age, 

but the pattern of deposition and the zircon age would 

indicate that the Nashoba zone belongs to the eastern 

basement, as indicated by Osberg (1978), rather than to 

North American basement or to a lower Paleozoic volcanic island arc.

Rocks of the Nashoba zone occupy the same strike belt 

as the Putnam Group of eastern Connecticut. To the 

north, the Nashoba Formation is correlated with the 

Gushing Formation of the Casco Bay Group in Maine 

although it is offset to the southeast in strike from the 

Gushing terrane (fig. 5). The Marlboro Formation is 

correlated with the Rye Formation in southeastern New 

Hampshire and Maine. Correlation with rocks in other 

lithotectonic belts of the Appalachians to the west across 

strike is uncertain. The rocks may be equivalent to units 

in the Bronson Hill anticlinorium and to parts of the 

Massabesic Gneiss Complex of southern New Hampshire 

and adjacent Massachusetts. If so, considerable structural complexity exists across this part of the orogen

(Goldsmith, this vol., chap. H). If not, or perhaps even 

so, the Nashoba zone represents an exotic, probably 

accretionary terrane that was formerly a part of northwest Africa.

MASSABESIC GNEISS COMPLEX (OZma)

By PETER ROBINSON 1

The Massabesic Gneiss Complex (OZma) of northernmost central Massachusetts and adjacent New Hampshire (figs. 1, 2) forms an isolated area of Proterozoic Z 

and Ordovician rocks within the broad belt of Silurian 

and Devonian strata of the Merrimack synclinorium (fig. 

5). The Massabesic Gneiss Complex consists of layered 

Proterozoic Z gneisses and Ordovician granitic gneisses. 

These rocks are abundantly intruded by fine-grained, 

pink biotite granite of Pennsylvanian-Permian age (Pgr). 

Practically none of the rocks in this region are related to 

the Fitchburg Complex (Dfgr), which is a complex of 

sheetlike plutons of Devonian age intruding Silurian and 

Devonian strata in a broadly synclinal region west and 

southwest of the area of exposed Massabesic Gneiss. The 

Massabesic is exposed in an anticline, produced during 

Acadian deformation, that plunges south, so that the 

Massabesic is exposed only at the north edge of the State 

bedrock map. On the State bedrock map, the Massabesic 

is assigned to the Nashoba zone of pre-Silurian strata to 

the east, rather than to the Bronson Hill zone to the 

west, for the following reasons: It is geographically 

closer to the Nashoba zone than to the Bronson Hill zone, 

and the lithology and tectonic position of the Massabesic 

vaguely resemble those of the Quinebaug Formation in 

the Willimantic dome of southeastern Connecticut; the 

Quinebaug is correlated with the Marlboro Formation of 

the Nashoba zone. The fault relationships described in 

the Willimantic dome (Wintsch, 1979b) have not, so far, 

been identified in the Massabesic, although, like the 

Willimantic, the Massabesic has the form of an anticline.

Stratigraphic classification of the Massabesic has long 

been controversial and was undergoing very rapid 

change as the State bedrock map was being compiled. 

Billings (1956) classified the Massabesic as part of the 

Fitchburg pluton of Devonian age. Later investigations 

of some of the gneissic Massabesic rocks in the Manchester, N.H., area (Besancon and others, 1977) yielded 

Proterozoic Z ages. In 1976-77, mapping in the 

Townsend (Mass.-N.H.) area (Robinson, 1978) and 

reconnaissance mapping in the Ashby (Mass.-N.H.) area 

(Peper and Wilson, 1978) showed all the granitoid rocks

Department of Geology, University of Massachusetts, Amherst, MA 01003.
STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS F19

as part of a Devonian plutonic complex (Fitchburg Complex). Such an interpretation seemed assured, on the 

basis of more detailed mapping, in the rest of Massachusetts (Grew, 1970; Hepburn, written commun., 1976; 

Tucker, 1977; Peper and Wilson, 1978), where the Fitchburg pluton was seen as a complex of granite and 

granodiorite-tonalite sills (Maczuga, 1981) intruding Silurian and Devonian strata (Tucker, 1977). A radiometric 

age of 390±15 Ma on zircon (Zartman and Marvin, this 

vol., chap. J, table 1) from the probably slightly younger, 

massive muscovite-biotite granite at Rollstone Hill, 

Fitchburg (Peper and Wilson, 1978; Maczuga, 1981), 

appears to support this view.

The Massabesic Gneiss Complex shown on the State 

bedrock map is based on the detailed mapping and 

radiometric dating of Aleinikoff (1978) in the Milford, 

N.H., area, just north of the Massachusetts-New Hampshire border; on field excursions in 1978 with Aleinikoff 

and J.B. Lyons in the Milford area and the adjacent 

Townsend, Mass.-N.H., area; and on reconnaissance 

reinterpretation in 1978 by Peter Robinson of previous 

work by G.R. Robinson (1978) and Peper and Wilson 

(1978) in the Townsend and Ashby areas.

The work of Aleinikoff (1978; Aleinikoff and others, 

1979) in the Milford area showed that the area mapped in 

the previous literature as Massabesic Gneiss contains no 

major amount of Devonian plutonic rocks but consists of 

three major components: (1) layered plagioclase-quartz 

gneisses believed to be metamorphosed felsic volcanic 

rocks, yielding zircon ages of around 645 Ma (Proterozoic 

Z), (2) massive coarse-grained pink granitic gneisses, 

yielding zircon ages of around 480 Ma (Ordovician), and 

(3) massive fine-grained pink biotite granite ("Milford, 

N.H., granite"), yielding zircon ages of around 275 Ma 

(Pennsylvanian-Permian). The first two are assigned on 

the State bedrock map to the Massabesic Gneiss Complex (OZma); the third is shown separately among the 

intrusive rocks of the Merrimack belt as pink and gray 

biotite granite (Pgr). Aleinikoff (1978) showed that a 

geochemical affinity exists between the Ordovician granitic gneisses and the late Paleozoic granites, suggesting 

the latter might be melting products of the former.

Reinterpretation of the mapping of G.R. Robinson 

(1978) in the Townsend area showed that almost none of 

the "Fitchburg Granite" as mapped resembles the Fitchburg Complex farther southwest in Massachusetts, but it 

can be divided amongst the three rock types identified by 

Aleinikoff. Specifically, most areas mapped as "granite 

gneiss" (Dfbg) by G.R. Robinson can be equated with the 

Proterozoic Z and Ordovician gneisses of the Massabesic 

Gneiss Complex proper. Although Proterozoic Z and 

Ordovician parts of the Massabesic have not yet been 

separately mapped in the area of the State bedrock map,

it is clear that both are present. Thus, unlike other areas 

labeled "OZ" on the State bedrock map, in the case of the 

Massabesic (OZma) the "OZ" does not denote uncertainty of age but, rather, indicates that the unit definitely contains rocks of both Ordovician and Proterozoic 

Z age. Massabesic Gneiss Complex proper (OZma) has 

also been identified (Robinson, 1978) in the extreme 

eastern edge of the Ashby quadrangle.

An interesting feature of the Massabesic of the 

Townsend area is its pronounced east-west mineral lineation, identical in orientation and character to the 

lineation in a wide area of Silurian and Devonian stratified and plutonic rocks to the west and southwest and 

clearly the product of an intermediate stage of the 

Acadian (Devonian) orogeny (Robinson, 1979).

The fine-grained nonfoliated part of the "Fitchburg 

granite" (Dfg) mapped by Robinson (1978) is nearly all 

identified as the late Paleozoic "Milford, N.H., granite" 

of Aleinikoff (1978). Field areas mapped in detail by G.R. 

Robinson show how this granite has intruded the 

Oakdale Formation in the Merrimack belt (So of the 

State bedrock map, Oqfg of G.R. Robinson) parallel to 

bedding or foliation to produce a series of elongate, 

slablike inclusions (Robinson and Goldsmith, this vol., 

chap. G). Misidentification of this fine-grained granite for 

Proterozoic Z parts of the Massabesic might lead to an 

interpretation based on these contact relations that the 

Oakdale is Precambrian in age.

In light of their geochemical affinities (Aleinikoff, 

1978), it is not surprising that the finer grained parts of 

the Ordovician granitic gneiss of the Massabesic are very 

difficult to tell in the field from the late Paleozoic granite 

(Pgr); hence contacts on the State bedrock map, modified 

from those of Robinson (1978), are highly tentative. Field 

separation of the Proterozoic and Ordovician parts of the 

Massabesic is even more difficult and was not accomplished even by Aleinikoff. An excellent exposure in a 

small abandoned quarry on the southwest shore of Potanipo Pond, Mass., shows the late Paleozoic granite 

(Pgr) truncating the foliation of the Massabesic Gneiss 

Complex (OZma).

A major problem yet to be solved is the nature of the 

contact between the Massabesic and surrounding stratified rocks, though the problem to the east could be less 

severe if the Oakdale Formation is Proterozoic (Barosh, 

1982; Lyons and others, 1982) or Ordovician(?)-Cambrian 

(Skehan and Murray, 1980, fig. 4). If the surrounding 

rocks are mainly Silurian and Devonian, as we think, 

then a search should be made for either an unconformity 

or possibly a deformed subhorizontal fault surface similar 

to others described in southeastern New England (Lundgren and Ebblin, 1972; Castle and others, 1976; 

Wintsch, 1979b; Goldstein, 1982).
F20 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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F22 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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Tucker, R.D., 1977, Bedrock geology of the Barre area, central 

Massachusetts: University of Massachusetts, Geology and Geography Department, Contribution No. 30, 132 p.

Wintsch, R.P., 1979a, Recent mapping in the Chester area, Connecticut, and its bearing on the Chester syncline [abs.]: Geological 

Society of America Abstracts with Programs, v. 11, no. 1, p. 60.

 1979b, The Willimantic fault: a ductile fault in eastern Connecticut: American Journal of Science, v. 279, p. 367-393.

Wintsch, R.P., and Hudson, M.R., 1978, Southeastward thrusting in 

eastern Connecticut [abs.]: Geological Society of America 

Abstracts with Programs, v. 10, no. 2, p. 91.

Wintsch, R.P., and Kodidek, K.L., 1981, Local and regional implications of recent mapping in the Essex area, Connecticut [abs.]: 

Geological Society of America Abstracts with Programs, v. 13, no. 

3, p. 184.

Zartman, R.E., and Naylor, R.S., 1984, Structural implications of 

some radiometric ages of igneous rocks in southeastern New 

England: Geological Society of America Bulletin, v. 95, no. 5, 

p. 522-539.

Zartman, R.E., Snyder, G.L., Stern, T.W., Marvin, R.F., and Buckman, R.C., 1965, Implications of new radiometric ages in eastern 

Connecticut and Massachusetts: U.S. Geological Survey Professional Paper 525-D, p. D1-D10.

Zen, E-an, editor, and Goldsmith, Richard, Ratcliffe, N.M., Robinson, 

Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 

of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 

scale 1:250,000.
Stratigraphy of the Merrimack 

Belt, Central Massachusetts

By PETER ROBINSON, UNIVERSITY OF MASSACHUSETTS, and RICHARD 

GOLDSMITH, U.S. GEOLOGICAL SURVEY

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-G

CONTENTS

Abstract............................................................................. Gl

Introduction........................................................................ 2

Ware, Gardner, Southbridge, and Wachusett Mountain

subbelts.................................................................... 8

Fitch Formation (Sfs, Sfss) ............................................. 8

Paxton Formation.......................................................... 12

Sulfidic schist and sillimanite quartzite (Spsq) .............. 12

Sulfidic quartzite and rusty schist (Spqr)..................... 14

Granofels member (Sp)............................................. 15

Amphibolite (Spa).................................................... 18

Bigelow Brook Member (Spbs, Spbc).......................... 18

Southbridge Member (Spso)...................................... 18

Sulfidic mica schist (Spss) ......................................... 18

Littleton Formation (Dl, D1+Ops, Dlf, Dlo, Dim)................ 19

Ware subbelt.......................................................... 20

Gardner subbelt...................................................... 22

Wachusett Mountain subbelt...................................... 22

Harding Hill syncline ............................................... 23

Nashua and Rockingham subbelts......................................... G24

Nashua subbelt........................................................... 25

Boylston Schist (SObo)........................................... 25

Tower Hill Quartzite (St, Sts).................................. 26

Oakdale Formation (So).......................................... 27

Worcester Formation (DSw).................................... 27

Coal Mine Brook Formation (Pcm)........................... 28

Age relations ........................................................ 29

Rockingham subbelt..................................................... 29

Vaughn Hills Quartzite (SOvh)................................. 30

Reubens Hill Formation (SOrh)................................ 30

Kittery Formation (SOk)......................................... 31

Eliot Formation (Se).............................................. 31

Berwick Formation (Sb, Sbs)................................... 31

Harvard Conglomerate (Ph).................................... 32

Age relations........................................................ 32

Discussion.................................................................. 33

References cited................................................................ 34

ILLUSTRATIONS

FIGURE 1. Map showing major divisions and structural features of the Merrimack belt and localities of Pennsylvanian strata.......... G4

2. Correlation chart of sedimentary and volcanic rocks of the Merrimack belt and their metamorphic equivalents............... 5

3-6. Geologic maps showing:

3. The Merrimack belt: A, western part; B, eastern part.............................................................................. 6

4. Gardner-Athol area............................................................................................................................. 9

5. Ware-Southbridge area........................................................................................................................ 10

6. Area of the Fitchburg plutons............................................................................................................... 11

7. Correlation charts of previous nomenclature of stratified units in the eastern part of the Merrimack belt: A, Nashua

subbelt; B, Rockingham subbelt.................................................................................................................. 16

8. Columnar sections for the eastern part of the Merrimack belt from eastern Connecticut to southeastern

New Hampshire and southwestern Maine ..................................................................................................... 26

9. Geologic map of the Worcester area................................................................................................................... 28

TABLE

TABLE 1. Distribution of Silurian-Devonian lithic units of the Massachusetts State bedrock map in western subbelts of the Merrimack 

belt............................................................................................................................................................. G3

in

THE BEDROCK GEOLOGY OF MASSACHUSETTS

STRATIGRAPHY OF THE MERRIMACK BELT, 

CENTRAL MASSACHUSETTS

By PETER ROBINSON l and RICHARD GOLDSMITH'

ABSTRACT

The Merrimack belt of central Massachusetts overlaps the junction 

between two zones of Ordovician and older rocks, the Bronson Hill zone 

on the west and the Nashoba zone on the east, and consists of Upper 

Ordovician, Silurian, Lower Devonian, and local Pennsylvanian strata. 

Distinctive groups of these strata form six subbelts. These are, from 

west to east, the Ware, Gardner, Southbridge, and Wachusett Mountain subbelts, in the western and central parts of the belt, and the 

Nashua and Rockingham subbelts, in the eastern part.

The Fitch Formation, a well-bedded, rusty-weathering, graphitebearing, calc-silicate granofels (Sfs) and subordinate rusty-weathering 

sillimanite-graphite-pyrrhotite-biotite schist, locally mapped separately (Sfss), is a thin unit confined to the Ware subbelt. The Fitch 

corresponds to the Fitch Formation in the Lovewell Mountain area, 

New Hampshire, and to the Francestown Formation (formerly a 

member of the Littleton Formation) in the Peterborough area and 

elsewhere in central and southern New Hampshire.

The Paxton Formation (Sp) is a predominantly gray-weathering, 

slabby quartz-plagioclase-biotite granofels widely distributed in the 

Gardner, Southbridge, and Wachusett Mountain subbelts. The Paxton 

is subdivided into members containing distinctive rock types. The 

lowest unit is a rusty-weathering, white to bluish-gray sulfidic and 

graphitic, highly magnesian schist and interbedded slabby-weathering, 

feldspathic, micaceous, and sillimanitic quartzite (Spsq) 30-60 m thick 

that forms a long belt along the west margin of the Gardner subbelt. 

The protolith was deposited under reducing conditions, characterized 

by sulfur-reducing bacteria and slow accumulation of fine-grained 

Fe-bearing silicate. The magnesian schist differs from underlying 

schists of the Middle Ordovician Partridge Formation and the Lower 

Silurian Rangeley Formation in the high magnesium content of the 

biotite, the abundance of rutile, and the lack of ilmenite and garnet. 

The unit resembles the Middle Silurian Smalls Falls Formation of 

northwestern Maine. Sulfidic quartzite and rusty schist (Spqr) forms a 

unit more than 250 m thick in the axial zone of a major refolded 

recumbent anticline in the Wachusett Mountain subbelt. Although 

graphitic, sulfidic, and rusty-weathering, the unit lacks the Mg-rich 

biotite and cordierite of the sulfidic schist and quartzite unit (Spsq) but 

may be a facies equivalent of it. Quartzites in the unit resemble Clough

Manuscript approved for publication November 16, 1987.

Department of Geology, University of Massachusetts, Amherst, MA 01003.

2U.S. Geological Survey.

Quartzite of the Connecticut Valley belt. It may be broadly equivalent 

to the Smalls Falls Formation or the uppermost part of the Perry 

Mountain Formation in Maine.

The granofels member (Sp) forms the bulk of the Paxton Formation. 

It is a well-layered, purple-gray quartz-calcic plagioclase-biotite granofels commonly interbedded with beds of green to pink calc-silicate 

granofels and rare beds of diopside-bearing marble. Rusty-weathering, 

graphitic, sillimanitic mica schist similar to schist of the Partridge and 

Rangeley Formations is locally interbedded with the granofels except 

in the western part of the Wachusett Mountain subzone. Tourmalinebearing pegmatites are common in the unit. The unit resembles the 

Warner Formation in New Hampshire and the Madrid Formation in 

Maine. It is lithically similar to the Vassalboro Formation in central 

Maine, the Eliot and Berwick Formations of southeastern New Hampshire, the Oakdale Formation of the Nashua subbelt, and the Hebron 

Gneiss of Connecticut. The Bigelow Brook Member (Spbs) consists of 

about equal proportions of thick-bedded granofels and sulfidic schist. It 

also contains a few marble beds and zones of calc-silicate granofels 

(Spbc). The Southbridge Member (Spso) consists of interbedded granofels and calc-silicate rock. Schist beds are lacking. Mappable areas of 

sulfidic mica schist (Spss) occur throughout the Paxton, but the largest 

area lies in the Gardner anticline above the granofels and below gray 

schist assigned to the Littleton Formation.

The Littleton Formation (Dl) occupies parallel narrow synclines in 

the Ware, Gardner, and Wachusett Mountain subbelts. It consists 

characteristically of gray-weathering schists that differ somewhat in 

composition in the different synclines. The Littleton schists typically 

contain sillimanite and garnet, and, at the highest grade of metamorphism, cordierite. In the westernmost Ware subbelt, Littleton resembles Littleton in the Connecticut Valley belt. East of the Hardwick 

pluton, it is quartz rich and resembles part of Rangeley C of the Maine 

sequence. West of the Coys Hill pluton, a belt of schist interbedded 

locally with fine-grained granofels and flanked on either side by Fitch 

Formation and characterized by large garnets is correlated with the 

uppermost part of the Warner Formation of New Hampshire. In the 

Ware subbelt, east of the Coys Hill pluton, the isoclinal synclines of 

Littleton lie between belts assigned on the State bedrock map to the 

Partridge Formation (Dl+Ops), some of which actually should be 

assigned to the Rangeley Formation. Schist in one syncline coincides 

with the Mount Pisgah Formation. In another, the schist coincides with 

gray schist formerly mapped as the upper part of the Hamilton 

Reservoir Formation. Two synclines contain a white quartz-feldspar 

gneiss (Dlf); another contains an orthopyroxene-biotite gneiss (Dlo). 

These units are considered to have igneous protoliths. In the Gardner

Gl
G2 THE BEDROCK GEOLOGY OF MASSACHUSETTS

subbelt to the east, Littleton overlying Paxton Formation in synclinal 

folds contains subordinate beds of calc-silicate rock. In the Wachusett 

Mountain subbelt, the gray schist of the Littleton shows little variety 

but locally contains lenses of calc-silicate rock. East of the Fitchburg 

Complex, the Littleton is at slightly lower metamorphic grade and 

contains andalusite and locally staurolite. Schist in a syncline at the 

southern edge of the Wachusett Mountain subbelt contains marble 

(Dim). An extension of this belt along the boundary of the Southbridge 

and Gardner subbelts into Connecticut was formerly mapped as part of 

the Bigelow Brook Formation.

The Boylston Schist (SObo), 1,000 m thick, at the base of the 

sequence in the Nashua subbelt, west of the Clinton-Newbury fault, is 

a rusty-weathering sillimanite schist containing subordinate calcsilicate rock. In fault contact with and above the Boylston, the Tower 

Hill Quartzite, 0-130 m thick, consists of thin-bedded quartzite (St) and 

schist and phyllite (Sts). The Tower Hill is considered to be the base of 

a turbidite sequence that includes the overlying Oakdale and Worcester 

Formations. The Oakdale Formation (So), possibly as much as 6,000 m 

thick, consists of ankeritic and actinolitic metamorphosed siltstone and 

interbedded calcareous phyllite and schist. In bulk composition it is 

similar to much of the Paxton Formation, but it contains more pelitic 

lenses and is finer grained and lower grade. The Oakdale may lie in an 

isoclinal syncline, but it is interpreted to lie in stratigraphic position 

between the Tower Hill and the Worcester Formation. The Worcester 

Formation (DSw), 2,000-^4,000 m thick, consists of gray and dark-gray 

interbedded phyllite, fine-grained metamorphosed graywacke, and 

rare calc-silicate rock and marble. Because the Worcester is conformable with the underlying Silurian Oakdale and yet lithologically resembles the Lower Devonian Littleton Formation, it is assigned a Silurian 

and Early Devonian age. Unconformably above the turbidite sequence 

is the Coal Mine Brook Formation (Pcm), 50-330 m thick, exposed at 

two places in down-faulted blocks near Worcester. At one, the Coal 

Mine Brook Formation consists of carbonaceous slate and phyllite and 

a 2-m-thick bed of metamorphosed anthracite. Plant fossils indicate a 

Middle Pennsylvanian age. At the other, conglomerate beds are 

interbedded with phyllite.

At the bottom of the sequence in the Rockingham subbelt, west of 

the Clinton-Newbury fault, are the Vaughn Hills Quartzite (SOvh) and 

Reubens Hill Formation (SOrh). The Vaughn Hills, 0-200 m thick, 

consists of thin-bedded quartzite, interbedded locally rusty-weathering 

phyllite and schist, and minor beds of calc-silicate rock. The Vaughn 

Hills is possibly equivalent to the Tower Hill Quartzite of the Nashua 

subbelt. The Reubens Hill Formation, 600 m thick, a metamorphosed 

igneous unit consisting of amphibolite and hornblende-plagioclase 

gneiss, has no counterpart in the Nashua subbelt. A fault separates the 

Vaughn Hills and Reubens Hill from the overlying turbiditic sequence 

that consists of the Kittery Formation (SOk), the Eliot Formation (Se), 

and the Berwick Formation (Sb, Sbs). These units, continuous with 

previously named units in southern Maine and New Hampshire, 

lithologically resemble the Oakdale and Paxton Formations in the 

subbelts to the west. The Kittery, 4,000 m thick, consists of thinbedded calcareous, commonly actinolitic metamorphosed siltstone, 

phyllite, and schist, and minor quartzite. The Eliot Formation, 300 m 

thick, consists of thin-bedded dark-gray to green slate and phyllite, 

commonly dolomitic, and metamorphosed siltstone, partly actinolitebearing. The Berwick Formation, 2,000 m thick, consists of thin- to 

thick-bedded calcareous, biotitic metamorphosed siltstone containing 

actinolite, or, at higher grade, diopside, minor garnet-mica schist, and 

feldspathic quartzite. Partly rusty-weathering schist and phyllite (Sbs) 

forms mappable lenses near East Pepperell and near Haverhill. Metamorphic grade increases from greenschist fades in the Kittery and 

Eliot Formation in the east to amphibolite facies in the Berwick 

Formation in the west, but metamorphic grade drops to greenschist 

facies again in the Oakdale Formation in the adjacent Nashua subbelt 

to the west. Unconformably on the older rocks, and correlated with the

Coal Mine Brook Formation, is the Harvard Conglomerate (Ph), 

estimated to be 100 m thick, consisting of polymict metamorphosed 

conglomerate interbedded with, and overlain by, gray, green, and 

purple chloritoid-bearing phyllite.

The pre-Pennsylvanian units in the Nashua and Rockingham subbelts were shown on the State bedrock map as Ordovician or Silurian, 

Silurian, and Early Devonian on the basis of correlation with lithologically similar rocks along strike in the Silurian fossiliferous sequence at 

Waterville, Maine, and the similarity of lithologies and sequences to 

those in the subbelts of the Merrimack belt to the west whose age 

assignments are based on continuity of lithologies to fossiliferous strata 

in New Hampshire and western Maine. Radiometric dating of the 

intrusive Newburyport Complex indicates that the Kittery Formation 

might be as old as Ordovician. Recent radiometric dating of intrusions 

into the Eliot and Berwick Formations in New Hampshire indicate that 

the stratified rocks in the sequences are Ordovician or older. Some 

workers in the region consider the sequences to be Late Proterozoic. 

The similarity of the sequences in the Nashua and Rockingham belts to 

sequences in the central and western part of the Merrimack belt raises 

questions that must be answered in future work.

INTRODUCTION

The Merrimack belt is here defined as the belt of 

Upper Ordovician, Silurian, Lower Devonian, and local 

Pennsylvanian strata east of the easternmost exposures 

of Lower Silurian Clough Quartzite and west of the 

Clinton-Newbury fault. The strata in the western part of 

the Merrimack belt closely resemble the Silurian and 

Lower Devonian strata of northwestern Maine (Moench 

and Boudette, 1970) and correlatives in central and 

southern New Hampshire (Hatch and others, 1983). 

Some firm correlations with these areas had been made 

at the time the Massachusetts bedrock geologic map (Zen 

and others, 1983; hereafter referred to as the State 

bedrock map) was compiled in 1980, but subsequent 

mapping has modified what is shown and requires further discussion. The strata in the eastern part of the belt 

are along strike from and resemble the fossil-bearing 

Silurian strata of the Waterville area, Maine (Osberg, 

1968), but specific correlations have not been established, partly because the stratigraphic position of the 

Waterville itself has been uncertain until recently 

(Osberg, 1980). In addition, some field interpretations of 

the contact relations of the Massabesic Gneiss Complex 

(Bothner and others, 1984) and some interpretations of 

the isotopic data on granitoid rocks in the Massabesic, as 

well as on plutons that intrude the sequence in the 

eastern part of the Merrimack belt (Zartman and Marvin, this vol., chap. J), suggest to some workers a 

pre-Silurian age for many of the strata of the belt (Lyons 

and others, 1982; Hatch and others, 1984; Bothner 

and others, 1984). Correlations within Massachusetts 

between the eastern and western parts of the belt are 

complicated by the fact that the western part is largely 

sillimanite grade or higher, whereas the eastern part is 

largely garnet grade or lower and also by the postmetamorphic faults of the Wekepeke system. Nevertheless,
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G3

TABLE I. Distribution of Silurian-Devonian lithic units of the Massachusetts State bedrock map (Zen and others, 1983) in western subbelts

of the Merrimack belt

Age Ware subbelt Gardner subbelt Southbridge subbelt Wachusett Mountain subbelt

Devonian Littleton Formation (Dl)

Feldspar gneiss member

(Dlf) 

Orthopyroxene gneiss

member (Dlo) 

Silurian Fitch Formation

Sulfidic calc-silicate (Sfs) 

Sulfidic schist (Sfss)

Littleton Formation (Dl)

Paxton Formation 

Sulfidic schist (Spss) 

Granofels (Sp) 

Sulfidic schist and quartzite 

(Spsq)

Littleton Formation (Dl)

Paxton Formation 

Granofels (Sp) 

Southbridge Member (Spso) 

Bigelow Brook Member

(Spbs) 

Calc-silicate (Spbc)

Littleton Formation (Dl) 

Ribbon marble (Dim)

Paxton Formation 

Sulfidic schist (Spss) 

Granofels (Sp) 

Quartzite and rusty 

schist (Spqr)

we find general correlations to be clear across the belt 

and to the north, with fossil-bearing rocks of Maine and 

New Hampshire. We find no compelling evidence for an 

important tectonostratigraphic boundary between the 

eastern and western parts of the belt.

The distribution of stratified rocks within the Merrimack belt, particularly the western part, is controlled by 

a very complex structural history that we are just 

beginning to understand. In brief, the outcrop pattern is 

now (1984) thought to have been produced by four 

episodes of folding and faulting and one of postmetamorphic faulting, as follows (Robinson, 1979): (1) Early 

nappes of 15- to 30-km amplitude, originally overfolded 

from east to west, produced numerous repetitions of 

stratigraphy but few hinges that can be recognized on a 

local scale. (2) Complex backfolding, in which the nappe 

axial surfaces were refolded in major nappelike folds 

directed from west to east, resulted generally in intense 

flattening of the rock units. During the later parts of the 

backfolding stage, prominent east-west mineral lineations formed parallel to minor fold axes and to the 

internal fabric of mylonites in west-dipping semiductile 

shear zones. (3) North-northeast-trending recumbent 

folds in foliation formed parallel to a strong mineral 

lineation that merges with the lineation pattern of the 

gneiss domes to the west. (4) A series of broad northtrending arches and depressions in foliation formed 

across the belt. The most important of these are the 

Gardner anticline, which makes a foliation anticline out of 

most of the western part of the Merrimack belt, and the 

Wachusett syncline, which runs near the center of the 

Fitchburg plutons and separates a broad area dominated 

by east dips from the dominant west dips in the eastern 

part of the belt. (5) Postmetamorphic normal faults 

formed as part of the Wekepeke fault system in the 

region east of the Fitchburg plutons.

For convenience of description, the Merrimack belt 

has been divided into stratigraphic-tectonic subbelts 

(Robinson, 1979), each with Silurian-Devonian sequences

of slightly different character or with a particular 

arrangement of stratigraphic and tectonic features (table 

1, fig. 1). These subbelts are described very briefly here 

to assist the reader with the stratigraphic description 

that follows. The reader should here be aware that the 

horizontal arrangement of rock units in the correlation of 

map units (fig. 2) is generally based on their present 

surface distribution from west to east and not on their 

horizontal distribution at the time of deposition, because 

the original order may have been reversed by recumbent 

folding.

The westernmost subbelt, here called the Ware subbelt (figs. 1, 3A), is characterized by gray-weathering 

schists assigned to the Littleton Formation exposed in a 

series of isoclinal synclines. The Ware subbelt is defined 

to include all those Silurian-Devonian strata that lie east 

of the west margin of the Hardwick pluton, or a line 

extended southward from it, and west of the westernmost exposures of the Paxton Formation, units Spsq and 

Sp (fig. 3A). Sulfidic calc-silicate rocks basal to the gray 

schist are here assigned to the Silurian Fitch Formation 

(Sfs). Since the final map compilation, several localities 

have been found that suggest that rocks assigned to the 

Littleton Formation (Dl) or the Partridge Formation 

(Op) might be better assigned to the Silurian Rangeley 

Formation, on the basis of similarity to rocks in the 

central New Hampshire sequence (Hatch and others, 

1983). The general scarcity of Silurian strata in the Ware 

subbelt may be ascribed to a pre-Devonian unconformity. In southern Massachusetts, the Ware subbelt 

includes strata mapped as the Mount Pisgah Formation 

and as gray-weathering schists in the upper schist member of the Hamilton Reservoir Formation by Seiders 

(1976) and Pomeroy (1977).

The next subbelt to the east, the Gardner subbelt (figs. 

1, 3A), is marked by the rather abrupt appearance of 

thick Silurian strata in the form of members of the 

Paxton Formation (Sp), dominated by calcareous grano-
G4 THE BEDROCK GEOLOGY OF MASSACHUSETTS

42°30'

42°00'

NEW HAMPSHIRE 

MASSACHUSETTS

20 MILES

20 KILOMETERS

FIGURE 1. Major divisions and structural features of the Merrimack belt and localities of Pennsylvanian strata.

fels, below the Devonian schists. The west margin of the 

subbelt is defined to include exposures of all members of 

the Paxton Formation. The east margin of the subbelt in 

northern Massachusetts is taken along the trace of a 

probable Mesozoic fault and its southern extension and in 

southern Massachusetts along the west margin of a belt 

of gray schist that is exposed in the center of the Gardner 

anticline (figs. 1, 3A). In the northern part of Massachusetts, the Gardner subbelt includes strata dipping both 

west and east on opposite limbs of the Gardner anticline. 

In southern Massachusetts, the Gardner subbelt includes 

some strata mapped here and by Emerson (1917) as part 

of the Paxton Formation but mapped as parts of the 

Bigelow Brook and Hamilton Reservoir Formations by 

Seiders (1976), Pomeroy (1975, 1977), Moore (1978), and

Pease (1972). Our detailed and reconnaissance mapping 

has shown that calcareous granofels of the Paxton Formation, previously mapped by Pease (1972) as part of the 

Bigelow Brook Formation, can be traced around the 

north end of the Oakham anticline directly into rocks 

previously mapped by Seiders as part of the upper gneiss 

member of the Hamilton Reservoir Formation.

The next subbelt, the Southbridge subbelt (figs. 1, 

3A), is a broad expanse of rather gently dipping strata of 

the Paxton and Littleton Formations; it represents an 

eastern extension of the Gardner subbelt in southern 

Massachusetts and is bounded on the east by the west 

margin of the Oakdale Formation. The Littleton Formation is confined to a single complexly refolded isocline 

that extends from the northeast corner of the subbelt
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G5

Worcester Basin Intrusive Rocks

Ph

Dht

Dchgr

SOn

EXPLANATION

Pcm 

Ph

Dl, Dlo, 

Dlf, Dim 

DI + Ops

DSw

Sfs, Sfss 

Sp, Spss, 

Spa, Spsq, 

Spqr, Spbs, 

Spso, Spbc 

So

Sb, Sbs 

Se

St, Sts 

SOvh

Stratified rocks

Coal Mine Brook Formation (Pennsylvanian) 

Harvard Conglomerate (Pennsylvanian) 

Littleton Formation (Lower Devonian)

Littleton Formation (Lower Devonian) and 

Partridge Formation (Middle Ordovician), 

undivided

Worcester Formation (Lower Devonian

and Silurian)

Fitch Formation (Upper Silurian) 

Paxton Formation (Silurian)

Oakdale Formation (Silurian) 

Berwick Formation (Silurian) 

Eliot Formation (Silurian) 

Tower Hill Quartzite (Silurian) 

Vaughn Hills Quartzite (Silurian or 

Ordovician)

SOk

SOrh

SObo

Pgr 

Dmgr

Dfgr

Dht 

Dchgr

SOn 

SOa

Dfgr

SOa

PENNSYLVANIAN

DEVONIAN

SILURIAN

ORDOVICIAN

Kittery Formation (Silurian or Ordovician)

Reubens Hill Formation (Silurian or 

Ordovician)

Boylston Schist (Silurian or Ordovician)

Intrusive rocks

Biotite granite (Pennsylvanian) 

Muscovite-biotite granite at Millstone

Hill (Lower Devonian) 

Fitchburg Complex (Lower Devonian or

younger)

Hardwick Tonalite (Lower Devonian) 

Coys Hill Porphyritic Granite Gneiss

(Lower Devonian) 

Newburyport Complex (Silurian and

Ordovician) 

Ayer Granite (Lower Silurian and

Upper Ordovician?)

FIGURE 2. Correlation of sedimentary and volcanic rocks of the Merrimack belt and their metamorphic equivalents and rocks 

intrusive into them that are referred to in text (modified from Zen and others, 1983).
G6 THE BEDROCK GEOLOGY OF MASSACHUSETTS

CONNECTICUT

VALLEY

BELT WACHUSETT MOUNTAIN SUBBELT / 42°45'N

42°30'N

ililffiipi

-^.v:;/;---^:;:/:::;.::;;,;^^/^// ^ m-:m^-rt^jso, 

 -^^ ^ ^ M^/m

72°15W / 72°W 

WARE /GARDNER /SOUTHBRIDGE SUBBELT 14 71°45'WNASHUA SUBBELT

FIGURE 3A. Geology of the western part of the Merrimack belt. Pre-Silurian rocks and Paleozoic intrusive rocks are labeled only

if discussed in text.
71°45'

42°45'

30'

10

71°00' 

15 MILES

10

1

15 KILOMETERS

WACHUSETT MOUNTAIN SUBBELT / NASHUA 

Massabesic Gneiss / SUBBELT 

Complex and Pgr /

ROCKINGHAM SUBBELT

ff/

SOn

(;//7N|

iC^fcl

Mb

SOa.

42°30'

' North Chelrris'ford

m

West Chclmsford

'*&#& ftirleyj

So,

;sovh ^

hPh,

'Harvard

Ph

-SOrh

1 Hudson

TPgp 

, \5ch_gr -

Xl / "

'SOnl x'

^o^r

Pcm-

-Ph

EXPLANATION

Paleozoic intrusive rocks

Biotite granite (Pgr)

Hardwick tonalite (Dht)

Coys Hill porphyritic granite gneiss (Dchgr)

Fitchburg Complex (Dfgr)

Newburyport Complex (SOn)

Ayer Granite (SOa) 

Coal Mine Brook Formation (Pcm) (Pennsylvanian) and Harvard Conglomerate (Ph)

(Pennsylvanian) 

Littleton Formation (Dl) (Lower Devonian)

and Worcester Formation (DSw) (Lower

Devonian and Silurian) 

Fitch Formation (Upper Silurian)

Oakdale Forma

Paxton Formatio

Undifferentiat

Sulfidic mica s

Sulfidic magne

sillimanite q

Sulfidic quartz

Berwick Formati

Metasandston

Sulfidic mica s

Eliot Formation 

Tower Hill Quart

Quartzite (St) 

Phyllite (Sts)

Vaughn Hills Q

Hill Formation 

Ordovician) 

^S0k | Kittery Formatio

Boylston Schist (

-SOrh

Pre-Silurian met

volcanic rocks 

Contact

Normal fault Ha

Thrust fault Saw

FIGURE 35. Geology of the eastern part of the Merrimack belt. Pre-Silurian rocks and Paleozoic intrusive rocks are labeled only if discussed in tex
G8 THE BEDROCK GEOLOGY OF MASSACHUSETTS

into the Gardner subbelt to the west. In southernmost 

Massachusetts, the Paxton Formation is divided into a 

western Bigelow Brook Member and an eastern Southbridge Member, on the basis of earlier work along the 

Connecticut State line (Barosh, 1974; Moore, 1978), 

following the usage of Pease (1972) in northern Connecticut.

The Wachusett Mountain subbelt (figs. 1, 3A), east of 

the Gardner subbelt and north of the Southbridge subbelt, contains strata well out on the east limb of the 

Gardner anticline and close beneath or intruded by the 

Fitchburg plutons. It is bounded on the west by a 

probable Mesozoic fault or its southern extension and on 

the east by the Wekepeke normal fault system of probable Mesozoic age. In the eastern frontal regions of the 

backfolded nappes, the strata of this subbelt are believed 

to be structurally higher than most of the rocks of the 

Gardner and Southbridge subbelts. If the folding were 

removed, we believe that these strata would occupy an 

intermediate position between the strata of the Ware and 

Gardner subbelts (see cross section D-D' of the State 

bedrock map).

East of the Wachusett Mountain and Southbridge 

subbelts is the narrow Nashua subbelt (fig. 3), which 

contains phyllite and metamorphosed calcareous metasiltstone of the Worcester (DSw) and Oakdale (So) 

Formations overlying an apparently basal phyllite and 

quartzite assemblage consisting of the Tower Hill 

Quartzite (St, Sts) and possibly the Boylston Schist 

(SObo). These rocks lie in a north-trending structural 

and metamorphic trough called the Nashua synclinal by 

Crosby (1880). The subbelt narrows to the south between 

the Southbridge subbelt and the Nashoba zone and 

extends into Connecticut south of Webster. Although the 

bulk of the strata are considered to be Silurian to 

possibly Devonian in age, Pennsylvanian rocks of the 

Coal Mine Brook Formation (Pcm) (Goldsmith and others, 1982) lie in fault-bounded blocks with older rocks at 

Worcester. The rocks in the Nashua subbelt are believed 

to occupy the down-faulted trough of an east-facing 

recumbent syncline (G.R. Robinson, 1981, p. 59-62), a 

major element of the backfolded nappe system mentioned above. This syncline may correspond structurally 

to the synclinal core of the postulated Colchester nappe 

(Dixon and Lundgren, 1968) in eastern Connecticut. The 

structural trough coincides with a metamorphic trough of 

lower grade than the surrounding subbelts (Thompson 

and Norton, 1968).

The Rockingham subbelt (fig. 35), containing strata of 

the Merrimack Group, forms a north ward-widening 

wedge between the Nashua subbelt and the Nashoba 

zone. Calcareous metasiltstone, phyllite, metasandstone, 

and quartzite of the Kittery (SOk), Eliot (Se), and 

Berwick (Sb, Sbs) Formations forming the Merrimack

Group are shown as Ordovician to Silurian on the State 

bedrock map, but they may be entirely Ordovician or 

older (Bothner and others, 1984). The strata for the most 

part are similar in composition and bedding style to the 

Paxton and Oakdale Formations of the Nashua trough, 

although at slightly higher metamorphic grade than the 

Oakdale. The boundary between the Rockingham subbelt and the Nashua subbelt is the contact between the 

Oakdale and the Berwick Formations. It is marked 

primarily by a contrast in metamorphic grade but also by 

a difference in expression of relict bedding. The bedding 

characteristics probably reflect premetamorphic differences in grain size at the time of deposition. In addition, 

the Merrimack Group in the Rockingham subbelt contains mappable zones of rusty-weathering schist and 

phyllite much like the Paxton Formation to the west in 

the Wachusett Mountain subbelt. However, rusty 

schists and phyllites are not found in the rocks of the 

intervening Nashua subbelt.

WARE, GARDNER, SOUTHBRIDGE, AND 

WACHUSETT MOUNTAIN SUBBELTS

In this section, rocks presumed to be Silurian (the 

Fitch and Paxton Formations) are discussed first, in 

generally west to east order, followed by discussion of 

rocks presumed to be Devonian (the Littleton Formation). Alternative interpretations and correlations are 

given, including some that appear to be more likely now 

than at the time the map was compiled. The stratigraphy 

of these rocks is impossible to discuss without at least a 

rudimentary description of the structural geology, but 

the emphasis in this chapter is on the stratigraphy. In 

discussing the distribution of units, repeated references 

are made to figures 4, 5, and 6.

FITCH FORMATION (Sfs, Sfss)

The dominant rock of the Fitch Formation (Sfs) in the 

Merrimack belt is in outcrop a slabby, rusty-weathering 

quartz-plagioclase-graphite rock. Biotite is scarce or 

absent, and broken foliation surfaces show abundant 

graphite platelets. The rusty-weathering character is 

due to the presence of pyrrhotite, which makes up 5-6 

percent of the rock in some beds. Mineralogically, the 

rock is a calc-silicate granofels (Field, 1975, table 4) 

usually containing 20-60 percent quartz, 35-55 percent 

calcic plagioclase (An65_80), 3-35 percent diopside, 1-2 

percent sphene, 0-2 percent graphite, and local actinolite, calcite, scapolite, clinozoisite, and biotite. Careful 

examination shows this unit to be well bedded, but 

bedding is usually obscured by the rusty weathering and 

the way the rock breaks into rectangular slabs. A
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G9

42°45'N

42°30'N

72°15'W 72° W

FIGURE 4. Geology of the Gardner-Athol area. Most Paleozoic intrusive rocks and pre-Silurian rocks are not 

labeled. Dl, Littleton Formation; Sfs, Sfss, Fitch Formation; Sp, Spsq, Spss, Paxton Formation. Circled 

letters and numbers are discussed in text.
G10 THE BEDROCK GEOLOGY OF MASSACHUSETTS

 Y:'-&.OV:^:V;;-::;;-:;l 5 KM

42<M5'N

72°15'W 72° W

42° N

FIGURE 5. Geology of the Ware-Southbridge area. Paleozoic intrusive rocks (except Coys Hill and Hardwick plutons) and pre-Silurian 

rocks are not labeled. Dl, Littleton Formation; Sfs, Fitch Formation; Sp, Spqr, Spsq, Spss, Paxton Formation. Circled letters and 

numbers are discussed in text. Cross section lines D-D' and F-F' are from State bedrock map (Zen and others, 1983).
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS Gil

'42°45'N

5MI

72°W 71°45'W

FIGURE 6. Geology of the area of the Fitchburg plutons. Paleozoic intrusive rocks and pre-Silurian 

rocks are not labeled. Dl, Dl?, Littleton Formation; Dim, marble in Littleton Formation; Sp, Spqr, 

Spss, Paxton Formation. Circled letters are discussed in text. Cross section line D-D' is from State 

bedrock map (Zen and others, 1983).
G12 THE BEDROCK GEOLOGY OF MASSACHUSETTS

subordinate rock type in the Fitch Formation is rustyweathering sillimanite-graphite-pyrrhotite-biotite schist 

(Sfss), which is very similar to the schist of the sulfidic 

schist-quartzite unit (Spsq) of the Paxton Formation. 

The Fitch sulfidic schist is locally interbedded with the 

Fitch calc-silicate and dominates the two lenses (labeled 

Sfss on fig. 4) near the eastern edge of the Ware subbelt.

Contacts of the Fitch Formation with adjacent units 

are poorly exposed but generally are sharp. However, 

in large exposures on the northeast side of Ragged 

Hill (Robinson and others, 1982a, Stop 5), slightly 

rusty interbedded schists and quartzites lie between 

Fitch calc-silicates and gray "big-garnet" schists of the 

Littleton.

Field (1975) estimated the thickness of the Fitch 

Formation in the Ware area to be 15 m. The wider belt in 

the southern part of the area is believed to have been 

caused by repetition of the unit thrice by isoclinal folding.

The mineralogy of the Fitch Formation suggests that 

its protoliths were calcareous and dolomitic shales and 

siltstones and locally interbedded aluminous shales. The 

abundance of graphite and pyrrhotite suggests deposition in a reducing environment of poor circulation with 

preservation of organic matter and much activity by 

sulfur-reducing bacteria.

The rocks assigned to the Fitch Formation in the 

Merrimack belt all lie within the Ware subbelt, and most 

are very close to the Coys Hill Pluton. As early as 1972 

Robinson realized that the most distinctive rock type, 

the rusty-weathering granofels, or quartzite, corresponds exactly to the distinctive lithology of both the 

"rusty quartzite member" (Dlr) of the Littleton Formation in the Monadnock quadrangle, New Hampshire 

(Fowler-Billings, 1949), and the Francestown Member of 

the Littleton Formation in the Peterborough quadrangle, New Hampshire (Greene, 1970). The rusty granofels 

is also identical to rocks assigned to the Fitch Formation 

at Gee Mill in the Lovewell Mountain quadrangle, New 

Hampshire, which overlie Clough Quartzite (Heald, 

1950; Thompson and others, 1968; Dean, 1976). On this 

basis, Field (1975) assigned the rocks in Massachusetts to 

the Fitch Formation with separate formational status, 

rather than to a member of the Littleton Formation. 

Now that the Francestown Formation has been assigned 

formation status in New Hampshire (Nielson, 1981; 

Hatch and others, 1983; Thompson, 1983), this rusty 

granofels in Massachusetts could as well be assigned to 

the Francestown as to the Fitch. However, the correlation with Fitch is also valid and is emphasized by the 

occurrence of similar sulfidic rocks in the Fitch of the 

Connecticut Valley belt in Massachusetts (Robinson, 

1963, p. 62; Hatch and others, 1988).

The most continuous outcrops of the Fitch Formation 

are in two extremely narrow belts (fig. 5, area C) west of

the Coys Hill pluton in the Ware quadrangle (Field, 

1975). These two belts merge southward into a single 

belt near Ragged Hill, suggesting that the Littleton 

Formation between them is an isoclinal syncline. The 

only other good exposures on the west side of the Coys 

Hill pluton are southwest of Barre (fig. 4, area C) and in 

a large railroad cut northwest of Baldwinville (Robinson, 

unpub. data). Three widely separated inclusions of Fitch 

Formation are present in the Hardwick pluton. Two 

lenses of Fitch Formation lie along the east contact of the 

Coys Hill pluton (fig. 5, area F), northwest of Warren, 

between it and the Partridge Formation, and two other 

lenses are along Littleton-Partridge contacts near the 

east edge of the Ware subbelt (fig. 4, area I).

PAXTON FORMATION

The name Paxton Formation is used here to describe 

all the stratified rocks of presumed Silurian age exposed 

in the Gardner, Southbridge, and Wachusett Mountain 

subbelts. The name Paxton Formation is a modification 

and expansion of the name Paxton Quartz Schist used by 

Emerson (1917) to describe all areas in central Massachusetts dominated by gray-weathering slabby quartzplagioclase-biotite granofels. The formation is named for 

the town of Paxton, Mass., where hundreds of stone 

walls are built of this slabby rock. In December 1976, 

Robinson and Tucker (Robinson and others, 1982a, Stop 

12A) rediscovered the large cascade exposure on the 

brook draining Eames Pond in the western part of 

Paxton, which probably was Emerson's type locality 

(Paxton Falls, fig. 6). In the future, as correlations with 

fossil-bearing strata in Maine improve, we imagine that 

various members will achieve formational status, and the 

name Paxton will either be dropped or be restricted to 

granofels equivalent to that exposed in Paxton.

In the following section, we have attempted to 

describe the various members in what we think is the 

correct stratigraphic order. Where rock units are considered roughly correlative but of different facies, they are 

discussed in geographic order from west to east without 

regard to original arrangement before folding. In many 

examples, the true stratigraphic facing direction and 

order are unknown, and evidence of complex recumbent 

folding makes several choices possible.

SULFIDIC SCHIST AND SILLIMANITE QUARTZITE (Spsq)

This unit forms a nearly continuous, contorted belt 

along the west margin of the Gardner subbelt (figs. 4, 5, 

area J) from just south of Route 2, in the northern part 

of the State, to well south of the Massachusetts Turnpike 

in the southern part. Since publication of the State 

bedrock map, Spsq has been traced several kilometers
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G13

into Connecticut (Berry, 1985). The unit was first defined 

by Field (1975) as the white schist member of the Paxton 

Formation of the Ware area and was mapped thence 

northward through Barre (Tucker, 1977) and into Templeton (H.B. Stoddart, written commun., 1978; D.E. 

Klepacki, written commun., 1978). In southern Massachusetts, Spsq was lumped with the upper schist member of the Hamilton Reservoir Formation (Seiders, 1976; 

Pomeroy, 1977) or mapped as schist within the upper 

gneiss member of the Hamilton Reservoir Formation. 

The unit changes very slightly in character from the 

north, where it contains some muscovite in the 

sillimanite-muscovite-K-feldspar zone, to the south, 

where the only mica is biotite in the sillimanite-garnetcordierite zone.

The sulfidic schist and quartzite unit tends to form 

large rounded to overhanging outcrops and holds up a 

series of relatively high ridges. The surface is extremely 

rusty, is commonly yellow orange, and generally has 

coatings of secondary limonite as much as 1 cm thick. 

Accumulations of secondary sulfates are common under 

overhangs. Soils in the vicinity commonly have an 

orange-red color, and sulfates in surface water have led 

to such names as "Alum Pond" and "Little Alum Pond." 

Smooth upper surfaces of outcrops commonly have 

widely spaced, rounded pits 3 to 8 cm across within which 

shiny pyrite is visible. Despite the robust appearance of 

the outcrops as a whole, the upper surfaces are generally 

friable, and collecting fresh specimens is extremely difficult. Beneath the crust of secondary limonite, the 

weathered rock commonly appears white due to the 

abundance of quartz, feldspar, sillimanite, and white 

mica and the absence of any dark mineral except discrete 

flakes of graphite. This characteristic led to the name 

"white schist member." However, really fresh rock is 

bluish gray. Interbedded with the typical schist is feldspathic, micaceous, and sillimanitic quartzite that forms 

hard, tough beds 5 to 8 cm thick, which weather into 

slabs. The quartzite is probably responsible for the fact 

that the unit forms prominent topographic ridges.

First encounters with this unit were in the sillimanitegarnet-cordierite zone many miles from the nearest 

stable occurrence of muscovite (Field, 1975). The abundance of what appeared to be white mica in the unit, as 

well as the sulfides, suggested that it might be a zone of 

secondary hydrothermal alteration. However, optical 

observations of the white mica consistently showed a 2V 

close to 0°, rather than the 30° characteristic of muscovite, and thus led to electron probe analyses and to 

determination that the white mica is pure Mg-biotite 

(Field, 1975; Tracy and others, 1976; Robinson and 

Tracy, 1977; Robinson and others, 1982b).

Thin sections of the unit (Field, 1975, table 6; Tucker, 

1977, table 4) show that it consists of 40-75 percent

quartz, 2-40 percent andesine, 6-30 percent orthoclase 

or microcline, 2-10 percent sillimanite, 1-8 percent magnesian biotite, 0-3 percent magnesian cordierite, tr 

(trace)-4 percent graphite, 0-3 percent pyrite, tr-1 percent pyrrhotite, and tr-1 percent rutile. Muscovite 

occurs near the northern end of the outcrop belt. Locally, 

the sillimanite has the form of pseudomorphs after 

andalusite. The biotites range from very pale reddishbrown iron-bearing varieties to colorless Mg end members and have Mg/(Mg+Fe) ratios between 0.75 and 

0.999 (0.04 weight percent FeO). Even the magnesian 

biotites have octahedral Al and as much as 0.074 Ti per 

11 oxygens, and thus they are not properly phlogopites. 

The cordierites, present only in the schist and not the 

quartzite, are charged with graphite and appear as black 

to bluish lumps. Some are essentially pure Mg end 

members with 0.00 weight percent FeO and only 0.08 

weight percent MnO. Even where charged with detrital 

zircons, these cordierites lack pleochroic halos, presumably because of lack of iron to be oxidized by alpha 

bombardment. The pure magnesian cordierites contain 

approximately 2 weight percent H2S, due apparently to 

the high sulfur fugacity of the pyrite-pyrrhotite assemblage in which they formed.

The key to understanding the mineralogy and ultimately the genesis of these rocks lies in the graphiteoxide-sulfide assemblages (for details see Robinson and 

Tracy, 1977; Robinson and others, 1982b). Specimens 

containing iron-bearing pale red-brown biotite and ironbearing cordierite invariably contain the assemblage 

graphite-rutile-pyrrhotite. In these bulk compositions, 

original pyrite is believed to have reacted with the Fe 

component of the hydrous silicates and graphite to 

produce pyrrhotite plus Mg-richer silicates plus H20 

plus C02 until the pyrite was exhausted. Specimens 

containing the nearly pure magnesian biotite and cordierite contain the assemblage graphite-rutile-pyritepyrrhotite. In these bulk compositions, originally richer 

in sulfide and poorer in iron-bearing silicate, the same 

reaction proceeded until the Fe component of the silicates was exhausted, with some pyrite still remaining. In 

one sample, from the lower grade north end of the belt, 

the sense of this reaction is shown by pyrrhotite rims 

growing around euhedral pyrite cubes. Robinson and 

others (1982b) showed that the primary deposition of 

such a unit requires not only reducing conditions and 

sulfur-reducing bacteria but slow deposition and a high 

proportion of fine detrital grains of Fe-bearing silicate 

capable of reacting with bacterially produced H2S in an 

open-system sedimentary environment. Sulfur-isotope 

data (Tracy and Rye, 1981) on several outcrops show 

very light sulfur values with 834S ranging from -25 to 

 29, identical to values obtained from modern muds in 

highly reduced deep zones of the Black Sea.
G14 THE BEDROCK GEOLOGY OF MASSACHUSETTS

In regions where the unit is relatively rich in pyrrhotite, contacts have been mapped by using a hand-carried 

magnetometer. The unit is characterized by extremely 

large-amplitude variations, whereas adjacent units are 

magnetically flat. This method was particularly successful in mapping the northern part of the Barre quadrangle, where magnetic data show that the unit forms 

several synclinal outliers capping the summits of hills 

(D.E. Klepacki, written commun., 1978). Elsewhere, 

where pyrite predominates over pyrrhotite, the unit is 

not detectable magnetically. As stated above, this unit 

was earlier mapped as part of the upper schist member of 

the Hamilton Reservoir Formation (Seiders, 1976; 

Pomeroy, 1977), which is now mainly included in the 

Partridge Formation. In the field the sulfidic schist and 

quartzite of the Paxton is distinguished from pyrrhotite 

schist of the Partridge Formation by the extremely pale 

Mg-rich biotite as compared to dark red-brown biotite in 

the Partridge, by the local presence of pyrite, by the 

abundance of rutile, and by the total absence of ilmenite 

and garnet, which characterize the Partridge. The mineralogy of the Partridge suggests that it contained a 

much higher proportion of detrital Fe-bearing silicate 

grains that were too coarse or too rapidly deposited to 

permit thorough reaction with biologically produced 

H2S.

The western contact of the sulfidic schist and quartzite 

unit with the Partridge Formation is nowhere exposed 

within an interval of 100 ft (30 m) but is presumed to be 

an unconformity on the basis of regional relations and the 

apparent lack of rock types correlative with the lower 

part of the Silurian section in Maine and central New 

Hampshire (Hatch and others, 1983). The maximum 

thickness of the unit is probably 30-60 m in south-central 

Massachusetts, where it lies between the Partridge 

Formation and the granofels member of the Paxton. The 

unit appears to pinch out beneath younger units to the 

north and to the south.

Those who have seen the units agree that the sulfidic 

schist and quartzite unit of the Paxton Formation in 

Massachusetts is a perfect lithic correlative of the Smalls 

Falls Formation of northwestern Maine (Moench and 

Boudette, 1970). Similarities include the magnesian biotite and cordierite (Guidotti and others, 1975, 1977), the 

abundant rutile and pyrrhotite, the andalusite grains or 

pseudomorphs, the abundance of quartzite beds, and the 

position in the sequence below a major unit of gray 

granulites and calc-silicates, which is the Madrid Formation in Maine (Moench, 1971) and the granofels member 

of the Paxton in Massachusetts.

In summary, the sulfidic schist and quartzite unit 

appears to be locally the basal Silurian unit in central 

Massachusetts. Its lithic character suggests that it consisted of clean, fine quartz sand mixed with fine-grained

detrital clay and organic matter, which was slowly 

deposited in a closed marine environment that permitted 

extensive reaction with biogenically produced H2S.

SULFIDIC QUARTZITE AND RUSTY SCHIST (Spqr)

This unit forms a single continuous belt of strata within 

the Wachusett Mountain subbelt (fig. 6, area S) and has 

not been specifically identified anywhere else. The belt is 

considered to form the axial zone of a major, presently 

northeast-directed, recumbent anticline, in which apparently younger rocks of the granofels member are both 

above (right side up) and below (upside down). The belt 

has been refolded about the Wachusett syncline and 

several other late folds to form a crude C-shaped outcrop 

pattern open to the northeast. The ends of the belt, at 

the points of the "C" northwest of Wachusett Mountain 

and southeast of the Fitchburg plutons, are interpreted 

as hinges of the postulated recumbent fold. The unit is 

generally not well exposed, but where thickest it does 

form the high ridges of Asnebumskit Hill (1,395 ft) (425 

m), and several hills to the north, as well as the top of a 

cliff on the west face of Stonehouse Hill. The unit occurs 

entirely within the sillimanite-muscovite zone of Acadian 

regional metamorphism.

Although the schists of this unit are graphitic, sulfidic, 

and rusty weathering, they do not apparently contain 

the extremely Mg-rich biotite and cordierite typical of 

the sulfidic schist and quartzite unit (Spsq) described 

above. However, lack of detailed petrographic work and 

lower metamorphic grade leaves this question somewhat 

open. In small outcrops and manmade excavations, in 

particular in a large quarry 3 mi (5 km) north of Asnebumskit Hill, mica schist is found on most broken surfaces. Only in well-weathered, glacially smoothed outcrops and in slabby float is it apparent that the unit is 

dominated by fine-grained to locally grit-sized feldspathic quartzite, with subordinate mica schist beds. No 

other rock in the western part of the Merrimack belt 

comes as close to resembling the Clough Quartzite of the 

Connecticut Valley belt.

In the northernmost outcrop in the belt, contact relations with the structurally underlying granofels member 

are well exposed. Here the typical quartzites of the unit 

are separated from the granofels by 3^4 m of sulfidic mica 

schist.

On the basis of our present structural and stratigraphic interpretation, the quartzite and rusty schist 

unit (Spqr) is probably a facies equivalent of the sulfidic 

schist and quartzite unit (Spsq), and the nappe in which 

it lies may be rooted at the west edge of the Gardner 

subbelt. Farther afield, the unit is quite similar to an 

unnamed rusty quartzite exposed on the bank of the 

Kennebec River in Maine (Osberg, 1980, Stop 3). Osberg
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G15

suggested that this unnamed unit may be equivalent to 

the Perry Mountain Formation, which overlies the 

Sangerville Formation and is overlain successively by 

the sulfidic schists of the Parkman Hill Formation 

(Smalls Falls) and the calcareous granofels of the Fall 

Brook Formation (Madrid). Thus, indications are that 

the sulfidic quartzite and rusty schist unit may be 

broadly equivalent to the upper part of the Perry Mountain Formation and the Smalls Falls Formation in Maine. 

Because the base of the unit is not exposed, only an 

estimate of a minimum thickness of 250 m can be given. 

The depositional environment appears to have been 

similar to that of the sulfidic schist and quartzite unit 

(Spsq), except that conditions for fixation of organic 

sulfur were less ideal.

GRANOFELS MEMBER (Sp)

The granofels member (Sp) contains the bulk of the 

Paxton Formation and its most characteristic rock types. 

The granofels occurs extensively in the Gardner and 

Wachusett Mountain subbelts (fig. 4, area M; fig. 5, area 

Y; fig. 6, areas R, W) and dominates the Southbridge 

subbelt. The outcrop pattern is crucial to major structural interpretations, and the unit appears to be lithically 

equivalent to the less metamorphosed Oakdale Formation in the Nashua subbelt to the east (fig. 7). Although 

the granofels member is recognized to have some facies 

variations, the distribution of these facies has not generally been mapped in detail.

The most characteristic rock type of the granofels 

member is well-layered slabby-weathering purple-gray 

quartz-plagioclase-biotite granofels with bedding thickness ranging from 2 cm to about 30 cm (Field, 1975; 

Tucker, 1977). The plagioclase is usually labradorite or 

bytownite. Commonly interbedded with the biotite granofels are beds 1-5 cm thick of green to pink calc-silicate 

granofels composed of quartz, plagioclase, diopside, 

actinolite, clinozoisite, and sphene, and locally scapolite, 

grossular garnet, graphite, and calcite. A few outcrops 

contain beds of diopside marble as much as 10 cm thick. 

Tourmaline-bearing pegmatites are extremely common 

in the Paxton granofels and dominate the outcrop in 

many areas. Commonly small pegmatites have been 

dismembered by shearing during metamorphism, so that 

boudins and individual feldspar fragments distributed 

through the outcrops form a sort of "popcorn rock" (D.R. 

Wones, oral commun., 1976).

Another important rock type in the granofels member 

is quartz-feldspar-mica-sillimanite-graphite-pyrrhotite 

schist. This schist is hardly distinguishable from schist of 

the Partridge Formation, although in general it lacks 

delicate bedding and forms more resistant outcrops than 

does the Partridge. The similarity of the two units

creates a serious mapping problem. Rusty schist interbedded with granofels is characteristic of the granofels 

member of the Paxton from the Barre area southward 

along the west limb of the Oakham anticline into Connecticut and also east of that anticline into Connecticut. 

In the very narrow synclines close to Wachusett Mountain, Robert Tucker (written commun., 1978) mapped 

schist and granofels separately (at a scale of 1:24,000), 

but they are all lumped as granofels member on the State 

bedrock map.

The granofels member is very poorly exposed in the 

center of the Gardner anticline (fig. 4, area M), but the 

rock that can be seen appears to be dominantly granofels. 

Emerson (1917) did not extend the Paxton very far north 

of Gardner, but abundant granofels and calc-silicate float 

in a gravel pit at Whitney Hill, Winchendon, led us to 

several localities including an excellent roadside exposure near East Rindge, N.H.

The granofels member in the western part of the 

Wachusett Mountain subbelt (fig. 6, area R), including 

the type locality at Paxton, seems to be largely free of 

sulfidic schist. The granofels also seems to show some 

internal stratigraphy: lower parts are dominated by 

well-bedded calc-silicate rocks, and upper parts close to 

the overlying Littleton Formation are dominated by 

micaceous granofels layers and gray schists. Similar 

internal stratigraphy has been described in the Madrid 

Formation in Maine (Moench, 1971) and the Madrid and 

Warner Formations in central and southern New Hampshire (Hatch and others, 1983; Thompson, 1983). Since 

the State bedrock map was published, a similar sequence 

has been seen in new outcrops of the granofels member 

low on the west face of New Ipswich and Pratt Mountains, N.H., northwest of Ashburnham (Peterson, 1984).

The Paxton Formation in the Wachusett Mountain 

subbelt, east of the Fitchburg plutons (fig. 6, area W) 

and west of the Wekepeke fault, as mapped by G.R. 

Robinson (1981) and Peck (1976), consists of thin-bedded 

feldspathic quartzite or metamorphosed siltstone interbedded with calc-silicate rock and subordinate beds and 

lenses of mica schist. The quartzite is a very fine grained, 

tan to brown, locally green-gray equigranular rock containing abundant reddish-brown biotite and some green 

chlorite. Greenish-gray calc-silicate beds are thin and 

commonly lenticular. Biotite schist layers, usually 

garnet and staurolite bearing, are only abundant near 

the Fitchburg pluton in the western part of the belt. 

According to G.R. Robinson (1981), bedding in the 

metasiltstone is expressed by differing modal proportions of quartz, plagioclase, biotite, and muscovite or 

actinolite. Less than 50 percent of the metasiltstone 

contains amphibole. Because of the immaturity of the 

sediments forming the metasiltstone, Peck (1976) suggested that they were derived in part from a volcanic
G16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

terrane. Thin, light-colored calcareous beds containing 

plagioclase, quartz, amphibole, and, in some beds, calcite 

compose less than 1 percent of the Paxton in this belt.

Beds average 2-12 cm in thickness and are rarely as 

much as 1 m thick. Graded beds are rare; the rocks in the 

staurolite-kyanite zone in the eastern part have a fine

NASHUA SUBBELT

Age

Pennsylvanian

Early Devonian 

and Silurian

Silurian

Silurian

or

Ordovician

Silurian (?), 

Ordovician, or 

Proterozoic Z

Ordovician or

Proterozoic Z

Emerson 

(1917)

Worcester 

Phyllite

Worcester 

Phyllite

Oakdale

Quartzite 

and 

Worcester

Phyllite

Oakdale 

Quartzite

Boylston

Schist,

Worcester

Phyllite

No rocks

assigned

Brimfield 

Schist

Gneisses and

schists of

undetermined

age

Hansen 

(1956)

Worcester

Formation

Harvard

Conglomerate 

Lentil of the 

Worcester

Formation

No rocks

assigned

Worcester

Formation

(phyllite

facies)

Vaughn Hills

Quartzite

Member of the

Worcester

Formation

Worcester 

Formation 

(mica schist 

facies)

Nashoba

Formation

Skehan 

(1967)

No rocks 

assigned

Oakdale

and

Worcester 

Formations

 fault 

Reubens Hill

amphibolite

Vaughn Hills

Formation

Nashoba

Formation

Grew 

(1970)

Worcester

Formation

Harvard 

Conglomerate

Holden 

Formation 

(part)

Oakdale 

Formation

Tower Hill

Quartzite 

Member of the 

Boylston

Formation

Boylston

Formation

not

discussed

Peck 

(1975, 1976)

No rocks

Units 3 and 4

Unit 2

Unit 1

 fault 

No rocks

assigned

 fault 

Reubens Hill

igneous complex

Vaughn Hills

Member of the

Tad muck Brook

Schist

Tadmuck

Schist

Zen and others 

(1983) 

State bedrock map

Coal Mine Brook 

Formation

Worcester 

Formation

Oakdale 

Formation

Tower Hill 

Quartzite

Boylston

Schist

Reubens Hill

Formation

Vaughn Hills

Quartzite

Tadmuck Brook 

Schist

Nashoba

Formation

FIGURE 7. Correlation of previous nomenclature of stratified units in the eastern part of the Merrimack belt: A, Nashua subbelt; B, Rockingham

subbelt.
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G17

lamination, locally showing crossbedding. These features 

disappear as the rocks become coarser grained to the 

west in the sillimanite zone around the Fitchburg Complex. The rock adjacent to, and as inclusions in, the 

Fitchburg Complex is a coarse quartzofeldspathic gneiss. 

Rusty schists have not been found in this belt of Paxton. 

The Paxton is overlain conformably by mica schist here 

assumed to be the Littleton Formation (G.R. Robinson, 

1981).

This belt of Paxton coincides in part with the original 

belt of Oakdale Quartzite of Emerson (1917) and corresponds to the western belt of Oakdale of Grew (1970) and 

the Oakdale Formation of G.R. Robinson (1978, 1981). 

Peck (1976) did not give the rocks a formal designation.

We have not followed these authors' or Emerson's exact 

nomenclature for rocks in this belt because it became 

clear in the compilation for the State bedrock map (see 

cross sections D-D', F-F') that the Paxton of area W 

(fig. 6) must be continuous around the Fitchburg plutons 

with the Paxton of area R (fig. 6), although this continuation is somewhat obscured by the Wekepeke fault 

south of Oakdale. In addition, this belt of Paxton contains 

less interbedded pelite than the rocks shown on the State 

bedrock map as Oakdale, east of the Wekepeke fault in 

the Nashua subbelt (fig. 7).

The narrow belt of Paxton (fig. 5, area AA) east of the 

Southbridge Member (Spso) (area Z) consists of rock 

similar to the Southbridge (described below) but finer

B ROCKINGHAM SUBBELT

Age

Pennsylvanian

Early 

Devonian

Silurian

Silurian 

or 

Ordovician

Silurian (?), 

Ordovician, or 

Proterozoic Z

Ordovician or 

Proterozoic Z

Emerson 

(1917)

No rocks 

assigned

Gneisses and 

schists of 

undetermined 

age

Merrimack 

Quartzite

No rocks 

assigned

Brimfield 

Schist

Gneisses and 

schists of 

undetermined 

age

Sundeen (1971), 

Sriramadas (1966)

No rocks 

assigned

Littleton 

Formation

Berwick 

Formation

Eliot 

Formation

not 

discussed

Novotny 

(1969)

No rocks 

assigned

Littleton 

Formation

Berwick 

Formation

Eliot 

Formation

Kittery 

Formation

Rye 

Formation

Zen and others 

(1983) 

State bedrock map

Harvard 

Conglomerate

No rocks 

assigned

Berwick 

Formation

Eliot 

Formation

Kittery 

Formation

Reubens Hill 

Formation

Group Merimack 

Vaughn Hills 

Quartzite

Tadmuck 

Brook 

Schist

 unconformity (?) 

Nashoba 

Formation

FIGURE 7. Continued.
G18 THE BEDROCK GEOLOGY OF MASSACHUSETTS

grained and more thinly and uniformly bedded. It corresponds to the Hebron Formation as mapped by Pease 

(1972) in the Eastford quadrangle, northeastern Connecticut, and was considered by Moore (1978) to be a 

formation beneath the Southbridge in his Paxton 

"group." On the geologic map, we have included it in the 

undifferentiated Paxton Formation (Sp).

The base of the granofels member is generally poorly 

exposed, but it apparently rests conformably on the 

sulfidic schist and quartzite unit (Spsq) or the quartzite 

and rusty schist unit (Spqr) or unconformably on the 

Partridge Formation. At the top, it either is overlain by 

separately mapped sulfidic schist or appears to grade 

into the Littleton Formation. Because of structural complexity, we have no reliable thickness figures. Tucker 

(1977) suggested a minimum thickness of about 200 m for 

the unit in the Barre area.

As hinted above, the granofels member (Sp) of the 

Paxton Formation, particularly where close to rocks 

correlated with the Smalls Falls Formation of Maine 

(Moench and Boudette, 1970) or the Littleton Formation 

of central New Hampshire (Hatch and others, 1983), is 

very similar in lithology and sequence to the Madrid and 

Fall Brook Formations of Maine and the Warner Formation of central New Hampshire, as well as the Fitch 

Formation of the Connecticut Valley belt. All of these 

units may be reasonably placed at the top of the Silurian 

section in their regions. The granofels member of the 

Paxton is also similar lithically to the Vassalboro Formation in central Maine (Osberg, 1980) and to the Eliot and 

Berwick Formations of southeastern New Hampshire, 

the Oakdale Formation in the Nashua subbelt in 

Massachusetts, and the Hebron Formation in eastern 

Connecticut.

For the derivation of the protoliths for the granofels 

member, the model for the Madrid Formation (Moench, 

1971) is adequate. The member originally consisted of 

interbedded feldspathic calcareous siltstone and impure 

calcareous shale and dolomitic limestone, possibly with a 

contribution of feldspar from volcanic ash. Interbedded 

schists were sulfidic black shales deposited when the 

volcanic and silt contributions were less.

AMPHIBOLITE (Spa)

Amphibolite is not a common rock type in the Paxton 

Formation, but a number of small lenses have been 

mapped. Some of these may have been contemporaneous 

volcanic deposits; others are probably mafic sills. Their 

structural and stratigraphic setting needs to be more 

thoroughly investigated.

BIGELOW BROOK MEMBER (Spbs, Spbc)

The Bigelow Brook Member is a subdivision of the 

granofels member of the Paxton in southern Massachusetts (fig. 5, area Y). The unit is defined as the eastern 

part of the rocks previously mapped as the Bigelow 

Brook Formation (Pomeroy, 1975; Seiders, 1976; Moore, 

1978), specifically excluding the gray-weathering aluminous schists and gneisses that we include in the Littleton 

Formation. This member consists of about equal proportions of granofels and sulfidic schist usually interbedded 

in thick beds, as can be seen by looking at a large 

Massachusetts Turnpike roadcut from the Route 149 

overpass. In addition, because of the high metamorphic 

grade, many aluminous beds within the granofels contain 

garnets as much as 3 cm in diameter. A few marble beds 

crop out, as at the spillway of Westville Dam in Southbridge. Within this interbedded unit a few zones dominated by calc-silicate granofels (Spbc) have been separately mapped.

SOUTHBRIDGE MEMBER (Spso)

The Southbridge Member (fig. 5, area Z) lies in sharp 

contact along the Black Pond fault (Peper and Pease, 

1976) with the Bigelow Brook Member (Spbs) to the 

west. In contrast to the Bigelow Brook Member, the 

Southbridge is free of schist beds and is almost pure 

interbedded granofels and calc-silicate. Because of either 

metamorphic grade or bulk composition, it does not have 

large garnets. The Black Pond fault becomes a beddingplane fault in southern Massachusetts and loses its 

identity northward, so that in the current state of 

mapping the Southbridge is not readily distinguished 

from the main granofels member of the Paxton. The 

Southbridge Member is most similar to the belt of Paxton 

between the Fitchburg plutons and the Wekepeke fault.

SULFIDIC MICA SCHIST (Spss)

Sulfidic mica schist is the designation used for mappable layers of sulfidic mica schist of the Paxton Formation 

wherever they have been separately mapped. Some of 

these layers have continuity and are stratigraphically 

significant, whereas others appear to be merely lenses in 

other members.

The largest areas of sulfidic mica schist occur at the top 

of the formation, above the granofels member, in the 

Gardner subbelt of the northern part of the State, on 

both limbs of the Gardner anticline. On the west limb of 

the anticline southwest of Gardner, the granofels member is directly overlain by gray schists of the Littleton 

Formation (H.B. Stoddart, written commun., 1977), but 

in giant highway excavations due west of Gardner a 

broad belt of sulfidic schists reappears rather abruptly 

below the Littleton (fig. 4, area L). The contact with the 

Littleton to the west is exposed in these highway cuts, 

but the rocks are so folded and sheared that significant 

interpretation cannot be made. The contact with the 

granofels member to the east is not exposed. The domi-
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G19

nant rock type is schist containing quartz, plagioclase, 

orthoclase, sillimanite, muscovite, biotite, garnet, 

graphite, ilmenite, and pyrrhotite typical of the 

sillimanite-muscovite-orthoclase zone and identical in 

character to many schists of the Partridge Formation. 

Assignment of this rock to the Paxton is encouraged by 

its stratigraphic position and more particularly by several large boudins as much as 20 m across of rather 

typical Paxton purple biotite granofels with spectacular 

green and pink calc-silicate beds. Also present in this 

exposure are boudins of a dismembered metamorphosed 

gabbro dike that cuts the sulfidic schist. North of the 

highway exposures this sulfidic schist belt is poorly 

exposed, but it has been tentatively traced north to New 

Hampshire.

On the east limb of the Gardner anticline is a much 

more extensive and wide belt (fig. 4, area N) of identical 

sulfidic schist that has only been studied in reconnaissance. These two belts probably correlate across the 

crest of the anticline, thus overlying the granofels member (Sp) and underlying the gray schist of the Littleton 

Formation (Dl) to the east. If our interpretation of the 

stratigraphic position of these sulfidic rocks is correct, 

they are possibly equivalent to sulfidic rocks in central 

New Hampshire (Malinconico, 1982; Hatch and others, 

1983) that locally lie between the gray granofels of the 

Warner Formation and the gray schists of the Littleton 

Formation.

As previously described, the narrow belts shown as 

Paxton granofels member (Sp) along the west side of the 

Fitchburg Complex actually consist of granofels and 

sulfidic schist that could be mapped separately at 

1:24,000. One of these belts has been tentatively traced 

north to the New Hampshire line. In this belt the 

granofels pinches out and the belt consists entirely of 

sulfidic mica schist locally containing exposures dominated by feldspathic quartzite. It is here labeled "Spss" 

(fig. 6, area T). Since the State bedrock map was 

compiled, Peterson (1984) has completely revised the 

map pattern near Ashburnham and has demonstrated 

that these sulfidic rocks are much more extensive than 

previously shown. At present they pose a stratigraphic 

dilemma. Their association with typical Paxton granofels 

at Wachusett Mountain suggests that they may be 

merely a more schist-rich facies of the granofels member 

(Sp). Their similarity to rusty schist at the top of the 

formation (Spss) near Gardner suggests a correlation 

with the uppermost Silurian. The similarity of the feldspathic quartzite to the quartzite and rusty schist unit 

(Spqr) of the Wachusett Mountain subbelt suggests a 

correlation with the lower part of the Silurian. If our 

structural interpretation of the Wachusett Mountain 

subbelt is correct and the narrow belts of Silurian strata 

are in anticlinal east-directed nappes from a western

region where the entire Silurian is thin, all three of the 

tentative correlations may be correct. Sulfidic schist 

inclusions within the Fitchburg Complex are identical to 

the sulfidic schists just discussed and are also labeled 

"Spss" (fig. 6).

Within the Paxton Formation southeast of the Fitchburg plutons, Grew (1970) mapped several isolated 

lenses of sulfidic schist, here shown as "Spss." Where 

exposed on the north side of Indian Lake (fig. 6), the rock 

is a very fine grained black mica schist, which is very 

magnetic due to abundant pyrrhotite and appears to be 

much lower in grade than any of the schists described 

above.

LITTLETON FORMATION 

(Dl, Dl+Ops, Dlf, Dlo, Dim)

The Littleton Formation in the Merrimack belt is an 

extension of gray-weathering schists of the Littleton in 

the eastern part of the Connecticut Valley belt. Unfortunately all the Littleton rocks of the Merrimack belt are 

exposed in narrow synclinal belts mostly representing 

folds of the nappe stage, so that physical tracing of 

stratigraphy across strike is impossible. That some of the 

belts are synclinal has been demonstrated by graded 

bedding, particularly in the syncline that runs through 

Mt. Pisgah west of Brimfield and Wales (Peper and 

Pease, 1976) and in two narrow synclines in the village of 

Barre and just east of it (Tucker, 1977). Many of the 

synclines in the Ware subbelt contain no recognized 

Silurian strata, and the gray schists assigned to the 

Littleton are in contact with rocks mapped as Middle 

Ordovician Partridge Formation. In some of these synclines, the gray-weathering rocks may not be Littleton 

Formation and perhaps should be assigned to some other 

gray-weathering unit in the central New Hampshire 

sequence. In the Gardner and Wachusett Mountain subbelts, the gray-weathering schists are in sequences with 

underlying rocks assigned with confidence to the Silurian, which makes their assignment to the Littleton more 

certain.

The different character of the rock in the different 

belts of Littleton Formation results from variations in 

the protoliths, as well as from metamorphic grade and 

degree of deformation during metamorphism. In the 

Ware and Gardner subbelts, the grade ranges from 

sillimanite-muscovite in the north to sillimaniteorthoclase-garnet-cordierite in the south. In the Wachusett Mountain subbelt, metamorphism seems to be in the 

sillimanite-muscovite zone over a broad area west of the 

Fitchburg plutons where sillimanite pseudomorphs after 

andalusite are abundant and assemblages of muscovitebiotite-garnet-cordierite are locally present (Peterson, 

1984). Inclusions in the Fitchburg plutons locally contain
G20 THE BEDROCK GEOLOGY OF MASSACHUSETTS

sillimanite-muscovite-orthoclase assemblages again with 

andalusite pseudomorphs. In the narrow belt of Littleton 

along the east margin of the Fitchburg plutons, metamorphic grade drops rather abruptly from sillimanitemuscovite assemblages with andalusite pseudomorphs at 

the western contact through andalusite-muscovite schist 

to garnet-mica schist locally containing kyanite at the 

eastern contact of the belt (Nelson, 1975; Peper and 

Wilson, 1978; Robinson and others, 1978). In the Southbridge subbelt, Littleton only occurs along the border 

with the adjacent Wachusett Mountain and Gardner 

subbelts.

WARE SUBBELT

The Littleton Formation in the western part of the 

Ware subbelt, west of the Hard wick pluton (fig. 4, area 

A), is identical in all aspects to the Littleton in areas 7 

(figs. 4, 5) and 8 (near Amherst in the Connecticut Valley 

belt (Hatch and others, 1988)). The unit is rather poorly 

exposed and commonly contains strongly foliated rock of 

mylonitic aspect in the sillimanite-muscovite-orthoclase 

and higher grade areas. The rock is commonly a grayweathering, dark, biotite-rich schist with fine sillimanite 

and garnet grains and conspicuous augen of feldspar. 

Field (1975) identified narrow, poorly exposed belts of 

gray schist on either side of the Hardwick pluton in the 

Ware area (fig. 5, area A) and suggested that the pluton 

may be in the middle of an isoclinal syncline of Littleton 

Formation. However, the distribution of Partridge Formation and Fitch Formation as inclusions and along the 

margins of the pluton near Petersham and Phillipston 

(C.K. Shearer, written commun., 1980) renders this 

suggestion improbable.

The next belt of gray schist, entirely east of the 

Hardwick pluton (figs. 4, 5, area B), appears to hinge out 

to the south and has been called the Ragged Hill syncline 

(Field, 1975; Robinson and others, 1982a, Stop 4). These 

rocks are conspicuously well bedded quartz-rich schists 

with abundant 0.5- to 1-cm grains of garnet, sillimanite, 

and commonly cordierite. Robinson and others (1982a), 

on the basis of the quartz-rich character, suggested a 

correlation with the Perry Mountain Formation of central New Hampshire, but J.B. Lyons and N.L. Hatch, 

Jr. (oral commun., 1982), suggested that these schists 

much more closely resemble the lower part of the Rangeley Formation. Correlation with the Rangeley is greatly 

enhanced by the fact that the belt of red-rusty schists 

containing calc-silicate pods directly to the west, originally mapped by Field (1975) with considerable uncertainty as the Lyon Road belt of Partridge Formation, is 

now considered by us to correlate with the upper part of 

member C of the Rangeley. Unfortunately both of these

belts, exposed over relatively broad areas near Ware, 

virtually pinch out to the south and north, preventing 

significant direct correlations.

The next belt of Littleton to the east, flanked on both 

sides by Fitch Formation north of Ware, has been called 

the Big Garnet syncline (figs. 4, 5, area C). The hinge of 

the syncline is located on the east face of Ragged Hill 

near Ware. Nearly the entire width of the syncline is 

exposed in a single set of cliffs on the northeast slope of 

Ragged Hill (Robinson and others, 1982a, Stop 5), and 

subsequent observation of this outcrop showed excellent 

graded bedding topping east on the west side and west 

on the east side. The characteristic rocks of the Big 

Garnet syncline are poorly exposed in Barre but superbly 

exposed in the southwest part of the town of Templeton.

The characteristic rock of the Big Garnet syncline is 

rather homogeneous, poorly layered, gray, medium- to 

fine-grained, sillimanite-biotite-cordierite-garnet schist. 

This schist is cut by vein networks, commonly deformed, 

of quartz, K-feldspar, plagioclase, and cordierite enclosing subhedral to euhedral garnets as much as 4 cm in 

diameter. The size difference between garnets in the 

matrix ( 1-3 mm) and in the feldspathic veins (1^1 cm), 

as well as the character and physical arrangement of the 

veins, strongly suggests that the vein garnets grew in 

the presence of a felsic silicate melt, probably a product 

of local fluid-absent melting (Robinson and others, 1982a, 

p. 341-342). In some exposures southwest of Templeton, 

the large garnets are set in a more massive matrix of 

quartz, cordierite, and plagioclase, suggesting that their 

original melt host may have been transported away 

during metamorphism. Electron probe analyses of these 

garnets (Richardson, 1975; Tracy and others, 1976; Robinson and others, 1982a) show that they are completely 

homogeneous except where their outer edges touch an 

adjacent grain of cordierite or biotite, permitting localized retrograde ion exchange. Commonly the big garnets 

contain internal zones of crystallographically oriented 

ilmenite plates parallel to and several millimeters inside 

crystal faces. In addition to ilmenite, these rocks also 

contain traces of magnetite, a mineral not found in any of 

the other types of Littleton. Field (1975) described two 

samples of this rock containing sillimanite pseudomorphs 

after andalusite.

Locally interbedded with the characteristic rock are 

beds as much as 2 m thick of fine-grained, light-purplegray biotite-feldspar granofels. This rock type, as well as 

the big-garnet rock itself, led N.L. Hatch, Jr., and J.B. 

Lyons (oral commun., 1982) to suggest a direct correlation with the uppermost part of the Warner Formation of 

central New Hampshire, essentially at its contact with 

the Littleton. Such a correlation, not shown on the State 

bedrock map, with the uppermost Warner, rather than
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G21

with the Littleton itself, seems eminently suitable for the 

rocks of the Big Garnet syncline and would help explain 

some of their peculiarities.

East of the Big Garnet syncline, the same big-garnet 

rock also occurs fairly consistently as a selvage a few 

meters thick between the Fitch Formation and the west 

contact of the Coys Hill pluton (figs. 4, 5, area D). This 

relationship supports the contention of Field (1975) that 

the pluton occupies a position in the stratigraphy equivalent to that of the Littleton Formation. In the southern 

part of the Ware area, the Coys Hill pluton converges on 

the east margin of the Monson Gneiss, so that all of the 

synclinal belts just described are squeezed into a very 

narrow, poorly exposed zone known to local workers as 

"The Slot" (fig. 5, near letter E).

East of the Coys Hill pluton, in the Ware subbelt, 

rocks assigned to the Littleton Formation occur mainly 

in four synclinal belts (figs. 4, 5): the Coys Hill syncline 

(area F), the Prouty Road syncline (area G), the Gilbert 

Road syncline (area H), and the Kruse Road syncline 

(area I). Along much of the east contact of the Coys Hill 

pluton, the granite is in direct contact with Partridge 

Formation or with lenses of Fitch Formation, but near 

Barre a thin strip of gray schist of the Littleton appears 

along the contact. Several miles farther north, in Barre, 

the Partridge of the intervening anticline hinges out so 

that the Coys Hill and Prouty Road synclines merge. In 

the northern part of the State, the exposure in this 

region east of the Coys Hill pluton is so poor that several 

areas have been designated on the State bedrock map 

only as "interfolded Littleton and Partridge" (Dl +0ps). 

From Barre south (fig. 5, area G), the Prouty Road 

syncline contains a large area of Littleton Formation 

including some unusual rock types interpreted as metamorphosed volcanic rocks. Area G includes the type area 

of the Mount Pisgah Formation of Peper and others 

(1975; see also Peper and Pease, 1976; Seiders, 1976; 

Pomeroy, 1977) shown as Littleton Formation on the 

State bedrock map. Graded bedding demonstrates the 

synclinal nature of the belt near Mt. Pisgah and also in 

Barre (Tucker, 1977). The Gilbert Road and Kruse Road 

synclines (figs. 4, 5, areas H and I) are the most 

continuous and consistent belts in the Ware subbelt. 

They have been traced tentatively from southernmost 

Massachusetts into New Hampshire. The Kruse Road 

syncline, though well exposed elsewhere, is hardly 

exposed at all in the Ware area, where Field (1975) did 

not show it. In southern Massachusetts the gray schists 

of these synclines were mapped as part of the upper 

schist member of the Hamilton Reservoir Formation. 

The synclinal nature of the Gilbert Road belt is shown by 

graded bedding in the Barre area (Tucker, 1977).

The gray schists of the Prouty Road syncline (area G) 

are conspicuously rich in garnet and sillimanite and also

commonly contain cordierite or muscovite at appropriate 

metamorphic grade. Graphite and ilmenite are the 

opaque minerals. In the Ware and Barre areas, quartzose beds are subordinate, and some are graded (Tucker, 

1977). Quartzose beds are conspicuous or even dominant 

near Mt. Pisgah in the southern part of the State. The 

schists commonly contain layers and concretionary 

lenses of gray biotite-plagioclase granofels or green to 

pink diopside-grossular calc-silicate granofels (Tucker, 

1977).

An abundant and locally predominant rock type in the 

Littleton of the Prouty Road and Coys Hill synclines 

(figs. 4, 5, areas G and F, respectively) of the Ware and 

Barre areas is white quartz-feldspar gneiss (Dlf), 

mapped as the feldspathic gneiss member by Field (1975) 

and Tucker (1977) and interpreted as metamorphosed 

felsic volcanic rocks. The gneiss commonly contains 

minor garnet and biotite, and locally sillimanite and 

secondary muscovite, and bears some resemblance to 

highly metamorphosed and deformed pegmatite and to 

the felsic upper part of the Ammonoosuc Volcanics 

(Robinson, 1963). The structural position of the gneiss 

and its locally intimate interlayering with gray schist 

favor its interpretation as volcanic rocks within the 

Littleton.

Also present in the Prouty Road syncline of the Ware 

area southwest of New Braintree (fig. 5), and commonly 

associated with the feldspar gneisses, are several lenses 

of coarse-grained, weakly foliated, brown-weathering 

orthopyroxene-biotite gneiss (Dlo), mapped as the orthopyroxene gneiss member by Field (1975). The rock 

consists of andesine or labradorite, intermediate orthopyroxene, cummingtonite, and biotite, with or without 

hornblende, quartz, and magnetite (Robinson and others, 1982a). Emerson (1917) published an analysis of 

this rock, which he mistakenly described as "wehrlite," 

showing 50 percent Si02 and the composition of a 

hypersthene-olivine andesite. Because of its stratigraphic position, the orthopyroxene gneiss is considered 

most probably to have been flows of andesite of unusual 

composition, although an intrusive origin cannot be ruled 

out.

The Littleton schists of the Gilbert Road syncline 

(Field, 1975; figs. 4, 5, area H) are gray-weathering, 

well-bedded sillimanite-garnet-biotite schists with 

quartzose beds 5-10 cm thick, showing excellent graded 

bedding in the Barre area. Garnets are typically 1-2 cm 

in diameter. Gray to white beds, 5-8 cm thick, of 

equigranular calc-silicate granofels are common in this 

belt and consist predominantly of quartz and calcic 

plagioclase with subordinate grossular garnet, diopside, 

clinozoisite, and sphene. Several zones of feldspathic 

gneiss (Dlf) were mapped separately in this belt. 

Pomeroy (1977) mapped these rocks as part ("husg") of
G22 THE BEDROCK GEOLOGY OF MASSACHUSETTS

the upper schist member of the Hamilton Reservoir 

Formation, and they are also mapped with certainty into 

outcrops of Littleton feldspathic gneiss north of Route 2 

in the Templeton area. The tracing of these rocks northward to New Hampshire and into slightly lower metamorphic grades is based on reconnaissance only. J.B. 

Lyons and N.L. Hatch, Jr. (oral commun., 1982), suggested that these rocks look more like the lower part of 

Rangeley member C of central New Hampshire than like 

typical Littleton, and the geology of central Massachusetts provides no solid evidence against this correlation. 

The Littleton schist of the Kruse Road syncline (fig. 4, 

area I) east of Barre (Tucker, 1977) typically is wellfoliated, gray-weathering, poorly layered garnetsillimanite-plagioclase schist, commonly with a streaked 

appearance due to the abundance of pegmatite veins. 

Garnets are 0.5 cm in diameter or smaller. This belt was 

mapped separately by Pomeroy (1977) and Seiders (1976) 

as the East Hill belt ("husn") of gray sillimanite schist 

within the upper schist member of the Hamilton Reservoir Formation. It can be found in two or three outcrops 

near New Braintree and West Brookfield. South of 

Barre, garnets tend to be larger, and cordierite has been 

locally identified. North of Barre, muscovite is abundant. 

The homogeneous, poorly bedded character and the local 

occurrence of schist of the Fitch Formation (Sfss) on one 

flank of this belt southeast of Templeton make correlation of these rocks with the Littleton Formation more 

probable than it was for the rocks of the Gilbert Road 

syncline.

GARDNER SUBBELT

The Littleton Formation of the Gardner subbelt occurs 

in three bands, the first in a complex syncline along the 

west limb of the Gardner anticline (figs. 4, 5, area K), the 

second in a deep, complexly refolded syncline that 

appears to trace westward from near Worcester across 

several anticlinal features to the vicinity of the Oakham 

anticline (figs. 4, 5, 6, area P), and the third in the 

Ashburnham area (figs. 4, 6, western part of area Q). 

The second band, here called the Harding Hill syncline 

("the Worm of central Massachusetts") after Tucker 

(1977), is discussed separately below. The third band, 

included in the Gardner subbelt on structural grounds, is 

better discussed in the stratigraphic context of the 

Wachusett Mountain subbelt.

The belt on the west limb of the Gardner anticline 

(area K) was described in detail by Tucker (1977), who 

identified it as a gray graphitic schist member of the 

Paxton Formation. In 1978 R.D. Tucker and Peter 

Robinson reinterpreted the belt as an isoclinal syncline of 

Littleton Formation, here called the Natty Pond syncline, on the basis of comparisons of the Barre and

Wachusett Mountain areas. The syncline appears to 

hinge out southward in a poorly exposed area northwest 

of Oakham between the sulfidic schist and quartzite unit 

(Spsq) and the granofels member (Sp) of the Paxton 

Formation. The belt is best exposed in the Hubbardston 

area and east of Templeton and has been traced in 

reconnaissance to New Hampshire.

Typical schists of the Natty Pond syncline (Tucker, 

1977, p. 36; Robinson, 1979, p. 156) are in the sillimaniteK-feldspar-muscovite zone. They are gray- to slightly 

brown-weathering schists and subordinate beds of calcsilicate granofels, a few with carbonate cores, and typically have quartzofeldspathic segregations 15-20 cm long 

strung out throughout the rock. The dominant minerals 

are quartz, plagioclase, biotite, muscovite, and garnet 

(crystals usually 1-3 mm) and minor orthoclase, sillimanite, graphite, and ilmenite. The schist is medium grained 

and evenly foliated and in places has slight compositional 

layering, but bedding is poorly displayed. Pegmatite and 

medium- to fine-grained garnetiferous granitic gneiss 

sills are generally more abundant than in adjacent units. 

These schists are quite similar to the Littleton of the 

Kruse Road syncline and in the Wachusett Mountain 

subbelt.

Contacts of the Littleton of the Natty Pond syncline 

with the sulfidic schist and quartzite (Spsq) of the Paxton 

to the west appear to be sharp and are easily mapped 

because of the dramatically different character of the 

rocks. Contacts with the granofels member (Sp) of the 

Paxton to the east are also generally distinct, but, in 

areas of particularly good outcrop, as at the top of the 

cliffs west of Riverside Cemetery east of Barre (Robinson and others, 1982a, Stop 14), biotite granofels and 

sulfidic schist typical of the Paxton granofels member 

appear to be interbedded with gray schist typical of the 

Littleton over several tens of meters. The contact with 

the sulfidic schist unit (Spss) of the Paxton is exposed in 

cuts on Route 2 in East Templeton, but pegmatites and 

complex folding make contact relations cryptic.

WACHUSETT MOUNTAIN SUBBELT

Gray-weathering schists mapped as Littleton Formation are the dominant stratified rocks of the Wachusett 

Mountain subbelt and the extreme eastern edge of the 

Gardner subbelt (figs. 3, 6). They form the envelope 

surrounding the belts of Paxton Formation that are 

interpreted as isoclinal anticlines and form inclusions in 

the Fitchburg plutons. These schists are exposed at the 

summits, or on high subsidiary peaks, and on the steep 

slopes of New Ipswich Mountain, Pratt Mountain, Mt. 

Watatic, Little Watatic, Wachusett Mountain, and Little 

Wachusett. West of the Fitchburg plutons the schists are 

in the sillimanite-muscovite zone of regional metamor-
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G23

phism; east of the plutons they appear in a rather steep 

contact gradient away from the plutons and may be as 

low as garnet zone.

West of the plutons (fig. 6, area Q), the gray schists 

show local variety but an overall monotony of aspect. 

Aluminous beds are fairly abundant and may contain 

sillimanite pseudomorphs (or retrograded pseudomorphs) after andalusite, which appear as projections on 

weathered surfaces and are referred to by local workers 

as "andalumps." Irregular quartzose beds, which do not 

display graded bedding, and poorly layered, gray quartzplagioclase-biotite-muscovite schists and gneisses are 

common. Minerals include quartz, plagioclase, sillimanite, biotite, muscovite, garnet (usually 1 to 3 mm or less 

in diameter), graphite, and ilmenite. Some outcrops are 

pitted, due to weathering of secondary chlorite that has 

replaced garnet. Structural and stratigraphic interpretation is hindered by several generations of quartz veins 

and pegmatites and by several generations of folding not 

easily deciphered. Another common feature of these 

rocks is "footballs" or zoned lenses of gray to green or 

pink calc-silicate granofels as much as 1 m in diameter. 

The lenses usually have an outer zone of gray biotitecalcic plagioclase granofels and an inner zone of spotted 

plagioclase granofels with diopside and grossular. Cores 

of the lenses locally contain preserved calcite.

A nearly continuous belt of gray mica schist (fig. 6, 

area U) occurs along the east margin of the Fitchburg 

plutons from Fitchburg south, and similar schist occurs 

as large (mappable) and small (unmappable) screens 

within the intrusions. In the Sterling area (northwest of 

Oakdale in fig. 6), Hepburn (written commun., 1976) 

called these schists the Bee Hill Formation, and similar 

rocks south of Maiden Hill (fig. 6, areas U and P) were 

called Holden Formation by Grew (1970). The southern 

part of Grew's Holden Formation is the Littleton of the 

Harding Hill syncline at the southern edge of the Wachusett Mountain subbelt. South of Fitchburg the gray 

schist along the east margin of the Fitchburg plutons is 

highly varied, depending on metamorphic grade, but 

locally contains pink or white andalusite. Some exposures show excellent bedding.

Near Fitchburg the gray schist unit consists of gray 

graded-bedded pelite and rare thin layers of white 

quartzite (Peper and Wilson, 1978). The schist is composed primarily of muscovite, quartz, biotite, and plagioclase. It commonly contains staurolite and garnet porphyroblasts and rarely kyanite porphyroblasts locally 

converted to fibrolite. Fibrolitic sillimanite is found 

primarily adjacent to granite of the Fitchburg plutons. 

The schist is apparently interbedded at the base with 

granofels of the underlying Paxton Formation (Peper 

and Wilson, 1978).

A thin section of an inclusion of gray schist in Fitchburg granite from the Quabbin Tunnel shows andalusite 

porphyroblasts that have been completely replaced by 

coarse sillimanite. What was apparently once a rim of 

muscovite around the andalusite has been completely 

replaced by fibrous sillimanite and orthoclase. The texture gives insight into possible complexities in the metamorphic history of this region.

The belt of gray schist along the west edge of the 

Massabesic Gneiss Complex is labeled "Dl?" (fig. 6, area 

V) but is not like the Littleton described elsewhere. The 

rock is rather massive, poorly layered schist containing 

abundant quartz and plagioclase and very coarse biotite 

and muscovite. It may correspond to rocks mapped as 

the lower part of Rangeley member C in central New 

Hampshire.

The Littleton in the narrow bands interspersed in the 

Paxton Formation in the Townsend area (fig. 6, area X) 

consists of lustrous mica schist containing few quartzofeldspathic layers (G.R. Robinson, 1981). Graded beds 

indicate that the mica schist overlies the Paxton (Robinson, 1981). The thickness of the schist in the Townsend 

area is from 0 to 235 m.

HARDING HILL SYNCLINE

The sinuous band of Littleton mapped along the axial 

surface of the Harding Hill syncline (figs. 4-6, area P) is 

discussed last because it is poorly exposed, because its 

connections are highly uncertain, and because it passes 

through or along the boundaries of several subbelts. The 

band is presently interpreted as the core of a refolded 

recumbent syncline overfolded to the east, with an 

inverted section of Paxton Formation structurally above 

and to the north and a right-side-up section of Paxton 

structurally below and to the south.

The schist in the syncline is traced westward from a 

postmetamorphic normal fault east of Holden and north 

of Worcester (fig. 6, area P) in a sinuous path along the 

southern edge of the Wachusett Mountain subbelt. The 

schist is well exposed where the band crosses the axis of 

the Gardner anticline west of Rutland in a cut on the 

abandoned Central Massachusetts Railroad known as 

Shannock's Folly. The schist in this eastern part of the 

band is typically gray, medium-grained biotitemuscovite-sillimanite schist, similar to the Littleton elsewhere in the Wachusett Mountain subbelt. Southeast of 

Holden, a narrow, poorly exposed strip of laminated 

siliceous marble (Dim) locally contains idocrase (Grew, 

1970; J.C. Hepburn, written commun., 1977).

From Rutland the band is traced southward (fig. 5, 

area P) through a zone of synmetamorphic faults and 

diorite intrusions into Connecticut. The band passes 

across the highly attenuated southern extension of the
G24 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Hubbardston syncline and thence northward along the 

east limb of the Oakham anticline. For some distance 

along this limb, the Paxton Formation is missing, and the 

Littleton is in direct contact with the Partridge Formation of the Oakham anticline, presumably along a synmetamorphic fault. From Rutland southward there is a 

progressive change in the character of the Littleton from 

medium-grained sillimanite-biotite-muscovite schist to 

coarse-grained sillimanite-garnet-cordierite-K-feldspar 

schist and gneiss within about 15 km of the Connecticut 

line. Within the higher grade parts, strongly foliated 

sillimanite-garnet pegmatites, locally containing graphite, are abundant. Granitic layers contain 1- to 2-cm 

aggregates of coarse sillimanite and biotite apparently 

pseudomorphous after cordierite. Within the schists are 

more feldspathic granulitic layers, several meters thick, 

that contain diopside-grossular calc-silicate layers 5-10 

cm thick. Sillimanite pseudomorphs of andalusite have 

been identified at several localities. In overall aspect 

these higher grade schists look very similar to the 

Littleton in the southern part of the Ware subbelt. 

Spectacular exposures in this area include the cut on the 

north side of the Massachusetts Turnpike where it is 

crossed by New Boston Road (Robinson and others, 

1982a, Stop 11) and outcrops near the southeast end of 

the pond in Bigelow Hollow State Park in Connecticut, 

where Seiders (1976) identified several mylonites. Overall these rocks (Dl on the State bedrock map) are the ones 

considered typical by Peper and Pease (1976; Peper and 

others, 1975) of the western part of their Bigelow Brook 

Formation.

Fairly extensive exposures of Littleton are present 

east of the northern oval of Partridge Formation in the 

core of the Oakham anticline (figs. 4, 5, area P), where 

Paxton Formation again appears between the Littleton 

and Partridge. These rocks are again lower grade 

muscovite-bearing schists. They appear to project northward into an area of no exposure along the line of 

the Quabbin Aqueduct Tunnel, in which Fahlquist 

(1935) identified them as the "Middle Member of the 

Paxton Formation." He characterized the "Middle Member" as mica schist overlain and underlain by typical 

purple granofels with calc-silicate of his "Upper" and 

"Lower Members." In the present context the "Middle 

Member" is interpreted as a recumbent syncline of 

Littleton structurally overlain and underlain by the 

granofels member of the Paxton Formation (Sp). Tracing 

of the recumbent syncline of Littleton around the north 

end of the Oakham anticline is based entirely on interpretation of Fahlquist's tunnel data and collection of 

specimens, because there are virtually no surface outcrops. The position of the synclinal hinge west of the 

Oakham anticline is conjectural.

NASHUA AND ROCKINGHAM SUBBELTS

The eastern part of the Merrimack belt (and the 

Merrimack synclinorium) consists of two subbelts: the 

Nashua subbelt, which lies east of the Southbridge and 

Wachusett Mountain subbelts and west of the Nashoba 

zone, and the Rockingham subbelt, which forms a wedge 

between the Nashoba zone and the Nashua subbelt in the 

northeast part of the Merrimack belt in Massachusetts 

(figs. 1, 35). The subbelts are distinguished partly on the 

basis of minor differences in rock type, metamorphism, 

and structure but mostly on historical differences in 

mapping of units and in terminology. The Nashua subbelt, named for the Nashua River, which drains the area, 

coincides with a metamorphic and structural(?) trough 

called the Nashua synclinal by Crosby (1880) and the 

Nashua trough by Smith and Barosh (1981). The Rockingham subbelt gets its name from the Rockingham 

anticlinorium, which projects into this area from New 

Hampshire and southern Maine (Billings, 1956; Hussey, 

1968). The western margin of the Nashua subbelt is 

essentially the Wekepeke and Pine Hill faults of the 

Mesozoic(?) Flint Hill fault system (Rodgers, 1970, p. 

107). The eastern margin of the Merrimack belt and the 

subbelts is defined by the Clinton-Newbury (also known 

as Essex) fault system and coincides with the western 

boundary of the Nashoba Formation of the Nashoba 

zone.

Appreciable mapping has been done in the Nashua and 

Rockingham subbelts since the early work of Burbank 

(1876), Crosby (1880), Emerson (1898), and Perry and 

Emerson (1903): Grew (1970, 1973, 1976) and Hepburn 

(written commun., 1976; 1978) in the Shirley and in the 

Worcester areas, Barosh (1974, 1976, 1977) and Dixon 

(1974; written commun., 1977 and 1978) in the Oxford 

and Webster area, and Peck (1975, 1976) in the Clinton 

area; Skehan (1967) and Skehan and Abu-Moustafa 

(1976) presented detailed data from the WachusettMarlborough tunnel at the east margin of the Nashua 

subbelt near Clinton. To the north, G.R. Robinson (1978, 

1981) mapped and described the rocks in the Lancaster 

and Pepperell areas, overlapping the subbelts; Gore 

(1976) studied the plutonic rocks near Ayer; Sriramadas 

(1966), Novotny (1969), Sundeen (1971), and more 

recently Shride (1976) mapped and described the rocks in 

the Rockingham subbelt. Correlation of previous nomenclature with that used on the State bedrock map is shown 

in figure 7. Barosh and others (1977) compiled the 

existing mapping for their map of the geology of the 

Boston I°x2° quadrangle. Their divisions are similar to 

those on the State bedrock map, but their unit designations are based largely on lithology.
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G25

NASHUA SUBBELT

On Emerson's (1917) geologic map of Massachusetts, 

the east part of the Merrimack belt coincides with the 

boundary between his "gneisses and schists of undetermined age," which he informally called "Bolton Gneiss" 

and is now called the Nashoba Formation (Hansen, 1956) 

to the east and his Worcester Phyllite, Oakdale Quartzite, Boylston Schist, Paxton Schist, and Merrimack 

Quartzite to the west (fig. 7), all of which he assigned to 

the Carboniferous on the basis of plant fossils found at 

Worcester. Grew (1970, 1973, 1976) showed that the 

fossiliferous strata at Worcester are isolated in a faultbounded block and that the Carboniferous age assignment is not applicable to the other rocks mapped as 

Carboniferous by Emerson and Hansen. Emerson's general arrangement of rock units has persisted, however, 

and for the most part only new age assignments and 

minor changes in nomenclature have been made during 

subsequent years (Goldsmith and others, 1982). However, the detailed stratigraphy of the rocks in the 

subbelts is still a matter of controversy because of 

differing interpretations of primary sedimentary features and minor structural features in the rocks and 

because of difficulty in correlating rock units across 

faulted boundaries.

Most of the rocks in the Nashua and Rockingham 

subbelts are interlayered calcareous metasiltstone, metapelite, and subordinate quartzite and marble that represent the eastern limit of outcrop of the largely Silurian 

and Devonian turbidite and fan basin-fill of the Merrimack synclinorium. In the Nashua subbelt, these strata 

consist of the Boylston Schist (SObo), Tower Hill Quartzite (St, Sts), Oakdale Formation (So), and Worcester 

Formation (DSw). Unconformably overlying the turbidite sequence is the Pennsylvanian Coal Mine Brook 

Formation (Pcm) in the Worcester area.

Primary sedimentary features are preserved in 

the less metamorphosed rocks. Graded bedding in the 

coarser beds and thin to cyclic laminar bedding in 

the finer grained beds, as well as local current bedding 

and scour, suggest turbidite deposition. Orthoquartzite and metaconglomerate in the lower part of the 

sequence are interpreted as proximal or shoal deposits, 

whereas the calcareous metasiltstone and pelite higher in 

the sequence are interpreted as proximal to distal deposits. Noncalcareous carbonaceous phyllite and schist, 

characteristic of the upper part of the sequence in the 

axial area of the Merrimack synclinorium, are less abundant in this eastern margin. On the basis of currentdirection studies, Peck (1976, p. 248) suggested that the 

sediment came from the west. Sulfidic schists are mostly 

lacking in the Nashua subbelt, except in the basal part of 

the sequence.

The Ayer Granite (Sagr) and its phases and granite at 

Millstone Hill (Dmgr) intrude or appear to intrude the 

sequence on the east, and the Fitchburg Complex (Dfgr) 

and its phases intrude the sequence on the west. The 

metamorphic trough lies between these two areas.

Metamorphism ranges from greenschist to amphibolite 

facies. Rocks are in the chlorite zone in the northern part 

of the Nashua subbelt but increase in grade to the south. 

However, the low-grade trough persists southward into 

eastern Connecticut (Thompson and Norton, 1968). 

Rocks on the flanks of the Nashua subbelt may reach 

andalusite-staurolite or locally sillimanite zone. Minerals 

typical of regional metamorphism in the metasiltstones in 

the chlorite and biotite zones are chlorite, biotite, ankerite, and locally calcite or dolomite. Actinolite and actinolitic hornblende, garnet, and locally diopside are found 

in rocks at higher grades. Epidote is present throughout. 

The mineralogy tends to give many rocks a gray-green 

color; however, biotite is characteristically brown or 

red brown and where abundant tends to give these rocks 

a purplish or brownish tinge on a fresh surface. Noncalcareous pelites tend to be gray to dark gray due to 

contained carbonaceous material, but at the higher 

grades of metamorphism they may be silvery and spangled with prominent muscovite. Pelitic layers in the 

middle grades of metamorphism may contain andalusite 

and staurolite. Metasiltstone appears to be thicker bedded and coarser grained (sand size rather than silt size) 

at higher grades than it is at the lower grades (G.R. 

Robinson, 1981, p. 27-36), and much of it at higher 

grades has a salt-and-pepper appearance. The coarse 

grain size in these areas could be in part a primary 

sedimentary feature as well as a result of recrystallization. A steep metamorphic gradient exists westward 

from the Nashoba Formation in the sillimanite zone 

through andalusite in the Tadmuck Brook Schist (fig. 7) 

into the rocks of the Nashua subbelt, which are in the 

chlorite and biotite zones.

BOYLSTON SCHIST (SObo)

The Boylston Schist (Emerson, 1917; Grew, 1970; 

Hepburn, 1978) consists of gray to dark-gray sillimanitebearing quartz-muscovite schist and gneiss, containing 

locally prominent quartzofeldspathic layers as much 

as 0.5 m thick, and subordinate rusty-weathering 

sillimanite-mica schist and tan to light-greenish-gray 

calc-silicate rock. Some beds of schist contain thin lenses 

of coticule. The Boylston is estimated to be 1,000 m thick 

(Grew, 1973; Hepburn, 1978). The distribution of the 

Boylston is essentially that shown by Emerson (1917, p. 

67-68). Grew (1970) included phyllite in his Boylston 

Formation (Unit B of Grew, 1973) that we now assign to 

the Worcester Formation. We now assign Grew's medial
G26 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Nashua subbelt Rockingham subbelt

Eastern 

Connecticut

Oxford-Webster 

area

WorcesterClinton area

Northeastern 

Massachusetts

Southeastern 

New Hampshire and 

southwestern Maine

Scotland 

Schist

Quartzite

Hebron 

Formation

< TatnicHill Formation ) 

Member

Fly Pond 

Member

Lower 

member

i-^\x^V/XXN^~/\/~^

^\

Worcester 

Formation

Oakdale 

and 

Paxton 

Formations

Boylston 

Schist

Nashoba 

Formation

~~ __

Tower Hill 

Quartzite^ _ ~~~"~~^

Coal Mine Brook 

Formation

Littleton^^j/^^ 

formation/

/Worcester 

/ Formation

Oakdale 

and 

Paxton 

Formations

"

Boylston 

Schist

Nashoba 

Formation

^^ -^

^^_

^^i^~~~

Harvard 

Conglomerate

flit!

Berwick 

Formation

Eliot Formation

Kittery 

Formation

Reubens Hill 

Formation

Vaughn Hills

Tadmuck 

Brook Schist

Nashoba 

Formation

^

 ~->~ 

Shapleigh 

Group of 

Hussey (1968) 

and Gonic 

Formation

,' 

Berwick 

Formation

Eliot Formation

Kittery 

Formation

Rye Formation

Upper 

(volcanic) 

member

Lower 

(sedimentary) 

member

Cape 

Elizabeth 

Formation

FIGURE 8. Columnar sections for the eastern part of the Merrimack belt from eastern Connecticut to southeastern New Hampshire and 

southwestern Maine. Dashed tielines indicate alternate interpretation. Queried tielines are conjectural.

calc-silicate unit to the Oakdale Formation and have only 

kept as Boylston his lowest unit, which coincides with 

part of the Science Park unit of Hepburn (1976). On the 

State bedrock map, the Boylston forms the base of the 

sequence from Worcester to Oxford (figs. 3A, 8). The 

Boylston lies in a stratigraphic position similar to that of 

the Tadmuck Brook Schist of the Nashoba zone and 

possibly part of the Vaughn Hills Quartzite of the 

Rockingham subbelt (fig. 8). The higher metamorphic 

grade of the Boylston, in contrast to the typically lower 

grade of the overlying Oakdale and Worcester Formations, suggests that it could actually be part of the 

Nashoba zone caught between strands of the ClintonNewbury fault system.

TOWER HILL QUARTZITE (St, Sts)

Quartzite (St) and schist and phyllite (Sts) between the 

Boylston Schist and the Oakdale Formation in the 

Worcester and Clinton areas (fig. 35) was named the 

Tower Hill Quartzite by Grew (1970; Unit C of Grew, 

1973). The Tower Hill Quartzite (St) consists of lightgray, finely crystalline quartzite in tabular beds 5 cm to 

1.5 m thick and minor interbedded dark-gray, carbonaceous laminated phyllite and mica schist that locally 

contain garnet and chloritoid. The phyllite (Sts) also 

forms sequences as much as 65 m thick above and below 

the main mass of quartzite. Beds of granule and pebble 

metaconglomerate are found in the Boylston area (Hepburn, 1976) on Green Hill, Tower Hill, and the hill 

crossed by Pierce St., West Boylston. The thickness of

the Tower Hill Quartzite ranges from 0 to 91 m (Peck, 

1976) or 130 m (Grew, 1973).

Some assignments of previously mapped units have 

been followed on the State bedrock map; others have not 

(fig. 7). The Tower Hill is Unit 1 of Peck (1976), which he 

considered to be the base of the turbidite sequence. The 

upper part of Grew's (1973) Unit B is shown on the State 

bedrock map as phyllite (Sts) of the Tower Hill in the 

Worcester area but is shown as Worcester Formation in 

the belt from Worcester to Webster (fig. 3A). In support 

of this assignment in the Worcester area, Grew (1973) 

described lenses of quartzite in Unit B that are as much 

as 25 m thick. The nature of the basal contact of the 

Tower Hill is not known. As mapped by Peck (1975), it is 

everywhere a fault except in Clinton (fig. 35) and on 

Green Hill to the south, east of Wachusett Reservoir (W, 

fig. 35), where the quartzite overlies Ayer Granite 

(Sagr). Emerson (1917, p. 224-226) noted the lack of 

evidence for intrusion of the Ayer into the Merrimack 

and Oakdale quartzites. Because of the extensive faulting in the Clinton-Worcester area, it is not certain 

whether the Tower Hill is equivalent to the Vaughn Hills 

Quartzite of the Rockingham subbelt or is a quartzite 

higher in the section. The Tower Hill lies above the 

Boylston according to the cross section drawn by Grew 

(1970, p. 117). Hepburn (oral commun., 1978) believed 

that the Tower Hill lies higher in the section and is 

equivalent to the quartzite at Franklin in eastern Connecticut, which lies beneath the pelitic Silurian and (or) 

Devonian Scotland Schist and overlies calcareous and
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G27

dolomitic metasiltstone and schist of the Ordovician and 

Silurian Hebron Formation.

OAKDALE FORMATION (So)

The Oakdale Formation (Emerson, 1917; Goldsmith 

and others, 1982), which conformably overlies the Tower 

Hill Quartzite (Peck, 1975), consists of interlayered 

brownish-gray to light-gray ankeritic metasiltstone and 

greenish-gray, gray, and dark-brown calcareous phyllite 

in beds 3 cm to 2 m thick. Beds are internally laminated 

and contain cyclic silt-clay couplets. In the actinolite zone 

of metamorphism, metasiltstone beds contain actinolite. 

The Oakdale in the Clinton area is 1,220-2,130 m thick 

(Peck, 1976); in the Townsend area, it is 2,000-6,000 m 

thick (G.R. Robinson, 1978, 1981).

The Oakdale forms a large part of the turbidite 

sequence in the Nashua subbelt and conforms to the 

eastern part of Emerson's Oakdale Quartzite. The 

Oakdale extends from Connecticut, where it has been 

mapped as part of the Ordovician and Silurian Hebron 

Formation, to the Hollis area, New Hampshire, where it 

has been mapped as Berwick Formation of the Merrimack Group (Sriramadas, 1966). The Oakdale is shown as 

stratigraphically underlying the Worcester Formation 

(DSw) on the State bedrock map. Pease (1972) and 

Barosh (1976) considered the sequence to be homoclinal 

and the Worcester to be a large pelitic lens in a westdipping and west-topping Oakdale-Paxton sequence. 

However, Grew (1973), G.R. Robinson (1981), and Hepburn (1976) gave evidence suggesting that the Oakdale 

lies in an isoclinal syncline.

The western belt of Emerson's (1917) Oakdale Quartzite from Oakdale north, which we have shown as Paxton 

Formation on the State bedrock map, was mapped as 

Oakdale by Grew (1970) and G.R. Robinson (1981) and as 

gray and brown granofels by Peper and Wilson (1978). 

However, Peck (1976) distinguished the rock west of the 

Wekepeke fault (his Unit 5; our Paxton) from the rock 

east of it (his Unit 2; our Oakdale) on the basis of 

differences in bedding character. He thought that they 

might be time-equivalent units but of different depositional regimes (Peck, 1976, p. 244-245). Peper and 

Wilson (1978) and G.R. Robinson (1981) drew the boundary with the Paxton where bed thickness and grain size 

increase. This boundary coincides approximately with 

the sillimanite-andalusite isograd (Peper and Wilson, 

1978) and supports the suggestion by Billings (1956) that 

the difference between Emerson's Paxton and Oakdale is 

one of metamorphic grade rather than bedding style or 

original grain size. Emerson (1917, p. 60) noted that the 

Oakdale and Paxton grade into one another west of 

Worcester. Crosby (1880) depicted the Oakdale and 

Paxton as equivalents on either side of his Nashua

"synclinal." We have elected to show the metasiltstone 

west of the Wekepeke in the belt north of Oakdale as 

Paxton for the reasons mentioned in the section on 

Paxton Formation. We have shown Oakdale west of the 

Wekepeke fault, however, in the area south of Worcester, following mapping by Barosh (1974, 1977) in which 

the Oakdale is distinguished from the Paxton by the 

greater amount of interbedded phyllite in the Oakdale.

WORCESTER FORMATION (DSw)

The Worcester Formation (Emerson, 1889, 1898), a 

nonsulfidic pelitic and arenaceous unit (figs. 35, 7A, 8), 

forms an extensive, overall gently west dipping belt 

north and west of Clinton and a narrower, moderately 

west dipping belt extending south from Worcester to 

Oxford. The Worcester Formation comprises most of the 

rocks mapped by Perry and Emerson (1903) and Emerson (1917) as Worcester Phyllite, except that the fossiliferous rocks of definitive Pennsylvanian age have all 

been found to be in a separate fault block from the rest of 

the rock Emerson called the Worcester. The fossiliferous 

rocks have been renamed the Coal Mine Brook Formation (Pcm) (Goldsmith and others, 1982).

The Worcester consists mostly of gray and dark-gray 

carbonaceous slate and phyllite, in part containing andalusite and chiastolite, and interbedded silt-sized metagraywacke, which is most abundant in the upper part. 

Beds of gray to brown impure marble, calc-silicate rock, 

and marble breccia are interbedded sparsely in the slate 

and phyllite. Beds are tabular and 0.6 cm to 1.25 m thick; 

the thicker beds are in the upper part of the formation. 

Slate and graywacke beds are internally laminated, and 

metagraywacke beds are locally crossbedded. The 

Worcester ranges in thickness from 3,050 to 4,270 m in 

the Clinton area (Peck, 1976) and from 2,000 to 3,000 m 

in the Townsend area (G.R. Robinson, 1978, 1981).

The exact stratigraphic position of the Worcester 

Formation is uncertain. On the State bedrock map, it is 

shown as overlying the Oakdale, with which it is conformable (Peck, 1975,1976). The Worcester Formation is 

truncated on the west by the Wekepeke fault, and no 

other formation is known to lie above it. If we assume an 

upward-younging sequence, the Worcester could be correlative with the Lower Devonian Littleton Formation 

(Dl) in the Merrimack synclinorium to the west, on the 

basis of similarity of lithology, or with all or part of the 

Lower Devonian Gonic Formation or Shapleigh Group of 

Hussey (1968) along strike in southern Maine. The 

stratigraphic position of the Worcester at the top of the 

sequence as indicated by Peck (1975) was questioned by 

Grew (1973) and G.R. Robinson (1981), who gave evidence from graded beds to suggest that the Worcester 

underlies the Oakdale. In support of this contention,
G28 THE BEDROCK GEOLOGY OF MASSACHUSETTS

G.R. Robinson (1981) and Hepburn (1976), as mentioned 

above, have described features suggesting that the 

Oakdale lies in an east-facing overturned syncline. This 

interpretation shifts Crosby's synclinal axis from the 

Worcester to the Oakdale Formation and places the 

Worcester stratigraphically lower in the section and 

equivalent to the Tower Hill Quartzite. G.R. Robinson 

(1981) suggested that the absence of quartzite in the 

Worcester is because the Worcester is a basinward facies 

in the overturned limb of this syncline, whereas the 

quartzite of Tower Hill represents a shoal facies in the 

normal limb. We have kept this correlation in mind but 

have shown the Worcester on the State bedrock map as 

lying above the Oakdale and as partly equivalent to the 

Littleton (fig. 8). An element of confusion may exist 

through the similarity of phyllite of the Worcester to 

phyllite of the Tower Hill (Sts). The Worcester Formation, the phyllite of the Tower Hill Quartzite, and even 

the Littleton Formation of the Wachusett Mountain 

subbelt may constitute three or two units or may be all 

one unit. Grew (1973) and Hepburn (1976) described thin 

beds of marble alternating with phyllite on the south side 

of the Wachusett Reservoir, an association recalling the 

Waterville Formation of Maine (Osberg, 1968). Peck 

(1976) and Robinson (1981), however, mapping to the 

north, did not mention beds of marble in their descriptions of the Worcester.

Uncertainty exists about the structural and stratigraphic position of thick pelite units in the turbidite 

section of the eastern part of the Merrimack synclinorium (see Osberg, 1978). Pelitic units such as the Worcester Formation, the Waterville Formation of Maine, and 

the Scotland Schist of Connecticut cannot be demonstrated everywhere as infolds of blanket pelites similar 

to the Devonian Littleton and Seboomook Formations of 

New Hampshire and Maine. These units may be major 

lenses of pelitic facies in the turbidite sequence, as 

suggested by Pease (1972) and Barosh (1976).

COAL MINE BROOK FORMATION (Pan)

The Coal Mine Brook Formation (Pcm) (Goldsmith and 

others, 1982) crops out in two localities near Worcester 

(fig. 9). One locality, containing an abandoned coal mine 

(Grew, 1976), is on the property of the Notre Dame 

Institute, Worcester, and the other is within the City of 

Worcester. Contact relations of the Pennsylvanian strata 

with the adjacent Oakdale and Worcester Formations 

are not visible at the coal mine locality, which is bounded 

by faults (Grew, 1970, 1973, 1976). Fossil flora found at 

the coal mine were identified as Middle to Late Pennsylvanian in age (Grew and others, 1970; Lyons and others, 

1976). The phyllite and conglomerate beds of the Coal 

Mine Brook at the other locality are poorly exposed and

71°45'

4215'

1 MILE 

J

SObo

OZn

' \

1 KILOMETER

EXPLANATION

Coal Mine Brook Formation 

Granite at Millstone Hill 

Worcester Formation 

Ayer Granite 

Oakdale Formation 

Tower Hill Quartzite (phyllite) 

Boylston Schist 

Nashoba Formation

----- Contact Dotted where concealed

 ~ - Fault Dashed where approximately 

located; dotted where concealed. 

D, downthrown block; U,upthrown 

block

FIGURE 9. Geology of the Worcester area. Adapted from Grew

(1970).
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G29

unfossiliferous. The conglomerate, however, contains 

clasts of the adjacent granite at Millstone Hill (Dmgr), 

which has been dated at 372 ±7 Ma and 365 ±15 Ma 

(Zartman and Marvin, this vol., chap. J, table 1). These 

strata must therefore be of Carboniferous age.

The Coal Mine Brook Formation at the coal mine 

consists of gray to dark-gray, very carbonaceous slate 

and phyllite containing a 2-m-thick bed of metaanthracite. The beds at the coal mine, some of which are 

graded, are 3 cm to 1 m thick and total 50 m in thickness. 

In Worcester, the Coal Mine Brook consists of interbedded carbonaceous phyllite, white arkose, and polymict 

conglomerate containing granules and pebbles of phyllite, mica schist, quartzite, granite, and vein quartz. 

Beds are from 2 mm to 2 m thick, and total thickness of 

the unit is 330 m.

These rocks have undergone low-grade metamorphism. Phyllites contain manganiferous garnet porphyroblasts and have a secondary slip cleavage, but they are 

not folded as complexly as the Silurian and Devonian 

strata. All the evidence indicates that the Coal Mine 

Brook lies unconformably on the older formations in spite 

of the lack of exposures of the basal contact.

AGE RELATIONS

The Boylston Schist has been assigned an Ordovician 

or Silurian age because of its position adjacent to the 

Proterozoic Z or Ordovician Nashoba Formation to its 

southeast, because of its high metamorphic grade, like 

that of the adjacent Nashoba, and because it seems to be 

part of the overlying turbidite sequence to its northwest. 

Emerson considered the Boylston to be contactmetamorphosed Worcester Phyllite. From the description of the Boylston, it is more likely correlative with the 

Tadmuck Brook Schist of the Nashoba zone (fig. 8) or the 

probably somewhat younger Vaughn Hills Quartzite of 

the Rockingham subbelt. The Tower Hill Quartzite (St, 

Sts) and the Oakdale Formation (So) are in contact with 

the Ayer Granite (Grew, 1973, p. 124; Peck, 1976, p. 242) 

of Late Ordovician and Early Silurian age (433±5 Ma; 

Zartman and Marvin, this vol., chap. J, table 1). If the 

Ayer is intrusive into the Tower Hill and Oakdale 

sequence, as we believe, and if the age of the Ayer is 

accepted as valid, the sequence is probably Ordovician. 

The sequence is shown as Silurian on the State bedrock 

map on the basis of regional correlations mentioned 

earlier. The Worcester Formation is intruded by small 

stocks of granite or granodiorite of the Early Devonian 

Fitchburg Complex, which place an upper limit on its 

age. The Fitchburg Complex has a U-Pb zircon age of 

390±15 Ma and an Rb-Sr age of 402±11 Ma (Zartman 

and Marvin, this vol., chap. J, table 1). Because the 

Worcester is conformable with and physically overlies

the Tower Hill and Oakdale, which have been assigned a 

Silurian age, and because of possible correlation with the 

Lower Devonian Littleton Formation of New Hampshire, the Worcester has been assigned a Silurian and 

Early Devonian age on the State bedrock map.

ROCKINGHAM SUBBELT

Partly calcareous metasiltstone, metasandstone, phyllite, and schist north and west of the Clinton-Newbury 

fault and east of the northern part of the Nashua subbelt 

(fig. 35) are assigned to the Kittery Formation (SOk), 

the Eliot Formation (Se), and the Berwick Formation 

(Sb) of the Merrimack Group (Hitchcock, 1877; Katz, 

1917; Billings, 1956; Hussey, 1968) of southeastern Maine 

and New Hampshire. These formations (fig. 75) project 

into Massachusetts from southeastern New Hampshire 

along the southerly plunging Rockingham anticlinorium 

(Billings, 1956; Sriramadas, 1966; Novotny, 1969; Sundeen, 1971). The distribution of the Kittery, Eliot, and 

Berwick Formations as mapped by Sriramadas (1966), 

Novotny (1969), and Sundeen (1971) has been modified 

extensively by the mapping of G.R. Robinson (1981) in 

the Pepperell area and of Shride (1976; written communs., 1977, 1979) in the Newburyport and Haverhill 

areas. The lithology of these units is in many respects 

similar to that of the Oakdale and Paxton Formations. 

Emerson (1917) called most of these rocks the Merrimack 

Quartzite, which he equated with his Oakdale Quartzite 

of the Worcester area. He identified the garnet-grade 

rocks of the Berwick as gneisses and schists of undetermined age. The Merrimack Group forms the bulk of the 

strata in the Rockingham subbelt with the addition of 

the Vaughn Hills Quartzite (SOvh) and its overlying 

Reubens Hill Formation (SOrh) and the Pennsylvania^?) Harvard Conglomerate (Ph), which unconformably overlies the sequence. The Vaughn Hills and 

Reubens Hill are included in the Rockingham subbelt 

because they appear to project beneath the Merrimack 

Group northeast of Clinton in the Clinton-Newbury fault 

zone. However, the two formations do not clearly form a 

sequence with the Merrimack Group because of the 

intervening Ayer Granite and faults. The Kittery Formation lies on the west flank of the Rye anticline, which 

has the Silurian, Ordovician, or older Rye Formation in 

its core, and thus apparently forms the base of the 

Merrimack Group. Structural studies indicate that the 

Eliot and Berwick successively overlie the Kittery to the 

west (Billings, 1956, p. 42; Hussey, 1968, p. 293).

Metamorphic grade increases progressively through 

chlorite and biotite zones in the east to garnet zone in the 

west, but a sharp gradient back to chlorite zone coincides 

with the contact of the Berwick Formation and the 

Oakdale Formation to the west in the adjacent Nashua
G30 THE BEDROCK GEOLOGY OF MASSACHUSETTS

subbelt. The boundary marks a change in bedding style 

and grain size from thin bedded and fine grained 

(Oakdale) to thick bedded and coarse grained (Berwick). 

The Tadmuck Brook Schist (SZtb) was assigned to the 

Nashoba zone on the State bedrock map because of its 

composition, a sulfidic pelite, and its high metamorphic 

grade, although the grade decreases westward within 

the unit. Because the metamorphism of the Nashoba 

Formation is considered to be no younger than Late 

Ordovician, the Tadmuck Brook is most likely Ordovician 

or older. However, as discussed in the chapter on the 

stratigraphy of the Nashoba zone (Goldsmith, this vol., 

chap. F), arguments exist for an unconformable relation 

between the Nashoba Formation and the Tadmuck 

Brook Schist (for example, Skehan and Murray, 1980, p. 

295). It is possible, then, that the metamorphism of the 

Tadmuck Brook is younger than Ordovician and the same 

age as the metamorphism in the adjacent Merrimack 

belt, which is considered to be Acadian (Late Devonian). 

If so, the Tadmuck Brook could be part of the turbidite 

sequence of the east side of the Merrimack belt, and 

some or all of the unit could be as young as Silurian.

VAUGHN HILLS QUARTZITE (SOvh)

The Vaughn Hills Quartzite (Hansen, 1956; Grew, 

1970; Peck, 1975; Goldsmith and others, 1982) forms the 

base of the sequence between Shrewsbury and West 

Chelmsford. Near Shrewsbury the Vaughn Hills and 

Tadmuck Brook Schist (SZtb) of the Nashoba zone 

project beneath a slice of Nashoba-zone rocks east of the 

Rattlesnake Hill fault of the Clinton-Newbury fault 

system (Hepburn, 1978).

The Vaughn Hills consists of cream- to flesh-colored, 

fine-grained, thin-bedded quartzite in beds 3 cm to 1.5 m 

thick, interbedded pale-green, locally rusty-weathering, 

laminated phyllite and mica schist, minor beds of calcsilicate-bearing metawacke, and, in the upper part, 

partly calcareous chlorite schist. Conglomerate beds are 

present between Chelmsford and Ayer (Jahns and others, 1959). The unit ranges in thickness from 0 to 100 m.

The identity and stratigraphic position of the Vaughn 

Hills Quartzite have been subject to several interpretations. Peck (1975) considered the Vaughn Hills to be the 

upper part of the Tadmuck Brook Schist; similarly, Bell 

and Alvord (1976) believed that the Tadmuck Brook 

grades up into the Vaughn Hills. Hansen (1956, p. 26) 

described gradation of his mica schist facies of the 

Worcester Formation (now mapped as Tadmuck Brook) 

into quartzite of the Vaughn Hills. The Vaughn Hills 

Quartzite is equivalent to part of Skehan and AbuMoustafa's (1976) unnamed units 1 through 12 east of the 

Rattlesnake Hill fault. Their unit 12, a quartzitic unit, is 

probably the Vaughn Hills, although Skehan (1969, p.

802) indicated that their unit 1, which is also a quartzite 

unit, might be the Vaughn Hills. Unit 1 lies below a 

sequence consisting largely of schist, units 2-9, that is 

the probable equivalent of the Tadmuck Brook Schist. 

Their quartzite unit is probably equivalent to the lens of 

quartzite near the base of the Tadmuck Brook shown by 

Bell and Alvord (1976, fig. 4).

A quartzite, phyllite, and pebble metaconglomerate 

unit lying west of the Tadmuck Brook Schist and below 

the Berwick Formation between West Chelmsford and 

Ayer was mapped by Jahns and others (1959) as Harvard 

Conglomerate. This unit has been shown as Vaughn Hills 

Quartzite on the State bedrock map because of its 

position adjacent to the Tadmuck Brook Schist. Alvord 

(1975) considered the conglomerate a breccia and 

mapped the unit as cataclastic rock of the ClintonNewbury fault.

The location of the top of the Vaughn Hills is somewhat 

uncertain. In the Clinton area, the Vaughn Hills is 

overlain by the Reubens Hill Formation. The chlorite 

schist described by Hansen at the top of the Vaughn Hills 

southeast of Bare Hill Pond (SOvh south of Harvard, fig. 

35) may be equivalent to part of the overlying Reubens 

Hill. Nearly everywhere, however, the Vaughn Hills is 

in fault contact with phases of the Ayer Granite (Sagr, 

Sacgr) so that it is difficult to establish a sequential 

relationship with the turbidite sequence west of the 

Clinton-Newbury fault zone. The Vaughn Hills was 

apparently included in the Brimfield Schist and Oakdale 

Quartzite by Emerson (1917).

REUBENS HILL FORMATION (SOrh)

The Reubens Hill Formation (Skehan, 1967; Goldsmith 

and others, 1982) lies between the Vaughn Hills Quartzite and the Ayer Granite and lies east of the Clinton fault 

of Skehan and Abu-Moustafa (1976) of the ClintonNewbury fault system. The Reubens Hill, about 590 m 

thick, consists of layered to massive metavolcanic and 

hypabyssal intrusive rocks. These are, in part, interlayered dark-greenish-gray chlorite-hornblende schist and 

amphibolite and gray to brownish-gray plagioclasebiotite-quartz schist and biotite-hornblende-plagioclase 

gneiss and, in part, massive greenish-gray metadiorite 

and dark-gray hornblende and actinolite rock poor in 

feldspar. Some amphibolite contains features that are 

possibly relic pillow structure.

The Reubens Hill has always been readily identifiable 

because its composition is different from that of adjacent 

units. It is equivalent to the Diorite of Crosby (1880), the 

Straw Hollow Diorite of Emerson (1917), the Reubens 

Hill amphibolite of Skehan (1967), units 13-15 of Skehan 

and Abu-Moustafa (1976), and the Reubens Hill igneous 

complex of Peck (1975). Units 13-15 of Skehan and
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G31

Abu-Moustafa contain massive phases principally composed of hornblende and actinolite that suggest a mafic or 

ultramafic protolith. Peck (1975) mentioned that some of 

the rock appears to have been derived from pillow lavas 

and that part of the formation is a massive metadiorite. 

Layered schist and gneiss in the complex are interpreted 

by both Peck (1976) and Skehan and Abu-Moustafa (1976) 

as volcanogenic deposits.

The Reubens Hill occupies only a small area near 

Clinton and the Wachusett Reservoir within the ClintonNewbury fault zone. The unit is apparently faulted out 

elsewhere along the east flank of the Merrimack belt, 

except possibly near the Vaughn Hills (see comments by 

Thompson and Robinson, 1976, p. 348). Possibly the 

Reubens Hill appears again in southeastern New Hampshire as the volcanic part of the Ordovician, Silurian, or 

older Rye Formation (see also Goldsmith, this vol., chap. 

F, figs. 5, 6).

KITTERY FORMATION (SOk)

The Kittery Formation (Kittery Quartzite of Katz, 

1917; Billings, 1956; Hussey, 1968) is primarily a thinbedded calcareous biotite metasiltstone. In detail, it 

consists of gray- to greenish-gray feldspathic, calcareous, biotitic metasiltstone; biotite phyllite and schist, 

which is commonly actinolitic; calc-silicate gneiss; and 

minor fine-grained quartzite and feldspathic quartzite. A 

few beds of fine-granule conglomerate lie at the base of 

thicker graded beds (Novotny, 1969). The finer grained 

rocks are thinly bedded to laminated; thicker beds show 

cross lamination, and some are graded. Small-scale 

scour-and-fill structures and ovoid calcareous concretions are common. The Kittery is 2,350-2,450 m thick 

(Hussey, 1968).

The Kittery appears to be intruded by the Newburyport Complex and is truncated to the south along the 

Clinton-Newbury fault. The Kittery probably rests disconformably (Hussey, 1968, p. 294) on the Ordovician or 

Silurian, or older, Rye Formation of southeastern New 

Hampshire, although this contact was shown by Novotny 

(1969) as a fault (Portsmouth fault) along the west side of 

the Rye. The Kittery appears to pass upwards or laterally (Billings, 1956, p. 39) into the more pelitic Eliot 

Formation, although Novotny (1969, p. 11) noted a 

disconformity at one place between the Eliot and the 

Kittery.

ELIOT FORMATION (Se)

The Eliot is a dominantly thinly layered pelitic unit as 

described by Hussey (1968) in Maine, but as mapped by 

Sundeen (1971) in the Haverhill area, New Hampshire, it 

includes metasiltstone sequences. The Eliot, 300 m thick, 

consists of thinly bedded, dark-gray to dark-green slate

and phyllite, commonly dolomitic, and light-gray to lightgray-green to brown, partly calcareous and actinolitebearing metasiltstone (Hussey, 1968; Novotny, 1969).

On the State bedrock map, the wide band of Eliot 

mapped by Sundeen in the Haverhill area has been 

narrowed to include only the primarily pelitic facies. The 

thicker bedded metasiltstone facies mapped by Sundeen 

has been assigned to the Berwick Formation. A.F. 

Shride (written commun., 1979) placed the EliotBerwick contact along the Merrimack River east of 

Haverhill.

BERWICK FORMATION (Sb, Sbs)

The Berwick in Massachusetts and New Hampshire 

consists primarily of thin to thick tabular and lenticular 

beds of calcareous metasiltstone, biotitic metasiltstone, 

and fine-grained metasandstone. Some layers contain 

quartz, biotite, and actinolite, others contain diopside, 

hornblende, and plagioclase. Interbedded with these 

rocks are small amounts of quartz-muscovite-garnet 

schist and feldspathic quartzite. Actinolite increases 

westward at the expense of biotite. In the sillimanite 

zone, the metasiltstones have a salt-and-pepper appearance on weathered surfaces. Two thick sequences of 

partly rusty-weathering pyrrhotite- or pyrite-bearing 

mica schist and phyllite (Sbs) have been shown separately on the State bedrock map (fig. 3.6). The eastern 

sequence (west of Newburyport) is a black to green 

pyritiferous phyllite (Sundeen, 1971). The western 

sequence, located midway between North Chelmsford 

and Townsend, is a massively bedded, locally cataclastic 

quartz-rich pyrrhotitic schist containing aggregates of 

biotite (G.R. Robinson, 1978, 1981). The Berwick is 

mostly in the garnet zone of metamorphism but in the 

western part of its outcrop area is in the sillimanite zone. 

Metamorphism has largely obscured primary sedimentary structures. The unit ranges in thickness from 1,850 

to 2,450 m (Hussey, 1968).

The pyritiferous phyllite as mapped by Sundeen 

(1971), forming the eastern sequence of Sbs, is a continuation of the Calef Member of the Eliot Formation of 

Billings (1956). A.F. Shride (written commun., 1976) 

reported that this phyllite occupies a narrower belt than 

that shown by Sundeen. Because the enclosing rock is 

now mapped as Berwick rather than Eliot, the pyritiferous phyllite is assigned to the Berwick Formation. 

Sundeen considered this phyllite to lie in the east limb of 

a north-trending anticlinal structure, whose axis would 

pass west of Haverhill. Shride traced Sundeen's phyllite 

unit into Massachusetts at Haverhill. The western 

sequence of Sbs in the east Pepperell area was mapped 

by G.R. Robinson (1981) as part of his Merrimack
G32 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Formation. These rocks have been assigned to the Berwick Formation on the State bedrock map.

HARVARD CONGLOMERATE (Ph)

The Harvard Conglomerate (Burbank, 1876) is nonfossiliferous; it rests unconformably on Ayer Granite 

(Thompson and Robinson, 1976) of Late Ordovician(?) 

and Early Silurian age (Zartman and Marvin, this vol., 

chap. J, table 1). The Harvard consists of polymict 

conglomerate containing mostly flattened and stretched 

pebbles of green and gray quartzite less than 10 cm in 

length but also pebbles of slate, phyllite, mica schist, and 

milky quartz, in an argillitic matrix. The conglomerate is 

in lenticular beds a few centimeters to 1 m thick. 

Interbedded with and overlying the conglomerate is 

gray, green, and pale-purple phyllite containing quartz, 

chloritoid, chlorite, and white mica. The thickness of the 

unit is indeterminate because of faulting.

The Harvard has traditionally been correlated with the 

conglomerate beds of the Pennsylvanian Coal Mine 

Brook Formation in Worcester because of its apparent 

unconformity on the older rocks. The Harvard crops out 

in two localities, both bounded by faults: the classic 

locality at Pin Hill near Harvard, described by Emerson 

(1917), Thompson and Robinson (1976), and Gore (1976), 

and at the northern of the two hills known as Vaughn 

Hills near Bolton, described by Hansen (1956). At Pin 

Hill (fig. 1), the conglomerate is clearly unconformable 

upon Ayer Granite, but at Vaughn Hills to the south, 

Hansen, who did not recognize the Clinton-Newbury 

fault, described the conglomerate as part of a sequence 

from Vaughn Hills Quartzite into phyllite and into Harvard Conglomerate. Reconnaissance by G.R. Robinson, 

Jr., and Goldsmith in the area of Vaughn Hills indicated 

not only that quartzite of the Vaughn Hills tongues out 

into phyllite similar to that of the Tower Hill Quartzite 

but that the polymict conglomerate mapped by Hansen 

as Harvard Conglomerate tongues out into the same 

phyllite. At that time, we interpreted the conglomerate 

in this area to be a submarine-channel slide deposit near 

or at the base of the turbidite sequence. The angularity 

of some rock fragments in the conglomerate was noted by 

Emerson (1917, p. 66, pi. VIIIB), Hansen (1956), and 

Thompson and Robinson (1976). However, in most places 

the quartzite pebbles are flattened, as is clearly seen in 

the roadside quarry south of Pin Hill. The greenish 

quartzite beds in the phyllite that is gradational with 

Harvard Conglomerate were compared by Hansen (1956, 

p. 25) with the quartzite beds of the Vaughn Hills. The 

quartzite clasts in the conglomerate beds are generally 

believed to have been derived from quartzites of the 

turbidite sequence (Currier and Jahns, 1952). Possibly 

some of the quartzite clasts in the conglomerate are

dismembered thin quartzite beds. Emerson (1917, p. 66), 

however, described the clasts as consisting of several 

kinds of quartzite (Oakdale quartzite, Westboro quartzite, and vein quartz) and several kinds of slate. The 

relations of rocks mapped as Harvard Conglomerate in 

the past, such as at Vaughn Hills and those mapped by 

Jahns and others (1959) southwest of West Chelmsford, 

need to be reexamined to see the differences between the 

Vaughn Hills and the Harvard and to determine if two 

conglomerates are present, one Ordovician and Silurian 

and the other Pennsylvanian.

AGE RELATIONS

The age of the rocks in the Rockingham subbelt is 

Ordovician and (or) Silurian and older. A Silurian age can 

be inferred on the basis of correlation with the Silurian 

Vassalboro Formation and associated units in central 

Maine (Hussey, 1968; Osberg, 1968; Ludman, 1976). The 

age assignments on the State bedrock map are based 

partly on this correlation and partly on the similarity of 

the strata with the Silurian formations, such as the 

Paxton, in central Massachusetts. An Ordovician or older 

age, however, has been determined for at least part of 

the Merrimack Group largely on the basis of the age of 

intrusions into the sequence (Zartman and Marvin, this 

vol., chap. J; Gaudette and others, 1984). Zartman and 

Marvin (this vol., chap. J, table 1) reported a U-Pb age of 

455 ±15 Ma on zircon from granodiorite of the Newburyport Complex. The Newburyport is believed to have 

intruded the Kittery and Eliot Formations (Novotny, 

1969), and the map pattern in the Newburyport East and 

West quadrangles (Shride, 1976) seems to support this 

interpretation. A search of the literature reveals no 

observation of an actual intrusive contact or of inclusions 

of Kittery or Eliot in the Newburyport Complex, nor 

does the pattern of metamorphic isograds in that area 

seem to conform to the shape of the complex. However, 

the U-Pb age of the Newburyport Complex and similar 

Rb-Sr isotopic ages on intrusions into the Kittery and 

Eliot Formations of the Merrimack Group farther north 

in New Hampshire (Gaudette and others, 1984) show 

that part of the Merrimack Group, and probably all of it, 

is Ordovician or older. D.R. Wones (oral commun., 1979) 

correlated the Merrimack Group with pre-Silurian rocks 

of similar lithology that he studied in the Penobscot Bay 

area of Maine (Stewart and Wones, 1974).

On the State bedrock map, the Kittery was assigned 

an Ordovician or Silurian age because of the possible 

Ordovician age of the Newburyport Complex. At the 

time the map was prepared, the isotopic age of the 

Newburyport Complex was less certainly Ordovician 

(445±15 Ma, Zartman and Naylor, written commun., 

1978), and the dating of intrusions farther north in New
STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS G33

Hampshire had not begun. The Eliot, even though it is in 

contact with the Newburyport Complex, was assigned a 

Silurian age through correlation with the Silurian Vassalboro Formation in central Maine. The Berwick was 

similarly assigned a Silurian age.

Both the Vaughn Hills Quartzite and the Reubens Hill 

Formation are assigned an Ordovician or Silurian age 

because of their position at the base of the section 

or below the turbidite sequence and because the 

Vaughn Hills seems to be in a sequence with the Tadmuck Brook Schist of Proterozoic Z, Ordovician, or 

Silurian age. Their relation to the Ayer Granite could not 

be established.

The radiometric ages discussed above suggest that the 

strata in the Nashua and Rockingham subbelts are 

Ordovician or older, which is older than expected from 

lithologic correlation with the Silurian strata to the north 

in Maine. No stratigraphic break is discernible between 

the rocks of the Nashua and Rockingham subbelts and 

the Silurian and Devonian rocks in the synclinorium to 

the west, however. Probable age range of the sequence 

at present is from Cambrian and (or) Ordovician to Early 

Devonian although Cambrian and (or) Ordovician to 

Silurian is most likely. J.B. Lyons and others (1982, p. 

54) suggested a Proterozoic age for the sequence on the 

basis of 1,188-Ma ages of detrital zircons from the 

Berwick Formation (Aleinikoff, 1978). Olszewski and 

others (1984) and Bothner and others (1984) supported a 

Proterozoic Z age for the sequence because of its apparent conformity with the Massabesic Gneiss Complex, 

which contains rock dated at 646 Ma (Aleinikoff and 

others, 1979; Lyons and others, 1982). The Massabesic 

Gneiss Complex is similar in part to rock in the Nashoba 

zone and has produced similar isotopic ages; however, we 

feel that the rocks of the Merrimack Group are a 

sequence that differs in lithology and depositional style 

from the rocks of the Nashoba zone and the Massabesic 

Gneiss Complex. The problem remains concerning the 

similarity in lithology and apparent continuity, in places, 

of the Merrimack Group sequence, if Proterozoic Z, with 

the Oakdale-Worcester and Paxton-Littleton sequences 

in the rest of the Merrimack belt, which are mapped as 

Silurian to Lower Devonian.

DISCUSSION

At the present time, uncertainties connected with the 

stratigraphy along the east flank of the Merrimack 

synclinorium are so numerous that one can only speculate 

as to the actual stratigraphic relations. On the basis of 

the structure and topping evidence available, several 

correlation schemes could be prepared. One is based on 

G.R. Robinson's (1978) evidence that the Worcester 

underlies the Oakdale and is equivalent to the Tower

Hill; another on Pease and Barosh's (1981) contention 

that the Scotland Schist (equals Worcester) is a pelitic 

lens in the Hebron Formation (equals Oakdale and 

Paxton). Our preferred scheme and that shown on the 

State bedrock map is shown in figure 8. We believe that 

the Tadmuck Brook, Vaughn Hills, Reubens Hill, and 

Boylston are the base of the section and are probably 

Ordovician or older in age. We believe that the BoylstonOakdale-Worcester sequence in the narrow belt from 

Worcester to Webster is a greatly compressed section of 

that north of Worcester. It is possible, however, that 

reexamination of this sequence might show that it is 

equivalent to the upper units of the Tatnic Hill Formation, so that the Oakdale is equivalent to the calcsilicate-bearing Fly Pond Member of the Tatnic Hill and 

the Worcester as mapped is equivalent to the twomica-schist-bearing Yantic Member. The Boylston would 

be equivalent to part of the lower member of the Tatnic 

Hill. The Boylston, as described, contains several different lithologies including calc-silicate-bearing beds (Grew, 

1973; Hepburn, 1976), as does the Vaughn Hills (Hepburn, 1976). Thus, lithologies seem to interfinger near 

the base of the section. The adjacent Nashoba Formation 

also contains calc-silicate-bearing zones (Bell and Alvord, 

1976). The Boylston Schist lithologically most resembles 

the Tadmuck Brook Schist, and the Tower Hill Quartzite 

resembles the Vaughn Hills Quartzite. As mapped by 

Peck (1975), the Tower Hill lies west of the ClintonNewbury fault zone and the belt of Ayer Granite, 

whereas the Vaughn Hills lies within and east of the 

Clinton-Newbury fault zone and the belt of Ayer Granite, so that the Tower Hill and the Vaughn Hills never 

meet. J.C. Hepburn (oral commun., 1979) suggested that 

the Tower Hill occupies a position similar to that of the 

Franklin Quartzite in Connecticut. The latter lies in 

upright position beneath graded metagraywacke beds of 

the Scotland Schist and above calcareous metasiltstone 

and phyllite assigned to the Hebron Formation. The 

Hebron Formation also lies above the Scotland, either in 

normal stratigraphic order (Pease and Barosh, 1981) or 

in an overturned section (Dixon and Lundgren, 1968). In 

the Clinton area, the Tower Hill lies in upright position 

beneath the calcareous metasiltstone and phyllite of the 

Oakdale Formation, and the Worcester Formation overlies the Oakdale in normal stratigraphic order (Peck, 

1976) or in an overturned section (G.R. Robinson, 1981). 

If the Scotland Schist is merely a pelitic lens in the 

Hebron Formation, as Pease and Barosh (1981) stated, 

the Scotland Schist could be anywhere in the Hebron 

section and the correlation of the Tower Hill with the 

Franklin Quartzite could be more reasonably made. The 

Reubens Hill Formation of the Rockingham subbelt must 

wedge out between the Tower Hill and the Oakdale, an 

interpretation for which there is no evidence, or, if the
G34 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Tower Hill does not correlate with the Vaughn Hills but 

lies higher in the section, as Hepburn believed, the 

Reubens Hill and the Vaughn Hills wedge out above the 

Boylston Schist. The Tower Hill, then, might correlate 

across into the Rockingham subbelt with the Kittery 

Formation (part of Emerson's (1917) Oakdale Quartzite 

with part of Katz's (1917) Kittery Quartzite). A correlation, not strongly held by us, of the Reubens Hill 

Formation, Vaughn Hills Quartzite, and Tadmuck Brook 

Schist of the Rockingham subbelt with the Rye Formation of southeastern New Hampshire and southwestern 

Maine (fig. 8) results from the position of the Rye with 

respect to the Kittery Formation. An alternative correlation is presented by Goldsmith (this vol., chap. F), in 

which the Rye is correlated with the Marlboro Formation 

of the Nashoba zone. The Reubens Hill Formation is a 

key unit, worth more study, in making a correlation 

between the Rye, the Marlboro, and the lower part of the 

sequence in the Rockingham subbelt.

On the basis of lithology and what little we know of the 

sequence, the rocks of the Nashua and Rockingham 

subbelts should not have different nomenclatures. The 

Paxton is much like the Berwick in lithology, and the 

Eliot and Kittery are somewhat like the Oakdale. The 

Eliot and Oakdale represent pelitic facies, rarely at high 

grade however, so that contained micas would not have 

been converted to less hydrous phases to produce a less 

schistose rock, like much of the Berwick.

The formations are largely distinguished on the basis 

of bedding style, grain size, and mica content, which 

probably represent differences in sedimentary facies, as 

well as in degree of metamorphism. A single formational 

name could very well apply to the whole sequence, in the 

same way as the Hebron Formation is used in Connecticut (Rodgers, 1985). Ideally, as the Merrimack terminology has historical precedence (Hitchcock, 1877; Katz, 

1917; Billings, 1956; Hussey, 1968) over Oakdale and 

Paxton (Perry and Emerson, 1903; Emerson, 1917) in 

Massachusetts and over Hebron Formation (Rice and 

Gregory, 1906; Gregory and Robinson, 1907) in Connecticut, it should be applied throughout the region. Unfortunately, the subunits of the Merrimack Group have not 

been clearly defined in southeastern New Hampshire in 

the zone between southeastern Maine and Massachusetts. Formation boundaries are drawn in different 

places by different people. Rather than use formational 

names for these units, the best approach might be to call 

the sequence Merrimack Formation, rather than Group, 

as G.R. Robinson (1981) has done. This formation could 

then be subdivided into members on the basis of differences in sedimentary facies characteristics recognized 

beneath the metamorphic overprint. A somewhat arbitrary boundary has been established on the State bedrock map between units certainly of the Merrimack

Group and units like the Oakdale and Paxton, which are 

well established by Emerson in Massachusetts. We have 

chosen to carry Emerson's terminology as far as possible, 

except that Emerson's western belt of Oakdale is now 

assigned to the Paxton.

We have not here suggested a revised nomenclature. 

A unified study of the stratigraphy and structure along 

the east side of the Merrimack belt in Massachusetts is 

needed in order to better understand the relations before 

the nomenclature can be revised. This study should be 

tied in closely with mapping in the same belt in southeastern New Hampshire and southern Maine.

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Seiders, V.M., 1976, Bedrock geologic map of the Wales quadrangle, 

Massachusetts and Connecticut: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1320, scale 1:24,000.

Shride, A.F., 1976, Preliminary maps of the bedrock geology of the 

Newburyport East and Newburyport West quadrangles, Massachusetts-New Hampshire: U.S. Geological Survey Open-File 

Report 76-488, 4 pis., scale 1:24,000.

Skehan, J.W., 1967, Geology of the Wachusett-Marlborough tunnel, 

east-central Massachusetts, a preliminary report, in Farquhar, 0., 

ed., Economic geology in Massachusetts Conference, Amherst, 

Proceedings: Amherst, Mass., Massachusetts University Graduate School, p. 237-244.

 1969, Tectonic framework of southern New England and eastern 

New York, in Kay, Marshall, ed., North Atlantic Geology and 

continental drift, a symposium: American Association of Petroleum Geologists Memoir 12, p. 793-814.

Skehan, J.W., and Abu-Moustafa, A.A., 1976, Stratigraphic analysis of 

rocks exposed in the Wachusett-Marlborough tunnel, east-central 

Massachusetts, in Page, L.R., ed., Contributions to the stratigraphy of New England: Geological Society of America Memoir 148, 

p. 217-240.

Skehan, J.W., and Murray, D.P., 1980, Geologic profile across southeastern New England: Tectonophysics, v. 69, p. 285-316.

Smith, P.V., and Barosh, P.J., 1981, Structural geology of the Nashua 

trough, southern New Hampshire [abs.]: Geological Society of 

America Abstracts with Programs, v. 13, no. 3, p. 178.

Sriramadas, Alaru, 1966, Geology of the Manchester quadrangle, New 

Hampshire: New Hampshire Department of Resources and Economic Development Bulletin 2, 92 p.

Stewart, D.B., and Wones, D.R., 1974, Bedrock geology of the 

northern Penobscot Bay area, in New England Intercollegiate 

Geological Conference, 66th Annual Meeting, Orono, Maine, Oct. 

12-13, 1974, Guidebook for field trips in east-central and northcentral Maine: Orono, Maine, University of Maine, p. 223-239.

Sundeen, D.A., 1971, The bedrock geology of the Haverhill 15' 

quadrangle, New Hampshire: New Hampshire Department of 

Resources and Economic Development Bulletin 5, 125 p.

Thompson, J.B., Jr., and Norton, S.A., 1968, Paleozoic regional 

metamorphism in New England and adjacent areas, in Zen, E-an, 

White, W.S., Hadley, J.B., and Thompson, J.B., Jr., eds., Studies 

of Appalachian geology Northern and maritime: New York, 

Interscience Publishers, p. 319-328.

Thompson, J.B., Jr., and Robinson, Peter, 1976, Geologic setting of the 

Harvard Conglomerate, Harvard, Massachusetts, in New England Intercollegiate Geological Conference, 68th Annual Meeting, 

Boston, Mass., Oct. 8-10, 1976, Geology of southeastern New 

England; a guidebook for field trips to the Boston area and vicinity: 

Princeton, N.J., Science Press, p. 345-351.

Thompson, J.B., Jr., Robinson, Peter, Clifford, T.N., and Trask, N.J., 

Jr., 1968, Nappes and gneiss domes in west-central New England, 

in Zen, E-an, White, W.S., Hadley, J.B., and Thompson, J.B., 

Jr., eds., Studies of Appalachian geology Northern and maritime: New York, Interscience Publishers, p. 203-218.

Thompson, P.J., 1983, Silurian-Devonian stratigraphy, Monadnock 

quadrangle, New Hampshire [abs.]: Geological Society of America 

Abstracts with Programs, v. 15, no. 3, p. 186.

Tracy, R.J., Robinson, Peter, and Field, M.T., 1976, Phase relations of 

natural pure magnesian cordierite and biotite from pyritesillimanite-orthoclase schist, central Massachusetts [abs.]: American Geophysical Union Transactions, v. 57, p. 338-339.

Tracy, R.J., and Rye, D.M., 1981, Origin and mobility of sulfur in 

graphitic schists, central New England [abs.]: Geological Society 

of America Abstracts with Programs, v. 13, no. 7, p. 569.

Tucker, R.D., 1977, Bedrock geology of the Barre area, central 

Massachusetts: University of Massachusetts Geology Department 

Contribution No. 30, 132 p.

Zen, E-an, editor, and Goldsmith, Richard, Ratcliffe, N.M., Robinson, 

Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 

of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 

scale 1:250,000.

Structural and Metamorphic 

History of Eastern Massachusetts

By RICHARD GOLDSMITH

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-H

CONTENTS

	Page

Abstract............................................................................ HI

Introduction....................................................................... 2

Nashoba zone east of the Merrimack belt................................ 4

Internal structure and metamorphism.............................. 4

Folds and minor structural features........................... 4

Metamorphism....................................................... 5

Faults................................................................... 8

Spencer Brook and Assabet River faults............... 9

Inclination of and movement on fault surfaces ....... 9

Structural position of the Nashoba zone............................ 9

Clinton-Newbury fault................................................... 11

Description............................................................ 11

Age of the Clinton-Newbury fault zone ...................... 16

Structural relations west of the Clinton-Newbury fault....... 16

Nashua trough....................................................... 16

Rockingham anticlinorium........................................ 18

Summary.................................................................... 20

Milford-Dedham zone .......................................................... 20

Structure and metamorphism in the basement blocks.......... 21

Milford antiform..................................................... 21

Salem block........................................................... 24

Melrose subblock.............................................. 24

Fault pattern in the Salem block......................... 26

Mystic fault..................................................... 26

Dedham and Foxborough blocks................................ 27

Fall River block ..................................................... 27

Metamorphism of the metavolcanic and plutonic

rocks........................................................ 27

Structural features in the New Bedford area......... 29

Assawompset Pond graben and related

structures.................................................. 32

Page

Milford-Dedham zone Continued

Structure and metamorphism in the basement blocks 

Continued 

Fall River block Continued

Joints ............................................................. H32

Summary and discussion.................................... 32

Proterozoic metamorphism............................................. 33

Structure and metamorphism of the basins in the MilfordDedham zone...................................................... 33

Boston basin.......................................................... 33

Newbury basins ..................................................... 36

Bellingham basin.................................................... 36

Narragansett basin................................................. 37

Norfolk basin......................................................... 40

Middleton basin...................................................... 41

Nantucket basin..................................................... 41

Mafic dikes.................................................................. 41

Summary of blocks and basins of the Milford-Dedham zone.. 42 

Bloody Bluff fault zone .................................................. 43

Burlington mylonite zone ......................................... 45

Wolfpen lens.......................................................... 46

Attitudes of fault surfaces and sense of movement in

the Bloody Bluff fault zone................................. 46

Age of the Bloody Bluff fault zone............................. 47

Tectonic events in eastern Massachusetts ............................... 47

Milford-Dedham zone .................................................... 47

Nashoba zone............................................................... 53

Newbury basins ........................................................... 53

East flank of the Merrimack belt..................................... 54

Zone boundaries........................................................... 55

Accretion.................................................................... 56

References cited................................................................. 58

ILLUSTRATIONS

Page

FIGURE 1. Index map showing major structural features of eastern Massachusetts.................................................................... H3

2. Map showing structural features of eastern Massachusetts...................................................................................... 6

3. Map showing major structural features of southeastern New England ...................................................................... 10

4. Geologic map of the Wachusett Reservoir area ..................................................................................................... 12

5. Geologic map of the east flank of the Merrimack belt, Massachusetts and New Hampshire, showing locations of cross

sections (A-A'-A", B-B', C-C' of figure 7).................................................................................................... 17

6. Schematic section near cross section C-C' across the Nashua "synclinal" as drawn by Crosby (1880, pi. III)................... 18

7. Schematic cross sections across the east flank of the Merrimack belt, Massachusetts and New Hampshire...................... 19

8-10. Maps showing:

8. Basins of the Milford-Dedham zone, eastern Massachusetts ........................................................................ 20

9. Form lines on foliation and direction of plunge of lineation in the Milford antiform, eastern Massachusetts......... 22

10. Structural features in and adjacent to the Salem block of the Milford-Dedham zone, eastern Massachusetts ....... 25

in
IV CONTENTS

FIGURE 11. Maps of the Fall River-New Bedford area, southeastern Massachusetts, showing: A, Structural features;

B, Interpretive cross sections; and C, Joints................................................................................................... H28

12-15. Maps showing:

12. Structural features of the Boston basin, eastern Massachusetts................................................................... 34

13. Structural features of the Norfolk and Narragansett basins, eastern Massachusetts ....................................... 38

14. Structural features along part of the Bloody Bluff fault zone, northeastern Massachusetts............................... 44

15. Tectonic events in eastern Massachusetts corresponding approximately to time intervals indicated on

tableS......................................................................................................................................... 50

TABLES

TABLE 1. Metamorphism, plutonism, and faulting in basement rocks of the Milford-Dedham zone, eastern Massachusetts..................... H21

2. Metamorphism and structure of cover rocks in basins of the Milford-Dedham zone, eastern Massachusetts............................ 42

3. Tectonic events in eastern Massachusetts ................................................................................................................... 48
THE BEDROCK GEOLOGY OF MASSACHUSETTS

STRUCTURAL AND METAMORPHIC HISTORY OF 

EASTERN MASSACHUSETTS

By RICHARD GOLDSMITH

ABSTRACT

Two terranes differing in stratigraphy, plutonism, and metamorphism compose most of eastern Massachusetts. On the bedrock geologic 

map of Massachusetts (hereafter called the State bedrock map), these 

are the Nashoba zone east of the Merrimack belt and the MilfordDedham zone. The Nashoba zone, bounded on the west by the 

Clinton-Newbury fault and on the east by the Bloody Bluff fault 

system, is a high-grade metamorphic terrane of northeast-trending, 

steeply dipping Proterozoic Z or Ordovician schist, paragneiss, and 

metavolcanic rocks intruded by synkinematic Ordovician S-type and 

postkinematic Silurian I-type plutons. The Nashoba zone may be 

synformal; only a few large-scale folds have been identified. The zone 

contains many small-scale, mostly easterly verging folds. The zone is 

cut by many northeast-trending longitudinal faults, many of which are 

younger than the Silurian plutons. Early faults, both within the zone 

and in the Clinton-Newbury fault system, are west-dipping thrusts and 

reverse faults characterized by mylonite in which movement sense is to 

the east and southeast. Later faults tend to be steep and characterized 

by brecciation and gouge. The Science Park block near Worcester on 

the east flank of the Merrimack belt is interpreted to be a detached slice 

of Nashoba-zone rock within the Clinton-Newbury fault zone. Structural features within the eastern flank of the Merrimack belt are 

believed to be primarily Acadian and can be related only in part to 

structures in the adjacent Nashoba zone.

The Clinton-Newbury fault zone extends from the MassachusettsConnecticut boundary to the Gulf of Maine. It dips steeply and 

truncates the rock units on either side of the zone at a low angle to the 

south and a high angle to the north. Within the fault zone are early, 

possibly Acadian, low-angle eastward-directed thrusts and reverse 

faults; later faults are younger than the Pennsylvanian strata in the 

Worcester area and may be in part Mesozoic.

The Milford-Dedham zone is that part of eastern Massachusetts east 

and southeast of the Bloody Bluff fault system. It is probably bounded 

on the east, off shore, by an extension of the Meguma terrane of Nova 

Scotia. The zone consists largely of a mostly crystalline Proterozoic Z 

plutonic-metamorphic basement in which older Proterozoic Z metasediments and mafic volcanic and plutonic rocks have been intruded by 

younger Proterozoic Z granite and granodiorite batholiths. In the lower 

and middle Paleozoic, the zone was intruded by plutons of Ordovician to 

Devonian gabbro and alkalic granite. The terrane has been broken by 

normal and reverse faults into upthrown blocks of basement rock and 

downthrown blocks containing sedimentary and volcanic cover rocks

Manuscript approved for publication November 16, 1987.

ranging in age from Proterozoic Z to Triassic-Jurassic. The Milford 

antiform, to the west, a northeast-plunging foliation arch, and the 

southern part of the Fall River block, which lies southeast of 

the Narragansett basin, contain variably gneissic granitoid rocks and 

amphibolite-facies metasediments in which less deformed areas are 

bounded by more intensely deformed shear zones. The Milford antiform 

is truncated to the north by the northeast-trending Bloody Bluff fault. 

Proterozoic Z plutonic rocks in the Salem, Dedham, and Foxborough 

blocks, in northeastern Massachusetts, and the northern part of the 

Fall River block are at most brittlely deformed in the greenschist 

fades. The Salem block is appreciably sheared near the Bloody Bluff 

fault zone, as for example in the 8-km-wide Burlington mylonite zone. 

A prebatholithic metamorphism in the Proterozoic Z metasedimentary 

and metavolcanic and plutonic rocks is greenschist facies to the east and 

amphibolite facies to the west.

The major basins between basement blocks of the Milford-Dedham 

zone contain strata of Proterozoic Z and Cambrian, Silurian and 

Devonian, Pennsylvanian, and Triassic-Jurassic age. Fossils in strata 

of Cambrian, Silurian, and Devonian age are of Acado-Baltic affinity. 

The unmetamorphosed Proterozoic Z marine strata in the Boston basin 

that lie unconformably on the batholithic rocks, and the coal-bearing 

alluvial Pennsylvanian strata in the Norfolk and Narragansett basins, 

have both been folded and cut by reverse faults that trend eastnortheast, indicating a synchronous compressional event probably 

Alleghenian in age. This event has been suggested as having been 

produced by a left-lateral shear system striking northeast. North-south 

faults and diabase dikes that cut the east-northeast-trending structures 

indicate a succeeding tensional regime.

The two narrow, fault-bounded Newbury basins between the 

Milford-Dedham and Nashoba zones in northeastern Massachusetts 

probably lie in a graben of Mesozoic age in the same fracture zone as the 

Triassic and Jurassic Middleton basin to the south. The Bellingham 

basin (also known as the Woonsocket basin) contains greenschist-facies, 

unfossiliferous, metamorphosed Pennsylvanian or possibly Proterozoic 

Z(?) rocks that lithologically resemble those of the Boston basin. The 

bounding faults of the basin are continuous with faults forming the 

southwest end of the Boston basin.

The Bloody Bluff fault system has a long history of movement. 

Mylonites in the system formed by ductile deformation at amphiboliteto greenschist-facies metamorphism have had superimposed brittle 

deformation. Shearing in the Burlington mylonite zone, part of the 

Bloody Bluff fault system in northeastern Massachusetts, is older than

HI
H2 THE BEDROCK GEOLOGY OF MASSACHUSETTS

the Early Devonian Peabody Granite, 1 but radiometric data from a 

continuation of the fault zone in Connecticut indicate an Alleghanian 

age for the latest movement in the Bloody Bluff system. At its north 

end, the Bloody Bluff system may be offset to the north along 

north-trending faults flanking the Middleton and Newbury basins. The 

Wolfpen lens, near Framingham, is a splinter of basement caught 

between branches of the fault zone. Fault surfaces in the Bloody Bluff 

system dip steeply westward in most places, and movement has been 

generally considered to be toward the east and southeast. There seems 

to be little evidence that the Bloody Bluff is a major strike-slip zone.

In the Milford-Dedham zone, a Proterozoic Z volcanic-plutonic arc 

was intruded by Proterozoic Z granite batholiths and was metamorphosed and deformed to at least greenschist facies in the east and 

possibly amphibolite facies to the west. A subsequent dilational event 

also in Proterozoic Z time led to development of a rift system 

accompanied by felsic volcanism and deposition of turbidite. Deposition 

of Cambrian shelf sediments ushered in a period of stability and 

subsidence. Throughout early and middle Paleozoic time, the MilfordDedham zone was in an extensional regime during which alkalic and 

gabbroic plutons were intruded. During Devonian to Pennsylvanian 

time, compressive movements tied to collision of the plate containing 

the Milford-Dedham zone with the North American plate resulted in 

east- to southeast-directed thrusting, uplift, erosion, and deposition of 

fluvial Pennsylvanian strata in a marginal basin. About this time 

deep-seated deformation under amphibolite-facies conditions along the 

margins of the zone produced orthogneisses in the Milford antiform and 

the New Bedford area and produced alteration and fracturing in the 

batholithic rocks in the center of the zone. Recent investigations in 

Rhode Island have led to the useful proposition that the MilfordDedham zone consists of two terranes of similar age but of dissimilar 

history. The gneissic terrane on the west appears to have underlain the 

Merrimack belt during the Acadian orogeny. The nongneissic terrane 

to the east, which contains features most like those on the Avalon 

peninsula of Newfoundland, was juxtaposed to the gneissic terrane 

later, in the Alleghanian. This proposition remains to be tested, 

however. Continued compression deformed the Pennsylvanian and 

older rocks in the basins. Renewed movement occurred along such 

preexisting zones of weakness as the Bloody Bluff and ClintonNewbury faults, during which the Newbury basins probably formed. 

Subsequent rapid uplift followed by east-west extension led to formation of the Triassic and Jurassic basins and widespread north-south 

faulting of basement and basin rocks. A period of subsidence resulted in 

Cretaceous overlap of coastal-plain deposits.

The Nashoba zone had a different history until the Pennsylvanian. 

The high-grade dynamothermal metamorphism of these rocks is penecontemporaneous with emplacement of the Silurian or Ordovician 

Andover Granite. Intrusive Silurian calc-alkaline granite and granodiorite, differing chemically from Paleozoic alkalic intrusives of the 

Milford-Dedham zone and from Ordovician to Devonian calc-alkaline 

and peralkaline intrusive rocks in the east flank of the Merrimack zone, 

were not metamorphosed. The rocks of the Nashoba zone project 

beneath the rocks of the Merrimack belt, as must those of the 

Milford-Dedham zone, and could be the basement for the strata of the 

Merrimack belt. The difference in nature of the plutonic rocks in the 

Nashoba and Milford-Dedham terranes indicates, however, that the 

two terranes were not connected until late in, or after, Devonian time. 

Both the origin and the placement of the Nashoba terrane are enigmatic. Features in the bounding fault zones suggest a long, complex

1The Peabody Granite is shown as Middle Devonian on the State bedrock map 

on the basis of Rb-Sr and K-Ar age determinations by Zartman and Marvin 

(1971). Refinement of isotopic age determinations since the map was compiled 

indicates an Early Devonian U-Pb age for the Peabody (Zartman and Marvin, this 

vol., chap. J, table 1).

history of movement in which pre-Devonian and post-Devonian ductile 

deformation preceded later brittle deformation. Evidence for Taconic 

or Acadian compressive deformation in the Milford-Dedham zone is 

lacking, but Acadian deformation is recognized on the east flank of the 

Merrimack belt and possibly the west flank of the Nashoba zone. The 

Ordovician dynamothermal event that affected the Nashoba zone is not 

necessarily in the same tectonic framework as the Taconic event of 

western Massachusetts. The original times of accretion of these terranes to each other and to North America is uncertain, but the present 

configuration of the terranes in eastern Massachusetts is a result 

largely of Alleghanian movements on which has been superimposed 

early to middle Mesozoic faulting.

INTRODUCTION

Eastern Massachusetts east of the Merrimack belt 

consists of two terranes2 of rock, the Milford-Dedham 

zone and that part of the Nashoba zone exposed at the 

surface east of the Merrimack belt (fig. 1). The term 

"Nashoba zone" is used in this restricted sense in this 

chapter. These terranes differ in stratigraphy and in 

metamorphic and plutonic history from each other and 

from rocks to the west (Goldsmith, this vol., chaps. E 

and F; Wones and Goldsmith, this vol., chap. I). The two 

terranes are bounded by major fault systems, and these 

fault systems also have differing metamorphic and tectonic histories. Thus, the terranes constitute distinct 

blocks of the Earth's crust. The Clinton-Newbury fault 

system on the west separates the Nashoba zone from the 

Merrimack belt. The Nashoba zone contains sedimentary 

and volcanic rocks of Proterozoic Z or possibly early 

Paleozoic age metamorphosed to high grade and intruded 

by Ordovician syn- or late-metamorphic S-type granite 

(Andover Granite) and by Silurian I-type postkinematic 

plutons of granodiorite and tonalite that are unique in 

composition in New England (Wones and Goldsmith, this 

vol., chap. I). The Bloody Bluff fault system separates 

the Nashoba zone from the Milford-Dedham zone to the 

east. The Milford-Dedham zone contains 590- to 630-Ma 

calc-alkaline granite batholiths that intrude medium- to 

low-grade metasedimentary, metavolcanic, and mafic 

plutonic and metaplutonic rocks of probable Proterozoic 

Z age. Overlying this basement, preserved in basins, are 

slightly metamorphosed to unmetamorphosed remnants 

of sedimentary and volcanic sequences of Proterozoic Z, 

Cambrian, Silurian and Devonian, and Pennsylvanian 

age. Plutons of alkalic granite of Ordovician and Devonian age and gabbro of Ordovician age intrude the

term "terrane" is used in a broad sense for an area of rock having a 

different lithostratigraphic, metamorphic, and deformational history from adjacent areas. It is equivalent to a zone or belt as these terms are used on the 

bedrock geologic map of Massachusetts (Zen and others, 1983) but is also used for 

a distinctive area within a larger unit; for instance, Putnam terrane in the 

Nashoba zone, ductilely deformed terrane in the Milford-Dedham zone. The 

rationale for use of the terms "zone" and "belt" on the State bedrock map is 

explained in Hatch and others (1984).
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H3

72° 70°

43° -

42°

Jewbury j&isan >7\7/. 

<Y Cape Arm1 <iVv 

V.'BostpH"-*B«ttffi 

basin '.

Lake Char faul^trc

! ' Narragansett." -'. ' ;: -' -. 

 . -.' '. .basin-'; ." ' .".' '/. '^_

±' J }' ' " ' ''. :-'-"-'-'--'.-/^'-'\- Fall River ] 

p:- :\-:--.-:::'-::':'i ^.y^^U^ block '

southeastern

iMassach usetts "1 ^

^iv^FaU River -batholith %

Covered

°s

%.

20

I

30 MILES

Cape Cod

&a&^-

Martha's 

Vineyard

30 KILOMETERS Nantucket ''

FIGURE 1. Index map and major structural features of eastern Massachusetts.

basement and the pre-Ordovician cover rocks. The eastern boundary of the Milford-Dedham zone may lie east of 

Cape Cod where a different terrane, possibly an extension of the Meguma terrane of Nova Scotia, is reflected in 

magnetic signatures (Klitgord, 1984).

The arrangement of this chapter is based largely on 

geographic distribution of the zones, which may reflect 

sequential docking onto the eastern edge of North America. Within each of the two terranes, the arrangement 

follows the relative age of recognizable tectonic events.
H4 THE BEDROCK GEOLOGY OF MASSACHUSETTS

A description of the structure and metamorphism of the 

Nashoba zone and of its western boundary, the ClintonNewbury fault system, begins the chapter. Because the 

oldest events in the Milford-Dedham zone are recorded in 

the Proterozoic basement rocks, the structure and metamorphism of each basement block within the MilfordDedham zone are described next, followed by the 

description of the overlying sedimentary basins in the 

zone and the nature of their boundaries. A description of 

the Bloody Bluff fault system concludes the section on 

the structure and metamorphism of the Milford-Dedham 

zone. A summary section attempts to place the tectonic 

events into a sequential development of accreted eastern 

North America, a concept implied on the State bedrock 

map (Zen and others, 1983).

The material described and discussed in this chapter is 

derived almost entirely from the work of others, and in 

only a few areas are the descriptions based on original 

observations of mine. The sources of data are many and 

include those listed on the State bedrock map as well as 

those referred to specifically in this chapter. The selection of structural features shown on the map was my 

responsibility. The selection process was aided by ideas 

resulting from discussions with colleagues involved in 

compilation of the State bedrock map and others involved 

in the geology of eastern Massachusetts, in particular 

Marland P. Billings, L. Peter Gromet, J.C. Hepburn, 0. 

Don Hermes, Richard S. Naylor, and James W. Skehan.

NASHOBA ZONE EAST OF THE MERRIMACK 

BELT

The Nashoba zone east of the Merrimack belt, between 

the Clinton-Newbury fault system on the west and the 

Bloody Bluff fault system and faults bounding the Newbury basins on the east (figs. 1, 2), is distinct from the 

Milford-Dedham zone to the east and the Merrimack belt 

to the west. The Nashoba zone consists of gneisses and 

schists of high metamorphic grade, primarily the Nashoba and Marlboro Formations, that are derived from 

sedimentary and volcanic protoliths (Goldsmith, this 

vol., chap. F), synkinematic plutonic rocks of Ordovician 

age, and distinctive postkinematic plutonic rocks of Silurian age (Zartman and Marvin, this vol., chap. J; Wones 

and Goldsmith, this vol., chap. I). The zone has a 

magnetic signature that contrasts with that of adjoining 

zones (Alvord and others, 1976; Castle and others, 1976; 

Harwood and Zietz, 1976). Foliation and layering dip 

steeply throughout the zone, and the rocks apparently 

have been folded and extensively cut and imbricated by 

faults, most of the traces of which are aligned parallel 

and subparallel to the trend of the rock units (Bell and

Alvord, 1976, fig. 4; Skehan and Abu-Moustafa, 1976). 

Metamorphism is at the upper limit of the amphibolite 

facies (sillimanite metamorphic zone) throughout most 

of the zone, dropping somewhat lower (andalusitestaurolite metamorphic zone) on the northwest flank. 

Intrusion of the Ordovician Andover Granite provides a 

younger (upper) limit for the age of the formations, but 

these formations may be as old as Proterozoic Z. Olszewski (1980) determined a U-Pb isotopic age of 730±26 Ma 

on zircon from the Fish Brook Gneiss. The main period of 

metamorphism is believed to have occurred close to the 

time of intrusion of the S-type Andover Granite. The 

complex internal folding and faulting within the zone 

make it difficult to determine the stratigraphic order of 

the units. The Nashoba and Marlboro Formations have 

been interpreted to be an upright homoclinal sequence 

facing west (Bell and Alvord, 1976; Skehan and AbuMoustafa, 1976; Skehan and Murray, 1980b); however, 

the map pattern of the lithologies (Bell and Alvord, 1976) 

and the magnetic pattern (Alvord and others, 1976) 

mentioned above suggest that this is not a homoclinal 

sequence. Some aspects of the folding and faulting within 

the Nashoba zone have been discussed in chapter F of 

this volume and will only be summarized here.

INTERNAL STRUCTURE AND METAMORPHISM

FOLDS AND MINOR STRUCTURAL FEATURES

Major folds in the Nashoba zone have been identified in 

only a few places, whereas most mappers have recognized widespread small-scale folding. Hansen (1956) 

mapped major folds on the west flank of the Nashoba 

zone and indicated an overall synformal structure for the 

Nashoba-Marlboro sequence; however, the west flank of 

this synform has been truncated, eliminating the Marlboro Formation. If the Reubens Hill Formation is equivalent to the Marlboro Formation, small remnants of the 

truncated west limb are preserved in small fault-bounded 

blocks in the Merrimack belt. Skehan and Abu-Moustafa 

(1976, fig. 1) showed a somewhat schematic set of two 

synforms and a central antiform in their cross section of 

the Wachusett-Marlborough tunnel, which extends from 

the Wachusett Reservoir, southwest of Bolton, to Marlborough. Foliation symbols on the maps of the Billerica 

and Westford quadrangles (Alvord, 1975) indicate zones 

of alternating steep east and west dips. As this foliation 

is parallel to compositional layering that represents 

modified bedding, one might deduce that the original 

bedding has been flattened into tight folds about steep 

axial surfaces. Such folds are suggested by the map 

pattern, by repetition of lithologic units, and by the 

aeromagnetic pattern (Alvord and others, 1976; Castle 

and others, 1976; Harwood and Zietz, 1976; Barosh,
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H5

1977), but they cannot be unequivocally mapped. Hinges 

of such folds would be difficult to identify in this highly 

deformed terrane. Traces of Hansen's large-scale folds 

are parallel to the general northeast trend of the gneissosity and schistosity in the zone. A synform at Rattlesnake Hill in Bolton (fig. 2) mapped by Hansen (1956) 

plunges gently northeastward and has a wavelength of 

about 0.8 km. Another synformal fold of his near Boxborough also plunges northeast, but steeply.

Most minor folds in the Nashoba Formation fold both 

bedding and a parallel foliation. Folds of layering, to 

which the foliation is axial planar, have not been 

recorded. Younger minor folds have locally been superposed on the earlier generation. Axial planes of the older 

folds described by Hansen (1956, p. 52) strike parallel to 

the trend of the rock units, dip to the west, and plunge 

moderately to the north, northeast, and southwest. 

Hansen has deduced from lengths of fold limbs that the 

major structure is synformal. Skehan and Abu-Moustafa 

(1976) described the same sense of folding in minor folds 

in their tunnel section. Locally in the tunnel, axial planes 

have been rotated into the horizontal. Many of the 

small-scale folds described by Skehan and Abu-Moustafa 

are related to faulting. On the southeast side of the 

Nashoba zone in the Marlboro Formation, Hansen (1956) 

observed that the minor folds trend obliquely to the 

trend of the units. Minor structures have a similar trend 

in the Marlborough area to the south (Hepburn and 

DiNitto, 1978). These minor folds range in size from 

minute wrinkles related to slip cleavage to broad undulations as much as a meter in wavelength. Their axial 

surfaces dip west or southwest, and their axes plunge 

steeply north to northwest. This geometry suggests 

either a significant lateral component of movement or 

rotation into the plane of a major thrust zone. Many folds 

here are broken or displaced by small thrust faults. 

Skehan and Abu-Moustafa confirmed these observations 

in the east end of the tunnel section near the MarlboroNashoba contact. These relationships suggest a structural discordance between the Nashoba and Marlboro 

Formations. Indeed, Hepburn and DiNitto (1978) 

mapped a fault along this contact. Hansen suggested that 

the Marlboro underwent a second period of folding, 

which is obscured in the Nashoba Formation by a difference in competency. Hansen (1956, p. 55) suggested, on 

the basis of orientation of superposed obliquely trending 

minor folds, that the Nashoba moved northeastward 

relative to the flanking formations. He noted (p. 57-58) 

that, at a few localities on the west side of the Nashoba 

zone, flow cleavage and schistosity in his mica schist 

facies of the Worcester Formation, now the Tadmuck 

Brook Schist, are axial planar to small isoclinal fold 

hinges in relatively competent quartzite layers.

In summary, the rocks of the Nashoba zone may be 

tightly folded on a large scale, but such folds have not 

been recognized in the map pattern of rock units. Vergence of the early minor folds is generally to the east and 

southeast, but the orientation of later folds, particularly 

on the eastern side of the zone, indicates subsequent 

lateral transport in a northeast to east direction.

The primary schistosity predates the emplacement 

of the Silurian Sharpners Pond Diorite and related 

plutons and also predates, but possibly not by much, 

the emplacement of the Ordovician Andover Granite. 

Hansen (1956, p. 55) observed that most of the major and 

minor folds were formed before feldspathization (that is, 

development of granitic and pegmatitic stringers and 

lenses, or partial melting). Most of these stringers and 

lenses lie in the plane of the foliation and are presumed to 

have formed while the regional stress field that produced 

the primary foliation in the Nashoba Formation still 

prevailed. This I interpret to mean that the peak of 

deformation preceded the thermal peak of metamorphism at sillimanite grade. The thermal peak was presumably close to the period of generation of the Andover 

Granite. The only deformation later than emplacement of 

the Silurian Sharpners Pond Diorite and the Ordovician 

Andover Granite appears to be related to faulting during 

movement on the regional Bloody Bluff and ClintonNewbury systems. In this regard, it is worthwhile to 

point out here that the Ordovician Andover Granite is 

considered to have been emplaced as a wet granite at 

considerable depth, whereas the younger, Silurian plutonic rocks such as the Sharpners Pond Diorite that also 

intrude the rocks of the Nashoba block are dry and 

presumably were intruded at a higher level in the crust 

(Wones and Goldsmith, this vol., chap. I).

METAMORPHISM

The Nashoba zone lies in a narrow north-south belt of 

high-grade metamorphism in southern New England 

(Thompson and Norton, 1968). Pelitic rocks of the 

Nashoba Formation, the Shawsheen Gneiss, and the 

eastern part of the Tadmuck Brook Schist contain 

sillimanite-muscovite and sillimanite-orthoclase mineral 

assemblages characteristic of the upper amphibolite 

facies. Pelitic rocks of the Marlboro Formation and the 

western part of the Tadmuck Brook Schist have assemblages containing staurolite and andalusite. These 

assemblages are truncated abruptly on the east by the 

Bloody Bluff fault and on the west by the ClintonNewbury fault. Formations to the west, in the eastern 

part of the Merrimack belt, lie in a trough (Nashua 

Trough) and contain garnet-, biotite-, and chlorite-zone 

assemblages. To the east, the metamorphic rocks of the 

Milford-Dedham zone are low to middle amphibolite
H6 THE BEDROCK GEOLOGY OF MASSACHUSETTS
42°00'

41°30'

Milford-Dedham zone Devonian to Proterozoic Z 

crystalline rocks

Milford-Dedham zone Cretaceous to Proterozoic Z 

sedimentary and volcanic rocks

Nashoba zone Silurian to Ordovician, 

Cambrian, or Proterozoic Z rocks

Merrimack synclinorium Pennsylvanian 

to Ordovician rocks

Contact

 yt<y. VAttleboro'. . .'.-->^-. ..' " . ....... ; . . '. './ '.£>?m^j*?. '. ' ^/?. .' ' '' '' '.' ' '. Assawornpset Pond ^^^ ^^ \ ' ' '.V^' ' ": ' ' .' '.''.'.'.' '. .'.' -graben. 

..^.;>7<V^' '. .' ' Narragah'settb'asin .'.-'.;.';.'-'\S

'^' \f:'-'\'^ 

 ^ //::--/::^V-:^:V:V^-^:"^---^ ^ ' -

r>w^.'.:^i AW U A .'.. .. '' * ! '?~\ ....... . ' ; . Providence . '.

» \

-g --- Fault Dashed where inferred; dotted where 

concealed; queried where uncertain. For Wekepeke fault only: U, upthrown side; D, downthrown side -1 ' Reverse fault Ticks on upthrown side 

Thrust fault Teeth on upthrown side

-> Anticline Dashed where inferred. Plunge 

indicated where known

-> Syncline Dashed where inferred. Plunge 

indicated where known

-^ Antiform Dashed where inferred. Shows 

plunge

-^ Synform Dashed where inferred. Shows 

plunge

- - Approximate location of shear zone of

O'Hara and Gromet (1984) 

Mylonite zone

20 MILES I

10 20 KILOMETEBS. - \ .I---'.. : ¥. ": :': ': A 'M*«~ix " Martha's Vineyard Coastal Plain cover

oH

a

O

a

O

a

3

CO

FIGURE 2. Structural features of eastern Massachusetts. Numbers refer to features or places mentioned in text. E, Esmond Granite.
H8 THE BEDROCK GEOLOGY OF MASSACHUSETTS

fades near the boundary and are greenschist fades 

farther east. The Silurian and Devonian Newbury Volcanic Complex is at subgreenschist facies, judging from 

the descriptions by Shride (1976b).

Abu-Moustafa and Skehan (1976) gave the most thorough description of the metamorphism in the Nashoba 

zone in their study of the Wachusett-Marlborough tunnel. They described pelitic assemblages containing 

sillimanite+orthoclase and sillimanite+muscovite. The 

orthoclase is commonly porphyroblastic. Garnet and clinozoisite are common. They placed the assemblages in 

the sillimamte-almandine-orthoclase subfacies of the 

almandine-amphibolite facies. The mineral associations 

suggested to them that the rocks of the Nashoba zone 

recrystallized at about 625-650 °C and at about 6 kbar, 

indicating a depth of cover of more than 23 km, assuming 

Pload=PH2o- Thompson and Norton (1968) placed the 

Nashoba zone east of and on the lower pressure side of 

the isobaric triple-point line for New England, on the 

basis of the assemblages. No one knows how much of the 

muscovite present is prograde dynamothermal and how 

much is later static hydrothermal. Examination of the 

rocks in some places suggests that some muscovite has 

replaced aluminum silicate. Presumably some muscovite 

was crystallized during late emplacement of the relatively hydrous Andover Granite (Castle, 1965a). Olszewski (1980) has shown from Rb-Sr data that a massive 

resetting of the Rb-Sr systems and a widespread fluid 

activity occurred about 450 Ma, about the time of 

emplacement of the Andover Granite (Handford, 1965); 

this then was apparently close to the peak of thermal 

metamorphism of the rocks. Emplacement of the Silurian 

Sharpners Pond Diorite and related rocks apparently 

had little metamorphic effect. Olszewski found no evidence for an Acadian metamorphic event, but he did 

recognize in his isotopic work the Carboniferous and 

Permian thermal event so widespread in southern New 

England (Zartman and others, 1970). The metamorphism 

we see in the Nashoba zone is, in part at least, penecontemporaneous with an early Paleozoic thermal-plutonic 

event. We do not know if an earlier (Proterozoic?), lower 

grade metamorphism was overprinted by the early Paleozoic metamorphism. The truncation of isograds by the 

bounding faults on the flanks of the zone indicates that 

the block was metamorphosed before moving to its 

present position.

FAULTS

Faulting within the zone has been fairly well documented and also widely inferred. A complex fracture 

pattern is shown on the bedrock geologic map of the 

Boston I°x2° quadrangle by Barosh and others (1977), a 

pattern that probably includes joints, faults, and other

lineaments (Barosh and others, 1974; Alvord and others, 

1976). Most of these features are not shown on the State 

bedrock map of Zen and others (1983), on which only 

faults having significant displacement are shown. Careful study by Skehan (1968) and Skehan and AbuMoustafa (1976) of the many major and minor faults 

cutting the Nashoba and Marlboro Formations in the 

Wachusett-Marlborough tunnel showed that faulting in 

the zone is indeed complex. Faults are most abundant on 

the flanks of the zone in the vicinity of the ClintonNewbury and Bloody Bluff faults, but some that Skehan 

and Abu-Moustafa considered significant are located in 

the tunnel section between the bounding faults. Most 

faults described by them are reverse and thrust faults 

having a sense of movement to the east. A few are highor low-angle normal faults. They recognized different 

ages of faults in the tunnel section: some control emplacement of pegmatite and granite; others cut and displace 

these masses. Some faults are characterized by mylonite 

and ultramylonite, and these, in places, are cut by later 

faults. Later faults are usually characterized by brecciation, alteration, silicification, and zones of gouge. The 

Marlboro Formation is particularly broken by faults of 

both types, and the number of faults increases toward 

the Bloody Bluff fault on the east side of the Marlboro. 

The faults generally dip to the west, and movement 

sense is generally to the east and southeast, in the same 

general direction as the vergence of the minor folds. A 

few faults show a vertical sense of movement, and one 

fault in the lower part of the Nashoba, unit N9 of Skehan 

and Abu-Moustafa (1976), has a right-lateral sense of 

movement. Basalt dikes in the lower part of the Marlboro 

noted by Skehan and Abu-Moustafa cut granite and 

minor folds and in places appear to be controlled by 

preexisting faults and fractures.

The faults observed by Skehan and Abu-Moustafa in 

the Wachusett-Marlborough tunnel have not been 

directly related to faults mapped on the surface in the 

Nashoba zone by Alvord (1975), Barosh (1976,1978), and 

Hepburn and DiNitto (1978) nor to faults deduced from 

the aeromagnetic pattern by Alvord and others (1976) 

and Castle and others (1976). Alvord and others (1976) 

showed unnamed thrust faults at the top and bottom of 

the Fort Pond Member of the Nashoba Formation; the 

upper thrust would pass through the middle of the 

Nashoba in the tunnel section. Alvord and others showed 

this fault as a continuation of a northeast-trending fault 

near Shrewsbury (fig. 2) that splays off the ClintonNewbury fault zone and truncates the upper part of the 

Nashoba. Castle and others (1976) showed the northeasttrending fault but did not extend it northward. Instead, 

they continued it to the northeast to end near Marlborough. I have shown the interpretation of Castle and 

others on the State bedrock map. This fault could coin-
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H9

cide with a fault zone that lies between units N18 and 

N17 of Skehan and Abu-Moustafa (1976), which they 

suggested may be regionally important.

SPENCER BROOK AND ASSABET RIVER FAULTS

The Spencer Brook and Assabet River faults (Alvord 

and others, 1976; Bell and Alvord, 1976) (fig. 2) are 

shown on the State bedrock map because as mapped the 

faults truncate the Shawsheen Gneiss and Fish Brook 

Gneiss. Castle and others (1976) did not show the Spencer Brook fault but did show a fault coinciding with the 

Assabet River fault. Skehan and Abu-Moustafa (1976) 

did not identify the Spencer Brook and Assabet River 

faults of Alvord and others as such in the tunnel section. 

North of the Concord area, I have shown the Spencer 

Brook fault as curving to the north following a continuous 

foliation pattern in the Andover Granite shown on Castle's (1964) maps of the Wilmington and Salem Depot 

quadrangles and connecting two areas of sheared rock 

shown by Castle and others (1976, pi. 1). This interpreted fault intersects the Clinton-Newbury fault south 

of Lawrence. I have shown the Assabet River fault as 

similarly following the curvilinear contact of the east 

edge of the main mass of Andover Granite. Castle (1964) 

showed no features crossing the foliation pattern that 

might suggest that the faults trend east-northeasterly to 

the Newbury area, as is shown by Barosh and others

(1977), to meet the east-trending faults forming the 

boundaries of the northern part of the Newbury basins 

(fig. 2). Nor do any features on the aeromagnetic maps 

indicate that such a connection exists.

The continuation of the Spencer Brook and Assabet 

River faults south of the Maryborough area is not clear. 

Barosh (1977) showed the faults coming together in an 

anastomosing pattern west of Marlborough near the 

intersection of 1-290 and 1-495 and continuing as one 

fault south to the Bloody Bluff-Lake Char fault zone. 

Castle and others (1976) showed an inferred fault coinciding with the Assabet River fault and merging south of 

Shrewsbury into an inferred fault near the top of the 

Sandy Pond Amphibolite Member of the Marlboro Formation, a fault not shown by Alvord and others (1976). 

Skehan and Abu-Moustafa (1976) showed a fault zone of 

probable significant displacement west of the contact of 

the Nashoba and the Marlboro, which may be the junction of the Spencer Brook and Assabet River faults. The 

mapping of Hepburn and DiNitto (1978) and Hepburn

(1978) suggests that the Assabet River fault in the 

Marlborough and Shrewsbury areas cuts downward 

below the basal schist of the Nashoba and truncates 

higher units of the Marlboro. The Tatnic fault (Dixon and 

Lundgren, 1968, p. 227; Wintsch and Hudson, 1978) in 

eastern Connecticut between the Quinebaug and the

Tatnic Hill Formations occupies a similar position in 

places between the basal rusty schist of the Tatnic Hill 

and the Quinebaug. Dixon (written commun., 1979), 

however, recognized no fault between the rusty schist of 

the Nashoba and the Marlboro in the Grafton quadrangle 

in her reconnaissance. Accordingly, on the State bedrock 

map I have shown the continuation of the Assabet River 

fault (1, fig. 2) as lying above the rusty schist of the 

Nashoba rather than below it. However, exposures are 

insufficient to locate the fault at either stratigraphic 

level, if a fault exists. Possibly it cuts down below the 

Nashoba as suggested by the mapping of Hepburn and 

DiNitto (1978) and cuts down to the Bloody Bluff-Lake 

Char fault as shown by Barosh (1977). If it does not do so 

south of Marlborough, it certainly must do so to the south 

near Oxford. Without detailed mapping in the Grafton 

and Worcester South quadrangles, the location of the 

southern continuation of the Spencer Pond and Assabet 

River faults can only be speculated upon.

INCLINATION OF AND MOVEMENT ON FAULT SURFACES

Little information is available concerning the amount 

of dip of the fault surfaces within the Nashoba zone. The 

inferred faults of Castle and others (1976) dip 60°-70° to 

the west, as computed from aeromagnetic data. This dip 

coincides with the dips of many of the younger faults in 

the Wachusett-Marlborough tunnel section. Some of the 

apparently older faults characterized by mylonite dip 

30°-^45° west. Observations in the tunnel are not sufficient 

to say which faults control the map pattern of the units. 

Several ages of faulting have clearly been superposed.

Barosh (1977) showed numerous faults in the Nashoba 

Formation in the area between Marlborough and Oxford 

and referred to the whole zone as the Nashoba thrust 

belt. The numerous faults observed in the WachusettMarlborough tunnel indeed show the zone as highly 

faulted. However, most of the faults in the tunnel appear 

to have insignificant displacement and accordingly are 

not shown on the State bedrock map. Major displacement 

is confined primarily to the faults at and near the flanks 

of the zone. Discussion of the stratigraphy in the zone 

(Goldsmith, this vol., chap. F) led to an interpretation 

that the lithologic units within the block are imbricated 

to some extent on such faults as the Spencer Brook and 

Assabet River faults. Movement on most faults, as 

described above, is east directed, west side up, and right 

lateral, at least at a late stage.

STRUCTURAL POSITION OF THE NASHOBA ZONE

The Nashoba zone (figs. 2,3) forms a lens-shaped block 

thinning to both north and south and tapering down dip 

to the west. If the trends of the two major fault zones
H10 THE BEDROCK GEOLOGY OF MASSACHUSETTS

43°00' 72°30' 72W 71°00' 70°30'

EXPLANATION

Cover rocks of the Milford-Dedham zone 

(Mesozoic to Proterozoic Z)

Rocks of the Merrimack synclinorium (Pennsylvanian to Silurian and Ordovician

Rocks of the Nashoba zone, Rye anticline, 

Tatnic Hill terrane, and Massabesic 

terrane (middle lower Paleozoic and 

Proterozoic(?))

Crystalline rocks of the Milford-Dedham 

zone (Paleozoic and Proterozoic Z)

Contact Dashed where inferred

Fault Dashed where inferred; dotted where

concealed; queried where uncertain 

Anticline or antiform Shows plunge where

known 

Syncline or synform

 Approximate location of shear zone of 

O'Hara and Gromet (1984)

Geologic features

Chester syncline

Unnamed fault between Nashua trough

and Rockingham anticlinorium 

Hunts Brook syncline 

Portsmouth fault 

Preston Gabbro

Science Park block 

Willimantic dome

42°00' -

41°30' -

0 10 20 MILES

 

0 10 20 KILOMETERS

FIGURE 3. Major structural features of southeastern New England.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS Hll

remain the same, the Nashoba zone lenses out somewhere in the Gulf of Maine. The Nashoba zone narrows 

considerably in southern Massachusetts near Webster 

but widens in outcrop width in Connecticut where it is 

represented by the Putnam terrane. The Nashoba Formation and the Fish Brook and Shawsheen Gneisses 

decrease in thickness southward from 8,765 m (I arbitrarily use one-half Bell and Alvord's (1976) thickness 

because of suspicion of repetition by faulting and folding) 

in the Westford-Billerica area to 3,740 m in the Wachusett tunnel section and to 1,701 m in the Tatnic Hill 

Formation in Connecticut (Dixon, 1965a; see Goldsmith, 

this vol., chap. F). Eastward along the Honey Hill fault, 

in southeastern Connecticut, the Tatnic Hill thins even 

more. Near Chester, Conn., less than 155 m of Tatnic 

Hill is present (Lundgren, 1963). The Marlboro Formation and the equivalent Quinebaug Formation in Connecticut are fairly uniform in thickness from north to 

south: 2,140, 2,702, and 2,215 m (Bell and Alvord's 

section has not been halved in these measurements 

because there appears to be no internal duplication of 

units). In southeastern Connecticut, the Quinebaug thins 

and is eventually cut out by eastward convergence of the 

Tatnic and Honey Hill faults (Rodgers, 1982). The Tatnic 

Hill Formation thins between the Tatnic fault and a fault 

at a still higher level, possibly the inferred ClintonNewbury, at the base of the Canterbury Gneiss, so that 

only the Yantic Member of the Tatnic Hill Formation can 

still be recognized in the vicinity of the Chester syncline 

(C, fig. 3) (Wintsch, 1979) in the keel of the Merrimack 

synclinorium. However, rocks lithologically similar to 

parts of the Tatnic Hill can be recognized in the Chester 

and Hunts Brook synclines (H, fig. 3) in the Proterozoic 

Z terrane of southeastern Connecticut (Lundgren, 1967; 

Goldsmith, 1967a,b). The Proterozoic Z basement is 

exposed (Snyder, 1964) in the Willimantic window (W, 

fig. 3) in central-eastern Connecticut beneath the Willimantic fault, which is equivalent to the Honey Hill fault 

(Wintsch, 1979). Here the Quinebaug equivalent is very 

much thinned and lies between the Willimantic-Honey 

Hill fault and a fault that is either the Tatnic fault or a 

higher level fault.

These observations indicate that the Nashoba zone is a 

downward-thinning wedge between the Milford-Dedham 

zone and the Merrimack belt. The shallowness of the 

discontinuity beneath the Nashoba zone is indicated by 

the trace of the Bloody Bluff, Lake Char, and Honey Hill 

faults in southeastern Connecticut and the presence of 

Milford-Dedham basement in the Willimantic window. 

As the Merrimack belt plunges north from southern 

Connecticut, sections across the Nashoba zone and Putnam terrane can only show the two terranes by projection as shallow westward-dipping wedges (cross section 

F-F' on the State bedrock map) beneath the rocks of the

Merrimack belt. The Science Park block of Nashoba 

(figs. 2, 4) mapped by Hepburn (1978) near Worcester is 

apparently an upthrust sliver of the Nashoba wedge 

caught in the Clinton-Newbury fault zone. The depth to 

the Nashoba zone or its equivalent beneath the Paxton 

Formation to the west is highly speculative (cross sections D-D' and F-F' on the State bedrock map). The 

Massabesic Gneiss Complex may be a larger upthrust 

slice of Nashoba-zone rocks than the Science Park block. 

The argument for low dips for the Clinton-Newbury and 

Bloody Bluff faults at depth, which is based on observed 

distribution of Proterozoic Z basement in Massachusetts 

and Connecticut, is in opposition to the argument for 

appreciable transcurrent movement for the faults postulated by Zen and Palmer (1981) and Zen (1983). It is 

possible that transcurrent movement has occurred, an 

idea more easily accommodated in the steeply dipping 

segment north of Ayer and perhaps the Rattlesnake Hill 

fault than to the south. Any transcurrent movement 

must be late. We do not yet have a clear picture of the 

three-dimensional disposition of the major blocks of rock 

in southeastern New England.

CLINTON-NEWBURY FAULT

DESCRIPTION

The Clinton-Newbury fault (fig. 2) forms the boundary 

between the Nashoba zone and the Merrimack belt. 

Northeast of Ayer, it truncates units of both the Merrimack belt and the Nashoba zone, but south of Ayer its 

trace coincides with the trend of units in both terranes, 

and it lies below and locally within what are apparently 

the lowermost stratigraphic units of the Merrimack belt. 

The fault forms the southern boundary of the Newburyport Complex in the Newburyport area, and the Ayer 

Granite lies west of the main trace of the fault throughout its extent.

Segments of the Clinton-Newbury fault were identified as early as 1880 (Crosby, 1880, p. 95-96; Clapp, 1921, 

pi. 1). Other workers suspected but could not identify a 

convincing break between the less metamorphosed rocks 

of the Merrimack belt on the west and the more metamorphosed rocks of the Nashoba zone on the east (Emerson, 1917, p. 77-78; Hansen, 1956, p. 20). The fault was 

first recognized as a major dislocation by Castle (1964; 

1965a,b) in the Lawrence-Groveland area (fig. 2) but was 

named by Skehan (1968, p. 281-283; Skehan and Murray, 

1980b) for the northeast-trending fault zone extending 

from the Wachusett Reservoir at Clinton, Mass., northeast to the vicinity of Newbury and Newburyport. Castle 

and others (1976) reviewed the history of identification of 

the zone and named the fault the Essex fault; however,
H12 THE BEDROCK GEOLOGY OF MASSACHUSETTS
Dl

Dl

EXPLANATION

OZn

/

OZn

Coal Mine Brook Formation (Middle

Pennsylvanian) 

Fitchburg Complex (Dfgr) and muscovite-biotite granite (Dmgr)

(Lower Devonian)

Littleton Formation (Dl) and 

marble in the Littleton Formation (Dim) (Lower Devonian)

Worcester Formation (DSw) and 

marble in the Worcester Formation (DSwm) (Lower Devonian 

and Silurian)

Ayer Granite (Lower Silurian and 

Upper Ordovician?)

Oakdale Formation (Silurian)

Tower Hill Quartette (Si) and 

phyllite associated with the Tower Hill Quartette (Sis) 

(Silurian)

Paxton Formation (Silurian)

Reubens Hill Formation (Silurian or

Ordovician) 

Vaughn Hills Formation (Silurian or

Ordovician

Boylston Schist (Silurian or

Ordovician) 

Tadmuck Brook Schist (Silurian?,

Ordovician, br Proterozoic Z)

Nashoba Formation (Ordovician 

or Proterozoic Z)

Muscovite granite (age uncertain)

Contact Dotted where concealed

Fault Dashed where inferred, 

dotted where concealed, queried where uncertain. U, upthrown block; D, downthrown 

block

Thrust fault Teeth on upper plate

Antiform Shows plunge

Synform

Strike and dip of bedding Ball 

indicates top of bedding is known from sedimentary structures 

Inclined 

Overturned

Strike and dip of foliation and 

compositional layering

3 KILOMETERS

IdQ

O 

» 

TJ

O 

»«JO1

o

w d

COCO

FIGURE 4. Geology of the Wachusett Reservoir area. Numbers refer to features mentioned in text. Adapted from Grew (1970), Peck (1975), Hepburn (1976, 1978), and the State

bedrock map (Zen and others, 1983).

ffi

h-' 

00
H14 THE BEDROCK GEOLOGY OF MASSACHUSETTS

by this time the term "Clinton-Newbury" had come into 

common use.

The Clinton-Newbury fault has fairly clear linear 

expression between Newburyport and Lowell where it 

consists of a single strand (Castle, 1964; Shride, 1976a). 

To the south, however, it is depicted as an anastomosing 

system of faults (Alvord, 1975; Peck, 1975; Gore, 1976b). 

The trace of the Clinton-Newbury fault on the branch 

southeast of the Tadmuck Brook Schist, southeast of 

Ayer, is based in large part on truncation of aeromagnetic pattern and lithic units in the Nashoba Formation 

along this line. Alternatively this truncation has been 

suggested to be an unconformity beneath the Tadmuck 

Brook Schist (Bell and Alvord, 1976).

Skehan (1968) identified a group of fault zones at the 

west end of the Wachusett-Marlborough tunnel in the 

Clinton area, which he later included in the ClintonNewbury fault zone. These are the Rattlesnake Hill fault 

zone, the Boylston fault zone, the Clinton fault zone, and 

the Wachusett Reservoir fault zone. Between the Rattlesnake Hill fault zone and the Boylston fault zone is the 

Science Park block, an apparently upthrust mass of 

Nashoba Formation (the Science Park unit of Hepburn, 

1976) and muscovite granite forming a lens that extends 

as far south as the Worcester area. South of Worcester 

the main trace is not clearly identified and is somewhat 

arbitrarily shown as separating the Nashoba Formation 

from the Boylston Schist of the Merrimack belt.

Castle and others (1976) argued that the ClintonNewbury fault is misnamed and should be called the 

Essex fault for its clear expression in Essex County 

where it dips steeply, because the Clinton fault, as 

identified, is a thrust. They felt that the main trace of the 

Essex fault is the steep Rattlesnake Hill fault of Skehan 

that bounds the allochthonous block of Nashoba near 

Shrewsbury on the east side (fig. 4) rather than the more 

gently dipping Boylston and Clinton faults that lie west 

of this block. However, the Rattlesnake Hill fault has not 

been traced as such south of Shrewsbury, and, as mentioned above, the name "Clinton-Newbury" is now well 

established in the literature for the whole zone. The 

Clinton fault of Skehan (1968) is not necessarily the main 

trace of the Clinton-Newbury zone even though it forms 

the lower limit of the masses of Ayer Granite. No Ayer 

Granite is presently identified in the Nashoba zone, and 

the Ayer appears to be entirely confined to the rocks of 

the Merrimack belt. The Tadmuck Brook Schist, the 

Vaughn Hills Quartzite, the Boylston Schist, and the 

Tower Hill Quartzite of the Merrimack belt lie west of 

the Rattlesnake Hill fault. Skehan and Murray (1980b, p. 

288-289) showed these units as part of the Nashoba zone. 

Hepburn (1978), however, showed a fault bounding these 

units on the east side (3, fig. 4) as part of the ClintonNewbury fault zone. This fault is shown on the State 

bedrock map.

The main trace of the Clinton-Newbury fault as 

defined above dips steeply in its northern part. A.F. 

Shride (oral commun., 1979) stated that the dip is steep 

in the Newburyport area, where the Clinton-Newbury 

coincides with Shride's (1971) Scotland Road fault. Mylonites close to the fault west of Groveland dip 60° to the 

west (Castle and others, 1976). Near Harvard the fault 

dips steeply and is characterized by zones of mylonite 

and mylonite gneiss, most conspicuous in Ayer Granite. 

Near Harvard the fault splits into two strands in the 

Tadmuck Brook Schist and forms a lens containing Ayer 

Granite and Harvard Conglomerate at Pin Hill in Harvard. Dips are steep in sheared Ayer Granite on both the 

west side of Pin Hill (Barosh, 1976, p. 311; Gore, 1976a, 

p. 106, and written commun., 1978) and the east side 

(Thompson and Robinson, 1976, p. 348). Farther south, 

the Rattlesnake Hill fault of Skehan (1968) dips 60°-65° to 

the west and was shown by Skehan and Abu-Moustafa 

(1976) as a reverse fault. The Clinton and Boylston faults 

to the west and those in the Carville basin of the 

Wachusett Reservoir have dips of 30°-^5° (Skehan, 

1968). Sense of movement on these faults is northwest 

over southeast. In the Wachusett-Marlborough tunnel 

section, steeper faults, such as the Rattlesnake Hill fault, 

locally fold and truncate the more shallowly dipping 

faults (Skehan and Murray, 1980b). The Boylston fault, 

however, according to Skehan and Abu-Moustafa (1976), 

is characterized by hydrothermal alteration, the presence of pyrite, and development of chlorite. This association suggests that it is a younger, shallower fault than 

the fault associated with ductile mylonites observed in 

the Harvard area to the north. G.R. Robinson, Jr. 

(written commun., 1984), observed, in the steeply dipping part of the Clinton-Newbury fault near Ayer, 

evidence for an earlier right-lateral displacement followed by left-lateral displacement.

The location of the principal strand of the ClintonNewbury fault system south of Clinton is arguable. The 

structural position of the block of rock (fig. 4) containing 

Hepburn's (1976) Science Park unit and the Rattlesnake 

Hill pluton that lies between the Rattlesnake Hill and the 

Clinton and Boylston faults is obscure (see Castle and 

others, 1976). I have interpreted the Science Park block 

as a slice of Nashoba Formation thrust up eastward along 

the Rattlesnake Hill fault to override a thin sequence of 

units that may lie unconformably on the Nashoba (Skehan and Murray, 1980b). To have been brought up from 

the west, the rocks of the Nashoba zone must extend at 

shallow depth an appreciable distance westward under 

the east flank of the Merrimack belt. Such a configuration is shown on cross section F-F' of the State bedrock 

map.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS HIS

The location of the Rattlesnake Hill fault south of 

Shrewsbury is uncertain. Hepburn's (1978) fault (3, fig. 

4) that diverges from the southeast margin of the Rattlesnake Hill pluton bounds a narrow wedge of Tadmuck 

Brook Schist and Vaughn Hills Quartzite (Skehan and 

Abu-Moustafa's (1976) units Ul through U10). Whether 

this fault rejoins the Rattlesnake Hill fault is not known. 

South of Shrewsbury, Castle and others (1976) showed 

the Rattlesnake Hill fault as offset to the west along a 

northeast-trending lineament and inferred fault (1, fig. 

4), coinciding with an abrupt cessation of north-trending 

magnetic anomalies. South of this offset they showed the 

Essex (Rattlesnake Hill) fault as continuing within the 

Nashoba Formation (4, fig. 4) to join the trace of the 

Clinton-Newbury near Oxford. They also raised the 

possibility that the fault trends directly south to join the 

Lake Char fault east of Oxford.

On the State bedrock map, I have extended the Rattlesnake Hill fault southwestward to the northeasttrending lineament (1, fig. 4), which incidentally is shown 

fairly well by a discordance in structural data on Hepburn's Shrewsbury map (unpub. data, 1979), and thence 

offset it along the lineament so that it eventually joins, 

near Lake Quinsigamond, the faults lying west of the 

Science Park block (fig. 4). On figure 4,1 have shown the 

fault as offset again to the north along the Pine Hill fault 

at Lake Quinsigamond. South of Worcester I have placed 

the main trace of the Clinton-Newbury fault at the 

boundary between the Boylston Schist and the Nashoba 

Formation. This location coincides with an unnamed fault 

of Castle and others (1976), lying between their Essex 

(Rattlesnake Hill) fault and their westernmost fault, the 

Clinton. In the Oxford-Webster area, I have placed the 

main trace at the base of the Ayer Granite (Sagr) 

following Barosh (1978) and Dixon (unpub. data, 1978). I 

have interpreted Castle and others' (1976) Clinton fault 

south of Worcester as a continuation of the high-angle 

Wekepeke fault (Peck, 1975) because it bounds the 

low-grade Worcester Formation on the west as the 

Wekepeke does farther north. The different generations 

of faults in the Oxford-Webster area have not been 

identified to my satisfaction. Somewhere in this area the 

north-south Flint Hill fault system (Rodgers, 1970; Skehan and Murray, 1980b, p. 294), characterized by brecciation, silicification, and hydrothermal alteration of 

probable Triassic and Jurassic age, of which the Wekepeke fault (Pine Hill fault of Castle and others) is a part, 

must connect with the similar fault system typified by 

the Lantern Hill fault in southeastern Connecticut (Goldsmith, 1985) and to the north-trending silicified and 

brecciated rocks that roughly coincide with the Lake 

Char ductile fault zone in eastern Connecticut (Dixon, 

1965b, 1968). The trace of the Clinton-Newbury fault 

southeast of Webster is speculative. Possibly it curves

southwestward to lie at the base of the Canterbury 

Gneiss (Ayer equivalent) in eastern Connecticut, as 

shown by Pease (1982, fig. 1). There it merges with the 

Honey Hill fault zone south of Colchester, Conn. In 

support of this idea, the base of the Canterbury Gneiss 

exposed along Connecticut Route 11 west of Salem Four 

Corners, Conn., has a cataclastic foliation dipping about 

20° to the northwest. On the other hand, Castle and 

others (1976) projected the Clinton-Newbury fault 

(Essex and Clinton faults) into faults within the Tatnic 

Hill Formation shown by Dixon (1974, 1982) in northeast 

Connecticut. The west-dipping faults like the Clinton of 

Skehan (1968) must be truncated by the steeply dipping 

Wekepeke fault, and their trace should theoretically be 

offset to the north by uplift on the west side of the 

Wekepeke. No such fault or faults have been mapped. 

Numerous thrusts involving Ayer Granite, however, are 

shown south of Worcester by Barosh (1974, 1977).

The Clinton-Newbury fault projects eastward into the 

Gulf of Maine where Simpson and others (1980) suggested that it is aligned with the trend of the South Atlas 

fault, or it may in some way connect with the CobequidChedebucto fault of Nova Scotia. It may connect with 

the Isleboro fault of eastern Maine, although the latter seems more identifiable with the Bloody Bluff fault 

from the nature of the rocks on either side of it. It 

does not appear to project into the Norumbega fault 

(Wones and Thompson, 1979) of eastern and southeastern Maine. The Norumbega seems rather to project 

toward the Massabesic terrane to the west, as suggested 

by Lyons and others (1982). Structural features of eastern Maine and northeastern Massachusetts are not 

directly correctable.

The north-northeast-trending Portsmouth fault of 

Novotny (1969), which lies between the Rye Formation 

of southern New Hampshire and the Kittery Formation 

of the Merrimack Group, does not project on the surface 

into the Clinton-Newbury as might be expected from its 

strike. Carrigan (1984a) found that the mylonites of the 

Portsmouth fault (P, fig. 3) are folded eastward around 

the south-plunging nose of the Rye anticline. This folded 

mylonite suggests that the Portsmouth fault may mark 

the reappearance to the north of the ductile portion of the 

Clinton-Newbury fault. The lithology and metamorphism of the Rye as described by Carrigan (1984b) 

resemble those of the Marlboro Formation in the Nashoba zone. Such an interpretation changes the position 

of the trend of the Clinton-Newbury and brings the fault 

closer to projected connections with the faults in Maine. 

An important fact to be emphasized is that the ClintonNewbury fault, although it appears to be a boundary 

between major basement blocks, coincides on the surface 

in Massachusetts with an apparent unconformity 

between the rocks of the Merrimack synclinorium and
H16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

the rocks of the subjacent Nashoba zone (Robinson and 

Goldsmith, this vol., chap. G). Typical Nashoba rocks, 

although disrupted by the Clinton-Newbury fault, 

project below the rocks of the Merrimack belt, as discussed above, and form part of the basement on which 

the Merrimack-belt strata are deposited. An alternative 

explanation is that part or all of the overlying Merrimack 

strata have moved into their present position on an early 

detachment surface.

AGE OF THE CLINTON-NEWBURY FAULT ZONE

The Clinton-Newbury is a composite fault zone, including both thrusts and reverse faults that have formed over 

a period of time. Some of the movement may have been 

as old as Acadian, particularly the low-angle thrusts, 

which may be related to the east-directed back-thrusting 

recorded by Robinson and Hall (1980) in the Bronson 

Hill zone to the west. Some of these thrusts contain 

sillimanite-grade mylonites, according to G.R. Robinson, 

Jr. (written commun., 1984). The deformation of the 

Devonian plutons suggests, however, that the bulk of the 

movement, particularly on the steeply dipping segments, 

is Late Devonian or younger. Faults in the ClintonNewbury zone are clearly younger than the metamorphism of the rocks in both the Nashoba zone and the 

Merrimack belt, and faults in the zone clearly cut the 

Devonian muscovite granite of the Rattlesnake Hill 

pluton. It is not certain how faults that enclose the 

Middle Pennsylvanian Coal Mine Brook Formation at 

Worcester are related to the Clinton-Newbury or to the 

probably Triassic and Jurassic faults of the Flint Hill 

fault system.

In summary, at least three different styles and ages of 

faulting are present in the Clinton-Newbury fault zone. 

The oldest is low-angle, east-directed thrusts and 

reverse faults. They are possibly of Acadian age but 

could be late Paleozoic. The next group is high-angle 

faults that truncate the former and that in places contain 

evidence of strike-slip movement. These are probably 

late Paleozoic. Both of these groups are ductile. The 

third style of faults is high-angle, apparently normal 

faults characterized by brecciation and silicification. 

They are probably Mesozoic in age.

STRUCTURAL RELATIONS WEST OF THE CLINTONNEWBURY FAULT

The Clinton-Newbury fault now forms much of the 

presently exposed east margin of the Merrimack belt. 

The east flank of the Merrimack belt, as here defined, is 

that area between the Wekepeke fault and the Eastford 

fault in Connecticut (figs. 3, 5), on the west, and the 

Nashoba zone, on the east. It is a structurally complex

area containing an assemblage of rocks whose sequence 

is uncertain but which are younger than the rocks of the 

Nashoba zone (Robinson and Goldsmith, this vol., chap. 

G). The area can be divided structurally into the Nashua 

trough (Crosby, 1880; Smith and Barosh, 1981), to the 

west and northwest, and the Rockingham anticlinorium 

(Billings, 1956), to the north and northeast. The boundary between the Nashua trough and the Rockingham 

anticlinorium in Massachusetts is in part a fault (F, fig. 3) 

coinciding with a metamorphic gradient, but near the 

New Hampshire border the boundary is primarily a 

metamorphic gradient and coincides with the contact 

between the Oakdale Formation and the Berwick Formation (or the equivalent units mapped by G.R. Robinson, 1978). The Rockingham anticlinorium is truncated 

diagonally by the Clinton-Newbury fault so that to the 

south, near Worcester, rocks of the Nashua trough lie 

adjacent to rocks of the Nashoba zone across the ClintonNewbury fault. The Nashua trough persists to the south 

into Connecticut through a complex system of faults near 

the Massachusetts-Connecticut border and appears to 

coincide with a low-grade metamorphic trough extending 

southward through eastern Connecticut (Thompson and 

Norton, 1968) that contains units such as the Scotland 

Schist (Dixon and Lundgren, 1968). The Rockingham 

anticlinorium and the Nashua trough contain similar 

lithologies, but differences in metamorphic grade between the two structural belts and the complex style of 

deformation make it difficult to establish stratigraphic 

sequences that could aid in determining the structure.

NASHUA TROUGH

The structure within the Nashua trough remains to be 

convincingly worked out. Crosby (1880) considered the 

Nashua trough to be synclinal (fig. 6) so that the Oakdale 

and Paxton Formations were equivalent units on opposite limbs and the Worcester Formation lay in the 

trough. Peck (1975) showed overturned beds in the 

Worcester Formation, but he also showed upright beds a 

short distance across strike so that the folding in these 

rocks may be primarily megascopic; his section, like that 

of Grew (1970), on the whole is upright and west facing. 

On the other hand, Grew (1973), Hepburn (1976), and 

G.R. Robinson (1981) presented evidence that suggests 

that the Oakdale Formation lies in an isoclinal syncline so 

that the Tower Hill Quartzite and the Worcester Formation are on opposite limbs. Crosby's Nashua "synclinal" 

possibly exists but in more complicated fashion than he 

figured. The whole section may be recumbently folded 

(G.R. Robinson, 1981, p. 59-63; fig. 7B,C). Robinson 

(1981) and Peper and Wilson (1978) both noted, in the 

area east of the Fitchburg Complex, a flat cleavage that 

deforms an earlier, relatively flat regional schistosity
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS HIT

30' 71°00'

43°00' -

EXPLANATION

Coal Mine Brook Formation and Harvard 

Conglomerate

Chelmsford Granite and Fitchburg Complex

Ayer Granite, Exeter Diorite, and 

Newburyport Complex

Littleton Formation

Worcester Formation

Oakdale Formation

Paxton Formation and Merrimack Group

Tower Hill Quartzite, Reubens Hill Formation 

Vaughn Hills Quartzite, and Boylston 

Schist

 Contact Dashed where inferred

 Fault Dashed where inferred 

A A'

Line of section Shown on 

figure 7

42°00' -

20 MILES

10 20 KILOMETERS

FIGURE 5. Geology of the east flank of the Merrimack belt, Massachusetts and New Hampshire, showing locations of cross sections (A-A'-A",

B-B', C-C' of figure 7).
HIS

 '« . :« 

 ' ''«'" ' '« ' 

» ' '.o: O;

EXPLANATION

Fitchburg Complex 

Ayer Granite 

Harvard Conglomerate

Worcester Formation (and 

phyllite fades of the 

Tower Hill Quartzite?)

Oakdale Formation and 

Paxton Formation

Tadmuck Brook Schist and 

Vaughn Hills Quartzite

Nashoba Formation

- Contact Dashed where 

projected

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

WEST

Nashua "synclinal" through Sterling and Hudson

EAST

WEST EAST

^ '^/ / ///. / //. . .v/ ':/:/ /' /'i I I 1 1 11 I I I '/fa

Nashua "synclinal" through Shirley and Harvard

FIGURE 6. Schematic section near cross section C-C" (see fig. 5 for location) across the Nashua "synclinal" as drawn by Crosby (1880, pi. III).

Unit names are those used on the State bedrock map (Zen and others, 1983).

accompanied by small-scale isoclinal folds. This deformation pattern suggests at least two stages of horizontally 

directed stress. Tucker (1978) described early stages of 

recumbent folding followed by open, upright folding to 

the west in the Wachusett Mountain area. These flat 

folds are indicated on cross section D-D' of the State 

bedrock map. The configurations shown in figure 7 are 

largely the result of the latest folding. Such inferences 

can be made to account for the discrepancies in topping 

evidence between Peck (1975) and Robinson (1981), 

presuming a regional northward plunge. However, no 

really satisfactory interpretation accommodates all the 

mapping. It is possible that submarine slides have disrupted the original depositional sequence. Furthermore, 

lateral stratigraphic facies changes can obscure equivalencies of units and thus lead to false inferences as to 

structure.

The formations in the Nashua trough are at lower 

metamorphic grade than are those in adjacent belts, and 

they coincide with a metamorphic trough (Thompson and 

Norton, 1968). The Nashua trough is bounded on its west 

side by a high-angle fault, the Wekepeke fault, that 

brings up higher grade metamorphic rocks on the west in 

and adjacent to the Fitchburg Complex and to the 

Massabesic Gneiss Complex in the Massabesic anticlinorium. The Worcester Formation contains andalusite in 

the western part of its outcrop area near the Wekepeke

fault and the Fitchburg Complex. The Wekepeke fault is 

part of the Flint Hill-Silver Hill fault system of probable 

Permian age (Lyons and others, 1982) and (or) Mesozoic 

age (Rodgers, 1970).

ROCKINGHAM ANTICLINORIUM

The Rockingham anticlinorium consists of folded but 

overall gently dipping low- to medium-grade metamorphic rocks of the Merrimack Group (figs. 5, 7). In New 

Hampshire, the Rye Formation of Proterozoic Z or early 

Paleozoic age is exposed in the easternmost anticline, the 

Rye anticline, but the Rye Formation does not reach 

Massachusetts on the surface. The formations are in open 

folds about steep axial surfaces on which are superimposed local zones of tight folding. The folding is tighter 

and more discordant to the west, and early isoclinal folds 

on a mesoscopic scale (not indicated in the section, fig. 

1A) are locally present. Existing sections imply that the 

strata are not severely deformed. G.R. Robinson's study 

(1978) suggested that the structure within the Merrimack Group in the Pepperell-Ayer area, at least, is not 

one of simple anticlines and synclines but that these are 

at least second-generation folds and that more than one 

earlier generation of folds is present. The cross sections 

of Sriramadas (1966) in the Nashua area of the Nashua 

trough indicate fairly tight folding, but in the Rockingham anticlinorium to the east the rocks of the Merrimack
en

fault Wekepeke 

River Nashua 

River Merrimack i 

fault Wekepeke . 

fault) Hill (Flint ' 

ivanxonaxs QNV omdaonvxaw AHOXSIH ao Naaxsva sxxasnHovssvw 6TH
H20 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Group appear to be less deformed and in more open folds 

(fig. 7A). However, an earlier generation of small-scale 

isoclinal folds seen in outcrops (Sundeen, 1971) suggests 

that the structure here too may be complex. Robinson 

(1978, section B"-B"') showed a unit equivalent to the 

Berwick Formation overlying a unit equivalent to the 

Oakdale Formation in the Pepperell area. The section is 

probably a combination of intertonguing facies and softsediment deformation that has subsequently been polydeformed.

Metamorphic grade increases across the Rockingham 

anticlinorium. The formations are in the chlorite and 

biotite zones in the east and reach garnet grade to the 

west. The garnet zone coincides with an area containing 

many plutons of granite and diorite of Silurian and 

Devonian age. A sharp gradient back to chlorite zone 

forms the boundary between the Rockingham anticlinorium and the Nashua trough in the Pepperell area. This 

boundary coincides with the contact of the Berwick and 

Oakdale Formations; it marks a change in bedding style 

and grain size from thin bedded and fine grained 

(Oakdale) to thick bedded and coarse grained (Berwick). 

The metamorphic zones, like the rock units, are truncated to the south by the Clinton-Newbury fault.

SUMMARY

In summary, the Nashoba zone is a lens of internally 

folded and faulted high-grade paragneiss and metavolcanic rock bounded by major fault systems. These faults 

separate the zone from terranes different in lithology, 

metamorphism, and plutonism. Regional relations indicate that the Nashoba zone projects beneath the Merrimack belt to the west. The paragneiss is intruded by 

late-metamorphic granite and quartz diorite of Ordovician age and postmetamorphic granite and granodiorite 

of Silurian and Devonian age. The main period of thermal 

metamorphism and folding is Ordovician or older. Faults 

range in age from Late Devonian (postgranite) to probably Triassic and Jurassic. East-directed thrusts and 

reverse faults are older than high-angle faults. The 

folding and thrusting on the east flank of the Merrimack 

belt are thought to be Acadian because the Upper 

Ordovician(?) and Lower Silurian Ayer Granite is 

involved.

MILFORD-DEDHAM ZONE

The Milford-Dedham zone has been broken by normal 

and reverse faults into upthrown blocks of Proterozoic 

plutonic and metamorphic basement rock and downthrown blocks containing sedimentary and volcanic cover 

rocks of Proterozoic Z, Cambrian, Silurian-Devonian, 

Pennsylvanian, and TriassicnJurassic age (figs. 1, 8). The

30 KILOMETERS

FIGURE 8. Basins of the Milford-Dedham zone, eastern Massachusetts.

major blocks of basement are the gneissic Milford antiform at the north end of the Rhode Island anticlinorium, 

the brittlely deformed Salem block to the northeast, and 

the southeastern Massachusetts batholith, subdivided 

into the Dedham, Foxborough, and Fall River blocks. 

Intervening basins are, from north to south, the Newbury, Middleton, Boston, Norfolk, Bellingham (also 

locally known as Woonsocket), Narragansett, and Nantucket basins. The Milford-Dedham zone east of the 

Bloody Bluff fault is at least 200 km wide. U.S. Geological Survey seismic line 5 (cited by Grow and others, 

1979) places the edge of the continental crust at about the 

northern edge of the East Coast magnetic anomaly 

(Klitgord and Behrendt, 1979) about 190 km south of 

Nantucket. Klitgord (1984) suggested that the boundary 

between the Avalon terrane (Milford-Dedham zone) and 

the Meguma terrane exposed in southern Nova Scotia is 

either within the Gulf of Maine platform or at the 

boundary of the Georges Bank rift basin. In the latter 

case, the edge of the Milford-Dedham zone would lie east 

of Nantucket Island and Cape Cod. In the former case, 

the boundary could not lie west of Cape Cod Bay and 

probably lies east of Cape Cod judging from the materialin cores drilled at Harwich, Mass. (Goldsmith, this vol. , 

chap. E; Wones and Goldsmith, this vol., chap. I).
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H21

TABLE l. Metamorphism, plutonism, and faulting in basement rocks of the Milford-Dedham zone, eastern Massachusetts

Fall River block

Milford antiform Salem block Dedham block Foxborough block

Northwest Southeast

Metamorphism

Paleozoic intrusive rocks

Rock type (In Rhode Island 

Devonian alkalic 

granite, partly 

gneissic).

Devonian to Ordo- Devonian to Ordovician alkalic vician alkalic 

granite and gab- granite, 

bro.

Devonian alkalic Not present 

granite.

Not present.

Cambrian strata

Rock type Not present Argillite and mar- Slate, argillite, 

ble. sandstone, and

Slate, argillite, and Not present 

quartzite.

Not present.

quartzite.

Contact metamor- Subgreenschist Subgreenschist 

phosed.

Proterozoic Z plutonic rocks

Rock type

Metamorphism

Orthogneiss and Granite to gabbro, Granite to gabbro, Granite to gabbro,

granite to dio- partly cataclastic partly cataclastic partly cataclastic

rite. and altered. and altered. and altered.

Amphibolite Greenschist to sub- Greenschist to sub- Greenschist to Subgreenschist. greenschist. greenschist.

Granite, partly cat- Orthogneiss, 

aclastic and granite, 

altered. and diorite;

locally cataclastic.

Greenschist to sub- Amphibolite. 

greenschist.

Proterozoic Z stratified rocks

Rock type Gneiss, schist, Gneiss, schist, Phyllite, quartzite, Phyllite and greenquartzite, and quartzite, and greenstone. stone, 

amphibolite. amphibolite, finegrained paragneiss and greenstone.

Metamorphism Amphibolite Amphibolite to Greenschist Greenschist

greenschist.

Faults

Phyllite and green- Gneiss and 

stone. schist.

Largely ductile Ductile and brittle Brittle Brittle

Greenschist

Brittle

Amphibolite.

Brittle and 

ductile(?).

STRUCTURE AND METAMORPHISM IN THE 

BASEMENT BLOCKS

MILFORD ANTIFORM

The largely batholithic rocks of the Rhode Island 

anticlinorium project into Massachusetts as the Milford 

antiform (Milford anticline of Hall and Robinson, 1982, 

fig. 1). The Milford antiform consists of gneissic plutonic 

rocks of the Proterozoic Z Sterling Plutonic Suite (Goldsmith, 1966; Goldsmith and others, 1982) of Connecticut, 

Rhode Island, and Massachusetts, the Proterozoic Z 

Milford Granite and adjacent unnamed granite (Zgr, on 

the State bedrock map) in Massachusetts, and the metasedimentary and metavolcanic rocks of the Plainfield and 

Westboro Formations and the Blackstone Group. A

large area of Devonian granite has been identified in 

central Rhode Island in what was formerly considered to 

be part of the Sterling Plutonic Suite (Hermes and 

others, 1981), but this rock does not extend into Massachusetts. Metasedimentary and metavolcanic rocks of 

the Plainfield and Westboro Formations and the Blackstone Group flank the antiform on the west, north, and 

east, respectively, and form septa within it. These rocks 

are mostly in the amphibolite facies of metamorphism 

(table 1). Rocks of the Proterozoic Z mafic metamorphicplutonic complex (Zv and Zdigb) of eastern Massachusetts are most abundant north of the antiform in the 

Salem block.

The Milford antiform is bounded on the west by the 

Lake Char fault. The antiform plunges northeastward in
H22

71°52'30" 

42°22'30"

THE BEDROCK GEOLOGY OF MASSACHUSETTS

45' 37'30" 30'

EXPLANATION

Approximate location of shear zone

of O'Hara and Gromet (1984) 

Fault Dashed where inferred

Anticline

Form lines on foliation Includes cataclastic foliation, which is mostly near 

major faults. Dashed where inferred

90°-80°

75°-50° 

45°-25° 

20°-0°

Lineation Showing direction of 

plunge. Mostly mineral and intersection lineation; rare fold 

axes in southeast portion of 

the map

15'

7'30"

42°00'

10 MILES

10 KILOMETERS

FIGURE 9. Form lines on foliation and direction of plunge of lineation in the Milford antiform, eastern Massachusetts. Data from McKniff (1964), 

Shaw (1967), Barosh (1974, 1978), Dixon (1974, written commun., 1977 and 1978), Nelson (1975a,b), Volckmann (1977), and Hepburn and 

DiNitto (1978).

the Marlborough-Framingham area where it is truncated 

obliquely by the Bloody Bluff fault. The trend of gneissosity and rock units in the Milford antiform forms a 

partial arc around a core of Milford Granite (figs. 2, 9). 

Internally, the antiform shows an interference pattern of 

fold structures evident in the East Douglas area. Low 

plunges of lineations combined with the complex map

pattern of units on the east side of the antiform near the 

Massachusetts-Rhode Island border suggest the presence of refolded nappe structures or of low-angle ductile 

deformation, which is reminiscent of features characteristic of metamorphic core complexes in the Western 

United States (see Coney, 1980). The west-projecting 

node at Oxford and Webster, called by Barosh (1982) the
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H23

Oxford anticline, is apparently younger than the Lake 

Char fault because it folds the fault.

The Proterozoic metasedimentary and metavolcanic 

rocks (Zp, Zw, Zb) within the Milford antiform are in the 

amphibolite facies and contain minor folds older than the 

gneissic fabric in the plutonic rocks. These folds are less 

noticeable in the west, where the plutonic rocks have a 

marked gneissic fabric, than to the east, where the 

plutonic rocks are less deformed, for example in and east 

of Hopedale. In most places, however, the foliation in the 

metamorphic rocks tends to parallel the foliation in the 

plutonic rocks, indicating that the two were deformed 

together.

In general the intensity of deformation increases from 

east to west across the Milford antiform toward the 

Bloody Bluff fault. However, within the antiform are 

zones of rock more gneissose than adjacent rock. Discrete zones of gneissic rock can be seen north and south 

of the Milford Granite; O'Hara and Gromet (1984) identified in northern Rhode Island a discrete zone of gneissosity (fig. 3), separating the Hope Valley Alaskite Gneiss 

(Zhg) to the west from the Ponaganset Gneiss (Zpg) and 

Devonian Scituate Granite to the east, that they believed 

to be the southward continuation of the Bloody Bluff 

fault. The gneissosity is more apparent in biotitic rocks, 

such as the Ponaganset Gneiss and the biotitic phase of 

the Milford Granite (Zmgd), than in the biotite-poor 

Hope Valley Alaskite Gneiss (Wones and Goldsmith, this 

vol., chap. I). The gneissosity is marked by preferred 

orientation of biotite, where present, and by flat lenses 

and laminae of quartz and of feldspar. This foliation is 

parallel to that in the metavolcanic and metasedimentary 

rocks, although the plutonic rocks can be seen in places to 

truncate layering and an earlier foliation in the metasedimentary rocks at a low angle. In places, the Hope Valley 

clearly cuts the other plutonic rocks. The fabric in the 

Milford Granite (Zmgr), in the core of the Milford 

antiform, is a lineation rather than a foliation (parallel 

planar metamorphic fabric), although the more biotitic 

phase (Zmgd) as mentioned above does have a foliation in 

places, particularly north of the central core. The Milford 

Granite and the unnamed granite (Zgr) to the north and 

east of the Milford are less deformed than are the 

orthogneisses to the west. They are characterized by 

rounded aggregates of bluish quartz and more equant 

feldspars, rather than flat aggregates of quartz and of 

feldspar characterizing the rocks to the west. To the 

northeast, the Hope Valley Alaskite Gneiss exposed in 

many roadcuts along the Massachusetts Turnpike in the 

Framingham area and within the Bloody Bluff fault zone 

is fine grained (which I believe is due to tectonic comminution of grain size) and has a marked foliation. East of 

the Milford Granite, in the Ashland-Holliston area, the 

foliation is less distinct. Southeast of the Milford Granite,

the unnamed granite (Zgr) and the Esmond Granite 

(Zegr) are not conspicuously deformed except for a 

north-northeast-trending zone of shear and cataclasis as 

much as 0.5 km wide in Zgr (2, fig. 2) along the northwest 

side of the Bellingham basin in Franklin and Blackstone. 

This cataclasis is little recrystallized and forms a mylonite gneiss rather than a blastomylonite like that present 

in the Bloody Bluff fault zone to the north. A thin 

east-trending strand of well-foliated Scituate Granite 

Gneiss (Zsgr) flanks the Blackstone Group in the Blackstone River valley on the south side of the less deformed 

Milford Granite (3, fig. 2). The structural significance of 

this narrow zone of penetrative deformation is not 

readily apparent. On the State bedrock map I have 

indicated that the gneissic terrane includes the Milford 

and the unnamed granite and that the division between 

the gneissic and nongneissic terranes falls about on a line 

from the southwest end of the Boston basin through the 

Bellingham basin. This line may be offset to the west 

along the Blackstone River valley and may lie at the west 

side of the Esmond Granite in Rhode Island (E, fig. 2). 

The Milford and the unnamed granite are included in the 

gneissic terrane because of their blastocataclastic fabric 

(blue quartz aggregates) and local zones of lineation 

and marked gneissosity. West of the line southwest from the Boston basin are variably penetratively deformed plutonic rocks (orthogneisses) and 

amphibolite-facies quartzite and paragneisses. East of 

the line are fractured, brittlely deformed, and partly 

hydrothermally altered plutonic rocks and greenschistfacies metasedimentary and metavolcanic rocks. The line 

coincides with faults that are a continuation of the 

northern boundary fault of the Boston basin and that 

project in the area of the Bellingham basin. The cataclastic zone in the unnamed granite (Zgr) west of the 

boundary is considered to be the result of deformation 

along this boundary. Sheared granite crops out sporadically elsewhere along the bounding fault and along the 

trace of the fault splaying southwestward off the boundary fault from west of Medway toward Milford. Gromet 

and O'Hara (1984) and O'Hara and Gromet (1984) 

believed that a discrete boundary separates the terrane 

characterized by the less foliated Milford Granite from 

the terrane characterized by the more foliated Hope 

Valley to the west. They placed this boundary west of 

the outcrop area of the Ponaganset Gneiss and thence 

north to intersect the Bloody Bluff fault (figs. 2, 3, 9) and 

thus would exclude the Milford Granite from the gneissic 

suite of rocks.

In summary, the Milford antiform is the northeastward-plunging nose of the polydeformed Rhode Island 

anticlinorium in which the foliation arcs around a more or 

less lineated and less deformed core and eastern flank. 

The Proterozoic Z plutonic rocks within the antiform
H24 THE BEDROCK GEOLOGY OF MASSACHUSETTS

are markedly gneissose on the west and north and 

decrease in gneissosity eastward. The decrease is not 

systematic, however, for the width of the zone and the 

intensity of deformation vary, and narrow zones of 

gneissic rock, like that described by O'Hara and Gromet 

(1984), flanked by less gneissic rock extend both eastward and northeastward. The zones of gneissosity 

decrease in prominence eastward to about a line extending from the Boston basin to the Bellingham basin, along 

which there is increased shearing and cataclasis.

The relations described above indicate a greater 

degree of pervasive deformation and recrystallization to 

the west than to the east. This distinction indicates not 

only a concentration of deformation but also a higher 

temperature regime to the west. The age of the deformation that produced the gneissosity is discussed below 

in the section on Proterozoic metamorphism. In the 

Milford antiform, the metasedimentary and metavolcanic 

formations are in the amphibolite facies of metamorphism (table 1). There is some evidence to indicate that 

they were metamorphosed before the intrusion of the 

plutonic rocks, but a subsequent differentially pervasive 

dynamic and thermal metamorphism involving the plutonic rocks has rendered such relationships obscure, 

particularly in the more highly deformed areas.

SALEM BLOCK

The basement north of the Milford antiform between 

Framingham and Newbury is here called the Salem block 

because of the extensive distribution of diorite and 

gabbro (Zdigb) (Salem Gabbro-Diorite, in part; Wones 

and Goldsmith, this vol., chap. I) within it (figs. 1, 2). It 

is bounded on the west by the Bloody Bluff fault, on the 

southeast by the northern border fault of the Boston 

basin, and on the south by faults and the northwesttrending rocks at the north end of the Milford antiform 

(4, fig. 2; fig. 10). The block consists primarily of mafic 

plutonic rocks (Zdi, Zgb, Zrdi, Zdigb), mafic to felsic 

metavolcanic rocks (Zv), and minor metasedimentary 

rocks (Zw). The diorite at Rowley (Zrdi) has a K-Ar age 

of 656±16 Ma (Zartman and Marvin, this vol., chap. J, 

table 1). These rocks have been intruded by the somewhat younger Proterozoic Z Dedham Granite and Topsfield Granodiorite and by early and middle Paleozoic 

alkalic granite and gabbro. The Lynn Volcanic Complex 

lies in the Melrose subblock of the Salem block. The 

Newbury basins containing Upper Silurian-Lower Devonian strata (Newbury Volcanic Complex), and the Middleton basin containing unmetamorphosed Triassic 

strata ("Re), flank the Salem block on the northwest side. 

The Boston basin lies to the southeast.

The metavolcanic rocks in the Salem block contain 

assemblages typical of the amphibolite facies (table 1). 

Northwest of Salem the rocks are primarily epidote- and

andesine-bearing amphibolite and feldspathic augen 

gneiss (Toulmin, 1964; Bell and Alvord, 1976). Nelson 

(1974, p. 10) noted the presence of sillimanite in his 

Claypit Hill Formation (included in Zv on the State 

bedrock map) in the Framingham area, and his units are 

within the garnet-amphibolite facies. Chlorite-bearing 

assemblages are probably partly retrogressive. The 

Proterozoic Z Salem Gabbro-Diorite intruded these 

mafic metavolcanic rocks (Toulmin, 1964, p. 69), and all 

these rocks are sheared in the Burlington mylonite zone 

(fig. 2; Castle and others, 1976, p. 33).

The Proterozoic Z plutonic rocks of the Salem block are 

not pervasively gneissic like those on the west side of the 

Milford antiform. They have, however, been subjected to 

a variably distributed cataclasis that moderately fractured quartz and feldspar grains and was accompanied by 

crystallization of epidote and chlorite-group minerals 

along fractures and shear planes. These assemblages 

indicate metamorphism no higher than greenschist 

facies. The deformation and retrogression are more 

pronounced toward the Bloody Bluff fault and within the 

Burlington mylonite zone and less pronounced to the 

east, as, for example, near Salem. The Topsfield Granodiorite is much more extensively altered than the Dedham Granite to the south.

In the Salem block then, we see evidence of an 

amphibolite-facies metamorphism in the stratified metavolcanic rocks before or during intrusion of the mafic 

plutonic rocks. A retrogressive greenschist-facies metamorphism was superposed on the mafic metavolcanicplutonic complex as well as on the younger Dedham 

Granite and Topsfield Granodiorite and may have accompanied the shearing along the major faults in the area.

MELROSE SUBBLOCK

Bell and Alvord (1976) and Kaye (1980) have outlined 

an area herein called the Melrose subblock bounded on 

the south by the northern boundary fault of the Boston 

basin, on the west by the Mystic fault (M, fig. 10), and on 

the north and east by the curvilinear Walden Pond fault 

(figs. 2, 10). This subblock, much of it in the town of 

Melrose, contains the outcrop area of the Lynn Volcanic 

Complex and the type area of the Middlesex Fells 

Volcanic Complex. I have applied the name Melrose to 

this subblock rather than using one of the rock unit 

names. Bell and Alvord (1976) showed the Walden Pond 

fault as intersecting the northward-trending Mystic fault 

at a sharp angle. Kaye (1980) modified this arrangement 

by dropping the Mystic fault north of the Melrose block 

and considered the remaining southern part of the Mystic 

fault to be a continuation of the curvilinear Walden Pond 

fault. This is approximately the configuration shown on 

the State bedrock map.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H25

7115' 71 "00' 70°45'-

42°45'

42°30'

Lawrence

 /- ' ' :* - --sr-T'-l / .-.: :--:H"i-.- y/A-'M'

10 MILES

0 5 10 KILOMETERS

FIGURE 10. Structural features in and adjacent to the Salem block of the Milford-Dedham zone, eastern Massachusetts. Basins are indicated

by pattern.

The subblock contains an assemblage of rocks somewhat different from that in the Salem block to the north. 

The most abundant unit is the Lynn Volcanic Complex, 

which does not crop out outside the Melrose subblock, at 

least to the north and west, except for outcrops on 

Marblehead Neck. The Lynn on Marblehead Neck and 

adjacent islands may actually be part of the Melrose

subblock offset to the north along a north-northeast 

trending fault through Marblehead harbor (C, fig. 10). 

Other units in the subblock are older and include the 

Proterozoic Z Dedham Granite, the Westboro Formation, and mafic metavolcanic rocks (Zv) of the Middlesex 

Fells Volcanic Complex. The older metavolcanic rocks 

are metamorphosed to amphibolite facies and contain a
H26 THE BEDROCK GEOLOGY OF MASSACHUSETTS

foliation parallel to layering. A common mineral assemblage, similar to that in the rest of the Salem block, 

includes hornblende, oligoclase and andesine, and epidote, the latter forming pods and lenses in some places 

(Bell and Alvord, 1976). These rocks are locally chloritized. Because the grade of metamorphism in the overlying Lynn Volcanic Complex is at most greenschist 

facies, a Proterozoic episode of amphibolite-facies metamorphism must have occurred before deposition of the 

Lynn Volcanic Complex. The rock assemblage in the 

Melrose subblock suggests that this subblock represents 

a shallower stratigraphic level than the rest of the Salem 

block; it is equivalent to the assemblage of rocks immediately beneath the Boston Bay Group in the Boston 

basin in the south. Significantly, C.A. Kaye (oral commun., 1979) identified a small area of Roxbury Conglomerate within the subblock, supporting the suggestion 

that the subblock contains shallow-level rocks. A sliver of 

the Melrose subblock may lie south of the northern 

border fault of the Boston basin in the Nahant area (see 

discussion of Boston basin below).

FAULT PATTERN IN THE SALEM BLOCK

The Salem block has been appreciably broken by 

faulting. Individual faults are difficult to locate because 

of intermittent exposures and similarity of rock types. 

Several different interpretations of the fault pattern 

exist (Bell and Alvord, 1976; Castle and others, 1976; 

Nelson, 1976; Barosh and others, 1977). Faults shown on 

the State bedrock map were taken from original quadrangle maps modified by recent observations by Kaye 

(1980) and A.F. Shride (written commun., 1979). Only 

the more significant faults have been shown. Shride (as 

reported in U.S. Geological Survey, 1980, p. 62) 

described the faults and fault zones in northeastern 

Massachusetts as varying greatly in their characteristics:

Some of the principal faults (that is, strike-slip faults of regional 

extent with displacement measurable in kilometers to tens of kilometers) [Bloody Bluff fault system and Burlington mylonite zone] are 

marked by zones that are narrow along lengths of many kilometers and 

are composed of cohesive mylonitic materials; these faults give way 

abruptly to bordering rocks that exhibit little cataclasis. Other regional 

faults are characterized by central zones whose widths are variable but 

approach 1 km, bordered by zones of pervasively shattered rock 

measuring hundreds of meters wide. Some central zones are dominantly mylonite or ultramylonite, whereas, in others, breccias make up 

considerable parts. Some of the weak and, therefore, rarely exposed 

fault zones are made up of thinly sheeted highly friable rock parted 

along innumerable slickensided shear surfaces; incoherent gouge, 

apparently, is a rare constituent. The cataclastic zones of secondary 

faults, those with displacements of no more than 1 to 2 km, are mostly 

only a few meters wide and sharply defined; a few exceptional zones are 

more than 250 m in width. Breccias seemingly are more characteristic 

of secondary faults than faults of regional extent.

The Walden Pond fault (fig. 10) is a steeply dipping 

fault, most likely a north-dipping reverse fault like the 

northern border fault to the south. Its curvilinear trace 

may actually mark the locus of a series of straight short 

faults that intersect at obtuse angles. The Melrose 

subblock has dropped down on these faults relative to the 

rest of the Salem block and moved up relative to the 

Boston basin. Although mapped, the Walden Pond fault 

itself has not been described by anyone.

MYSTIC FAULT

Bell and Alvord (1976, fig. 1) showed the Mystic fault 

(M, fig. 10) extending northeastward from the Boston 

basin at Arlington to the structurally complex area near 

Lynnfield, where the Burlington mylonite zone and the 

Bloody Bluff fault meet and lose their identities. The 

southern segment of the Mystic fault joins the Walden 

Pond fault on the State bedrock map. Bell and Alvord's 

Mystic fault, if projected to the northeast, would come 

close to or truncate the northwest margin of the main 

pluton of Peabody granite at Peabody. From there the 

fault could reasonably be drawn on the basis of the 

aeromagnetic pattern as the fault shown by Castle and 

others (1976) (E, fig. 10) striking north to the Bloody 

Bluff fault zone near Lynnfield. However, neither Castle 

and others (1976) nor A.F. Shride (oral commun., 1979) 

found evidence in surface exposures for a connection 

between the Mystic fault at the edge of the Melrose 

subblock and a fault north of the main pluton of the 

Peabody Granite. I have chosen arbitrarily to shift 

Castle and others' (1976) fault to the west where it passes 

through the serpentinite mass at Lynnfield (S, fig. 10) 

and thence continues southward west of the Peabody 

pluton to intersect the Walden Pond fault. The northsouth fault through the serpentinite is connected on the 

State bedrock map with a fault that extends across the 

Melrose subblock from the Boston basin. Other northsouth faults of small displacement (A,B, fig. 10) that 

extend from the Melrose subblock into the Boston basin 

have been mapped by Kaye (1980,1983). The significance 

of an east-west fault (F, fig. 10) shown by Castle and 

others (1976) cutting across the serpentinite mass from 

the Burlington mylonite zone to the pluton at Peabody is 

not apparent.

The north-south faults shown on the State bedrock 

map that extend from the Boston basin into the Melrose 

subblock (Billings, 1976a,b; Kaye, 1980, 1983) are apparently the youngest faults in northeastern Massachusetts; 

they probably are the same age as the fault bounding the 

Triassic Middleton basin and perhaps the faults bounding 

the Silurian-Devonian Newbury basins.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H27

DEDHAM AND FOXBOROUGH BLOCKS

Southeast of the Salem block and the Milford antiform 

in the Milford-Dedham zone is a terrane containing the 

Proterozoic Z southeastern Massachusetts batholith. 

This terrane is divided into the Dedham block, the 

Foxborough block, and the Fall River block (figs. 1, 2). 

The first two blocks are discussed here, and the third in 

the next section. Metasedimentary and metavolcanic 

rocks of Proterozoic age are not abundant in the Dedham 

and Foxborough blocks and primarily form inclusions and 

screens within the Proterozoic Z plutonic rocks. The 

Dedham block can be construed as continuing southwestward to northeastern Rhode Island, where it contains 

the Blackstone Group, Esmond Granite, and associated 

rocks east of the Bellingham basin. To the north it is 

interpreted to abut the Salem block beneath the Boston 

basin (fig. 2).

The metasedimentary and metavolcanic rocks (Zb, 

Zw) in the Dedham and Foxborough blocks are primarily 

amphibolite, phyllite, and quartzite, in the upper 

greenschist- and epidote-amphibolite facies of metamorphism (table 1). In the Norwood area, metabasalt contains green amphibole, and metarhyolite contains saussuritized plagioclase and green biotite variably altered to 

chlorite (Chute, 1966). Scattered exposures of quartzsericite phyllite locally containing pyrite cubes are 

present in the Dedham block to the southwest. In 

northern Rhode Island there is evidence for a period of 

regional dynamothermal metamorphism in the Blackstone Group before intrusion of the Esmond and Dedham 

Granites (Coyle and others, 1984). North of Woonsocket, 

the Ordovician-Silurian granite (SOqgr) clearly intrudes 

folded and metamorphosed (epidote-amphibolite facies) 

Blackstone Group. Lack of evidence for early Paleozoic 

metamorphism in the zone indicates that the metamorphism is probably Proterozoic.

Textures in the plutonic rocks of the Dedham and 

Foxborough blocks are similar to those in the less 

deformed parts of the Salem block, but the rocks on the 

whole are less fractured and altered. Most of the cataclasis and alteration is concentrated along north- to 

north-northwest-trending faults that transect the boundaries of the blocks. The rocks of the Foxborough block 

are appreciably shattered and altered at its southwestern end where the Norfolk and Narragansett basins 

merge. The diorite and gabbro (Zdigb) retains its igneous 

texture, but some feldspars are saussuritized and mafic 

minerals altered to chlorite and epidote. Where diorite or 

gabbro has been intruded by Dedham Granite, hornblende has been altered to biotite, and in places a hybrid 

rock has been produced. The alteration to chlorite and 

sericite in the Dedham is primarily along small shear 

fractures most numerous near faults. Small veinlets 

containing epidote and quartz are common in these

altered areas. The batholithic rocks in the eastern part of 

the Foxborough block are little altered.

Numerous north-trending faults chop the rocks into 

slices, as shown by the crenulate contact of the south side 

of the Foxborough block. The principal fault of this sort 

is the Stony Brook fault, which projects into the Boston 

basin (fig. 2) and is further described in the section on 

that basin. A similar north-south fault set is present in 

the basement beneath the Pennsylvanian strata in the 

eastern part of the Narragansett basin (Williams and 

Willey, 1973) and in the exposed basement in the northern part of the Fall River block (Koteff, 1964). Vein 

quartz is associated with these north-trending faults 

(Lyons, 1977). The border faults along the edges of many 

of the blocks are discussed in sections describing the 

structure of these basins.

FALL RIVER BLOCK

Structural features and metamorphism in the Fall 

River block have not hitherto been described and have 

been mapped only in two places: the Assawompset Pond 

area (Koteff, 1964) and the Tiverton area (Pollock, 1964). 

Accordingly, features in this block are described more 

completely in the following pages than has been done for 

the other areas.

The structural and metamorphic features in the northern part of the Fall River block (fig. 11) are in general 

similar to those in the Foxborough block north of the 

Narragansett basin. The batholithic rocks closely resemble those north of the basin, not only in composition and 

texture but also in degree of deformation and alteration. 

Like them, they are not foliated and lack gneissosity. 

The Proterozoic metavolcanic-metasedimentary septa 

are in the greenschist facies of metamorphism. In the 

southern part of the Fall River block, however, the 

plutonic rocks tend to be gneissose, and the metavolcanic 

rocks are in the amphibolite facies (table 1).

METAMORPHISM OF THE METAVOLCANIC AND PLUTONIC ROCKS

Metamorphism of the Proterozoic metaigneous rocks 

(Zgs, Zgn) in the Fall River block appears to be relatively simple. The gneiss and schist (Zgs) have a principal foliation parallel to the layering, which is locally 

crenulated by a later slip cleavage. Foliation in both 

gneiss and schist is marked by a preferred mineral 

orientation, primarily of the micas. Lineation is prominent only in the noses of the major folds in biotite gneiss. 

It appears to be an intersection lineation marked by 

streaks of biotite and rodding of quartz and feldspar. 

Splintery, lineated rock in other areas may mark noses of 

isoclinal or tight folds, but no minor folds of this sort 

were seen in outcrop. The slip cleavage strikes to the 

northwest and dips steeply.
H28

42°00'

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

71°00' 70°45'

tyLong ;^7 i

a-j^rf - ?i °lu

^ XZpgr/^

NEW 

BEDFORD ^

41°30

FIGURE 11 A. Structural features of the Fall River-New Bedford area, southeastern Massachusetts. Pr, Rhode Island 

Formation; Pp, Pondville Conglomerate; Zfgr, granite of the Fall River pluton; Zpgr, porphyritic granite; Zagr, 

alaskite; Zdi, diorite; Zgg, granite, gneiss, and schist, undivided; Zv, metamorphosed mafic to felsic flow, 

volcaniclastic, and hypabyssal intrusive rocks; Zgs, gneiss and schist near New Bedford; Zgn, biotite gneiss near 

New Bedford.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H29

FA w

EXPLANATION

Contact Dashed where approximately located;

dotted where concealed 

Fault Dashed where inferred; dotted where

concealed

Antiform Showing plunge

Synform Showing plunge 

Strike and dip of bedding

Strike and dip of foliation 

Inclined 

Vertical

Strike and dip of foliation and compositional 

layering

Strike and dip of flow foliation in plutonic 

rock 

Inclined 

Vertical

Strike and dip (vertical) of axial surface 

of minor fold of foliation

Strike and dip of cleavage

Strike and dip of shear fracture in 

plutonic rock 

Inclined 

Vertical

Strike and dip of mylonite 

Inclined 

Vertical

Lineation Showing bearing and plunge. 

Mostly elongation or streaks of minerals such as biotite, quartz, feldspar, and hornblende

Minor fold axis Showing bearing and 

plunge

FIGURE 11 A Continued.

The textures in the gneiss and schist indicate that a 

static thermal phase succeeded an earlier dynamothermal stage. The thermal phase could be attributed to the 

emplacement of the plutonic rocks. However, because 

the plutonic rocks described below also seem to be 

involved in the thermal progression, the thermal phase 

may have been superposed much later, in the same way 

as the late Paleozoic (Alleghanian) thermal event was 

superposed on the Proterozoic rocks of southeastern 

Connecticut and southern Rhode Island (Lundgren, 

1966; Zartman and others, 1983). Mineral assemblages in 

the metaigneous rocks of southeastern Massachusetts 

indicate greenschist-facies metamorphism to the north 

and amphibolite-facies metamorphism to the south. To 

the north, around Long Pond and Assawompset Pond 

(fig. 11 A), inclusions and septa in the granites are in the 

greenschist facies. Koteff (1964) mapped a sericitequartz-feldspar schist on the west side of Long Pond and 

noted a fine-grained inclusion in granite to the west that 

contains quartz, sericite, and subordinate epidote, 

opaque minerals, and apatite. A metadacite porphyry 

near Rock (fig. 11 A) contains relic igneous texture but 

greenschist-facies mineral assemblages: saussuritized 

plagioclase, interstitial epidote, and biotite altered to 

chlorite. Primary indicators of the amphibolite-facies

metamorphism to the south are the coexistence of epidote, calcic oligoclase, and blue-green hornblende. No 

truly pelitic assemblages are known here. The biotite 

gneiss along the shores of Buzzards Bay contains abundant discontinuous pegmatite patches and veins, locally 

exhibiting a swirled pattern indicating plasticity and 

local melting.

The plutonic rocks in the northern and western parts 

of the Fall River block are little deformed except by 

faulting. They locally contain an indistinct foliation. 

From about Long Pond southward and southwestward, 

the plutonic rocks show widely spaced, discrete shear 

zones containing feldspar augen and reoriented streaks 

of mafic minerals. These zones become more numerous 

southward, so that the rocks develop a pervasive gneissic 

fabric and become augen and flaser gneiss. This transition is best seen in the porphyritic granite (Zpgr) that 

extends from Long Pond to Acushnet near New Bedford. 

Granite of the Fall River pluton (Zfgr) to the east is 

equigranular and poor in mafic minerals, and in it the 

transition is not well demonstrated, although zones of 

shear can be recognized locally. South of Fall River in the 

Tiverton area, the local phase, the Bulgarmarsh Granite, 

is relatively unfoliated, but, to the southeast near Westport Point, Zfgr is gneissic. Foliation symbols in the Long 

Pond area shown on figure 11A represent recognizable 

zones of gneissosity in the granite of the Fall River 

pluton and in the porphyritic granite. These zones have a 

general west to west-northwest trend.

The gneissic terrane is primarily east of a prominent 

topographic lineament (the inferred North Dartmouth 

fault, fig. 11 A) that marks the contact between the 

granite of the Fall River pluton and the belt of gneiss and 

schist extending from the south end of Long Pond 

south-southwest to Westport Factory, North Dartmouth. From there the belt of gneiss and schist extends 

southwestward in a vaguely defined zone toward the 

Rhode Island State line west of Center Village, Westport. The east-west change is more abrupt than the 

north-south change and is in part attributed to a difference in competency between the relatively maficmineral-poor granite of the Fall River pluton and the 

porphyritic granite to the west. Mafic-mineral-poor 

rocks such as the alaskitic granite (Zagr) are gneissic, 

however, in the Westport-New Bedford area. The 

increase in gneissosity southward and southeastward is 

somewhat similar, but in reverse direction, to the 

increase in gneissosity on the west side of the Rhode 

Island anticlinorium in western Rhode Island and eastern Connecticut.

STRUCTURAL FEATURES IN THE NEW BEDFORD AREA

All the units in the New Bedford area are complexly 

folded, as shown by the trend of gneissosity and schis-
H30 THE BEDROCK GEOLOGY OF MASSACHUSETTS

KILOMETERS 

SEA LEVEL 

Pr Zfgr Zpgr Pr Zpgr

B

KILOMETERS 

SEA LEVEL -

1-

2-

3-

4

Pr ; ;

Zfgr/

'Zfgr

Acushnet South Dartmouth

synform ; antiform A'

} . : , V .---7--. MILES

; : Z^gr Zdi / ; j Zagr Zgn,.-'' Zagr ''-.. Zgn Zagr

LEVEL

B'

Zfgr

Zfgr-

/ Zagr ; Zgs x x Zagr_____~~ Q. Zfgr Zdi

MILES 

 SEA LEVEL

c

KILOMETERS

SEA LEVEL -

1 

2-

3-

4-

Pr Zfgr Zfgr Pr Pr Zfgr Zpgr Pr Zgg

-

-

" "

Zfgr

"Zfgr

Zfgr

\l\r

MILES

- SEA LEVEL

-1

-2

0 5 10 MILES

i 1 i i 1

i i i i 1 |

0 5 10 KILOMETERS

EXPLANATION

- - - - Contact Dashed where inferred; dotted where projected

 - - - Fault Dotted where projected

FIGURE 115. Interpretive cross sections of the Fall River-New Bedford area, southeastern Massachusetts.

tosity as well as by contacts between units. Lineation is 

prominent in fold hinges, particularly in the alaskitic 

granite in the South Dartmouth antiform. Foliation in 

the plutonic rocks is formed by parallel orientation of 

biotite and flattened feldspar megacrysts. Lineation is 

formed primarily by elongated biotite clots and by quartz 

and feldspar rods.

The orientation of these major folds is not consistent. 

Axial surfaces strike east-northeast, and vergence seems 

to be to the south (fig. 115). The South Dartmouth 

antiform (fig. 11 A,B) extending from Sconticut Neck 

through South Dartmouth to South Westport plunges to 

the west. However, the diorite at Acushnet (Zdi) to the 

north forms a phacolithic body in the variably northeastplunging Acushnet synform (fig. 11A,B); to the southwest, in the vicinity of Westport Point, lineation in 

gneissic granite plunges to the southeast. A zone of 

interfingering units and probably of tight folding and 

rotation, possibly associated with east-trending strikeslip faults, lies between the Acushnet synform and the

South Dartmouth antiform. A shear zone may separate 

the South Dartmouth antiform from the area of southeast 

plunges near Westport Point, but exposures are poor in 

this area. Two mylonite seams are exposed on the shores 

of Buzzards Bay in the Slocum River area. The more 

prominent zone, striking N. 65° W. and dipping 65° S., is 

well exposed on Potomska Point, Dartmouth. Asymmetry of feldspar augen tails and folding of sheared pegmatite indicate right-lateral displacement along the zone. A 

thin seam is exposed in ledges on a road to the east in 

rock containing a lineation S. 65° E. plunging 25°. The 

relatively steep plunges of lineation in the New Bedford 

region suggest a significant component of lateral movement between structural domains.

The systematic increase in gneissosity from the Long 

Pond area to the New Bedford area may be more 

apparent than real because of lack of continuity of 

exposures. The west- to west-northwest-trending shears 

in the porphyritic granite in the Long Pond area have the 

same trend as the cleavage in the metamorphic rocks.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H31

42°00' 70°45'

41°30'

FIGURE 11C. Joints in the Fall River-New Bedford area, southeastern Massachusetts.
H32 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Neither is parallel to the axial surfaces of the major folds 

in the New Bedford area. These shears may be younger 

than the gneissosity, and movement on them may have 

caused the discrepancy in direction of plunge in the 

different fold domains. The gneissic terrane may have a 

relatively sharp northeast-southwest boundary, but in 

reconnaissance in the area no sharp boundary was 

observed.

ASSAWOMPSET POND GRABEN AND RELATED STRUCTURES

A downdropped block containing Rhode Island Formation was mapped by Koteff (1964) south of Middleboro in 

the Assawompset Pond area (figs. 2, 1L4.). Slickensides 

in the northeast corner of the quadrangle plunge steeply 

down dip, and the contact of the porphyritic granite 

(Zpgr) with granite of the Fall River pluton (Zfgr) is not 

greatly offset. Koteff cited seismic studies that indicate 

that two buried preglacial channels across Mason Road in 

Freetown and North Avenue in Rochester coincide with 

the position of the faults. Koteff believed that the faults 

do not extend far south of the quadrangle; however, 

some evidence indicates that the western bounding fault 

continues south to the New Bedford area. Here, easttrending units appear to be slightly offset across the 

Acushnet River, and, at the Route 140 interchange with 

Phillips Street north of New Bedford, an outcrop of 

crushed and broken granite is laced by quartz veins 

trending both N. 15° E. and N. 50° W. and dipping 85° 

and 35° N., respectively. The veins are cut by steeply 

dipping fractures N. 40° E. and N. 10° W. This outcrop is 

about on the probable trend of the western bounding 

fault. The continuation of the eastern bounding fault is 

less clear. It either dies out or, as shown on figures 2 and 

11, turns southeast and merges with an inferred fault 

along a pronounced topographic break and an abrupt 

cessation of outcrop east of a line from Rock through 

Snipatuit Pond to Mattapoisett Harbour. I have interpreted the eastern bounding fault to project northward 

east of the inlier at Middleboro as Lyons (1977) showed 

it, rather than to the west as Williams and Willey (1973) 

suggested. This interpretation eliminates what would be 

a long, narrow horst east of the graben that at its north 

end would encompass the inlier.

The North Dartmouth fault trending south-southwest 

from Long Pond to North Dartmouth is largely inferred 

from topography, although the linear topographic low 

may be entirely or in part due to differential erosion 

along a belt of gneiss and schist (Zgs). This valley serves 

to separate altered but not pervasively gneissic plutonic 

rocks from mostly gneissic plutonic rocks. This fault is 

inferred to continue south down the east branch of the 

Westport River, although a more southwesterly trend 

would align it with a northeast-trending magnetic lineament east of Kirby Corner and Central Village, Westport (U.S. Geological Survey, 1971a,b). A northwesttrending fault through Hixville that splays off the North 

Dartmouth fault is inferred because of apparent offset of 

the gneiss and schist and granite of the Fall River pluton. 

Granite exposed in a gravel pit south of Hixville is partly 

crushed and sheared and has an anastomosing shear 

fabric varying from N. 60° W. to N. 70° E. Other small 

northwest-trending faults are shown at Fall River, 

where some rock is sheared in a northwest direction; 

along the west branch of the Westport River, for which 

there is little evidence except topography and an aeromagnetic lineament; and at the north end of the Fall 

River pluton at Lakeville. The fault at the west end of 

Assawompset Pond is based primarily on the basement 

configuration shown by Williams and Willey (1973) and 

by the observation that much of the granite exposed on 

Route 140 to the south is veined with quartz.

JOINTS

Joint sets in the plutonic rocks (fig. 11C) reflect shear 

and fault directions. One set of joints strikes westnorthwest to east-northeast generally parallel to shear 

directions in the rocks. This set does not seem to be 

reflected in the Pennsylvanian rocks to the west nor to 

noticeably affect the boundary between the plutonic 

rocks and the Pennsylvanian strata. Another set strikes 

northeast, about parallel to the east boundary of the 

Narragansett basin. Still another strikes north in the 

same direction as faults that cut the Narragansett basin. 

The north- and northeast-striking joints in many places 

contain vein quartz. Quartz-filled fractures are abundant 

in exposures along State Route 140 southwest of 

Lakeville. Northeast-trending joints along the west side 

of the Fall River pluton near Assonet are parallel to 

crushed and altered zones. The generally east-trending 

sets are considered to be older than the northeast- and 

north-trending sets and to have formed in a more compressive environment. They are possibly related to the 

stresses producing the east-west or west-northwest 

shears in the basement rock.

SUMMARY AND DISCUSSION

The metasedimentary and metavolcanic rocks in the 

Fall River block range in metamorphic grade from 

greenschist facies in the north to amphibolite facies in the 

south. As well as can be determined, the metamorphism 

preceded or was contemporaneous with the intrusion of 

the batholithic rocks. The foliation is folded and is cut 

locally by a northwest-striking slip cleavage. The plutonic rocks become gneissic to the south and are folded 

along with the metasedimentary and metavolcanic rocks 

into a complex pattern. This deformation apparently took
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H33

place at relatively high temperature, because there is 

little apparent retrogressive metamorphism in the plutonic rocks of the New Bedford area, in contrast to the 

situation to the north. Therefore I infer a late source of 

heat to the south and stresses producing structures 

oriented in an east-northeast to east direction superimposed on structures trending northerly. The relations are 

similar to those west of the Narragansett basin in 

southern Rhode Island and southeastern Connecticut.

The age of the metamorphism of the metavolcanic 

rocks is not clear. To the west in the Newport, R.I., area 

(fig. 2), metavolcanic rocks probably of the same age as 

those in the Fall River block were metamorphosed to low 

grade before intrusion of the Newport Granite (Kay and 

Chappie, 1976; Rast and Skehan, 1981). The metavolcanic rocks in the Fall River block were presumably 

metamorphosed at about the same time, before intrusion 

of the granite of the Fall River pluton; however, later 

deformation has obscured the relations.

PROTEROZOIC METAMORPHISM

The metamorphism and structure in basement rocks of 

the Milford-Dedham zone are summarized in table 1. 

Evidence exists in several places that the basement 

rocks were involved in a Proterozoic Z episode of metamorphism. The Cambrian strata have undergone no 

greater metamorphism than nearby Carboniferous 

strata, or, in the Boston area, than Proterozoic Z basin 

fill. I have cited evidence in my discussion of the Milford 

antiform and Salem blocks that there was at least a 

low-grade greenschist-facies metamorphism to the east 

and a middle-grade amphibolite-facies metamorphism to 

the west in the Proterozoic metavolcanic and metasedimentary rocks before intrusion of the 630-Ma Dedham 

and related granites. However, we do not know to what 

extent the amphibolite-facies metamorphism in the western and the extreme southeastern parts of the zone was 

produced during the Proterozoic metamorphic event and 

how much was superimposed during a late Paleozoic 

event known to be more intense to the south and less 

intense to the north. The Proterozoic rocks described by 

Nelson (1974), Volckmann (1977), Bell and Alvord (1976), 

and Goldsmith (this vol., chap. E, table 3) in the less 

intensely metamorphosed area of the Salem and Dedham 

blocks are all in the amphibolite facies, although in many 

places they contain retrogressive assemblages that 

include epidote, chlorite, and sericite (Nelson, 1974; Bell 

and Alvord, 1976). As the Dedham and related granites 

are also involved in this retrogressive metamorphism, it 

must be post-Dedham. The overprinted amphibolitefacies metamorphism in these blocks is pre-Dedham. In 

the Milford antiform, where the batholithic rocks tend to 

be gneissic, the metasedimentary and metavolcanic

rocks are also in the amphibolite facies. There the 

post-Milford Granite deformation is more intense and is 

amphibolite rather than greenschist facies, so no retrogressive assemblages are found in this area. Devonian 

alkalic granite in Rhode Island is partly gneissic (O'Hara 

and Gromet, 1984), as are the metamorphic rocks in the 

New Bedford area. In parts of the Dedham and Foxborough blocks, and in the northern part of the Fall River 

block, the metamorphosed Proterozoic rocks are in the 

greenschist facies, although some uncertainty exists as 

to the proper age assignment for the phyllites (Zb, Zbs) 

and volcanic rocks (Pwv, Zv) in the Wrentham, Dedham, 

Medfield, and Plympton areas. The interpreted pattern 

of Proterozoic metamorphism is shown on the State 

bedrock map. The western part of the Salem block could 

have been shown in the amphibolite facies, but the 

superimposed Paleozoic greenschist-facies metamorphism is pronounced in this area and masks the older 

metamorphic mineral assemblages.

STRUCTURE AND METAMORPHISM OF THE BASINS IN 

THE MILFORD-DEDHAM ZONE

Analysis of the structures affecting basins containing 

cover rocks that overlie the Proterozoic basement helps 

us interpret the tectonic events that affected both basement and cover in the Milford-Dedham zone. The major 

basins (fig. 8) are the Boston basin, occupied by Proterozoic Z sedimentary and volcanic rocks, the Newbury 

basins, occupied by Silurian and Devonian sedimentary 

and volcanic rocks, the Bellingham basin, occupied by 

Pennsylvanian (or Proterozoic Z) sedimentary and volcanic rocks, the Norfolk basin, occupied by Pennsylvanian 

sedimentary rocks, and the Narragansett basin, occupied by Cambrian and Pennsylvanian sedimentary rocks. 

A small basin containing Triassic sedimentary rocks, the 

Middleton basin, has been identified in the LynnfieldMiddleton area (Kaye, 1983). A larger basin containing 

Triassic and Jurassic sedimentary and volcanic rocks, the 

Nantucket basin, is located in the subsurface beneath 

Nantucket and Nantucket Sound. These basins, and the 

structures within and bounding them, will be described 

in order of decreasing age of contained rocks. Discussion 

of the ages of the fold and fault systems in the basins is 

given in the section on blocks and basins of the MilfordDedham zone.

BOSTON BASIN

The structure of the Boston basin has been described 

and summarized by M.P. Billings in a series of papers 

(Billings, 1929; 1976a,b; 1979) based on engineering 

studies by him and others in tunnels in the Boston area, 

on surface mapping by him and his students, and on
H34 THE BEDROCK GEOLOGY OF MASSACHUSETTS

42°30 7115' 71°00' 70°45'

42°15

where concealed. U, upthrown side;

D, downthrown side (where known)

Reverse fault Hachures on uothrown side

I 

10 MILES

I I 

0 5 10 KILOMETERS

FIGURE 12. Structural features of the Boston basin, eastern Massachusetts. A, B, C, Wl, and W2 are alternative interpretations of faults and

fault blocks referred to in text. M, Medford Diabase.

earlier mapping by W.O. Crosby (cited by Billings) and 

LaForge (1932). Billings' studies have been recently 

supplemented by Bell (written commun., 1976), who 

mapped primarily on the periphery of the basin, and by 

Kaye (1980; written commun., 1978, 1979).

The Boston basin contains unmetamorphosed Proterozoic Z sedimentary and volcanic rocks that are in gentle 

to tight open folds and are cut by mostly reverse faults. 

Both faults and folds trend east-northeast. The basin is 

bounded on the north and northwest by the northern 

border fault (figs. 2, 12) and on the south by the Mount 

Hope and Blue Hills faults. West of Hingham, in the 

Hingham anticline (fig. 12), Roxbury Conglomerate rests 

nonconformably on Dedham Granite. The southwest end 

of the basin is terminated by north- to northwesttrending high-angle normal faults that are younger than 

the reverse faults. Folds are open to the north but

become tighter and higher in amplitude to the south. 

Plunge of the major folds averages 8°-15° east-northeast 

to east, steepening to as much as 18° toward the southern 

part of the basin. However, folds between the Mattapan 

anticline and the Blue Hills fault plunge gently to the 

southwest (Billings, 1976a, p. 42). The Hingham anticline, which contains Dedham Granite in its core, plunges 

steeply to the west. High-angle faults cut off the limbs of 

some of the folds in the southern half of the basin so that 

they are almost coincident with synformal axes named by 

Billings (1976a) (not shown on fig. 12, but see cross 

section C-C'" on the State bedrock map). Drag along 

these faults has produced locally steep dips. Axial surfaces of the folds dip steeply north. Cleavage is developed locally in the Mattapan Volcanic Complex and 

Boston Bay Group and also dips steeply north. However, 

the cleavage is not axial planar to the folds in the basin
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H35

(Billings, 1929, p. 101-102) and does not seem to be 

present in the older rocks (Nelson, 1976, p. 1379).

The northern border fault is a reverse fault that 

thrusts the Melrose subblock and Salem block southward 

and southeastward over the Boston Bay Group. It is at 

least 38 km long, extending from south of Natick into 

Massachusetts Bay near Nahant. As exposed in the 

Maiden tunnel (Billings and Rahm, 1966), the fault dips 

about 55° north and is knife sharp. Billings (1976a, p. 41) 

suggested that the stratigraphic throw is not great. My 

reconstruction of the geology in cross section C-C' on the 

State bedrock map suggests a throw of about 1.3 km. The 

border fault has customarily been drawn at the base of 

the topographic scarp near Medford and Saugus. However, south of this scarp, the Nahant peninsula and the 

adjacent shore at Revere Beach contain Cambrian 

strata, rather than strata of the older Boston Bay Group, 

and contain the Ordovician Nahant Gabbro, the only 

Paleozoic intrusive rock in the basin. These observations 

suggest that the Nahant Gabbro and adjoining rocks are 

in a displaced segment (A, fig. 12) of the Melrose 

subblock from north of the northern border fault as 

suggested by Billings (1979, fig. 7; written commun., 

1979). C.A. Kaye (written commun., 1978) suggested 

that the fault shown on the State bedrock map in the 

Boston basin in Watertown (Wl, fig. 12) continues east 

to pass south of Nahant (W2, fig. 12). These interpretations are not shown on the State bedrock map because of 

their speculative nature.

The location of a projection of the border fault into 

Massachusetts Bay is uncertain. The fault appears to be 

offset along the northeast-trending fault from Nahant 

Bay through Marblehead Harbor (C, fig. 12). To the west 

and southwest, the northern border fault loses its identity near Watertown and may splay off to join the 

Weston fault. South of this juncture, Nelson (1975a) 

showed the border fault as a normal fault having throw to 

the south (fig. 12). On the other hand, it may be 

represented in the system of faults trending southwesterly and forming the west side of the Boston basin 

through Wellesley and Natick. The possibility that it is 

part of the system of faults extending toward Woonsocket that separates the Milford block from the Dedham 

block has been mentioned above. The northern border 

fault is cut by the Mesozoic Medford Diabase (J"fcd), 

which places an upper limit on its age.

A series of east-northeast-trending faults, the Mount 

Hope, Neponset, and Blue Hills faults, within the basin 

parallel the northern border fault (fig. 12). They dip 

steeply, and downthrow is to the north. Billings 

(1976a,b) believed that these faults were originally 

northward-directed thrusts dipping less than 45°, which 

have since been rotated to near vertical. He calculated 

throw on the Mount Hope fault from two sites to be 183

m and 350 m, the greater throw to the east. Throw on the 

Neponset fault is about 610 m. The Mount Hope fault 

extends southwest to join with northerly trending faults 

enclosing the southwest end of the Boston basin. To the 

east, the Mount Hope fault extends into Massachusetts 

Bay along the north limb of the Mattapan anticline. The 

Neponset fault cuts off the southeast limb of the Hyde 

Park syncline and the north limb of the narrow Milton 

anticline. The Neponset fault becomes indistinct to the 

west where it cuts into the crystalline rocks. To the east, 

the Neponset fault extends into Quincy Bay, where it 

appears to merge with the Mount Hope fault. Billings 

(1982) presented convincing arguments for the existence 

of the Blue Hills thrust fault, despite contrary strong 

arguments by Kaye and Zartman (1980, p. 258). North of 

Hingham the Blue Hills fault juxtaposes members of the 

Dedham Granite and Roxbury Conglomerate on the 

south with the Cambridge Argillite on the north. West of 

Milton the Blue Hills fault swings to the southwest, 

where it is disrupted by north-trending faults and cannot 

be recognized in the crystalline Dedham Granite. The 

fact that Cambrian and Ordovician rocks to the south are 

thrust over the older Proterozoic Z Boston Bay Group to 

the north on the Blue Hills fault was explained by 

Billings (1982) as due to an earlier Ordovician cauldron 

subsidence of the complex in the Blue Hills consisting of 

the Quincy Granite and Blue Hills Granite Porphyry. 

These rocks and their overlying Carboniferous cover on 

the south have since been thrust northward, rotated, and 

tilted to the south (Billings, 1982, p. 919). Billings 

credited W.O. Crosby (1900) with first suggesting the 

southward tilting of the Blue Hills block. The amount of 

cauldron subsidence was postulated by Billings to be on 

the order of 5,000 m. He suggested that some of the 

north-south faults in the Blue Hills may have been 

originally the bounding faults of the Ordovician cauldron 

subsidence. These north-south faults are clearly postPennsylvanian and are possibly Mesozoic but could have 

followed the loci of the Ordovician faults. He also suggested that the southwest-trending part of the Blue Hills 

fault west of Milton may coincide with a former ring 

fault.

The fault shown on the State bedrock map near 

Watertown (Wl, fig. 12) brings up conglomerate and 

tuffaceous beds on its south side. Billings (1929) formerly 

placed a fault here but in a later paper (1976a, p. 43) 

indicated that this conglomerate is a tongue of Roxbury 

projecting into the Cambridge Argillite at a fairly low 

horizon and that a fault is not needed. I have preferred to 

keep the fault, because the exposure is close to the axis 

of the Charles River syncline where lower stratigraphic 

units would not be expected to appear on the surface. 

Kaye's (written commun., 1978) fault W2 (fig. 12), 

deduced from review of the tunnel data of Billings, may
H36 THE BEDROCK GEOLOGY OF MASSACHUSETTS

be a continuation of this fault, as mentioned above. A 

parallel fault deduced by Kaye is shown as B on figure 12. 

The orientation of the folds in the Boston Bay Group 

indicates to me that they are related to the eastnortheast-trending faults in the basin and that the 

stresses were compressional.

North- and northwest-trending faults shown by Billings (1976a,b) and Kaye (1980) truncate the eastnortheast-trending faults and folds. Some of these 

younger faults offset the Pennsylvanian strata in the 

Norfolk basin and are therefore post-Pennsylvanian in 

age. The largest of these is the Stony Brook fault, which 

Billings (1976a) considered a normal fault along which the 

west side has been downthrown 640 m. It appears to be 

part of an en echelon system extending from the Narragansett basin to north of the northern border fault.

NEWBURY BASINS

The Newbury Volcanic Complex lies in two faultbounded, wedge-shaped and lenticular basins between 

the Nashoba zone and the Milford-Dedham zone in 

northeastern Massachusetts. A very small area near 

Lynnfield shown as Newbury strata by Castle and others 

(1976) is probably the Triassic and Jurassic Middleton 

basin. The exact nature and attitudes of the faults 

bounding the Newbury basins are uncertain. The eastnortheast-trending faults that flank the east-trending 

segment of the northern basin (fig. 10) were shown by 

Shride (1976a) to continue southwest to the Andover 

area. The northernmost, called the Parker River fault by 

Shride (1976b, fig. 1), has more recently been shown by 

Shride (written commun., 1979) to merge with the 

Clinton-Newbury fault near Lawrence and is shown thus 

on the State bedrock map. The southern fault cannot be 

carried far with confidence into the Nashoba zone to the 

west. Movement sense appears to be right lateral on both 

faults (Shride, 1976a). Barosh and others (1974, 1977) 

and Bell and Alvord (1976) showed these two faults as 

continuous with the Spencer Brook and Assabet River 

faults, respectively, of the Nashoba zone. However, 

there seems to be little evidence for such a connection in 

the maps of Castle (1964). Castle and others (1976) limit 

the faults to the arcuate borders of the basins and show 

them to be offset by north-south faults. The northern 

basin contains steeply dipping strata overturned to the 

east and southeast and topping into the basement rocks, 

thus requiring a fault on the southeast side. The eastern 

fault forming the boundary of the southern, wedgeshaped part of the northern basin truncates units of the 

Newbury Volcanic Complex more clearly than the northtrending fault on the west.

The smaller basin southwest of Topsfield (fig. 10) 

mapped by Toulmin (1964) is probably also fault bounded

on both sides. The strata strike northeast about parallel 

to the contacts and dip 40°-75° to the northwest. It is not 

known whether or not the strata are overturned. If they 

are right side up, a fault is required on the west side, as 

Toulmin (1964, p. A71) hypothesized. Shride (1976b, p. 

151) argued that the eastern boundaries of both basins 

are faults. Castle and others (1976) and Shride (1976b, 

fig. 1) showed faults on both sides of the basin at 

Topsfield.

Because of the attitude of the Silurian-Devonian strata 

in the basins, the beds must have been appreciably 

rotated since their deposition, yet the rocks of the basins 

are little metamorphosed. Shride (1976b, p. 151) noted 

the lack of cataclasis in the Newbury as opposed to the 

relatively pervasive cataclasis and alteration of the adjacent Proterozoic Z Topsfield Granodiorite. In the adjacent Nashoba zone to the west, the paragneisses and 

schists are in the upper amphibolite facies of metamorphism. Because the contacts are tectonic, it is difficult to 

say where the rocks of the Newbury Volcanic Complex 

were in relation to the adjacent Nashoba-zone rocks at 

the time of their metamorphism or to the Proterozoic 

rocks to the east at the time of their alteration and 

cataclasis. As the Newbury is subgreenschist facies at 

best and is not sheared, it most likely lies in a graben 

possibly of Mesozoic age. The location of the bounding 

faults of the basins could well be predetermined by the 

presence of the older Bloody Bluff fault system.

BELLINGHAM BASIN

The Bellingham basin contains poorly exposed metasedimentary and metavolcanic rocks, which have been 

considered to be of Pennsylvanian age but in which no 

fossils have been found. It could possibly be all, or in 

part, of Proterozoic Z age, like the rocks in the Boston 

basin (Goldsmith, this vol., chap. E). The basin is 

bounded by faults that are an extension to the southwest 

of the northern and western bounding faults of the 

Boston basin and the Mount Hope fault. The attitude of 

the faults bounding the Bellingham basin is not certain, 

but the linearity of the basin boundaries suggests that it 

is probably steep. However, the granite west of the basin 

is pervasively sheared in a zone about 0.5 km wide, as 

noted in the section on the Milford antiform above. 

Foliation in the granite dips moderately to shallowly to 

the west. A shear zone this wide is not characteristic of 

faults in and flanking the Boston basin. I believe that the 

shallowly dipping shear zone is older than the apparently 

steep faults now forming the west side of the basin. A 

narrow, steeply dipping shear zone in granite is exposed 

on the east side of the basin near Woonsocket, but the 

few exposures of alkalic granite on the flanks of the basin 

to the north are not noticeably sheared.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H37

The strata in the Bellingham basin are metamorphosed 

in the greenschist fades. Pebbles and cobbles in metaconglomerate are extremely to moderately flattened in 

the plane of the foliation, which in turn is locally folded 

and crenulated. Dips of bedding and schistosity range 

from moderately flat to steep. According to Rose and 

Murray (1984), the earliest deformation produced a 

northwest to west-northwest schistosity dipping north. 

Axes of later folds and crenulations trend northeast to 

east-northeast. The western boundary of the Bellingham 

basin effectively forms the boundary between the largely 

brittlely deformed terrane to the east and the variably 

gneissic, more ductilely deformed terrane to the west. 

Phyllite assigned to the Blackstone Group east of the 

basin is at lower metamorphic grade than rocks of the 

Blackstone in the Milford antiform to the west.

NARRAGANSETT BASIN

The structure of the Narragansett basin (figs. 2, 13) 

has received considerable attention because of interest in 

the coal-bearing Rhode Island Formation. Early observations were made principally by Woodworth (in Shaler 

and others, 1899); Skehan and others (1979) and Skehan 

and Murray (1980a,b) summarized the structure more 

recently. In the central and eastern parts of the northern 

Narragansett basin, Massachusetts, the Pennsylvanian 

strata lie in broad, east-trending, open folds and are not 

metamorphosed (Hepburn and Rehmer, 1981). In the 

northwest corner of the basin, the structure is complicated by reverse faults and thrusts and north- to 

northeast-trending folds. Farther south, in Rhode 

Island, the structure again becomes complex (Mosher, 

1983), and the rocks reach sillimanite grade. The following description deals only with that part of the basin in 

Massachusetts.

The strata within the northern part of the Narragansett basin in Massachusetts east of Attleboro and 

between Mansfield and the Taunton River lie in broad, 

open, east-northeast-trending folds (Lyons, 1977) and 

are cut by a single cleavage. The Dighton Conglomerate, 

the highest unit in the Narragansett Bay Group (Skehan 

and others, 1979), occupies the synclines. There is no 

convincing evidence for more than one period of deformation in this area (Woodworth, in Shaler and others, 

1899, p. 157), in contrast to the more complex history 

found in the southern Narragansett basin (Murray and 

Skehan, 1979; Burks and others, 1981; Mosher, 1983, for 

example). In the Attleboro area, at the northwest corner 

of the basin, the Pennsylvanian strata are folded about 

north-northeast axes and have been cut by northtrending, but curvilinear, reverse faults as well as by the 

ubiquitous north-south normal faults (Lyons and Chase, 

1976; Lyons, 1977). The Blake Hill fault block (Woodworth, in Shaler and others, 1899, p. 183) near Plainville 

(A, fig. 13) is a gently to moderately dipping block of rock 

that appears to have been thrust northward over steeply 

dipping strata. To the south, a large north-trending 

synclinal fold passes south and east of the inlier of 

Hoppin Hill (Lyons, 1977). East-directed thrusts lie on 

the east side of the syncline. The Hoppin Hill inlier 

contains Dedham Granite and its fossiliferous Cambrian 

cover. The shallowness of the basin in this area is 

affirmed not only by the exposures at Hoppin Hill but 

also by the presence of a small inlier of granite exposed 

at the southwest end of the Manchester Pond Reservoir 

(J.P. Schafer, oral commun., 1979) southeast of Hoppin 

Hill. The complexity of deformation in the northwest 

corner of the basin is probably due to adjustments of 

basement blocks near their junctions during compressive 

post-Pennsylvanian deformation of the Narragansett and 

Norfolk basins.

The shape of the Narragansett basin is only partly 

determined by observed faults. Depositional contacts of 

the Pennsylvanian strata on the Proterozoic Z basement 

are present on the north and southeast sides. Predepositional or syndepositional faults are inferred to be 

present around much of the basin, however. On the north 

margin of the basin, basal Pennsylvanian strata can be 

seen to rest nonconformably on the Dedham Granite in 

several places. North of Mansfield, weathered granite 

regolith passes upward into bedded arkose. At another 

site near Plainville, moderately north-dipping red arkosic sandstone fills a fracture in the granite. Dips in the 

Pennsylvanian strata near the contact are moderately 

steep to the south, but basinward dips are flatter, 

indicating that the basement block moved during or after 

deposition. Woodworth (in Shaler, 1899, p. 128) noted 

that from Mansfield to Brockton, the dip of the basal 

beds becomes steeper than that of the higher beds as the 

contact is approached. To explain this discrepancy, he 

suggested that an unconformity exists between the basal 

Pennsylvanian red beds and the overlying gray carbonaceous beds typical of the Rhode Island Formation. However, he also conceded that the steeper dips along the 

border could be attributed to drag upward along the 

edges of the basin due to downfaulting of rocks in the 

basin. Woodworth described several places where small 

northeast-trending faults can be seen near the edge of 

the basin. The attitudes of the Pennsylvanian strata are 

the opposite of what would be expected in listric faulting. 

An inferred fault concealed beneath the strata at the 

surface is shown along this contact in cross section C-C'" 

on the State bedrock map.

A north-to-south gravity traverse by Peter Sherman 

(Weston Observatory, Boston College, 1976, fig. 6B) 

from Mansfield to Assonet that crosses the northern part 

of the basin indicates what might be the general config-
H38 THE BEDROCK GEOLOGY OF MASSACHUSETTS

42°15' 71°00' 70°45'

42°00' -

41°45'

 :r^^:;- ;;/^^;^^;:.;/.;;;>\.:^^;^: ;/.;:::: ;-^^^V.^^V^v ^^ ^^^^ - ' ':"-: ^ ^,: ^

' ' : ' ' -I ' '. '- . .'-* ' .'.'. ' '. ' ' .-'. .' '. '. '. '.' : :.'.'.'' ' .Suver-p

'"'"'' ' "*!'^'-:-: .";^

: : ' ^ ' :'-\-::'-:'---J-^. \ .-:-

- ."- .-: ; - . - -'.- Middleborb^"^/ '-\ 

 ' : -- - .:- .- .-. .- ' -.-/- * /. ' \ ; :-v::-:\---/:-^v.v-:-v.^^v^y: : .\--.Vx

 -'.' ' .' /Assawompsetl/. 

''.-'.-:. "/ Pond &'-' -*&' "

0 10 MILES

0 5 10 KILOMETERS 

FIGURE 13. Structural features of the Norfolk and Narragansett basins (stippled) in eastern Massachusetts.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H39

EXPLANATION

Contact

High-angle fault Dashed where

inferred; dotted where concealed.

D, downthrown side; U, upthrown

side (where known) 

Reverse fault Hachures on upthrown

side 

Thrust fault Teeth on upthrown

side 

Anticline

Syncline 

Geologic features

Blake Hill fault block

Diamond Hill

Hoppin Formation at West Wrentham

Sheared gabbro

Basement inlier at Pondville

Mesozoic mafic dike at 

North Middleboro

FIGURE 13. Continued.

uration of the floor of the basin and the possible slope of 

the margins. The profile indicates a little more than 2 km 

of downthrow on the northern and southern margins 

along fault planes that dip moderately into the basin. 

Skehan and Murray (1980b, fig. 4) showed a normal fault 

downthrown to the south within the basin and south of 

the northern contact with the crystalline rocks to account 

for an abrupt thickening of the Pennsylvanian strata. On 

the basis of the abrupt thickening and lack of observed 

faulting at the predepositional to syndepositional surface, a fault that does not reach the surface is shown 

along the northern margin of the basin on cross section 

C-C'" on the State bedrock map. Between Rehoboth and 

Dighton, the floor of the basin is about 1 km shallower 

than it is to the north, but the basin deepens toward the 

south edge. A moderately dipping normal fault is inferred in the profile on the north side of this block. The 

gravity profile indicates that a normal fault is present on 

the south side of the basin near Assonet (figs. \\A,B\ 13). 

Yet at the contact here, as on the north side of the basin, 

the Pennsylvanian strata seem to rest nonconformably 

on the granitic basement because, near Assonet, shallow 

drill holes basinward from the contact indicate that 

reworked regolith lies at the base of the Carboniferous 

beds (J.A. Sinnott, oral commun., 1979). Thus, if a fault 

of the amount of displacement indicated by the gravity 

profile exists, it must be farther out in the basin. Even 

so, the granite of the Fall River pluton just east and 

south of Assonet shows fracturing and alteration. The 

existence of a northeast-trending fault here is supported 

by the abrupt increase in depth to top of basement shown 

(Williams and Willey, 1973) in the hydrologic map of the 

Taunton River drainage area west of the inlier of basement located north of the town of Middleboro (Woodworth, in Shaler and others, 1899; Hartshorn, 1960; 

Lyons, 1977). On the basis of the above data, an inferred 

fault has been drawn along the southeast side of the 

Narragansett basin from the Fall River area to the 

North Plympton area. It may be farther away from the 

contact of the Pennsylvanian strata with the basement 

than shown on cross section C-C'" and may be in part or 

all pre- or syn-Carboniferous, like the fault inferred at 

the north side of the basin. From Fall River south, 

Pennsylvanian pebble conglomerate rests nonconformably on the Proterozoic Z basement. The fault, if present, 

presumably lies to the west. The existing scarp at the 

boundary of the basin may be due solely to difference in 

rock resistance to weathering and erosion. This inferred 

northeast-trending fault is along the same trend as the 

Beaverhead fault (Murray and Skehan, 1979) of southern 

Rhode Island and may represent a continuation of it or 

one of a parallel zone of faults (Barosh and Hermes, 1981; 

Burks and others, 1981, p. 268). This fault may be offset 

by faults bounding a small horst of basement shown by 

Quinn (1971) on the north end of Aquidneck Island west 

of Tiverton. Skehan and Murray (1980b, fig. 6) showed a 

series of west-directed thrust faults south of this locality, 

which have moved the crystalline rocks westward over 

the basin strata. In this case, the inclination of the 

Assonet fault may actually be to the southeast.

The shape of the east end of the Narragansett basin is 

largely controlled by north-trending faults in part determined by mapping and in part inferred from the hydrologic maps of Williams and Willey (1973) and Williams 

and others (1975). The most notable feature is the 

Assawompset Pond graben mapped by Koteff (1964) and 

described above (figs. 2, 11). Similar faults of lesser 

magnitude offset the contact of the basin to the northeast. The north-trending faults that offset the 

Pennsylvanian-basement rock boundary in the Hanover 

area are based on basement configuration, well information, surface topography, and alignment of boulders of 

vein quartz observed by Lyons (1977). A northeasttrending fault extending from Silver Lake to Marshfield 

is inferred from topography and outcrops of shattered 

and silicified rock and veined rock near Marshfield 

(Chute, 1965).

The extension of the Narragansett basin to the area 

north of the Marshfield Hills is based on drilling in 1979 

by the U.S. Geological Survey (E.G.A. Weed, unpub. 

data, 1979). Two shallow drill holes along the North 

River, one north of North Marshfield and the other near 

the coast south of Scituate, passed through red sandstone of either the Wamsutta Formation or the Rhode 

Island Formation. The Marshfield Hills and adjoining 

areas to the south proved to be underlain by granite. The 

boundaries of the basin shown on the State bedrock map
H40 THE BEDROCK GEOLOGY OF MASSACHUSETTS

in this area are only approximate, but the east-west 

reach of the North River south of Scituate surely lies 

within the basin.

The west side of the Narragansett basin lies primarily 

in Rhode Island, where the boundary is considered to be 

a fault (fig. 13). This fault, the Diamond Hill fault, 

projects northward north of the Massachusetts-Rhode 

Island State line and the mass of vein quartz at Diamond 

Hill (B, fig. 13) (Quinn, 1971, p. 45). The area north of 

Diamond Hill coincides with the faulted southern termination of the Norfolk basin and its junction with the 

Narragansett basin. The small outcrop of Cambrian 

Hoppin Formation south of West Wrentham (C, fig. 13) 

appears to lie in a fault wedge at this juncture. A splay of 

the western boundary fault of the basin appears to 

project to the north into a fault shown by Volckmann 

(1977) that extends from Franklin through the Dedham 

area toward the Boston basin. A sliver of sheared gabbro 

is exposed along this trace northwest of West Wrentham 

(D, fig. 13).

NORFOLK BASIN

The Norfolk basin is a partly fault-bounded, eastnortheast-trending synclinal basin between the Dedham 

and Foxborough blocks (fig. 13). Basin fill consists of the 

alluvial Pondville Conglomerate and the Wamsutta Formation. These Pennsylvanian strata are folded and 

cleaved but are unmetamorphosed (Hepburn and Rehmer, 1981). Cazier (1984) mapped two sets of minor folds 

in the Pennsylvanian strata, both having horizontal axes 

that strike roughly east-northeast parallel to the trend of 

the basin margins. The first is isoclinal and has a related 

pressure-solution cleavage dipping about 40° N.; the 

second is open and has an axial planar cleavage dipping 

about 50° N. Cazier noted that thrust faults along the 

southeast margin of the basin deform both cleavages. 

The north side of the basin appears to be an unconformity 

that now dips steeply. The south side of the basin is the 

clearly defined Ponkapoag3 fault. The basin is offset 

along its length by the faults of the late north- to 

north-northwest-trending system that offset the bounding faults of the Narragansett and Boston basins.

The Ponkapoag fault separates the Pennsylvanian 

strata from the Proterozoic Z Dedham Granite of the 

southeastern Massachusetts batholith. East of Braintree, the Pennsylvanian rocks are cut out by the fault so 

that the Ordovician and Silurian Quincy Granite and the 

Cambrian Braintree Formation lie against the Dedham. 

Near Hingham, the fault cuts into the batholithic rocks,

3Although the town near this fault is labeled Ponkapog on the State bedrock 

map, the name of the fault was spelled Ponkapoag by Billings (1976a, 1982), and 

this usage is followed here.

isolating a patch of Dedham Granite north of the fault 

and nonconformably overlying Roxbury Conglomerate in 

the Hingham anticline. The Ponkapoag fault appears to 

dip steeply throughout its extent. Near Weymouth the 

fault dips about 80° N.; farther west it dips about 60° N., 

although Cazier (1984) described thrust faults along the 

southeast margin of the basin that dip 25°. Billings 

(1976a,b) computed the stratigraphic throw near Hingham to be about 410 m. He believed then that the fault 

was originally a northward-directed thrust, which had 

been rotated back to the south, like some of the faults in 

the Boston basin. Billings (1982, p. 919) more recently 

proposed that the fault is normal and that the downthrown block is to the north.

The north side of the Norfolk basin in the vicinity of 

the Blue Hills is not a fault but an unconformity (Chute, 

1969). Here the basal Pondville Conglomerate dips about 

60° S., slightly less steeply than the underlying Blue 

Hills Granite Porphyry (SObgr) in the Blue Hills (Billings, 1982, p. 917). These relations led Billings to conclude that the block between the Blue Hills fault and the 

Norfolk basin has been tilted about 60°, up to the north 

and down to the south, since deposition of the Pennsylvanian strata.

Elsewhere along the north side of the Norfolk basin, 

the relations are less clear. West of the Stony Brook fault 

(fig. 13), Chute (1964, p. 41-12) cited truncation of units 

as evidence for a southeastward-directed thrust. I suggest that the discontinuous nature of Pondville Conglomerate along the flanks of the basin need not indicate 

faulting but could be due to nondeposition. No basal 

conglomerate is present to the south, on the north side of 

the Narragansett basin, but lenses of conglomerate 

indicating channels are present within the Rhode Island 

Formation above the base of the section (Lyons, 1977). 

However, on the State bedrock map I have shown 

Chute's (1964) fault on the north side of the basin as 

continuing south into the Franklin area, because the 

geometry there seems to require it.

The southwestern end of the Norfolk basin is appreciably broken by faults and is flanked in part by felsic 

volcanic rocks of uncertain age. The Sharon Syenite just 

east of the Ponkapoag fault exposed on 1 195 south of 

Franklin is appreciably altered and fractured, but elsewhere faults themselves are not exposed. A horst or 

inlier (E, fig. 13) near Pondville containing Dedham 

Granite and fine-grained granite flanked by steeply 

dipping Pennsylvanian strata splits the southwestern 

end of the basin into two parts. The Wamsutta Formation in the southern part is continuous with Wamsutta in 

the Narragansett basin. The Wamsutta in the northern 

part, near Weymouth, is cut off against the Ponkapoag 

fault.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H41

MIDDLETON BASIN

The Middleton basin, containing Upper Triassic and 

Lower Jurassic red beds (Kaye, 1983), lies in the same 

north-northeast-trending structural zone as the Newbury basins and is close to the southern Newbury basin 

(figs. 2,10). The Middleton basin is estimated to be about 

5.7 km long and no greater than 0.5 km wide; however, 

the strata are actually exposed in only one place. A 

normal fault striking northeast, exposed on the southeast 

margin of the basin, separates the basin from sheared 

rock of the Proterozoic basement. Kaye (1983) estimated 

the throw on this fault to be greater than 500 m. The 

western contact of the basin is not exposed. Beds of 

arkosic conglomerate contain cobbles of crystalline rocks 

of the area, and the beds are clearly unconformable on 

the crystalline rocks. The Triassic and Jurassic red beds 

are not sheared but dip to the west, more steeply at the 

bounding fault than away from it. If the dips do not 

reverse, a fault should lie at the west side of the basin 

also, but Kaye indicated that the beds probably do dip to 

the southeast on the west side. This contact, then, which 

is shown on the State bedrock map as a fault, could 

instead be an unconformity.

The basin lies at a location where the Bloody Bluff fault 

and the Burlington mylonite zone cease to be readily 

identified and where the contact between the Nashoba 

zone and the Milford-Dedham zone turns northerly from 

its general northeast trend. The fault at the southeast 

side of the basin is clearly post-Late Triassic and indicates that Mesozoic faulting has occurred in the area. The 

faults bounding the Newbury basins in this same zone 

are quite possibly, in part at least, Mesozoic. This 

younger faulting would account for the lack of pervasive 

mylonitic texture in the Newbury rocks such as characterizes the basement rocks in the Bloody Bluff and 

Burlington mylonite zones, which have had a longer 

history of deformation (see section below on the Bloody 

Bluff fault zone).

NANTUCKET BASIN

A northeast-trending basin containing sandstone and 

basalt of Triassic and Jurassic age is buried beneath the 

Coastal Plain sediments on Nantucket Island and under 

Nantucket Sound (Weed, in Goldsmith, this vol., chap. 

E; cross section E-E' of the State bedrock map). This 

basin trends northeast and is inferred to be about 25 km 

wide and 100 km long (Austin and others, 1980). The 

northern boundary, beneath the middle of Nantucket 

Sound, is interpreted as a southerly dipping normal fault, 

on the basis of seismic-reflection profiles (Ballard and 

Uchupi, 1975). The strata within the basin appear to dip 

north. The shape of the floor of the basin is not known, 

nor is the nature of the southern boundary. The deposits

are at least 1 km thick. A smaller northeast-trending 

basin extending from the lower arm of Cape Cod across 

Cape Cod Bay has been postulated by Ballard and 

Uchupi (1975), on the basis of seismic-reflection profiles 

and magnetic data.

The Nantucket basin, like the Middleton basin, is part 

of the system of Mesozoic rift basins of the Eastern 

United States and maritime Canada (Klitgord and 

Behrendt, 1979; Grow and others, 1979), a swarm of 

which are present to the north in the Gulf of Maine and 

the Bay of Fundy (Ballard and Uchupi, 1975; Klitgord, 

1984).

MAFIC DIKES

The orientations and ages of mafic dikes in eastern 

Massachusetts provide clues to the pattern of regional 

stress through time. The larger dikes such as the Medford Diabase and the dike in the Bellingham basin are 

shown on the State bedrock map. A few smaller dikes are 

shown elsewhere, as on islands in Boston Harbor and in 

the Marlborough area. One dike noted by Lyons (1977) is 

shown cutting the Rhode Island Formation at North 

Middleboro (F, fig. 13). Most dikes trend north-south and 

are considered to be of Mesozoic age. A few thin northsouth dikes not shown on the map can be seen in the 

crystalline basement southwest and west of Boston in 

highway cuts along 1-495 and 1-90. Dikes are more 

numerous in the Milford-Dedham zone than is shown on 

the State bedrock map, and they are not all of Triassic or 

Jurassic age. LaForge (1932) and Ross (1981) described 

dikes of diabase and lamprophyre of several ages in the 

Boston area. Ross, who studied the dikes most recently, 

found from isotopic work that most are probably Triassic 

and Jurassic in age, but they range in age from Devonian 

to Jurassic. The orientation of the dikes in relation to age 

has not been measured systematically. LaForge (1932), 

however, divided the dikes into an older group, striking 

west to northwest and dipping variably, and a younger 

group, striking north and dipping steeply. Kaye (oral 

commun., 1981) noted that diabase dikes present on some 

of the islands in Boston Harbor are sill-like. The older 

west-striking dikes are presumably of late Paleozoic age, 

whereas the younger north-striking dikes are Mesozoic. 

Some of LaForge's older dikes are probably related to 

the Brighton Melaphyre and therefore would be Proterozoic Z in age rather than post-Pennsylvanian. No data 

are available on these dikes. Some lamprophyre dikes 

could be lower and middle Paleozoic and related to the 

alkalic plutons in the area; others may be younger.

Dikes and faults of similar ages have similar orientations in the Boston basin area. The Triassic and Jurassic 

dikes are aligned in the general north- to north-
H42 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 2. Metamorphism and structure of cover rocks in basins of the Milford-Dedham zone, eastern Massachusetts

Rocks

Age

Metamorphism

Oldest bounding

faults.

Major deformation and trend

of stress.

Age of major

deformation.

Newbury basins

Mudstone, siltstone, rhyolite,

and basalt.

Silurian and

Devonian.

Low greenschist

East-northeast

and northnortheast; highangle, eastnortheast faults

could be

reverse faults

(rotation

toward basin).

North-northwest

compression;

rotation of

strata.

Post-Early Devonian.

Middleton basin

Arkose, conglomerate, and

shale.

Triassic and

Jurassic.

None

Northeast, highangle, normal.

West-northwest

tension; tilting

of strata.

Jurassic(?)

Boston basin

Slate, argillite,

conglomerate,

and volcanic

rocks.

Proterozoic Z to

Cambrian.

Subgreenschist

East-northeast

reverse faults,

movement

toward basin.

North-northwest

compression;

open to tight

folds.

Alleghanian

Bellingham basin

Phyllite, metaconglomerate, and

volcanic rocks.

Pennsylvanian or

Proterozoic Z.

Greenschist

North-northeast

to northeast,

high-angle, may

be reverse

faults toward

basin.

West-northwest

to northwest

compression;

fold flattening.

Alleghanian

Norfolk basin

Metasandstone,

conglomerate,

and volcaniclastic rocks.

Pennsylvanian

Subgreenschist

East-northeast

reverse fault

toward basin on

south side. Part

of north side

high-angle, possibly reverse

fault toward

basin.

West-northwest

compression;

rotation of

strata.

Alleghanian

Narragansett basin

Sandstone, conglomerate, meta-anthracite,

and volcanic rocks.

Pennsylvanian.

Subgreenschist to

greenschist (in

Massachusetts).

North-south, highangle, normal on

west side; none

known on north side

unless buried. Possible northeasttrending buried fault

on southeast side.

North-south, highangle on east side.

West-northwest compression, open folds;

reverse faults and

thrusts in northwest

corner.

Alleghanian.

northwest trend of the young faults that cut the basin 

margins. The older east- to northeast-trending dikes are 

parallel to the east-northeast-trending faults bounding 

and within the Boston basin.

SUMMARY OF BLOCKS AND BASINS OF THE MILFORDDEDHAM ZONE

Rocks in the basins of the Milford-Dedham zone are 

folded and faulted but, except for those in the Bellingham 

basin, are little metamorphosed (table 2). The Proterozoic Z strata in the Boston basin and the Cambrian strata 

there and at Hoppin Hill are no more metamorphosed 

than are the strata in the Silurian and Devonian Newbury basins and the Pennsylvanian strata in the Norfolk 

basin and northern part of the Narragansett basin. The 

Triassic-Jurassic rocks of the Middleton basin have only 

been faulted and are not metamorphosed at all. Some of 

the strata in the Bellingham basin are no more deformed 

and metamorphosed than strata in the Boston basin. The 

other greenschist-facies rocks mapped within the Bellingham basin may actually be equivalent to the Proterozoic Z Blackstone Group rather than the Pennsylvanian 

Bellingham Conglomerate. The evidence thus indicates 

that little deformation or metamorphism occurred within 

the zone between the Proterozoic Z time and the late 

Paleozoic. The folding and metamorphism seem to be

related to compressive crustal movements in the late 

Paleozoic Alleghanian orogeny. Stresses at that time 

consisted primarily of west-northwest-east-southeast 

compression that broke up the crystalline basement and 

preserved covering strata in differentially down-dropped 

blocks flanked by reverse faults. Farther south in Rhode 

Island, Alleghanian events described by Mosher (1983) 

and Murray and Mosher (1984) involved early east-west 

compression producing westward-directed folds and 

thrusts, followed by a regional strike-slip component 

producing folds oriented slightly more northeasterly and 

verging eastward. Their third phase of folding is localized along an east-northeast-trending shear related to a 

regional east-west megashear system to the south. It is 

not clear how these structures relate to the deformation 

observed to the north in Massachusetts. Probably the 

major control in eastern Massachusetts was preexisting 

zones of weakness in the Proterozoic Z basement, which 

were reactivated during crustal movements in the late 

Paleozoic. Such preexisting zones of weakness may have 

controlled the original shapes and locations of the basins 

and may have controlled the formation of the Boston 

basin during a Proterozoic Z rifting event.

The Pennsylvanian strata were deposited in an alluvial 

plain possibly in a broad intermontane or rift basin that 

formed following the Devonian extensional volcanicplutonic events. McMaster and others (1980) postulated 

sinistral shearing on northeast-trending faults to form
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H43

the basins, followed by dextral shearing on the same set 

of faults in the Permian to cause the post-Pennsylvanian 

compressional events and north-south extension, permitting intrusion of the east-trending Narragansett Pier 

Granite. The abundant north- to north-northwest- and 

locally north-northeast-trending faults are clearly later 

than the faults and folds of Alleghanian age and are 

ascribed to early Mesozoic rifting.

The north-south faults are clearly younger than deposition of the Pennsylvanian deposits in the Narragansett 

basin, as they offset the contact with the crystalline 

rocks at the northeastern and northern edges of the 

basin. Some of these faults, such as those at Diamond 

Hill and in the Hanover area, are associated with vein 

quartz. Brecciation and veining by quartz is typical of 

faults of Triassic and Jurassic age in eastern New 

England (Rodgers, 1970), and I consider the generally 

north-south faults in eastern Massachusetts to be Mesozoic in age. The Assawompset Pond graben is probably a 

Mesozoic structure. The west- and northwest-trending 

faults are possibly of similar age or older. The northeasttrending Beaverhead fault of southern Rhode Island 

must be post-Pennsylvanian because it cuts out Pondville 

Conglomerate on southern Conanicut Island. The 

inferred Assonet fault, however, may be an older fault 

in the basement that partly determined the location of 

the margin of the Narragansett basin before deposition of the Pennsylvanian strata. Apparently, postPennsylvanian movement here has not been great. The 

steeply dipping, west-trending shear zones in the crystalline rocks of the Fall River block are possibly prePennsylvanian because no such shearing has been 

reported in the Pennsylvanian strata in the basin. The 

age of the northwest-striking thin mylonite zones in 

South Dartmouth is not known. No clear relationship is 

seen between structures in the crystalline complex of the 

Fall River block and polydeformation recorded in the 

pre-Pennsylvanian rocks in the southern Narragansett 

basin to the east (Kay and Chappie, 1976; Rast and 

Skehan, 1981). The gneissic plutonic rocks in the New 

Bedford area were deformed at higher temperatures 

than were the rocks at Newport, R.I., but these are in a 

different block. It is most likely that the Fall River block 

was relatively rigid during Alleghanian deformation. 

Possibly the primary foliation formed earlier, at the same 

time as the Si cleavage in the greenschist-facies Proterozoic Z metasedimentary and metavolcanic rocks at 

Newport, R.I., described by Kay and Chappie (1976) and 

Rast and Skehan (1981). The east-northeast-trending, 

southward-verging fold pattern in the crystalline rocks in 

the New Bedford area could be related to development of 

S2 cleavage in the Newport rocks, but the supposition is 

weak. In the Pennsylvanian rocks of the southern Narragansett basin, the oldest deformation is visualized as

east-west compression in which the Fall River block 

moved westward (Burks and others, 1981, p. 272). 

Possibly the east-west shears in the block are tears that 

developed during this process.

In the Boston, Norfolk, and northern Narragansett 

basins, vergence of folding and thrusting has been to the 

south and generally oriented north-northeast. The strata 

in the Newbury basins are tilted into orientations 

approximating the trends of the bounding faults, which 

are east-northeast in the northern part of the northern 

basin and north-northeast in the southern part and in the 

southern basin. The east-northeast trends in the Boston, 

Norfolk, and northern Narragansett basins seem to me 

to form a pattern of deformation that, because it affects 

Pennsylvanian rocks, indicates the deformation is a 

reflection of Alleghanian crustal movements. This deformation increases in intensity from north to south. The 

greater complexity of structures at the northwest end of 

the Narragansett basin near its junction with the Norfolk 

basin is probably due to complications arising from the 

adjustments in cover rocks as basement subblocks were 

jostled during Alleghanian deformation. The controlling 

stress could be the left-lateral shear system striking 

northeast mentioned above.

BLOODY BLUFF FAULT ZONE

The Bloody Bluff fault (Cuppels, 1961, p. D46; Skehan, 

1968, p. 282) forms the boundary between the MilfordDedham zone and the Nashoba zone in eastern Massachusetts from near Westborough to Lynnfield (figs. 1, 

14). The history of recognition of the fault and its 

configuration are described by Castle and others (1976). 

The boundary between the Milford-Dedham zone and the 

Nashoba zone from south of Marlborough to Oxford is the 

Lake Char fault, which on the State bedrock map is 

shown as coinciding with the trace of the Bloody Bluff. 4 

The fault is not well exposed in this interval. In Connecticut the Lake Char fault forms the boundary between the 

gneissic part of the Milford-Dedham zone in the Rhode 

Island anticlinorium and the Putnam terrane as far south 

as the Preston Gabbro (PG, fig. 3) in North Stonington, 

Conn., where the boundary turns west as the Honey Hill 

fault.

The main zone of the Bloody Bluff fault is a polydeformed zone of ductile deformation and cataclasis as 

much as 3.2 km wide near Framingham (Nelson, 1976) 

and 5 km wide in the Burlington area where it is the 

Burlington mylonite zone of Castle and others (1976). 

However, at the type locality of the fault at Bloody

4Since the State bedrock map was prepared, O'Hara and Gromet (1984) have 

identified a shear zone east of the Lake Char fault, which they consider to be the 

southward continuation of the Bloody Bluff fault (see figs. 3, 9).
H44 THE BEDROCK GEOLOGY OF MASSACHUSETTS

10 MILES

10 KILOMETERS

EXPLANATION

 Fault Dashed where inferred

A B D Alternative fault interpretations

not shown on the State bedrock

map

T Topsfield Granodiorite 

S Serpentinite at Lynnfield 

C Anticline in Burlington mylonite zone

FIGURE 14. Structural features along part of the Bloody Bluff fault zone, northeastern Massachusetts.

Bluff (fig. 14) (Cuppels, 1961) and other places, brittle 

deformation structures are common. This complexity 

indicates that the Bloody Bluff zone has had a long 

history of deformation. Within the zone of mylonitic 

rocks are markedly layered blastomylonites of felsic and 

mafic rock and mylonites of quartzite. Similar rocks can 

be seen in the Proterozoic Z Waterford Group in the 

lower plate of the Honey Hill fault zone southeast of the 

Preston Gabbro in southeastern Connecticut (Goldsmith, 

1980, 1985).

Southwest of Westborough, mylonitic rocks form a 

relatively narrow zone. The fault is exposed on 1-495 

east of Westborough. From Westborough northeast, the 

Bloody Bluff-Lake Char fault system widens and splits 

into several east-trending branches. The central splay is 

the main Bloody Bluff fault of Cuppels (1961, p. D46; 

Skehan, 1968, p. 282). A southern splay, trending east 

toward the Boston basin, was called the Weston fault by 

Nelson (1975a,b; 1976). The northern splay is arcuate and 

less well documented. Castle and others (1976) ended the 

Bloody Bluff fault against (A, fig. 14) a fault (B, fig. 14) 

extending from Westborough to Framingham, which 

they called the Lake Char fault. The splay off the Bloody

Bluff fault from near Millbury to the Framingham area 

shown on the State bedrock map is largely interpreted 

from discordance of foliation pattern. O'Hara and 

Gromet (1984) placed their split between the Lake Char 

and Bloody Bluff faults south of Millbury (5, fig. 2). The 

location of some of these faults is interpretive, and other 

fault patterns could be drawn in northeastern Massachusetts. Castle and others (1976) and Nelson (1976) 

based much of their interpretations of the location of 

major faults on aeromagnetic patterns. Because the 

faults are zones of crushed rock rather than discrete 

surfaces, they could have been shown more accurately on 

the State bedrock map as patterned areas, rather than as 

discrete fault traces. On the State bedrock map the 

significant fault in the Bloody Bluff system is considered 

to be the westernmost fault, the one that separates 

recognizable Milford-Dedham rock assemblages from 

Nashoba assemblages.

The Bloody Bluff fault ends as a discrete entity in the 

Danvers-Lynnfield area of Essex County, at the southern end of the southern Newbury basin and the Middleton basin. This locality appears to be a structural nexus, 

for it is about here that the Burlington mylonite zone is
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H45

no longer recognized, and the postulated Mystic fault of 

Bell and Alvord (1976, fig. 1; fig. 10) intersects the 

Bloody Bluff fault. This nexus probably represents the 

intersection of fault systems of somewhat different 

trends and ages. The identity of the Bloody Bluff fault 

northeast of the Lynnfield-Danvers area is questionable, 

and the fault may change character. The Proterozoic Z 

plutonic rocks in this area have a nonsystematically 

oriented crushed fabric and a retrogressive mineral 

assemblage, unlike the directed, well-recrystallized fabric of the mylonite zones to the southeast. The western 

boundary of the Milford-Dedham zone from Lynnfield 

north is marked by narrow fault zones, such as those 

forming the boundaries of the Newbury basins and at 

least one side of the Middleton basin (Kaye, 1983). The 

Bloody Bluff and the Burlington mylonite zone have 

northeasterly trends, whereas the faults bounding the 

Newbury basins have more northerly trends. The northtrending faults bounding the basins are later than, and 

have truncated and offset, the zones of more pervasive 

northeasterly deformation in the Bloody Bluff and Burlington zones. Possibly a splay of the Bloody Bluff continues northeast along the southern boundary of the 

Topsfield Granodiorite (T, fig. 14), to the Gulf of Maine 

where it lies under the Quaternary sands of Plum Island.

The Weston fault of Nelson (1975a,b) splays easterly 

from the Bloody Bluff fault near Westborough toward 

the northern border fault of the Boston basin in the 

vicinity of Weston (fig. 14). This area is so broken by 

faults that it is difficult to determine the actual relationship of the Weston fault to the border fault; however, the 

north-northeast-trending Burlington mylonite zone 

projects southwestward into the trace of the Weston 

fault, and I have accordingly shown the eastern boundary of the zone as a fault branching off the Weston fault 

near Weston. The Burlington mylonite zone is described 

below.

The fault shown on the State bedrock map splaying off 

the Lake Char fault in the vicinity of Millbury is drawn to 

connect with an east-northeast-trending fault mapped by 

Nelson (1975a) south of Framingham. This fault offsets 

rock units as much as 6 km in a right-lateral sense. 

Southwest of Framingham near Marlborough, Nelson's 

fault was not recognized by Barosh (1978) or by Hepburn 

and DiNitto (1978). Although its existence southwest of 

the Framingham quadrangle is questionable, it is drawn 

on the State bedrock map along contacts of units aligned 

parallel to its possible projected trace and parallel to the 

gneissic foliation in the plutonic rocks in this area.

BURLINGTON MYLONITE ZONE

The Burlington mylonite zone of Castle and others 

(1976) extends from Weston to Lynnfield and Danvers

(figs. 2, 14). They visualized the northwest side of the 

zone as the primary locus of dislocation on the Bloody 

Bluff fault. As described by Castle and others (1976), the 

mylonite zone is about 1.5 km thick although it has been 

folded and faulted to produce an outcrop width of as 

much as 5 km. Castle (1964) showed a north-plunging fold 

of foliation in the zone southwest of Lynnfield (C, fig. 14). 

The rocks range from ultra-fine-grained laminated mylonites to coarse augen and flaser gneisses. Mylonitic 

quartz schist and blastomylonitic, thinly layered, leucocratic, siliceous rocks are seen in thin section to have a 

ribbon-like fluxion structure. The augen and flaser 

gneisses tend to be near the center of the zone, and the 

more laminated rocks are on the flanks. This division 

may reflect original differences in rock composition as 

well as degree of deformation. Within the zone are areas 

of layered mafic nonmylonitic rock and areas of massive 

mafic rock sheared only around the margins. The latter 

form large-scale augen of competent rock around which 

the strain was distributed. Bands and seams of chlorite 

schist are common in sheared mafic rock. The zone of 

serpentine mapped by Castle (1964) near Lynnfield (S, 

fig. 14) is mostly antigorite. Kaye (1983, p. 1076) suggested that it was ultramafic material squeezed up in a 

shear zone. The ductile mylonites are cut by later brittle 

faults (Castle and others, 1976).

The Burlington mylonite zone does not seem to persist 

as a discrete feature northeast of Lynnfield nor southwest of Weston. To the northeast, its last good expression seems to be at the south end of the Newbury and 

Middleton basins. Castle and others (1976, p. 17) concluded that the Burlington mylonite zone does not extend 

seaward toward the Gulf of Maine but turns northward 

and is cut out against, or dips beneath, weakly magnetic 

plutonic rocks (Sharpners Pond Diorite and related 

rocks) and the Newbury Volcanic Complex. Kaye (1983) 

did not show the mylonite zone extending beyond the 

Lynnfield and Danvers area but did suggest (oral commun., 1980) that it swings to the east following the 

southern edge of the Topsfield Granodiorite (T, fig. 14). 

However, as the Topsfield is part of the Milford-Dedham 

assemblage, this zone cannot be the main boundary 

between the Nashoba and Milford-Dedham blocks, which 

must lie beneath the Newbury basins. To the south, the 

Burlington mylonite zone ends near the Weston fault of 

Nelson (1976). On the State bedrock map, I have shown 

the eastern boundary of the mylonite zone as a fault 

continuing south to merge into the Weston fault near 

Weston. This fault line does not represent a discrete fault 

surface but a boundary between mostly sheared and 

mostly unsheared rock.

It is quite likely that the Burlington mylonite zone is of 

composite age. However, Castle and others (1976) stated 

that the Burlington mylonite zone is intruded by the
H46 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Paleozoic alkalic granites, presumably in this case the 

Early Devonian Peabody Granite (Dpgr). This provides 

an upper (young) age limit for some of the deformation, 

at least in this part of the zone.

Mylonitic and cataclastic rocks of the kind described by 

Castle and others (1976) in the Burlington zone and by 

Nelson (1976) along the Bloody Bluff fault zone are 

repeated to the south, although often in a narrower zone, 

along the Lake Char and Honey Hill faults in eastern and 

southeastern Connecticut (Lundgren and Ebblin, 1972; 

Goldstein, 1982; Dixon, written commun., 1983; Goldsmith, 1985) and in the zone east of the Lake Char fault 

identified by O'Hara and Gromet (1984). The wide, 

nonuniform distribution of faults and shear zones, such as 

the Burlington mylonite zone in northeastern Massachusetts, may be the culmination at a shallower level of the 

more pervasive, ductile strain evident in the western 

side of the Rhode Island anticlinorium to the south. The 

irregular distribution of the deformation is probably in 

large part caused by the distribution of rocks of different 

competencies; in addition, this area is a major boundary 

between different terranes in which movement is distributed over a fairly wide zone forming a typical anastomosing cataclastic pattern, albeit on a regional scale. The 

gabbroic rocks of the diorite and gabbro suite act as 

resistant knots around which the strain was distributed, 

as is the case with the Preston Gabbro in Connecticut. 

The major strain appears to have been accommodated by 

the quartzitic and quartzofeldspathic rocks of the Westboro Formation and the overlying volcanic units to 

produce the quartzitic mylonites noted by Castle and 

others (1976).

WOLFPEN LENS

A lens of rock called the Wolfpen lens (figs. 2, 14) lies 

between the northern branch and the main trace of the 

Bloody Bluff north of Framingham and east of Marlborough. It consists of sheared amphibolite and metaplutonic rock that was called the Wolfpen Tonalite by 

Emerson (1917) and "altered and sheared rocks" by 

Hepburn and DiNitto (1978). A question exists as to 

whether these rocks belong to the Milford-Dedham zone 

or to the Nashoba zone. Nelson (1975b) showed the 

Wolfpen lens as much smaller than is shown on the State 

bedrock map. To the west of the boundary of Nelson's 

lens, he mapped a poorly exposed unnamed granite, 

which in its eastern part contains an inclusion of his 

Proterozoic Z Claypit Hill Formation. The western part 

of this granite, which Nelson showed as intruding the 

Marlboro Formation, is now mapped as granodiorite of 

the Indian Head pluton (igd on the State bedrock map) 

and assigned to the Nashoba zone. If, as mapped by 

Nelson, there is only one granite, then this is the only

place in southeastern New England where plutonic rock 

straddles the boundary between the Milford-Dedham 

zone and the Nashoba zone. Because the Claypit Hill is a 

formation in the Milford-Dedham zone, the granite that 

encloses it must also belong to the Milford-Dedham zone. 

Either the rocks mapped as Claypit Hill Formation are 

misidentified and are actually Marlboro Formation, or 

two different granites exist here. Mapping by Barosh 

(1978) and Hepburn and DiNitto (1978) in the Marlborough area indicates that Nelson's granite may be two 

different plutonic rocks. Hence I have drawn the northwest boundary of the lens along a topographic lineament 

aligned with Barosh's and Hepburn and DiNitto's faults 

bounding the zones so that the Wolfpen inclusion of 

Claypit Hill is within the Milford-Dedham zone. This lens 

ends near Sudbury where its northern bounding fault 

rejoins the main strand of the Bloody Bluff fault. The 

sharp bend in the trace of the fault north of the junction 

was shown by Castle and others (1976) as an offset on a 

transecting north-south fault. Barosh and others (1977) 

also showed a north-south fault here (D, fig. 14) that 

offsets the Bloody Bluff fault to the south. They continued the Bloody Bluff to intersect the main trace near 

Lincoln. Shride (written commun., 1979), however, did 

not recognize a fault here and believed the trace of the 

Bloody Bluff fault is only folded. Following Shride, the 

north-south fault is not shown on the State bedrock map.

ATTITUDES OF FAULT SURFACES AND SENSE OF 

MOVEMENT IN THE BLOODY BLUFF FAULT ZONE

The attitudes of fault surfaces in the Bloody Bluff fault 

system have been measured directly in several places. 

The sense of movement is less well determined. The 

Lake Char fault in Connecticut dips about 25° to the west 

in northeastern Connecticut, as determined by measurements of mylonitic fabric. Dips steepen to about 45° as 

this fault is traced into Massachusetts and steepen to 60° 

and more where the fault becomes the Bloody Bluff and 

starts to curve easterly in the Westborough area. Castle 

and others (1976) computed dips of 60° on the basis of 

magnetic data in the Bloody Bluff zone near Marlborough. Mylonite on the west edge of the Burlington zone 

dips 60°-85° to the west. Nelson (1975a) showed dips of 

60°-65° on branches of the Bloody Bluff fault in the 

Framingham area. Castle and others (1976) stated that 

the Bloody Bluff dips more gently at about 20° northwest 

where the fault turns from northeasterly to northerly 

near Lynnfield.

The sense of movement on the main Bloody Bluff fault 

system has been generally considered to be a thrust or 

high-angle fault with movement toward the southeast 

and east (Skehan, 1969; Nelson, 1976). The opposite 

sense of movement has been proposed at least for the 

latest movements on the Lake Char fault (Goldstein,
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H47

1982) and the related Honey Hill fault (Lundgren and 

Ebblin, 1972) in Connecticut. A strike-slip component of 

movement on the Bloody Bluff was proposed by Bell (as 

cited in U.S. Geological Survey, 1969, p. A21-A22) at 

least from Framingham north. At the type locality, 

Smith and Barosh (1983) recognized dextral displacement that deformed an earlier foliation aligned parallel to 

the fault trend. O'Hara and Gromet (1984) stated that 

movement sense in the shear zone east of the Lake Char 

fault is dextral. However, Castle and others (1976) 

pointed out that "b" lineations in minor folds associated 

with the mylonites have gentle plunges. Any translational movements like those noted by Smith and Barosh 

(1983) may have been relatively late. Nelson (1976) saw 

little evidence for lateral displacement in the ductilely 

deformed rocks.

Some transcurrent movement has been reported for 

minor and probably young faults in the zone. Northeast 

of Lynnfield, where the major fault zones are not 

exposed, sense of movement on northeast- and northnortheast-trending faults flanking the basins (fig. 10) is 

right lateral, according to Bell and others (1977). Other 

and younger minor faults (D, fig. 10) strike northwest 

across the trend of the rocks and dip at high angles. In 

the Cape Ann and Ipswich area, Dennen (1975) showed 

the northeast-trending faults as having right-lateral 

transport and also a vertical component, up on the 

northwest. Dips are not indicated but are presumably 

steep to the northwest.

AGE OF THE BLOODY BLUFF FAULT ZONE

The metamorphic mineral assemblages and textures of 

the ductilely deformed rocks within the Bloody Bluff 

zone indicate that they were formed initially at pressures 

and temperatures prevailing at medium metamorphic 

grade. According to Nelson (1976), movement may have 

commenced during or after the regional metamorphism 

of the metasedimentary and metavolcanic rocks. This 

metamorphism probably occurred before intrusion of the 

Proterozoic Z Dedham Granite. Retrogressive mineral 

assemblages, cataclasis, and hydrothermal alteration 

indicate that deformation continued, or occurred later, 

within the fault zone at lower temperature and pressure. 

The Ordovician Andover Granite in the adjacent Nashoba block is mylonitized (Nelson, 1976, p. 1383). An 

upper age limit on at least some of the movement is 

provided by Castle and others (1976), who, as mentioned 

above, observed that the Early Devonian Peabody Granite intrudes and is not deformed by the Burlington 

mylonite zone. Nelson (1976, p. 1382) mapped undeformed pegmatite cutting cataclastic rock of the Bloody 

Bluff zone, but its age is unknown. The contrast in 

deformation between the Newbury Volcanic Complex

and the adjacent Topsfield Granodiorite suggests a postProterozoic, pre-Late Silurian movement on the Bloody 

Bluff in that area. However, a growing weight of evidence indicates that extensive crustal movements 

occurred in the late Paleozoic during the Alleghanian 

orogeny in eastern New England (Rast and Skehan, 

1983; Gromet and O'Hara, 1984; Hermes and Zartman, 

written commun., 1984; Murray and Mosher, 1984). 

From this evidence, some of the deformation along the 

Bloody Bluff fault zone might have occurred in the late 

Paleozoic. Because the fault that bounds the Triassic 

Middleton basin on its south side cuts the mylonitic 

rocks, one can conclude that Mesozoic movement has 

occurred in other places also along the Bloody Bluff zone. 

Therefore, I conclude that the Bloody Bluff fault zone 

had a long history starting with postmetamorphic pervasive ductile deformation, before the Late Silurian and 

Early Devonian, and concluding with shallow, brittle 

deformation probably ending in the Mesozoic.

TECTONIC EVENTS IN EASTERN 

MASSACHUSETTS

Eastern Massachusetts consists of all or parts of three 

lithotectonic zones or belts distinguished by differences 

in stratigraphy and in metamorphic, plutonic, and deformational history (Goldsmith, this vol., chaps. E, F; 

Robinson and Goldsmith, this vol., chap. G; Wones and 

Goldsmith, this vol., chap. I). These three are the 

Milford-Dedham zone, the Nashoba zone east of the 

Merrimack belt, and the east flank of the Memmack 

belt. The Newbury basins, described as part of the 

Milford-Dedham zone (Goldsmith, this vol., chap. E), are 

not readily assignable to either of the adjacent zones and 

are treated as a separate terrane for purpose of discussion in the following section. The present positions of the 

zones are largely determined by post-Pennsylvanian 

tectonic events. Before that time the terranes had differing geologic histories (table 3, fig. 15). The sources of 

these terranes and their times of mutual accretion are 

only partly understood. Questions and uncertainties 

about the tectonic events summarized in table 3 and 

figure 15 are discussed below, as are speculations on the 

history of accretion of the terranes.

MILFORD-DEDHAM ZONE

The Milford-Dedham zone is fragmented by faults into 

blocks of exposed basement and blocks containing Proterozoic Z to Mesozoic cover rocks. This fragmentation 

took place primarily in the Permian and TriassicJurassic, but earlier events probably predetermined loci 

of later deformation. Earlier events include the forma-
H48 THE BEDROCK GEOLOGY OF MASSACHUSETTS 1u1i$s0)

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TABLE 3. Tectonic events in eastern Massachusetts Continued

Milford-Dedham zone Nashoba zone

Event Representative feature Event Representative feature

CQH

»d

O 

Hd

East side of Merrimack belt P

Event Representative feature J> 

^

Cambrian Transgressive littoral and shallow shelf 

deposition on passive continental margin.

Proterozoic Z Uplift, erosion. Rifting and rift-basin 

turbidite filling; continued volcanism, mainly mafic. First delineation of basins 

of Milford-Dedham zone? 

Felsic volcanism; granitic calc-alkalic plutonism of batholithic dimensions.

Volcanism and plutonism dominantly mafic 

to intermediate calc-alkalic. Greenschist 

to lower amphibolite-facies dynamothermal metamorphism in compressive 

tectonic (subduction) regime.

Volcanic arc (island or continental margin) 

littoral to shelf sedimentation. Volcanic 

component increasing upward.

Nonconformity at base of No record 

Hoppin Formation.

Boston Bay Group, including the mafic volcanic 

rocks of the Brighton 

Melaphyre.

Mattapan Volcanic Complex, Westwood Granite 

(579 Ma); Dedham Granite, Milford Granite (630 

Ma).

Mafic and felsic metavolcanic rocks; gabbro, diorite (diorite at Rowley 

656±16 Ma).

Sedimentation and volcanism in a basin west of a 

volcanic arc.

Tadmuck Brook Schist, 

Nashoba Formation, 

Marlboro Formation. 

730±26-Ma zircon in Fish Brook Gneiss (646- 

Ma zircon from Massabesic Gneiss Complex in 

New Hampshire) (Aleinikoff and others, 1979).

Westboro, Plainfield Formations (1,500-Ma detrital zircons in Westboro).

No record.Not known.

KHH 

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H50 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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 .: .;:';vr

25 MILES

I 

25 KILOMETERS

C. Ordovician D. Silurian and Devonian

FIGURE 15. Tectonic events in eastern Massachusetts corresponding approximately to intervals of time indicated on table 3. Nashoba and 

Milford-Dedham zones are exotic terranes accreted to the North American craton during the Paleozoic. Base is present-day arrangement of 

lithotectonic units. Explanation for figure 15 follows on p. H52.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H51

72° 

43°|-f42'

70°

0 25 KILOMETERS

E. Late Devonian and Mississippian F. Pennsylvanian

G. Permian H. Triassic and Jurassic

FIGURE 15. Continued.
H52 THE BEDROCK GEOLOGY OF MASSACHUSETTS

EXPLANATION

Deposition and igneous activity 

Sedimentation

Sedimentation and volcanism 

Sedimentation, volcanism, and plutonism

Metamorphic grade 

Subgreenschist facies or lower

Greenschist facies 

Amphibolite facies 

Upper amphibolite facies

Plutonism (may be combined) 

S-type

Faults Dashed where inferred; queried where uncertain

High-angle; U, upthrown block: D, downthrown block. 

Some pre-Triassic faults may be transcurrent

Reverse; teeth on upthrown block

Low-angle; teeth on upper block. Movement sense may 

be thrust or normal

Antiform

 t Synform

FIGURE 15. Continued.

tion of a Proterozoic Z magmatic arc, probably on a 

continental margin, eventual rifting of this margin, and a 

poorly documented post-Devonian, pre-Pennsylvanian 

erogenic (collisional?) event, which led in the Pennsylvanian to uplift, erosion, and subaerial sedimentation from 

an eastern source no longer present. Compressive deformation and dynamothermal metamorphism probably 

related to subduction, recorded in the stratified rocks of 

the Proterozoic basement, appear not to have occurred 

again until the Late Pennsylvanian and Permian. In the 

interim, through much of the Paleozoic, a static, essentially extensional, intracratonic plutonic-volcanic regime 

prevailed.

The actual timing of the Proterozoic metamorphism 

and the subsequent events that led to development of the 

present map pattern are not entirely clear. The stratified 

rocks were metamorphosed before intrusion of the batholiths, but how much earlier is not certain. Because the

metamorphosed mafic and felsic volcanic rocks (Zv) are 

considered to be precursors to, or penecontemporaneous 

with, the mafic plutonic rocks (Zgb, Zdi, Zdigb), the time 

of metamorphism could lie between the times of their 

emplacement, as indicated by the 656-Ma date on the 

diorite at Rowley, and the emplacement of the Dedham 

Granite at 630 Ma (table 3). However, it is possible that 

the Westboro Formation and its equivalents and the 

overlying mafic volcanic rocks (Zv) were metamorphosed 

earlier, before emplacement of the mafic plutonic rocks. 

The dated diorite at Rowley is little deformed and may 

represent a fairly young phase of the mafic plutonism.

The metamorphic rocks are at greenschist facies 

throughout much of the zone. They are, however, in the 

amphibolite facies along the west side of the Rhode 

Island anticlinorium and the Milford antiform and in the 

New Bedford area. In these areas, the Proterozoic Z 

plutonic rocks have also been deformed in the amphibolite facies (O'Hara and Gromet, 1984). Therefore, the 

time of amphibolite-facies metamorphism on the west 

and southeast sides of the Milford-Dedham zone is open 

to question. I have shown this metamorphism on the 

State bedrock map as Proterozoic in age. However, other 

possibilities are that the greenschist-facies Proterozoic 

metamorphism has been overprinted by an amphibolite 

facies at the time of metamorphism of the Nashoba zone, 

at the time of the widespread Permian dynamothermal 

event of southern New England, or in the Late Devonian. The first seems rather unlikely because the steep 

metamorphic gradient between the two zones and their 

distinctly different Paleozoic plutonic signatures preclude the two zones being in juxtaposition before and 

during the early Paleozoic. The Permian event is more 

likely in view of the mounting evidence that an intense 

thermal metamorphism affected basement and cover 

alike in southern Rhode Island and southeastern Connecticut (see, for example, Skehan and Murray, 

1980a,b). This event extended northward into central 

Rhode Island where Day and others (1980) implied fairly 

shallow northerly dipping isotherms. However, there is 

no record of a Permian amphibolite-facies metamorphism 

affecting the Nashoba zone. The somewhat higher grade 

of metamorphism on the west and southeast flanks of the 

Milford-Dedham zone as opposed to the generally lower 

grade of metamorphism in the center could be interpreted primarily as resulting from greater uplift along 

the western and southeastern parts of the zone during 

the Permian, which exposed deeper levels of the crust, in 

similar fashion to the greater uplift inferred along coastal 

Rhode Island and Connecticut (Lundgren and Ebblin, 

1972). There is a good possibility, though, that the 

deformation that transformed the Proterozoic plutonic 

rocks into orthogneisses may have begun earlier than the 

Permian. Castle and others (1976) noted that the Early
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H53

Devonian Peabody Granite intrudes but is not deformed 

by the Burlington mylonite zone. Nelson's (1976) observation that pegmatite cuts the mylonite in the Bloody 

Bluff fault zone suggests a pre-Devonian age for the 

mylonite. To the south along the zone boundary, Pignolet 

and others (1980) have dated cataclasis in the Honey Hill 

fault zone (fig. 3) and deformation in the Silurian and 

Devonian Canterbury Gneiss of the upper plate as Devonian, although O'Hara and Gromet (1983) questioned 

their interpretation of the data. Losh and Bradbury 

(1984) recognized both pre-424-Ma and Acadian movements in the Honey Hill fault zone. The Proterozoic 

rock units in southeastern Connecticut are multiply 

deformed, producing an interference pattern (Goldsmith, 

1985). This deformation involves a Carboniferous alkalic 

granitoid (R.E. Zartman, oral commun., 1981) formerly 

assigned to the New London Gneiss (the Joshua Rock 

Member). It seems likely that a post-mid-Devonian metamorphism, probably Carboniferous, ranging from 

amphibolite to greenschist facies has been superimposed 

on the earlier Proterozoic greenschist- or possibly lower 

amphibolite-facies metamorphism. Although the age of 

earlier movements is not well documented, all agree that 

latest major movements on the Lake Char and Honey 

Hill faults in this region are at greenschist facies and are 

of Permian age. Because the same age and kind of rocks 

having similar styles of metamorphism and cataclasis 

exist along the Milford-Dedham zone boundary at the 

north end of the Rhode Island anticlinorium as in southeastern Connecticut, I feel somewhat confident in using 

the conclusions reached in southeastern Connecticut 

to support the conclusions reached from observations 

made on the boundary of the Milford-Dedham zone in 

Massachusetts.

NASHOBA ZONE

The Nashoba zone is a relatively homogeneous terrane 

containing high-grade metamorphic rocks derived from 

volcanic and sedimentary rocks of Proterozoic Z or early 

Paleozoic age (table 3; shown as Proterozoic on fig. 15A). 

Compositional layering, metamorphic foliation, and unit 

contacts dip steeply throughout the zone. The metamorphic rocks are intruded by foliated peraluminous granite 

of Ordovician age and nonfoliated diorite, granodiorite, 

and granite of Silurian and perhaps also Devonian age. 

The zone is cut by numerous faults of late Paleozoic and 

probably Mesozoic age. The timing of the high-grade 

dynamothermal metamorphism in the Nashoba zone 

(table 3) is not clearly defined. The parallelism of the 

foliation in the Ordovician Andover Granite to the metamorphic foliation in the host Nashoba and Marlboro 

Formations indicates that intrusion of the Andover was 

paracontemporaneous with the regional metamorphism. 

Accordingly the metamorphism is shown as Ordovician

on figure 15C. However, there could also have been an 

earlier metamorphism of Proterozoic Z age. As mentioned above, the relatively steep metamorphic gradient 

between the Nashoba zone and the Milford-Dedham zone 

indicates that the two zones were not juxtaposed at the 

time of high-grade metamorphism in the Nashoba zone. 

The fact that the Nashoba contains a suite of Paleozoic 

plutonic rocks distinct from those in the Milford-Dedham 

zone (Wones and Goldsmith, this vol., chap. I) indicates 

that the two were not juxtaposed through the Silurian 

and perhaps the Devonian. The Silurian intrusive rocks 

of the zone (fig. 15D) are of a type associated with 

continental volcanic arcs (Wones and Goldsmith, this 

vol., chap. I). Some aplite and pegmatite in the Andover 

Granite are younger than the bulk of the Andover 

Granite (Wones and Goldsmith, this vol., chap. I) and are 

possibly of Silurian and (or) Devonian age. Evidence for 

Alleghanian metamorphism and plutonism is lacking in 

the Nashoba zone. Retrogressive assemblages have not 

been developed to any great extent (see Abu-Moustafa 

and Skehan, 1976) except for an episode of hydration, 

which is probably associated with Andover plutonism. 

There is, however, a considerable amount of late Paleozoic faulting. Within the Nashoba zone, the older dynamothermal metamorphic fabric has been cut in more or 

less imbricate fashion by later shearing and faulting that 

strikes subparallel to the bounding faults (Skehan, 1967, 

1968) so that the whole block could be considered a zone 

of shear, the Nashoba thrust belt of Barosh (1982), 

between the Bloody Bluff fault zone and the ClintonNewbury fault zone. However, we can only speculate as 

to whether the movement sense was dominantly lateral 

or dominantly inclined. The Clinton-Newbury fault zone 

itself contains both moderately dipping and steeply dipping fault surfaces, which seem to be of different ages 

and textural fabrics and which have not been sorted out 

satisfactorily. The Bloody Bluff fault zone likewise contains differently dipping fault surfaces and both ductile 

and brittle fabrics. Most of the faults within and on the 

flanks of the Nashoba zone appear to be fairly steep, 

judging from surface observations and the data of Castle 

and others (1976). The movements could be largely 

translational; however, the regional relations require 

that the faults flatten with depth (cross sections D-D' 

and F-F' of the State bedrock map) and that earlier 

movements were thrusts. Any strike-slip or normal 

movement is probably late.

NEWBURY BASINS

The Newbury basins are fault bounded, so that their 

relationship to the adjacent Nashoba and MilfordDedham zones is uncertain. We do not see the basement 

for the Newbury Volcanic Complex. The rocks of the
H54 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Newbury Volcanic Complex are at a much lower metamorphic grade than the rocks of the adjacent Nashoba 

zone and are much less deformed and altered than the 

adjacent Proterozoic rocks of the Milford-Dedham zone. 

The Newbury basins are apparently wedges of higher 

level material emplaced between the two larger bounding blocks. Presumably the Newbury lies unconformably 

on one or the other of them, most likely on the MilfordDedham Proterozoic Z basement, as seems to be indicated by correlation with similar units in the Coastal 

Volcanic belt of eastern Maine. The Newbury Volcanic 

Complex is correlated on the basis of similar AcadoBaltic faunal assemblages with the Leighton Formation 

of the Pembroke Group in the Eastport area, Maine; the 

Leighton, however, contains no volcanic rocks (Shride, 

1976b). Volcanic rocks of similar age are present in the 

Milford-Dedham zone in the East Greenwich Group of 

central Rhode Island (Quinn, 1971; Hermes and others, 

1981). These rocks are alkalic, like the lower and middle 

Paleozoic plutonic rocks of the Milford-Dedham zone and 

like those in the Gulf of Maine (Hermes and others, 

1978). The Newbury Volcanic Complex is also comparable to the Silurian and Devonian Castine Volcanics of the 

Penobscot Bay area, Maine, described and dated by 

Brookins and others (1973). The Castine consists of 

bimodal volcanic rocks and sedimentary rocks that rest 

unconformably on metamorphosed Ells worth Schist 

(Stewart and Wones, 1974). The Castine, like the Newbury, lies in an area of appreciable faulting that has 

produced many blocks containing different sequences of 

rock. The Castine is intruded by the mildly alkalic 

Bays-of-Maine Igneous Complex of Chapman (1962) of 

roughly comparable age. If the Castine and Newbury are 

equivalents, then the Newbury could be related to the 

alkalic and peralkalic rocks of the plutons of the MilfordDedham zone by correlating Chapman's Bays-of-Maine 

with the alkalic rocks from the Gulf of Maine. It would be 

helpful in placing the Newbury in context if one could 

correlate the lower Paleozoic Ellsworth Schist on which 

the Castine rests with a rock unit in eastern Massachusetts. This correlation has not been made to date. The 

Silurian plutonic rocks of the Nashoba zone are unique to 

New England (Wones and Goldsmith, this vol., chap. I) 

and are probably not genetically related to the Newbury; 

comparisons of trace elements may help prove or disprove this relationship. The Newbury Volcanic Complex 

most likely belongs to the Milford-Dedham zone rather 

than the Nashoba zone but may be the remnant of an 

entirely separate terrane. The present position of the 

Newbury basins and the Newbury Volcanic Complex can 

only be attributed to post-Early Devonian faulting. 

There has, of course, also been post-Triassic faulting in 

this area (Kaye, 1983).

EAST FLANK OF THE MERRIMACK BELT

The east flank of the Merrimack belt, consisting of the 

Nashua trough and the Rockingham anticlinorium (Robinson and Goldsmith, this vol., chap. G), has a somewhat 

uncertain history primarily because the age of the stratified rocks is uncertain. However, the arguments presented below indicate that the rocks of the Nashoba zone 

underlie, at least in part, those of the Merrimack belt and 

in fact may be the basement upon which the Merrimackbelt rocks were deposited. The Nashoba zone projects 

beneath the Merrimack belt west of the ClintonNewbury zone as indicated by the "cross-sectional" map 

view of the Merrimack synclinorium exposed in southeastern Connecticut (fig. 3; see Rodgers, 1970). There 

the Tatnic Hill and Quinebaug Formations, which are the 

equivalents of the Nashoba and Marlboro Formations in 

the Putnam terrane, extend westward beneath the turbidite section of the Merrimack belt and above the 

Proterozoic Z basement. The Science Park block of the 

Nashoba zone (figs. 2, 4) must have been thrust up from 

beneath the Merrimack-belt rocks to the west. Whether 

the Putnam terrane extends to the Bronson Hill anticlinorium is a matter of controversy (Rodgers, 1981; Pease, 

1982; Robinson and Tucker, 1982; London, 1984). In 

eastern Connecticut, the rocks of the Putnam terrane 

pinch out westward between the Honey Hill fault and the 

probable extension of the Clinton-Newbury fault. A 

terrane similar to the Nashoba, containing the Massabesic Gneiss Complex, forms a structural high within the 

Merrimack synclinorium. To the north in Maine, a terrane similar to the Nashoba, the Passagassawakeag, 

containing the Casco Bay Group, is shown on the latest 

bedrock geologic map of Maine (Osberg and others, 1984) 

as resting upon, rather than underlying, the turbidite 

sequence of the Kearsarge-central Maine synclinorium, a 

division of the Merrimack synclinorium. However, a 

preliminary seismic profile across this terrane (Stewart 

and others, 1986) suggests that the Passagassawakeag 

terrane structurally underlies the turbidite sequence.

The rock units of the Merrimack belt exposed in the 

Clinton-Newbury fault zone near the contact with the 

Nashoba zone are quartzites and conglomerates and are 

probably basal to the sequence in the Merrimack belt. A 

strong possibility exists that the Vaughn Hills Quartzite, 

the Reubens Hill Formation, the Kittery Formation, and 

possibly the Tower Hill Quartzite are Ordovician (or 

older) in age and lie unconformably on the Ordovician or 

Proterozoic Z Nashoba Formation (Robinson and Goldsmith, this vol., chap. G). The Clinton-Newbury fault 

zone thus appears to coincide with a hinge line because 

littoral facies of the Merrimack sequence appear within 

the zone. The Merrimack belt and the Nashoba zone have 

been parts of a single terrane from at least early Paleo-
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H55

zoic time if the apparent unconformity is correctly interpreted. However, subsequent deformation and metamorphism have clearly affected both terranes. Arguing 

against an early docking of the Nashoba zone and the 

east flank of the Merrimack belt is the steep metamorphic gradient between them (Thompson and Norton, 

1968) and the apparent difference in type of early and 

middle Paleozoic plutonism (Wones and Goldsmith, this 

vol., chap. I), which suggests that the two terranes were 

not together until after the Devonian. The metamorphic 

and structural style of the Nashoba zone differs from that 

of the Merrimack belt. This difference is in part because 

the Nashoba zone appears to have undergone an earlier 

period of early Paleozoic or older metamorphism. Staurolite and andalusite porphyroblasts in the Merrimackbelt rocks and in the Tadmuck Brook Schist of the 

Nashoba zone were formed in a primarily static thermal 

environment, either at the time of intrusion of the 

Silurian and Devonian granites of the Merrimack belt or 

during the Permian. However, some of the Ayer Granite 

and rocks of the Fitchburg Complex, as well as the 

Canterbury Gneiss of Connecticut, have a fabric suggesting that they were emplaced during a period of dynamothermal metamorphism that is the Late Devonian metamorphism indicated on the State bedrock map. This 

event (fig. WE) probably culminated in the orogenic 

events that preceded and accompanied the deposition of 

the Pennsylvanian strata. The Middle Pennsylvanian 

Coal Mine Brook Formation at Worcester is metamorphosed to garnet grade, so clearly a post-Pennsylvanian 

metamorphism affected the rocks of the eastern flank of 

the belt also (fig. 15G). Successive metamorphic overprints may have smoothed out the steep gradient 

between the high-grade Nashoba zone and the low-grade 

sequence in the east flank of the Merrimack belt. PostPennsylvanian faulting (fig. 15G,H), however, has upset 

the preexisting pattern of isograds, bringing together 

rocks earlier metamorphosed at different levels.

ZONE BOUNDARIES

The two major zones, the Milford-Dedham and the 

Nashoba, were separate at least through the end of the 

Devonian and did not reach their present relative positions until after the Permian events. The earliest possible time for docking of the Milford-Dedham zone to the 

Nashoba zone would be after intrusion of the unique 

Silurian granites and diorites. The first bridge is the 

probably synchronous deposition of the Middle Pennsylvanian Coal Mine Brook Formation in the Merrimack 

belt to the west and the stratigraphically equivalent 

Rhode Island Formation in the Milford-Dedham zone to 

the east (fig. 15F). Subsequent late Paleozoic and early

Mesozoic faulting and erosion have disrupted the Pennsylvanian depositional blanket (fig. 15G,H).

The major zones of dislocation have been active over 

an appreciable period of time and under different 

regional stress systems. Castle and others (1976) and 

Nelson (1976) indicated evidence for at least preDevonian deformation along the Bloody Bluff fault zone, 

and there is clear evidence for Permian movement from 

both field and isotopic work (O'Hara and Gromet, 1983). 

In addition, Mesozoic faulting is present in the Bloody 

Bluff fault zone in the Middleton area and apparently also 

in the Worcester-Clinton area on the east flank of the 

Merrimack belt. In places, low-angle thrusting and later 

high-angle faulting under ductile conditions have been 

succeeded by high-angle tensional normal faulting. We 

have then a reactivation of zones of crustal weakness 

over a period from possibly early Paleozoic to Mesozoic 

under differing conditions of regional stress.

In the Penobscot Bay region of Maine, terranes similar 

to those in eastern Massachusetts have been separated 

by faults, one of which is dated as pre-Middle Devonian 

because it was cut by a pluton of that age (Stewart and 

Wones, 1974). Other faults in this region are younger 

(Wones and Thompson, 1979). Farther north in New 

Brunswick and Newfoundland, the major faults range 

from early Carboniferous to post-Pennsylvanian (Zen, 

1983, and references therein). In Massachusetts we lack 

the evidence of plutonic intrusion across faults to delimit 

the time of faulting.

The major fault zones of eastern Massachusetts have 

been interpreted as thrust and reverse faults (Harwood 

and Zietz, 1976; Barosh and others, 1977; Skehan and 

Murray, 1980a) and as possible parts of a strike-slip fault 

system of regional extent on the basis of paleomagnetic 

data (Irving, 1977; Kent and Opdyke, 1978, 1979, 1980; 

Brown, 1980). The descriptions of the fault zones indicate 

that the ductile zones tend to be earlier and flatter and 

that the brittle faults tend to be later and steeper, so that 

possibly both views are correct. The attitudes of the 

ductile fault surfaces vary from place to place, however, 

although the zones appear to be relatively linear. The 

Bloody Bluff fault has both steeply and shallowly dipping 

segments. The Lake Char fault is typically shallow 

dipping. Such variations are most likely due to subsequent folding. The Rattlesnake Hill fault of Skehan 

(1968), which seems to be the principal fault separating 

the Nashoba zone from the Merrimack belt (Castle and 

others, 1976; Gore, 1976b; Skehan and Murray, 1980b), is 

relatively steep and is younger than the flatter ductile 

mylonite zones like the thrusts in the Clinton-Wachusett 

Reservoir area or parts of the Bloody Bluff. Several 

proposals have been made for the direction of movement 

on the flatter faults. The Lake Char fault (fig. 3) of 

Connecticut has been proposed to be a thrust bringing
H56 THE BEDROCK GEOLOGY OF MASSACHUSETTS

the higher grade Putnam Group over the Proterozoic Z 

basement (Dixon and Lundgren, 1968; Wintsch, 1979) 

and a decollement or "normal" fault in which the Putnam 

Group has slid off the Proterozoic Z basement (Lundgren 

and Ebblin, 1972; Goldstein, 1982; Danforth and Owens, 

1984). The decollement concept might be likened to the 

movement within the metamorphic core complexes of the 

Great Basin in the Western United States (Coney, 1980). 

By analogy, one would expect the ductile part of the 

Bloody Bluff fault to have a sense of movement similar to 

that of the Lake Char, although O'Hara and Gromet 

(1984) believed these to be two separate faults in northern Rhode Island. However, the normal fault movement 

conflicts with the southeast vergence of folds and thrusts 

seen in the Nashoba and Putnam terranes and in the 

New London anticlinorium (fig. 3) of southeastern Connecticut (Goldsmith, 1985). The normal sense of movement thus is probably only a late phenomenon.

The Permian pattern of deformation (fig. 15G) can 

probably be tied to a single stress field. It could be 

argued that the east-northeast pattern of compressive 

Permian deformation in the Milford-Dedham zone is the 

result of major northeast-trending right-lateral movement along the Bloody Bluff and Clinton-Newbury 

zones. Mosher (1983), Murray and Mosher (1984), and 

McMaster and others (1980) postulated large-scale rightlateral transcurrent movement in the Alleghanian to 

account for the structural configuration in the southern 

Narragansett basin. However, no clear evidence exists 

to prove significant strike-slip motion along the major 

faults of eastern Massachusetts, although the steep dips 

in places of some of the major fault surfaces are suggestive. The steep lineations in the Nashoba zone could be 

indicators of strike-slip movement, but they also could 

have been produced by intersection of later structural 

features of slightly different orientation on already 

steeply dipping rocks. Eberly (1984) commented on the 

paucity of faults showing strike-slip movement in a 

traverse across eastern Connecticut. Nevertheless, the 

paleomagnetic data of Kent and Opdyke (1978, 1979, 

1980) and of others indicate considerable lateral displacement of terranes in New England in the Late Pennsylvanian to Permian, and a number of reconstructions have 

been made to accommodate the data (Van der Voo, 1983; 

Le Fort, 1983, for example). On the other hand, recent 

articles such as that of Irving and Strong (1985) suggest 

caution in reconstructing plate positions from paleomagnetic data.

ACCRETION

The Nashoba and Milford-Dedham zones are exotic 

terranes that have been accreted to the North American 

craton during the Paleozoic. It is generally agreed that

the Taconian suture marking the closing of the lapetus 

Ocean and accretion of an island arc lies west of the 

Merrimack synclinorium (Stanley and Hatch, 1988) and 

that further collisional events occurred in Acadian time. 

The nature and time of amalgamation of the terranes east 

of the Taconian suture are less certain. Accreted terranes in the Appalachian region, with specific reference 

to eastern Massachusetts, have been discussed recently 

by Williams and Hatcher (1983) and Zen (1983). From the 

evidence presented in this chapter, and in chapter E 

(Goldsmith, this vol.), it is possible that Nashoba-type 

rocks were basement for the Merrimack strata by at 

least Ordovician time and that the Milford-Dedham zone, 

or at least the gneissic part of it, was joined to the 

Nashoba zone by Carboniferous time; this join thus may 

mark the site of an Acadian suture. Crustal adjustments 

in the Permian and later in the Mesozoic disrupted this 

amalgamation and produced the arrangement we see 

now. Gneissic rocks similar to those in the Milford 

antiform reappear in the Willimantic dome in eastern 

Connecticut and the Pelham dome in central Massachusetts. These gneissic rocks have apparently been 

involved in the Acadian orogeny that affected central 

Massachusetts, whereas there is little or no evidence of 

an Acadian orogeny in the eastern, nongneissic part of 

the Milford-Dedham zone. The gneissic terrane on the 

west may be considered to have underlain the Merrimack 

and Nashoba rocks at an early stage, certainly before the 

Pennsylvanian and apparently before th^ Late Devonian. The eastern, nongneissic part the MilfordDedham zone is most like the rocks of the Avalon 

Peninsula in Newfoundland. O'Hara and Gromet (1984) 

suggested from evidence in Rhode Island that the 

gneissic, western part of the Milford antiform is actually 

a separate block from that containing the Dedham and 

Esmond batholiths to the east, even though the two 

contain rocks of the same age. They suggested that the 

two parts were joined during the Alleghanian, whereas 

the gneissic rocks were deformed primarily earlier in the 

Acadian orogeny. This suggestion seems to resolve several problems in treatment of the Milford-Dedham zone 

as a single entity on the State bedrock map, particularly 

the discrepancy in metamorphic grade and style of 

deformation between the ductilely and brittlely deformed terranes and the uncertainty of the timing of the 

amphibolite-facies metamorphism in the gneissic terrane. It permits the gneissic terrane, or at least that part 

beneath the Merrimack synclinorium, to have been 

involved in Acadian or earlier deformation without 

requiring Acadian deformation in the rest of the MilfordDedham zone. However, if two terranes are present, the 

significance of the Clinton-Newbury fault is called into 

question.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H57

The two distinct boundaries in eastern Massachusetts 

on the surface are those between the Milford-Dedham 

and Nashoba zones and between the Nashoba zone and 

the Merrimack belt. The first boundary, the Bloody Bluff 

fault system in Massachusetts and the Lake Char and 

Honey Hill fault systems in Connecticut, may mark the 

site of a cryptic suture. Osberg (1978) suggested that the 

Paleozoic volcanic-plutonic assemblage (Newbury Volcanic Complex and alkalic plutons) of eastern Massachusetts and coastal Maine is part of an arc formed adjacent 

to a suture; this may be the cryptic Acadian suture of Le 

Fort (1983) between an African plate, in which he 

included eastern Massachusetts but not the Avalon Peninsula of Newfoundland, and an Avalon prong to the 

northwest. Le Fort's Silurian and Devonian volcanicplutonic arc represents an Andean- or Cordilleran-type 

margin developed on the African plate during the closing 

of the Theic Ocean on an east-dipping subduction zone. 

How the Nashoba zone fits into this scheme is not clear. 

I consider the Nashoba to be equivalent to part of the 

Gander zone of Newfoundland (Goldsmith, this vol., 

chap. F). There is no evidence of ocean-floor material in 

or near this suture zone unless one considers the serpentinite at Lynnfield or the Reubens Hill Formation on the 

east flank of the Merrimack belt as described by Peck 

(1975) to be such.

The Milford-Dedham zone of eastern Massachusetts 

has been suggested to be formerly part of a microcontinent lying between an African plate and the North 

American plate (Schenk, 1971; Skehan and others, 1978; 

Strong, 1979; Rast, 1980) and to have close affinity to 

rocks in northwest Africa (Hurley and others, 1974; 

Skehan and others, 1978; Olszewski, 1980; Simpson and 

others, 1980). The 730±26-Ma zircon age on the Fish 

Brook Gneiss and the 1,500-Ma age on detrital zircon 

from the Shawsheen Gneiss (Olszewski, 1980), both of 

the Nashoba zone, form a pattern that has a counterpart 

in northwest Africa. However, the original spatial relation of the Nashoba to the Milford-Dedham zone is 

unknown. The two may have been originally from different environments on the same plate, as suggested by 

Skehan and Murray (1980b, p. 313), equivalent to the 

northern apron of the Avalon platform of Rast (1980), or 

they may have been parts of two different plates. On the 

other hand, following the model of O'Hara and Gromet 

(1984), perhaps the gneissic terrane of the MilfordDedham zone has been attached to the Nashoba terrane 

longer than has the nongneissic terrane.

If the Nashoba and Milford-Dedham zones were joined 

together and accreted to North America before the 

Pennsylvanian, then the Permian deformation and metamorphism were produced not by collision of the MilfordDedham zone with the terranes to the west but by 

movement and rotation of these terranes along preexisting sites of juncture or other zones of weakness as a 

result of intraplate adjustments or impact of another 

plate or microplate that lay south or east of the already 

accreted terranes. This zone of Permian movement is the 

Variscan orogenic belt of Rast and Skehan (1984). The 

rotation in a clockwise direction of Africa (the MilfordDedham zone) against the Nashoba (and Putnam) zone to 

the west in the Permian proposed by Wintsch and Le 

Fort (1983) does not necessarily mean the closing of a 

suture but could reflect continued or renewed movement 

along a zone in which the suture had already formed. The 

Clinton-Newbury fault zone can easily be construed as an 

intraplate movement zone of this sort. The collision and 

slip hypothesis developed to explain the features in the 

southern Narragansett basin (McMaster and others, 

1980; Mosher, 1983; Murray and Mosher, 1984) seems 

reasonable. Broad-wavelength magnetic anomalies 

(Zietz and others, 1980) and regional gravity patterns 

(Haworth and others, 1980) suggest an east-northeasttrending zone of discordance between crustal domains 

south of the New England coast, which may be a 

complementary part of the regional northeast strain 

pattern. Rast and Skehan (1984) using other arguments 

also speculated on a dislocation in Long Island Sound. 

Whether this is an interplate boundary or intraplate 

boundary is unknown.

Although the sequence of events in eastern Massachusetts is not entirely clear, it appears that the accretionary sequence is Nashoba-type basement beneath the 

Merrimack belt, Nashoba zone east of the Merrimack 

belt, and Milford-Dedham zone. The Nashoba-type rocks 

and the gneissic Milford-Dedham rocks appear to have 

been present beneath the Merrimack belt at the time of 

Acadian deformation and thus correspond perhaps to the 

craton X basement of Zen (1983). Most of the exposed 

Nashoba, east of the Merrimack belt, and the nongneissic Milford-Dedham rocks show no evidence of 

Acadian deformation, either because they were in place 

but far removed from the center of Acadian orogeny, as 

Zen suggested, or because they were moved into place 

later. The rotation and subgreenschist-facies metamorphism of the Newbury Volcanic Complex might be 

construed as Acadian, but there is no evidence for 

metamorphism of this age in the adjacent structural 

blocks, whereas there is for Alleghanian deformation. 

The sequence of events discussed above does fit in a 

general way the west-to-east, oldest-to-youngest accretionary model of Williams and Hatcher (1983).

The arrangement of terranes resembling the Nashoba 

and Milford-Dedham zones along the coast of New England and the Maritime Provinces of Canada indicates that 

they are now wedges or splinters of once more continuous terranes extending to Europe and Africa (see, for 

example, Rast and others, 1976). Reconstructing the
H58 THE BEDROCK GEOLOGY OF MASSACHUSETTS

whole requires careful consideration of the details of 

stratigraphy and structure in each area and careful 

correlation of these details between areas.

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1980, Magnetic anomaly map of the Appalachian orogen: Memorial 

University of Newfoundland Map 2a, scale 1:1,000,000.

Intrusive Rocks of 

Eastern Massachusetts

By DAVID R. WONES, VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY, 

and RICHARD GOLDSMITH, U.S. GEOLOGICAL SURVEY

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-1

CONTENTS

	Page

Abstract............................................................................ II

Introduction.......................................................................^ 1

Intrusive rocks of the Milford-Dedham zone............................ 2

Proterozoic Z mafic plutonic rocks................................... 2

Diorite and gabbro (Zdigb).... ................................. 8

Gabbro (Zgb)........................................................ 10

Diorite (Zdi) .......................................................... 10

Diorite at Rowley (Zrdi).......................................... 10

Sharon Syenite (Zssy) ............................................. 11

Cumberlandite (cu) ................................................. 12

Serpentinite (u)...................................................... 12

Discussion of the mafic plutonic rocks ........................ 12

Proterozoic Z batholithic rocks........................................ 12

Dedham batholith................................................... 12

Dedham Granite (Zdgr, Zdngr)........................... 12

Topsfield Granodiorite (Ztgd).............................. 15

Esmond Granite (Zegr)...................................... 17

Grant Mills Granodiorite (Zgmgd)........................ 17

Westwood Granite (Zwgr).................................. 22

Fine-grained granite (fgr) .................................. 23

Granite of the Fall River pluton (Zfgr)................. 23

Porphyritic granite (Zpgr).................................. 23

Alaskite(Zagr)................................................. 25

Granite, gneiss, and schist, undivided (Zgg).......... 26

Plutonic rocks of the Milford antiform ........................ 29

Hope Valley Alaskite Gneiss (Zhg)...................... 29

Scituate Granite Gneiss (Zsg) ............................. 29

Ponaganset Gneiss (Zpg).................................... 30

Milford Granite (Zmgr, Zmgd)............................ 31

Biotite granite (Zgr).......................................... 33

Paleozoic intrusive rocks................................................ 33

Nahant Gabbro (Ongb) ............................................ 33

Quincy Granite (SOqgr)........................................... 36

Blue Hills Granite Porphyry (SObgr)......................... 38

Cape Ann Complex................................................. 38

Alkalic granite and quartz syenite (SOcgr)............ 38

Beverly Syenite (SOcb)...................................... 40

Squam Granite (SOcsm)..................................... 40

Micrographic rhyolite of the Newbury Volcanic

Complex (DSnr) ............................................... 40

Intrusive rocks of the Milford-Dedham zone Continued 

Paleozoic intrusive rocks Continued

Alkalic granite in Franklin (DOgr)............................

Peabody Granite (Dpgr)..........................................

Wenham Monzonite (Dwm)......................................

Cherry Hill Granite (Dcygr)....................................

Granite of the Rattlesnake Hill pluton (Drgr).............

Mesozoic intrusive and silicified rocks .............................

Diabase dikes (Jd)..................................................

Massive quartz and silicified rock (q).........................

Discussion of the intrusive rocks of the Milford-Dedham

zone .................................................................

Relation to extrusive rocks......................................

Interpretation of igneous events...............................

Intrusive rocks of the Nashoba zone .....................................

Andover Granite (SOagr)..............................................

Sharpners Pond Diorite (Ssqd).......................................

Straw Hollow Diorite and Assabet Quartz Diorite (Ssaqd).. 

Granodiorite of the Indian Head pluton (igd)....................

Orange-pink rusty-weathering granite (Sgr).....................

Light-gray muscovite granite (mgr)................................

Discussion of the intrusive rocks of the Nashoba zone........

Intrusive rocks of the eastern part of the Merrimack belt.........

Newburyport Complex.................................................

Tonalite and granodiorite (SOngd)............................

Granite (Sngr).......................................................

Ayer Granite (Sagr).....................................................

Clinton facies (Sacgr) .............................................

Devens-Long Pond facies (SOad)..............................

Diorite and tonalite (DSdi)............................................

Chelmsford Granite (Dcgr)............................................

Muscovite-biotite granite at Millstone Hill (Dmgr).............

Fitchburg Complex (Dfgr).............................................

Discussion of the intrusive rocks of the eastern part of the 

Merrimack belt...................................................

Intrusive rocks of eastern Massachusetts and plate-tectonic

models....................................................................

Regional relations..............................................................

References cited................................................................

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ILLUSTRATIONS

FIGURE 1. Map showing distribution of intrusive rocks of eastern Massachusetts............................................................................ 13

2. Correlation diagram of intrusive rocks and selected sedimentary and volcanic rocks and their metamorphic equivalents in

eastern Massachusetts..................................................................................................................................... 4

3. Map showing distribution of Proterozoic Z batholithic rocks in the Milford-Dedham zone.................................................. 8

in
IV CONTENTS

FIGURE 4. Map showing distribution of Proterozoic Z mafic plutonic and volcanic rocks in the Milford-Dedham zone....................... 19

5. Quartz-plagioclase-K-feldspar-mafic minerals diagram of modes of Proterozoic Z intrusive rocks of the Milford-Dedham

zone......................................................................................................................................................... 13

6. Plot showing CaO and Na20+K20 against Si02 for Proterozoic Z intrusive rocks of the Milford-Dedham zone ................ 14

7. Ternary plot of normative albite, anorthite, and orthoclase for Proterozoic Z intrusive rocks of the Milford-Dedham

zone......................................................................................................................................................... 15

8. Ternary alkalis-FeO-MgO plot of Proterozoic Z intrusive rocks of the Milford-Dedham zone......................................... 15

9. Chondrite-normalized plot of rare-earth elements in Proterozoic Z mafic plutonic rocks of the Milford-Dedham zone ........ 17

10. Plot of thorium against uranium for Proterozoic Z intrusive rocks of the Milford-Dedham zone..................................... 17

11. Map showing distribution of Proterozoic Z batholithic rocks in the Milford-Dedham zone ............................................. 18

12-14. Quartz-plagioclase-K-feldspar-mafic minerals diagram of modes of:

12. Dedham Granite and Topsfield Granodiorite............................................................................................. 19

13. Westwood Granite; Westwood Granite in Plymouth quarry, Weymouth; and Milford Granite........................... 20

14. Fall River pluton including the porphyritic granite.................................................................................... 21

15. Chondrite-normalized plot of rare-earth elements in Proterozoic Z batholithic rocks, northeastern Massachusetts: 

A, Dedham Granite; B, Dedham Granite north of Boston, Westwood and Esmond Granites, and Topsfield 

Granodiorite.............................................................................................................................................. 30

16. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks in the Milford-Dedham zone,

southeastern Massachusetts......................................................................................................................... 32

17. Map showing locations of drill holes encountering bedrock in Cape Cod and the adjacent mainland................................ 33

18. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks, Milford antiform............................ 34

19. Map showing distribution of Paleozoic intrusive rocks in the Milford-Dedham zone ..................................................... 35

20. Geologic map and cross section for Andrews Point, Cape Ann................................................................................. 39

21. Ternary quartz-plagioclase-K-feldspar plot of modal composition of Dedham Granite as compared with fields of

composition of Dedham Granite of Dowse (1949) and Lyons (1969), Milford Granite, and Westwood Granite ............. 42

22. Plot of a part of the ternary system Na20-K20-Al203 in molecular percent for intrusive rocks of the Milford-Dedham

zone......................................................................................................................................................... 43

23. Ternary plot of normative albite, anorthite, and orthoclase for Paleozoic intrusive rocks of the Milford-Dedham zone....... 43

24. Ternary alkalis-FeO-MgO plot of Paleozoic intrusive rocks of the Milford-Dedham zone showing field of Proterozoic Z

intrusive rocks of the Milford-Dedham zone.................................................................................................... 43

25. Chondrite-normalized plot of rare-earth elements of Paleozoic intrusive rocks............................................................ 44

26. Map showing distribution of intrusive rocks in the Nashoba zone............................................................................. 46

27. Map showing distribution of intrusive rocks in the eastern part of the Merrimack belt................................................ 51

TABLES

Page

TABLE 1. Ages and age relations of intrusive rocks of the Milford-Dedham zone............................................................................. 17

2. Modes of representative plutonic rocks of the Proterozoic Z mafic volcanic-plutonic complex, Milford-Dedham zone................ 10

3. Major-oxide, normative-mineral, and trace-element compositions of plutonic rocks of the Proterozoic Z mafic volcanic-plutonic

complex.......................................................................................................................................................... 11

4. Major constituents determined by point-count of stained slabs of intrusive rocks of the Milford-Dedham zone ....................... 16

5. Modes of plutonic rocks of the Dedham batholith in northeastern Massachusetts............................................................... 22

6. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks, Milford-Dedham zone,

northeastern Massachusetts................................................................................................................................ 24

7. Modes of plutonic rocks of the Dedham batholith, southeastern Massachusetts.................................................................. 25

8. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks, southeastern

Massachusetts.................................................................................................................................................. 26

9. Modes of plutonic rocks of the Milford antiform ........................................................................................................... 27

10. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks in the Milford antiform........ 28

11. Modes of some Paleozoic intrusive rocks of the Milford-Dedham zone.............................................................................. 36

12. Major-oxide, normative-mineral, and trace-element compositions of some Paleozoic plutonic rocks, Milford-Dedham zone ........ 37

13. Ages and age relations of intrusive rocks of the Nashoba zone and the eastern part of the Merrimack belt............................ 47

14. Modes of intrusive rocks of the Nashoba zone.............................................................................................................. 47

15. Major-oxide, normative-mineral, and trace-element compositions of granodiorite of the Indian Head pluton........................... 49

16. Modes of intrusive rocks of the Merrimack belt east of the Wekepeke fault...................................................................... 52
THE BEDROCK GEOLOGY OF MASSACHUSETTS

INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS

By DAVID R. WoNES 1 ' 2 and RICHARD GOLDSMITHS

ABSTRACT

The intrusive rocks of eastern Massachusetts east of the Wekepeke 

fault form three distinctly different assemblages coinciding with the 

three lithotectonic terranes that form eastern Massachusetts: the 

Milford-Dedham zone, the Nashoba zone, and the eastern part of the 

Merrimack belt. In the Milford-Dedham zone, a largely mafic volcanicplutonic complex of Proterozoic Z age is intruded by 630-Ma batholiths 

of calc-alkaline granite to granodiorite. These batholithic rocks occupy 

two regions. To the east, the Dedham-Fall River batholith consists of 

partly altered but mostly nongneissic granites and granodiorites; to 

the southwest, the Milford antiform consists of gneissic to partly 

gneissic granites and granodiorites. The batholithic rocks in the two 

regions are similar in age and chemistry. Slightly younger granite is 

associated with a Proterozoic Z rifting(?) event that produced a 

bimodal volcanic suite in the area of the Boston basin. During the early 

and middle Paleozoic, the Proterozoic Z rocks were intruded by 

discrete alkaline and peralkaline plutons ranging from gabbro to 

granite, which lie primarily in a northeast-trending belt northwest of 

the Narragansett basin. No late Paleozoic intrusive rocks are known in 

the Milford-Dedham zone in Massachusetts, although they are present 

in southern Rhode Island. Mesozoic intrusive rocks are confined to 

mafic dikes associated with early Mesozoic rifting.

Intrusive rocks of the Nashoba zone are all Paleozoic; they range 

from the peraluminous, S-type, partly gneissic Andover Granite of 

Late Ordovician to Silurian age but including some rock of Devonian 

age, to postmetamorphic I-type quartz-diorite and to granite of Silurian 

age. Since the bedrock map of Massachusetts (Zen and others, 1983) 

was prepared, the latter granite has been found to include rock of 

Devonian age. The Silurian plutonic rocks contrast in composition with 

the Andover Granite and with the Paleozoic rocks of the MilfordDedham zone and are I-type granite associated with volcanic arcs. Data 

on the shallow intrusions of the nearby Late Silurian and Early 

Devonian Newbury Volcanic Complex are insufficient to determine 

whether they are chemically related to the Silurian plutons of the 

Nashoba zone. Some of the granite that has recently been determined 

to be Devonian in age is peraluminous, like the older Late Ordovician 

and Silurian Andover Granite. These changes in composition with time 

imply a changing crustal source for the material.

The intrusive rocks of the eastern part of the Merrimack belt 

constitute a different assemblage. These rocks are primarily calcManuscript approved for publication November 16, 1987.

1Deceased.

2Virginia Polytechnic Institute and State University, Blacksburg, Va.

3U.S. Geological Survey.

alkaline granodiorite and granite of Silurian to Ordovician age and 

include small masses of diorite and, in one pluton, norite. Some of these 

rocks, such as parts of the Ayer Granite, are gneissic and appear to 

have been deformed during Acadian metamorphism and intrusion. 

Younger muscovite-bearing granite of Devonian age forms small 

aligned plutons that are probably satellitic to the larger Fitchburg 

Complex west of the area but that extend into Connecticut as the 

Canterbury Gneiss. The Ordovician and Silurian Newburyport Complex bears mineralogical resemblance to some of the Silurian intrusions 

of the Nashoba zone. Recently determined ages of the intrusions in the 

eastern part of the Merrimack belt indicate that some or all of the 

strata in this part of the Merrimack belt are somewhat older than 

shown on the State bedrock map.

The differences in the compositions, styles of emplacement, and 

times of intrusion of the intrusive rocks of the three lithotectonic belts 

of eastern Massachusetts are a basis for attempting to establish a 

history of accretion of crustal blocks during the Paleozoic. The MilfordDedham zone was a cratonic element during the Paleozoic and was not 

joined to the Nashoba zone until after intrusion of the Devonian 

granites. The Nashoba zone could have been close to the east flank of 

the Merrimack belt somewhat earlier. The two seem to share a 

Devonian magmatic event and possibly an earlier one as well. The three 

belts clearly were assembled by the Pennsylvanian, although their 

present boundaries involve Permian events.

The Proterozoic Z rocks of the Milford-Dedham zone consist of many 

more intrusive rocks and many fewer supracrustal rocks than the 

Avalon terrane of Newfoundland. The Proterozoic Z rocks of the zone 

bear some resemblance to rocks in the Charlotte belt of North and 

South Carolina in both age and volume of intrusive rocks, although the 

intrusive rocks in the Charlotte belt are more mafic. The Ordovician 

and Silurian intrusive rocks of the Nashoba zone resemble intrusions in 

the lithotectonically equivalent Passagassawakeag terrane in southeastern Maine. The intrusive rocks of the east flank of the Merrimack 

belt in Massachusetts are in part older and are more deformed and 

metamorphosed than the intrusive rocks in the Merrimack belt to the 

north in Maine. The Ordovician to Silurian rocks of the east flank of the 

Merrimack belt in Massachusetts do not have obvious counterparts 

along strike to the north beyond New Hampshire. The Devonian 

intrusions of the Merrimack belt in Massachusetts are compositionally 

similar to those in eastern Maine but were generally more affected by 

Acadian dynamothermal metamorphism than were the mostly highlevel, postmetamorphic plutons of northeastern New England.

INTRODUCTION

Eastern Massachusetts is divided into three lithotectonic terranes: a part of the Merrimack belt east of the

n
12 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Wekepeke fault and the Fitchburg Complex and northwest of the Clinton-Newbury fault; the Nashoba zone, 

between the Clinton-Newbury fault and the Bloody Bluff 

fault; and the Milford-Dedham zone, southeast of the 

Bloody Bluff fault (fig. 1). These three terranes have 

distinctive strata and distinct intrusive, deformational, 

and metamorphic histories (Zen and others, 1983; Hatch 

and others, 1984). Fossils are present in the supracrustal 

rocks of the Milford-Dedham zone, but they are found 

only in the Pennsylvanian strata at Worcester in the 

eastern part of the Merrimack belt and are lacking in the 

Nashoba zone. Therefore, lithologic correlations within 

each terrane have been made in varying degrees on the 

basis of comparisons of the strata alone (Goldsmith, this 

vol., chaps. E and F; Robinson and Goldsmith, this vol., 

chap. G). The oldest known rocks are in the MilfordDedham terrane. The stratified rocks of this zone consist 

of metasedimentary, metavolcanic, and metaplutonic 

rocks of probable Proterozoic Z age (fig. 2). These are 

intruded by calc-alkaline batholithic rocks of Proterozoic 

Z age, which form a basement for weakly metamorphosed to unmetamorphosed sedimentary and volcanic 

rocks ranging in age from latest Proterozoic Z and 

Cambrian to Pennsylvanian.

The strata of the Nashoba zone are high-metamorphicgrade pelitic schists and gneisses, calc-silicate rocks, 

minor marble, feldspathic gneisses, and amphibolite. The 

protoliths of some of these rocks contained a large 

proportion of mafic volcanic or volcaniclastic material. 

Their high grade of metamorphism contrasts sharply 

with the lower grade strata in the Milford-Dedham zone 

on the east and in the east part of the Merrimack belt on 

the west, indicating that the faults bounding these 

terranes are of significant magnitude. The age of these 

rocks is somewhat uncertain. On the geologic bedrock 

map of Massachusetts (Zen and others, 1983; hereafter 

referred to as the State bedrock map), they are shown as 

Ordovician or Proterozoic Z.

The strata in the eastern part of the Merrimack belt 

are primarily a low- to middle-metamorphic-grade turbidite sequence of calcareous metasiltstone and phyllite 

containing minor quartzite and rare conglomerate beds 

near the base. These rocks, equivalent to the Berwick, 

Eliot, and Kittery Formations of the Merrimack Group of 

southeastern New Hampshire and the Oakdale Formation of Massachusetts, are of uncertain age. On the State 

bedrock map, they were considered to be of Silurian age 

and were correlated with the Paxton Formation of 

central Massachusetts, but radiometric dating of intrusive rocks in Massachusetts (Zartman and Marvin, this 

vol., chap. J) and recent radiometric dating and mapping 

in southern New Hampshire (Lyons and others, 1982; 

Bothner and others, 1984) have indicated that these

strata may be much older, and they may belong to a 

separate terrane.

The intrusive rocks of the three terranes (fig. 1) also 

have histories and compositions that are quite distinct 

from each other. In this report, we describe the intrusive 

rocks of each terrane and explain the reasons behind the 

particular groupings and splits that were used on the 

State bedrock map. We also offer an interpretation of the 

thermal history of eastern Massachusetts. This analysis 

depends greatly on the radiometric determinations discussed and summarized by Zartman and Naylor (1984), 

Zartman and Marvin (this vol., chap. J), and Hermes and 

Zartman (1985). We have been greatly aided in the 

preparation of this chapter by reviews by 0. Don Hermes, J. Christopher Hepburn, G. William Leo, and 

David B. Stewart. Because of the death of the senior 

author during preparation of this manuscript, material is 

included in it that was intended for a future paper 

developing more fully some ideas and concepts regarding 

the intrusive rocks of eastern Massachusetts. Thus, 

these ideas and concepts as expressed in the present 

chapter may be somewhat fragmentary, but the data are 

presented for others to use as they see fit.

INTRUSIVE ROCKS OF THE MILFORD-DEDHAM

ZONE

The intrusive rocks of the Milford-Dedham zone (table 

1) range in age from Proterozoic Z to Jurassic, but they 

form four major groups. The first is a suite of Proterozoic 

Z rocks consisting of gabbro, diorite, and syenite (Sharon 

Syenite) that form the plutonic part of a largely mafic 

volcanic-plutonic complex, the volcanic part of which is 

described in chapter E of this volume (Goldsmith, this 

vol.). The second consists of calc-alkaline granitic to 

granodioritic rocks of Proterozoic Z age that form two 

batholithic masses intruding the mafic plutonic rocks and 

older metasedimentary and metavolcanic rocks. The two 

batholithic masses are the Dedham batholith, in which 

we include the granite of the Fall River pluton and 

associated rocks in southeastern Massachusetts, and the 

plutonic rocks of the Milford antiform (fig. 3). The third 

major group consists of plutons of alkalic granite and 

gabbro of Ordovician to Devonian age. The fourth group 

consists of dikes of Mesozoic age.

PROTEROZOIC Z MAFIC PLUTONIC ROCKS

The Proterozoic Z mafic plutonic rocks occupy a large 

area in northeastern Massachusetts (fig. 4) where there 

are also numerous masses of metavolcanic rocks. South 

and west of Boston, the mafic plutonic rocks occupy 

scattered roof pendants and enclaves in the granitic
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 13

72°00' 70°30'

42°45' -

BOSTON 

BASIN

NARRAGANSETT 

BASIN

EXPLANATION

Permian granite

Devonian to Ordovician granite

Devonian and Silurian diorite

: Fall River: 

Devonian rpiuton: to Ordovician alkalic

granite and gabbro 

Silurian diorite to granite

Silurian or Ordovician

peraluminous granite 

Proterozoic Z granite and granodiorite 

Proterozoic Z diorite and gabbro

jjjfteissfeterrane

V 

Buzzards

Contact Dotted where concealed

 - Fault Dashed where inferred; 

dotted where concealed

42°00' -

41°30r -

FIGURE 1. Distribution of intrusive rocks of eastern Massachusetts.
14 THE BEDROCK GEOLOGY OF MASSACHUSETTS

MERRIMACK BELT

Sedimentary and

volcanic rocks and

their metamorphic

equivalents

Worcester 

basin

# Pcm Ph

Sp

DSw

1

So

St

SObo

5Ovh

SOrh

Sb

Se

SOk

Intrusive rocks

SOngd DSdi

\Sngr

SOad

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i I

NASHOBA ZONE

Sedimentary and

volcanic rocks and

their metamorphic

equivalents

SZtb

OZn

OZf

OZsh

Ozm

OZmg

* Fossil-bearing unit

FIGURE 2. Correlation of intrusive rocks and selected sedimentary and volcanic rocks and their 

metamorphosed equivalents in eastern Massachusetts; adapted from Zen and others (1983).
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 15

MILFORD-DEDHAM ZONE

Sedimentary and

volcanic rocks and

their metamorphic

equivalents

Narragansett 

basin

Newbury 

basin

ZP Zw Zv

Zgs Zgn

Intrusive 

rocks

AGE

Lower Jurassic

Pennsylvanian

Devonian

Silurian

Ordovician

Cambrian

Cambrian to 

Proterozoic Z

Proterozoic Z

FIGURE 2. Continued.
16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 17

TABLE 1. Ages and age relations of intrusive rocks of the Milford-Dedham zone

[Radiometric age data from Zartman and Marvin, this vol., chap. J, table 1. Ages are interpreted to be within 5 percent of primary age of the intrusion except as noted.

bio, biotite; hbl, hornblende; rieb, riebeckite; wr, whole rock. , no data]

Map unit Name Intrudes Age (Ma), method

Mesozoic and younger rocks

Jd 

u

Medford Diabase 

Serpentinite

194±6, K-Ar/bio.

Paleozoic rocks

Dpgr

Dwm 

Drgr 

DOgr 

SOcgr, 

SOcb, 

SOcsm.

SOqgr

SObgr 

Ongb

Peabody Granite

Wenham Monzonite 

Granite of the Rattlesnake Hill pluton 

Alkalic granite in Franklin 

Cape Ann Complex

Quincy Granite

Blue Hills Granite Porphyry 

Nahant Gabbro and gabbro at Salem 

Neck.

Zdigb; Zv, metamorphosed mafic to 

felsic volcanic rocks.

Zdigb, Zv 

Zdgr

Zdigb, Zv

Cbw, Braintree and Weymouth 

Formations.

 bw

359±24, Rb-Sr/wr, 395±20, U-Pb/ 

zircon.

395±20, U-Pb/zircon. 

382±14, K-Ar/rieb.

426±6, Rb-Sr/wr, 450±25, UPb/zircon

450±25, U-Pb/zircon, 466±14, KAr/rieb.

450±13, Rb-Sr/bio, 483±21, K-Ar/bio.

Batholithic plutonic rocks of Proterozoic Z age

Zdgr Dedham Granite

Zdngr Dedham Granite north of Boston

Ztgd Topsfield Granodiorite

Zwgr Westwood Granite

Zegr Esmond Granite

Zgmgd Grant Mills Granodiorite

fgr Fine-grained granite

Zfgr Granite of the Fall River pluton

Zpgr Porphyritic granite

Zagr Alaskite

Zgg Granite, gneiss, and schist, undivided

Zgr Biotite granite

Zmgr Milford Granite

Zmgd Milford Granite, mafic phase

Zhg Hope Valley Alaskite Gneiss

Zsg Scituate Granite Gneiss3

Zpg Ponaganset Gneiss

Zdigb; Zv; Zdi; Zb, Blackstone Group;

Zw, Westboro Formation. 

Zdi, Zgb, Zdigb, Zv, Zw 

Zdigb, Zrdi, Zv 

Zdgr 

Zb 

Zb

Zgs, gneiss and schist near New Bedford.

Zgs

Zgs; Zgn, biotite gneiss near New Bedford.

Zdi; Zgb; Zw; Zvf, metamorphosed felsic volcanic rocks. 

Zb 

Zb 

Zsg; Zp, Plainfield Formation

Zb

630±15, U-Pb/zircon.

595±17, Rb-Sr/wr, 640±14,K-Ar/hbl.

579±28, Rb-Sr/wr. 1 

621 ±8, U-Pb/zircon. 2

630±15, U-Pb/zircon.

630±15, U-Pb/zircon.

630±15, U-Pb/zircon, 601 ±5, U-Pb/

zircon. 2 

630±15, U-Pb/zircon.

Proterozoic Z mafic plutonic rocks

Zrdi

Zdi

Zdigb

Zgb

Zssy

cu

Diorite at Rowley Zv

Diorite 

Diorite and gabbro 

Gabbro 

Sharon Syenite 

Cumberlandite 

656±16, K-Ar/hbl.

Interpreted to be within 10 percent of primary age of intrusion.

2Radiometric data from Hermes and Zartman (1985).

3The Scituate Granite Gneiss of the State bedrock map, not the Devonian Scituate Granite.

batholithic rocks. It is possible that not all the rocks 

included in this category on the State bedrock map 

(Zdigb, Zgb, Zdi, Zssy) are of Proterozoic Z age. Some

gabbros, for example, may be found to be of Ordovician 

or Devonian age and may be associated with the alkalic 

plutons. However, contact relations for many bodies
18 THE BEDROCK GEOLOGY OF MASSACHUSETTS

72W 30' 71°00' 70°30'

42°30'

42°00'

41°30'

JVEW_HAMPSHIRE ___ / 

MASSACHUSETTS 

10 20 30 MILES 

_J I I I I

0 10 20 30 KILOMETERS

EXPLANATION

Pennsylvania!! to Proterozoic Z cover rocks

Mattapan and Lynn Volcanic Complexes

Proterozoic Z batholitic rocks 

Nongneissic rocks

Proterozoic Z metamorphic and mafic plutonic 

rocks and granites of Paleozoic plutons

 Contact

 Indefinite boundary of New Bedford 

gneissic terrane

 Fault

FIGURE 3. Distribution of Proterozoic Z batholithic rocks in the 

Milford-Dedham zone.

indicate that the mafic rocks are older, possibly only 

slightly older, than the granite to granodiorite batholiths. Modes for some of these rocks are given on table 2.

DIORITE AND GABBRO (Zdigb)

The "diorite and gabbro" (Zdigb) designation was used 

on the State bedrock map for masses of diorite and 

gabbro, separated by many screens of stratified, largely 

mafic, metavolcanic rocks, that cannot be distinguished 

separately on the scale of the map. These masses are 

mostly northwest and north of Boston. In addition, 

small-scale intrusions of granitic rock of Proterozoic Z 

age are scattered through the unit. All of the rocks 

formerly mapped as Newburyport Quartz Diorite and all 

of the rocks that earlier workers (Emerson, 1917; Toulmin, 1964; Dennen, 1981) assigned to the Salem GabbroDiorite, except the gabbros at Salem Neck, have been 

included in Zdigb. Also included are intrusive mafic dikes 

and sills and light-colored dikes that may be contemporary or younger. Probably most of the rock mapped as 

Zdigb is gabbro rather than diorite, as for example the 

rock labeled diorite (table 3, no. 4) by Emerson. The two 

diorites (table 2, nos. 1-2) have a quartz content in the 

gabbro range. The Sharon Syenite (table 3, no. 3) is 

actually closer to a monzonite than a syenite.

The suite of mafic plutonic and metavolcanic rocks is 

well exposed in roadcuts along Route 2 in Arlington and 

Lexington (fig. 4). In Forest River Park, Salem, volcanic 

fabrics are preserved, and several ages of dikes are 

evident. These dikes may be equivalent to the Ordovician 

Nahant Gabbro, the Proterozoic Z Brighton Melaphyre, 

the diorite at Rowley, or the metavolcanic rocks.

The urbanization of the region and the superposed 

Pleistocene glacial deposits, coupled with several periods 

of fracturing and alteration, make distinguishing these 

mafic rocks a formidable task. Dennen (1975, 1981) 

suggested that much of the diorite and gabbro in the 

Cape Ann area is coeval with the Cape Ann Complex, the 

gabbro at Salem Neck, and the Nahant Gabbro. However, because much of the gabbro and diorite west of 

Cape Ann is intruded by the Dedham Granite and its 

equivalents in age, we have chosen to classify most of 

these rocks as Proterozoic Z. The 656-Ma age of the 

diorite at Rowley (Zrdi) (table 1) and the 630-Ma age 

established by Zartman and Naylor (1984) for the granitic rocks at Lexington and Saugus (Zdngr, Zdgr) that 

in this area intrude gabbro and diorite and amphibolite 

(Zdigb, Zv) indicate that much of the material called 

Salem Gabbro-Diorite by earlier workers is Proterozoic 

Z in age. Careful geologic mapping of individual outcrops 

at large scales, coupled with geochronologic studies, 

will be required to sort out the mafic rocks within this 

map designation. The wide variations in reported geochronologic ages may be true, even the suspicious K-Ar 

age of 886±22 Ma (Zartman and Marvin, this vol., chap. 

J, table 1) for hornblende from a gabbroic pegmatite in 

Lexington.
72W

INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 

30' 15' 71°00'

19

70°30'

42°45' -

_ _____ _NEW HAMPSHIRE

MASSACHUSETTS

".\'*yy. Arlington'.' '

 *^-r * . . * '-

Zgb^

Zbv-p4"--'I ':& .'. .-; /.--NARRAGANSETT .. :' .' .: .'.

Pennsylvanian to Proterozoic Z 

cover rocks

Proterozoic Z mafic plutonic rocks

Proterozoic Z mafic and felsic 

metavolcanic rocks -'-V-V: : .-'-'.-Fall River

Cumberlandite

Contact Dotted where concealed

Fault Dotted where concealed

; New Bedford *.

42°00' -

45' -

41°30' -

FIGURE 4. Distribution of Proterozoic Z mafic plutonic and volcanic rocks in the Milford-Dedham zone. Zrdi, Diorite at Rowley; Zv, 

metamorphosed mafic to felsic volcanic rocks; Zdigb, diorite and gabbro; Zdi, diorite; Zgb, gabbro; Zssy, Sharon Syenite; Zbv, greenstone 

and amphibolite of the Blackstone Group.
no THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 2. Modes, in percent, of representative plutonic rocks of the 

Proterozoic Z mafic volcanic-plutonic complex, Milford-Dedham

zone

[Samples 1-4 are single thin sections of representative samples; sample 5 is an 

average of modes from 6 thin sections (Lyons, 1977); figures in parentheses 

give range of modes. Zdi, diorite; Zssy, Sharon Syenite. Sample localities 

shown by field number in Wones and others (1986). n, andesine; nr, not 

reported; tr, trace]

Sample no. .....

Unit ..........

Points

counted ......

Field no. .......

Quartz ........

Plagioclase ....

Microcline. ....

Biotite........

Epidote. ......

Hornblende . . .

Chlorite. ......

Magnetite .....

Titanite.......

Apatite .......

Allanite. ......

Pyroxene .....

Other. ........

1

. Zdi

. 1223

F-32

. 2.2

. 7.4**

. 0

. 26.8

. 34.4**

. 23.1

. .2

. 2.0

. 3.5

. .3

. 0

. 0

nr

2

Zdi

1210

We-9*

4.4

42.8n

0

13.7

3.6

33.6

.3

.7

.1

.8

0

0

nr

3

Zdi

1185

NBN-21a*

11.0

36.0

.2

14.0

7.8

26.8

2.9

.4

.7

.2

0

0

nr

4

Zssy

1528

W-^0*

2.9

22.0

64.7

.7

0

.6

0

.9

0

.3

tr

7.9

nr

5

Zssy

1.4 (0.2-2.8)

nr

87.3 (82.8-94.4)

nr

nr

5.0 (0.8-7.0)

nr

1.8 (0.5-3.3)*

nr

nr

nr

3.6 (1.0-6.6)

.8 (0-1.3)

* Chemical analysis in table 3.

** Most plagioclase converted to epidote.

* Reported as opaque minerals.

GABBRO (Zgb)

Gabbro (Zgb) of the State bedrock map comprises 

masses of gabbro that intrude older stratified rocks and 

have been intruded by younger Proterozoic rocks. Some 

masses are equivocal in their age limits and could be 

Paleozoic in age. Age relations are especially well preserved in the Holliston area (fig. 4; Volckmann, 1977), 

where the older stratified rocks, the intrusive gabbro, 

and the intruding Milford Granite can all be found. In the 

large roof pendant in this area, layering is well preserved 

in a gabbro that has been partly uralitized. A small mass 

of anorthositic gabbro is spatially associated with cumberlandite (cu) east of Woonsocket, R.I., but the contact 

relations are not known (Rutherford and Carroll, 1981; 

Rutherford and Hermes, 1984).

DIORITE (Zdi)

The diorite (Zdi) consists of undifferentiated masses of 

plagioclase-hornblende rock (tables 2, 3; fig. 4) that 

formed intrusive masses of intermediate composition; 

some recrystallized older volcanic rocks may be included 

in the category. The modal variations in the diorite are 

large. Some are quite rich in plagioclase, whereas others 

are enriched in amphibole or biotite. Much of the rock 

northeast of Boston that earlier workers assigned to the 

Newburyport Quartz Diorite has been included in this 

unit.

Many of the diorite masses are intruded by the Proterozoic Z batholithic assemblage of granite and granodiorite. In the large roof pendant near Randolph, and west 

of Spot Pond, Stoneham, recrystallization of the older 

volcanic rocks to dioritic- and quartz dioritic-appearing 

rocks in contact zones of the Dedham Granite is well 

exhibited. At Canton, between Randolph and Sharon, 

intrusive dikes of diorite can be found in the older 

volcanic rocks. We are unaware of any localities where 

diorite (Zdi) intrudes the Dedham Granite or its equivalents. A mass of altered diorite (Zdi) (table 2, no. 1; fig. 

4) straddling the Massachusetts-Rhode Island State line 

southwest of Wrentham has septa of metavolcanic rocks 

of the Blackstone Group and apparently has intruded 

these rocks. Several masses of diorite crop out in the 

New Bedford area of southeastern Massachusetts (fig. 

4). One of these forms a phacolithic mass well exposed in 

a local crushed-rock quarry (Warren Brothers quarry) at 

Acushnet. Two other partly lenticular masses crop out in 

scattered exposures near the mouth of Slocums River 

and along the shore at Horseneck Beach, Westport. The 

rock in the plutons ranges from gabbroic diorite to quartz 

diorite (table 2, nos. 2-3; table 3, nos. 1-2). The diorite is 

dark gray, medium grained, and mostly nongneissic to 

faintly gneissic but is markedly foliated in and near shear 

zones. The rock consists mostly of hornblende, some of 

which contains relic pyroxene cores, and plagioclase and 

subordinate biotite and quartz. The plagioclase is a 

zoned, lathlike calcic andesine to labradorite in the 

Horseneck Beach pluton but is saussuritized in the 

Acushnet pluton. Accessory minerals are pyrite, titanite, apatite, and secondary chlorite and epidote.

Relations with the adjacent granitic rocks are not 

everywhere clear, and more than one age of diorite may 

be present. Most of the diorite masses of southeastern 

Massachusetts appear to be older than the granitic rocks. 

At Slocum's Neck, in the Slocums River area, diorite 

containing dark greenstone inclusions is cut by dikes of 

granite, and in one place an intrusion breccia consisting 

of fragments of diorite in a granitic matrix is exposed. On 

a point across the river to the east, however, a finegrained dike of diorite sharply cuts coarse-grained 

inequigranular granite. This dike is interpreted to be of 

Paleozoic or Mesozoic age. At Horseneck Beach, diorite 

is cut by alaskite and biotite granite. In the Warren 

Brothers quarry at Acushnet, diorite is intruded by 

even-grained granite or granodiorite that has a contact 

zone of spotted rock containing clots of biotite and 

hornblende. A few aplite and pegmatite dikes intrude all 

the rocks in this quarry.

DIORITE AT ROWLEY (Zrdi)

The diorite at Rowley (Zrdi) is a 5-km2 circular stock of 

hornblende diorite that intrudes the metavolcanic rocks
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 111

TABLE 3. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of plutonic rocks of the Proterozoic Z 

mafic volcanic-plutonic complex

[Major-oxide compositions for samples 1-3 from X-ray spectroscopy by Paul 

Hearn and Susan Wargo; all Fe reported as Fe203 ; sample 4, whole-rock 

analysis by H.S. Washington (in Emerson, 1917). Trace-element abundances 

from instrumental neutron activation analyses by L. J. Schwartz except Rb and 

Sr determined by X-ray spectroscopy by G.A. Sellars and B. McCall. Trace 

elements not determined for sample 4. nd, not determined]

Sample no. ............... 1 2 3 4

Unit .................... Zdi Zdi Zssy Zdigb

Major-oxide composition, in weight percent, 

and alkali-alumina ratio

Si02 .................... 51.7 52.5 58.8 51.52

A1203 ................... 16.5 16.3 15.4 17.06

Fe203 .................. 9.46 8.37 9.65 1.97

FeO.................... nd nd nd 8.60

MnO. ................... .15 .15 .33 0

MgO.................... 6.2 7.2 .1 4.87

CaO.................... 8.1 7.4 3.7 8.59

NagO................... 2.6 2.9 3.6 3.44

K20.................... 1.44 .71 4.92 1.77

Ti02 .................... 1.32 .85 .84 2.45

P205 .................... .35 .16 .18 nd

(NaaO+KjjOVAlgOg. ...... .24 .22 .55 .30

Normative-mineral composition, in weight percent, 1 

and differentiation index (DI)

Qtz..................... 2.2 3.8 5.1 0

Cm .................... 0 0 0 0

Or. ..................... 8.5 4.2 29.1 10.4 

Ab..................... 21.9 24.5 30.4 29.1

An..................... 29.1 29.4 11.3 25.8

Di...................... 7.3 5.3 3.3 8.0

Hd..................... 0 0 0 5.8

Fo ..................... 0 0 0 4.5

Fa ..................... 0 0 0 4.2

En ..................... 13.7 16.6 .2 5.1

Fs...................... 13.6 13.0 16.9 4.3

Ilm..................... 2.5 1.6 1.6 4.6

Ap ..................... .8 .4 .4 0

DI ..................... 33 32 65 39

Trace-element abundances, in parts per million, 

and selected ratios

Rb ..................... 52 26 51

Cs...................... 1.8 1.2 3.7

Sr...................... 469 395 113

Ba ..................... 477 216 2603

Rb/Cs .................. 29 22 14

Rb/Sr. .................. .1 .06 .4

Sc...................... 22.6 25.8 20.3

Cr. ..................... 187 371 nd

Co ..................... 38.8 33.4 1.2

Zn ..................... 115 122 132

La ..................... 24 21 31

Ce ..................... 50 65 41

Nd..................... 26 21 42

Sm..................... 6 4 8

Eu ..................... 1.60 1.33 7.33

Gd ..................... 4.2 3.8 5.7

Ho..................... .6 .6 .7

(Zv) in the town of Rowley. Contact relations with the 

adjacent Topsfield Granodiorite are not known. The only 

radiometric age from this group of rocks, 656 Ma (table 

1), comes from this stock. The diorite consists of plagioclase, hornblende, and biotite; it has been fractured and 

silicified, and the minerals have been altered to epidote 

and chlorite (Dennen, 1981).

SHARON SYENITE (Zssy)

The Sharon Syenite forms a long narrow mass along 

the southeast side of the Norfolk basin (fig. 4) of gray to 

dark-gray syenite and minor ferrogabbro. Both rock 

types are intruded by the Dedham Granite (Zdgr), but 

their relationship to other Proterozoic Z gabbros is 

unknown. Near Sharon, the map pattern suggests that 

the syenite is intruded by diorite (Zdi). 

The syenite is gray and relatively unaltered; it is 

marked by ubiquitous minor hornblende (table 2, nos. 

4-5; table 3, no. 3). Its modal composition is closer to that 

of a monzonite than of a syenite because of the amount of 

plagioclase. Pyrite is common and contributes to the

TABLE 3. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of plutonic rocks of the Proterozoic Z 

mafic volcanic-plutonic complex Continued

Sample no. ............... 1 2 3 4

Unit .................... Zdi Zdi Zssy Zdigb

Trace-element abundances, in parts per million, 

and selected ratios Continued

Tb ..................... .81 .70 1.07

Tin..................... .32 .29 .47

Yb ..................... 2.1 2.3 2.7

Lu ..................... .29 .34 .45

La/Yb.................. 11 9 11

Hf...................... 3.7 3.4 2.4

Zr. ..................... nd nd nd

Th2 ..................... 5.0 3.4 2.3

Th ..................... 4.6 3.2 2.6

U ...................... 1.4 1.0 nd

1Pe203 calculated as FeO, except sample 4. 

2Delayed neutron reactivation determination by H.T. Millard, Jr., and C. McFee.

Description of samples 

Sample localities shown in Wones and others (1986) 

1. We-9. Diorite; biotite hornblende diorite cut by alaskite and granite; 

ledges east end of East Horseneck Beach, 200 m east of Horseneck 

Road. UTM grid: N45968-E3312. 

2. NBN-21a. Diorite; biotite hornblende quartz diorite; Warren Brothers quarry, Main St., 1 km south of Acushnet center. UTM grid: 

N46151-E3414. 

3. W-40. Sharon Syenite; loose blocks at water tank at top of Knuckup 

Hill, 1.1 km south of Wrentham center. UTM grid: N46585-E3069. 

4. Diorite and gabbro; diorite; Peaches Neck, Salem; from Emerson 

(1917, p. 180, no. 1 in table), H^O also reported.
112 THE BEDROCK GEOLOGY OF MASSACHUSETTS

characteristic deep rusty weathering of the Sharon Syenite (Chute, 1966) and its high iron content. Lyons (1969) 

described the syenite and found the ranges in modal 

composition shown in table 2. The mafic-mineral content 

of these rocks ranges between 7 and 16 percent.

The Dedham Granite intrudes the Sharon Syenite in 

outcrops along Route 1-95 southwest of Sharon, where 

small dikes of Dedham lithology have become desilicated 

and crystallized as quartz syenite. These syenites are 

distinctly different from the dark, coarse-grained, 

sulfide-bearing rocks typical of the Sharon Syenite.

CUMBERLANDITE (cu)

Cumberlandite (cu, fig. 4), actually a mela-troctolite, is 

an unusual cumulate rock that crops out in northeastern 

Rhode Island (Rutherford and Carroll, 1981; Rutherford 

and Hermes, 1984); it is presumably genetically related 

to the adjacent anorthositic gabbro (Zgb), which intrudes 

the Blackstone Group (Zb). The contact relations of the 

cumberlandite itself with the Blackstone Group or with 

the adjacent Esmond Granite (Zegr) of Rhode Island are 

not known (O.D. Hermes, written commun., 1984). Hermes has found chemical evidence to suggest that the 

gabbro could be related to the Paleozoic alkalic magmatic 

rocks rather than to the Proterozoic Z suite. The cumberlandite is a black, dense rock that contains tabular 

plagioclase in a preferred orientation. Rutherford and 

Carroll (1981) reported a mode of 49 percent olivine, 32 

percent titaniferous magnetite, 15 percent plagioclase, 

and accessory ilmenite and Al-rich spinel.

SERPENTINITE (u)

Serpentinite forms a poorly exposed, ill-defined mass 

(u, fig. 4) within the undifferentiated diorite and gabbro 

(Zdigb) in the town of Lynnfield. It may be part of the 

Proterozoic Z plutonic and volcanic complex, but, as it 

lies along a highly faulted zone (Goldsmith, this vol., 

chap. H), assigning an age is difficult. Kaye (1983, p. 

1076), for example, suggested that it is ultramafic material injected into a shear zone. If so, it is more likely to be 

middle to late Paleozoic in age, or possibly Mesozoic. We 

feel, however, that it is more likely a highly sheared and 

altered ultramafic rock of the Proterozoic Z mafic complex.

DISCUSSION OF THE MAFIC PLUTONIC ROCKS

The diorites (Zdi, Zdigb, tables 2 and 3) appear to be 

normal mafic rocks of a calc-alkaline suite (figs. 5-8) and 

to have normal rare-earth-element (REE) patterns (fig. 

9). The Sharon Syenite, on the other hand, differs from 

others of the suite and from the Proterozoic Z batholithic 

rocks. The Sharon is off the calc-alkaline trend in the

Peacock diagram (fig. 6) and has a positive rather than a 

negative europium anomaly (fig. 9), suggesting cumulate 

plagioclase or K-feldspar. It does, however, fall on trend 

with the diorites in thorium-uranium ratio (fig. 10).

PROTEROZOIC Z BATHOLITHIC ROCKS

The Proterozoic Z batholithic rocks fall into two major 

groups, those in the Dedham batholith and those in the 

Milford antiform (figs. 3, 11). The former tend to be 

nongneissic, primarily brittlely deformed, whereas the 

latter tend to be gneissic or partly gneissic and to have 

been ductilely deformed. An exception to this grouping is 

the plutonic rocks at the south end of the Dedham 

batholith, southeast of the Fall River pluton, which tend 

also to be gneissic. The Topsfield Granodiorite is considered to be an outlier of the Dedham batholith. All these 

rocks are approximately the same age, about 630-580 Ma 

(table 1). These batholithic rocks have been sampled 

more extensively (Wones and others, 1986) than the 

Proterozoic Z mafic plutonic rocks, and so more mineralogical and chemical data are available. Major constituents of these rocks (derived from point-counts of stained 

slabs) are listed in table 4, and their relative amounts are 

plotted in figures 5 and 12-14. Modes from point-counts 

of thin sections from some of these rocks and chemical 

compositions of some of these rocks are listed in tables 

5-10.

DEDHAM BATHOLITH 

DEDHAM GRANITE (Zdgr, Zdngr)

The Dedham Granite (Zdgr) occupies a large area 

within several regions in the Milford-Dedham zone (fig. 

11). The Dedham north and west of the Boston basin 

consists of both granite and granodiorite (Zdngr; table 5, 

no. 6; table 6, nos. 7-8). The Dedham intrudes rocks of 

the Blackstone Group (Zb), the Westboro Formation 

(Zw), and the mafic volcanic-plutonic complex (Zdigb, 

Zdi, Zgb, Zv).

Contacts with older rocks are particularly well 

exposed in Saugus, Stoneham, Canton (C, fig. 11), and 

Cohasset, although dikes of granitic rocks similar to the 

Dedham are found throughout the Proterozoic Z terrane 

southeast of the Bloody Bluff fault zone. Igneous breccias 

in which the Dedham encloses blocks of mafic rock are 

well exposed west of Spot Pond in Stoneham and in 

Cohasset. Near Arlington, granite dikes grade into syenite where they terminate within the older masses of 

amphibolite and gabbro. Small masses of pink granite 

intruding diorite and gabbro north of the Boston basin 

are considered to belong to the northern, more mafic 

phase of the Dedham (Zdngr); this pink granite is the 

most mafic of the rocks mapped as Dedham.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 113

Quartz

K-feldspar

OA

O C

o

y

O

A

O A

7\

AO 

O^ A 

O A 

O

V v V

Plagioclase

Mafic minerals 

EXPLANATION

 Dedham Granite, Esmond Granite, and

Westwood Granite (table 5) 

O Granites of southeastern Massachusetts

(table 7) 

A Plutonic rocks of the Milford

antiform (table 9)

 Proterozoic Z mafic plutonic

rocks (table 2) 

D Sharon Syenite (table 2)

FIGURE 5. Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Proterozoic Z intrusive rocks of the Milford-Dedham 

zone from tables 2, 5, 7, and 9. Mafic minerals used are biotite, hornblende, pyroxene, iron oxides, and chlorite.

The Dedham Granite is typically medium to coarse 

grained, is commonly fractured, and usually has a salmon 

color mottled by green clots of epidote and chlorite 

formed by alteration. Fresh rocks that do not contain 

closely spaced (less than 1 m) joints are gray. Weathered

surfaces commonly have prominent knobs of quartz, the 

so-called "hob-nailed boot" texture. Modes of Dedham 

Granite (tables 4-6) and ternary plots (figs. 5, 12) 

show that the composition ranges mostly from granite 

to granodiorite. A few are quartz monzodiorite to
114 THE BEDROCK GEOLOGY OF MASSACHUSETTS

15 ! ! j 

EXPLANATION

 Proterozoic Z batholithic rocks (tables 6, 8, and 10)

 Proterozoic Z mafic plutonic rocks (table 3) 

D Sharon Syenite (table 3}

50 55 60 65 

PERCENT SiO2

70 75 80

FIGURE 6. CaO and Na-jO+KaO plotted against Si02 for Proterozoic Z intrusive rocks of the Milford-Dedham zone. Lines are approximate fit

to data.

monzonite. The K-feldspar crystals are perthitic and 

subhedral, and they appear to have formed with plagioclase and quartz early in the crystallization sequence. 

The plagioclase is partly saussuritized; chemical analyses 

(table 6) indicate that the plagioclase probably ranged 

from albite to andesine but was mostly oligoclase. Biotite 

is the dominant mafic mineral, although there are trace 

amounts of hornblende in some rocks. Titanite is ubiquitous as euhedral primary crystals and as anhedral products of biotite alteration. Apatite, zircon, magnetite, and 

other opaque minerals are common accessories. Plagioclase has been altered to the assemblage albite-sericiteepidote, and biotite and hornblende have altered to 

chlorite, epidote, and titanite. A hematite dust in the 

alkali feldspars gives the rock its characteristic salmon 

color.

REE analyses show a general enrichment in light 

REE and a negative europium anomaly (fig. 15), 

although two samples (2 and 3, table 6) show a lower and 

flatter pattern of heavy REE with respect to chondrite 

than the others, and sample 9 is less enriched in light 

REE. The profile of the mafic sample of Dedham Granite 

north of Boston is similar to the other profiles but is 

elevated above them, suggesting an overall enrichment 

of some sort.

The Dedham Granite is enriched in anorthite, hornblende, and biotite where it is close to the older mafic 

rocks, confirming Crosby's (1913) ideas on assimilation of 

wall rocks. Most of the modes that plotted in the granodiorite and tonalite fields (figs. 5,12) were collected from

rocks close to the older rocks. The K-feldspar content 

increases from north to south within the Dedham Granite 

and its related rocks. This increase may be an actual 

gradient, or perhaps a deeper erosional level is exposed 

in the south and assimilated roof pendant material has 

contaminated the shallower portions of the batholith 

exposed toward the north.

The Dedham Granite also contains porphyritic varieties, not distinguished in table 4, that contain alkali 

feldspar megacrysts 1-4 cm long and 0.5-2 cm wide. 

Lyons (1969) identified a separate pluton of this variety 

in the area north of Mansfield that he named the 

Barefoot Hill quartz monzonite (no. 1, table 6). The 

Barefoot Hill quartz monzonite of Lyons is included in 

the Dedham Granite on the State bedrock map because it 

is in gradational contact with the Dedham in the region of 

Brockton (Chute, 1950) and is herewith referred to as the 

porphyritic variety of Dedham. The REE pattern for the 

porphyritic granite is only slightly richer in heavy REE 

than samples 2 and 3 (table 6). Porphyritic rocks occupy 

large areas near Brockton and Assawompset Pond. 

According to Lyons, the same rock appears in the inlier 

of basement near Middleboro (fig. 11) and is probably the 

porphyritic granite (Zpgr) of the Fall River-New Bedford area. There is an apparent concentration of this 

variety of the Dedham Granite to the southeast in the 

Milford-Dedham zone.

The Dedham Granite as mapped on the State bedrock 

map may include small masses of younger granite, such 

as the Westwood Granite, whose boundaries have not
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 115

EXPLANATION

 Plutonic rocks of northeastern

Massachusetts (table 6) 

o Plutonic rocks of southeastern

Massachusetts (table 8) 

& Plutonic rocks of the Milford

antiform (table 10)

 Proterozoic Z mafic plutonic 

rocks (table 3)

FIGURE 7. Ternary plot of normative albite (Ab), anorthite (An), and 

orthoclase (Or) for Proterozoic Z intrusive rocks of the MilfordDedham zone. Fields of silica-saturated rocks from O'Connor (1965) 

as modified by Barker (1979).

been determined. The rock that Chute (1965b) mapped as 

Westwood Granite in the Scituate and Marshfield areas 

was believed by Wones to be a phase of the Dedham and 

not the typical Westwood. It is possible, but not certain, 

that the terrane underlain by the Dedham Granite is a 

composite of many smaller plutons. Bateman and others 

(1963) demonstrated that the Sierra Nevada batholith is 

a composite batholith but that individual plutons within it 

are commonly larger than the 1,400 km2 estimated for the 

Dedham Granite. The cover of glacial deposits, urban 

development, and extensive faulting in eastern Massachusetts have made it difficult to resolve this question.

TOPSFIELD GRANODIORITE (Ztgd)

The Topsfield Granodiorite (Ztgd), first described by 

Toulmin (1964), occupies an area of 80 km2 between 

Middleton and Newbury (fig. 11). It intrudes the diorite 

and gabbro (Zdigb) and mafic and felsic metavolcanic 

rocks (Zv). It is bounded on the northwest by faults 

bordering the Newbury and Middleton basins and northward extensions of the Bloody Bluff (and Mystic?) fault 

zones (Goldsmith, this vol., chap. H), and on the southeast by a splay(?) off the Bloody Bluff fault. Although the 

Topsfield Granodiorite has not been dated, we consider it 

to be part of the Proterozoic Z batholithic assemblage 

because it is similar in mineralogy and alteration to the 

Dedham Granite and because it intrudes the Proterozoic 

Z mafic complex. It is probably overlain by the Silurian 

and Devonian Newbury Volcanic Complex (Dennen, 

1975). The contact is now a fault.

The composition of the Topsfield ranges from granite 

to tonalite. The only sample shown in figure 12 happens 

to be a tonalite. The Topsfield is typically altered to red 

and green. The plagioclase is altered to sericite and 

epidote, and it is strongly dusted with hematite. In 

places, blue quartz forms large anhedral grains or mosaics of grains. Much of the rock is transected by recrystallized mylonite bands that truncate and incorporate 

altered grains, implying that some of the deformation 

postdates the alteration. The one sample described here 

(table 4; table 6, no. 9) is a typical tonalite. The REE 

profiles (fig. 15) are flat and are approximately three 

times the values for chondrites, quite unlike rocks of 

similar bulk composition (Frey and others, 1978). The 

thorium content is unusually low relative to uranium, 

which may reflect the relatively high degree of alteration 

of the rock.

FeO

MgO

EXPLANATION

 Batholithic rocks of northeastern

Massachusetts (table 6) 

0 Batholithic rocks of southeastern

Massachusetts (table 8) 

a Batholithic rocks of the Milford

antiform (table 10)

 Mafic plutonic rocks (table 3)

FIGURE 8.-Ternary AFM (alkalis-FeO-MgO) plot of Proterozoic Z 

intrusive rocks of the Milford-Dedham zone.
116 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 4. Major constituents, in percent, determined by point-count 

of stained slabs of intrusive rocks of the Milford-Dedham zone

[1,200-2,000 points counted per slab. Totals may not sum to 100 because of 

rounding. Sample localities shown by field number in Wones and others (1986)]

Sample UTM grid Mafic P - Quartz K-feldspar Plagioclase . ,

no. N. E. minerals

Dedham Granite south of the Boston basin (Zdgr)

DMA-30 .... 46770 3220 33.0 28.5 33.6 5.0 

DMA-32 .... 46781 3196 29.4 23.8 45.6 1.2 

DMA-33 .... 46779 3197 29.6 35.8 32.6 2.1 

DMA-34 .... 46767 3196 10.2 15.3 62.5 12.0 

DMA-34A... 46766 3190 29.7 29.4 37.5 3.4 

DMA-41 .... 46481 3050 46.7 30.5 22.5 .3

DMA-93 .... 46813 3242 31.0 38.1 24.3 6.6 

DMA-94 .... 46603 3158 34.4 36.7 24.7 4.5

DMA-98.... 46729 3360 30.9 38.4 29.1 1.5 

DMA-99 .... 46729 3360 25.7 39.7 31.9 2.7 

DMA-100 . . . 46717 3361 20.0 42.1 30.9 7.0 

DMA-101 . . . 46701 3369 31.2 20.0 43.2 5.6 

DMA-102 . . . 46677 3358 25.0 29.8 39.3 5.9 

DMA-104 . . . 46718 3387 22.4 44.4 24.6 8.5 

DMA-105 . . . 46723 3388 19.6 10.8 64.5 5.1 

DMA-108 . . . 46621 3429 30.0 35.8 32.5 1.6 

DMA-109B . . 46720 3425 30.9 29.3 35.2 4.5 

DMA-116 . . . 40787 3421 15.4 30.8 44.0 9.8 

DMA-117A.. 46798 3462 21.0 18.4 51.2 9.2

DMA-119 . . . 46799 3475 20.3 33.5 39.6 6.7

DMA-120 . . . 46804 3485 29.4 35.4 29.4 5.8 

DMA-121 . . . 46807 3494 19.4 22.4 45.0 13.2 

DMA-123 . . . 46773 3524 29.7 36.8 28.8 4.7 

DMA-129 . . . 46597 3568 30.2 47.6 19.9 2.3 

DMA-132 . . . 46566 3571 30.1 45.0 22.3 2.6 

DMA-138 . . . 46601 3552 38.5 32.8 23.4 5.3 

DMA-140... 46588 3536 35.0 11.1 49.8 4.1 

DMA-143 . . . 46618 3456 37.0 28.3 33.3 1.4 

DMA-146 . . . 46644 3392 30.0 28.0 38.9 3.1 

DMA-147 . . . 46646 3390 24.4 15.0 48.9 11.6

DMA-150 . . . 46632 3353 26.1 28.5 41.4 4.0 

DMA-151 . . . 46649 3346 29.2 37.9 30.9 2.0

DMA-153B . . 46653 3316 30.8 39.2 27.9 2.0 

DMA-157 . . . 46614 3248 26.9 20.4 46.3 6.3 

DMA-158 . . . 46590 3231 32.1 29.8 33.9 4.2 

DMA-159 . . . 46587 3205 29.7 24.9 34.1 6.3 

DMA-160 . . . 46613 3182 34.7 41.6 19.3 4.4 

DMA-166 . . . 46609 3295 26.7 24.4 43.6 5.2 

DMA-196 . . . 46558 3009 27.9 35.0 34.0 3.0 

DMA-201 . . . 46695 3018 33.9 39.1 23.5 3.6 

DMA-203 . . . 46663 3048 34.1 39.4 24.8 1.7 

DMA-204 . . . 46654 3018 29.5 33.2 35.4 1.9 

DMA-208 . . . 46805 3150 26.7 39.0 30.0 4.3

F 1 >I£KQQ OAAA OQ 1 O£ 1 O/l O £!

W-3. ....... 46576 3042 2.6 51.4 34.0 11.9

W-4. ....... 46568 3059 24.9 28.4 40.0 6.7

W-6. ....... 46567 3061 35.3 28.3 28.3 8.1

Dedham Granite north of the Boston basin (Zdngr)

DMA-65 .... 47038 3328 29.0 12.1 41.9 17.1 

DMA-66 .... 47040 3329 26.4 32.5 36.9 4.2 

DMA-67 .... 47052 3321 26.1 2.8 59.1 17.0 

DMA-127 . . . 46995 3148 27.6 29.9 35.9 6.6

Topsfield Granodiorite (Ztgd)

DMA-60 .... 47334 3480 30.6 0 47.5 21.9

Dedham Granite west of the Boston basin (Zdgr)

DMA-1 ..... 46925 3134 36.6 31.7 27.5 4.2 

DMA-3 ..... 46907 3128 37.0 20.2 37.1 5.7 

DMA-5 ..... 46882 3089 31.8 30.0 34.1 3.8 

DMA-6 ..... 46883 3089 33.4 17.9 43.3 5.5 

DMA-7 ..... 46851 3105 34.2 39.5 25.9 .4

TABLE 4. Major constituents, in percent, determined by point-count 

of stained slabs of intrusive rocks of the Milford-Dedham zone 

Continued

Sample UTM grid Mafic

F Quartz K-feldspar Plagioclase . . 

no. N. E. minerals

Dedham Granite west of the Boston basin (Zdgr) Continued

TIM A C /1ASQQ QnBQ 9QQ Xn A IS A 9 Q

DMA-9 ..... 46817 3060 34.8 7.2 48.9 8.8

DMA-10 .... 46827 3064 4.7 30.4 53.0 10.9 

DMA-13 .... 46813 3022 28.8 35.9 28.2 7.1 

DMA-19 .... 46891 2984 39.9 26.1 25.9 8.1 

DMA-20 .... 46885 2987 28.8 23.5 40.8 7.0 

DMA-174 . . . 46844 3038 38.2 39.1 18.8 3.9

Westwood Granite (Zwgr)

DMA-29 .... 46772 3218 26.2 35.9 33.6 4.3 

DMA-31 .... 46790 3184 31.5 36.5 28.4 3.6 

DMA-36 .... 46744 3182 35.0 29.3 32.5 3.2 

DMA-44 .... 46687 3315 28.5 35.7 30.9 3.9 

DMA-70 .... 46688 3318 31.9 34.2 28.9 5.0 

DMA-206 . . . 46794 3182 30.7 33.4 32.4 3.6

DMA-209 . . . 46774 3124 28.5 37.3 32.1 2.1 

DMA-109 . . . 46720 3425 28.8 35.0 32.9 3.3 

DMA-110 . . . 46725 3427 19.6 35.0 41.6 3.7 

DMA-117B . . 46790 3462 26.3 47.5 25.1 1.0 

DMA-137 . . . 46715 3414 24.5 33.4 38.4 3.8

Milford Granite (Zmgr; Zmgd where noted)

DMA-14 .... 46837 2970 38.8 36.3 18.7 6.2 

DMA-22 .... 46709 2926 32.3 38.3 26.3 3.1 

DMA-27.... 46729 3072 28.1 34.8 33.4 3.2 

DMA-28 .... 46703 3108 28.4 29.5 38.7 2.9 

DMA-179 . . . 46726 2957 28.6 43.7 25.6 2.2

DMA-183... 46775 2960 32.0 26.1 32.5 9.3 

DMA-194... 46865 2972 17.9 55.3 24.1 2.6 

Mi-3 ....... 46702 2931 36.1 33.4 23.2 7.3

Mi-4 (Zmgd). 46718 2918 37.0 38.4 17.3 7.3 

Mi-5....... 46727 2910 38.4 4.9 42.1 14.2

Granite of the Fall River pluton (Zfgr)

A-2........ 46313 3338 30.5 34.0 32.2 3.2

A-3........ 46310 3315 30.9 31.4 36.2 1.5

A-5b....... 46290 3309 38.2 34.2 24.7 2.8

A-7........ 46300 3306 33.9 35.2 28.7 2.2

A-8........ 46285 3291 24.8 41.4 28.1 5.7

A-9........ 46283 3294 36.5 33.0 28.9 1.7

A-10 ....... 46242 3276 32.5 39.4 24.2 3.9

A-ll ....... 46253 3263 37.1 38.4 21.7 2.7

AP-la ...... 46269 3371 32.6 28.9 33.7 4.8

AP-4....... 46319 3362 25.3 9.1 51.7 13.9

AP-6 ....... 46331 3341 33.8 34.9 25.6 2.7

AP-7....... 46293 3374 31.5 35.3 29.4 3.7

FR2....... 46165 3224 19.5 42.8 31.7 6.0

T7iT> o /I£1£Q OIQi? OO Q O1 A QQ C £ Q

FR-8....... 46187 3221 35.9 35.9 20.2 7.9

FR-12 ...... 46184 3202 42.0 40.1 15.6 2.3

FRE-3 ..... 46189 3237 33.9 30.8 28.3 4.4

FRE-4 ..... 46231 3234 36.3 33.8 19.8 10.1

FRE-11. .... 46234 3252 33.2 38.1 23.1 5.6

r K.& Jus. .... 4o<sll o&OJl o(.<s oo.y &0.& o. (

T-l ........ 46099 3178 40.0 28.1 27.2 5.0

Porphyritic granite (Zpgr)

NBN-9 ..... 46146 3411 30.6 28.0 34.1 7.3 

We-2....... 45968 3240 16.2 55.1 21.8 5.9

We-4....... 45987 3270 25.4 45.3 20.7 8.5

NBN-22 .... 46179 3422 30.0 25.4 27.4 14.0*

* Plus 3.1 percent muscovite.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 117

HI

100

90

80

70

60

50

cc.

UJ 40

t 30

CC

O

2

O

O 20

O 

O

CC

10

9

8

EXPLANATION

Diorites (samples 1 and 2)

 - Sharon Syenite (sample 3)

La Ce Nd Sm Eu Gd Tb Tm

FIGURE 9. Chondrite-normalized plot of rare-earth elements in Proterozoic Z mafic plutonic rocks of the Milford-Dedham zone

from table 3.

Yb Lu

Sample numbers

40

30

20

10

E 

a 

a

0.5

EXPLANATION

Proterozoic Z batholithic rocks 

Proterozoic Z mafic plutonic rocks 

Sharon Syenite

I i i

01 23456 789 10 11 12 

U (ppm)

FIGURE 10. Plot of Th (thorium) against U (uranium) for Proterozoic 

Z intrusive rocks of the Milford-Dedham zone.

ESMOND GRANITE (Zegr)

The Esmond Granite (Zegr) occupies nearly 100 km2 of 

Rhode Island. It forms an elongate pluton southeast of 

Woonsocket of which only the northern part is in Massachusetts (fig. 11). A similar pluton lies west of Woonsocket, and phases of the biotite granite (Zgr) and the 

Milford Granite (Zmgr) resemble it. In fact, Quinn (1971) 

thought the Esmond and the Milford might be the same 

rock. A coarse-grained facies of the Esmond intrudes the 

Blackstone Group and a related tonalite. A fine-grained 

facies intrudes both the tonalite and the coarse-grained 

facies (Hermes and Zartman, 1985). Contact relations 

with the Milford Granite are not known. The age of the 

Esmond (621 ±8 Ma) appears to be close to that of the 

Milford (table 1). The Esmond Granite is mottled red and 

green, like the Dedham Granite, and is massive except 

where foliated by local deformation. The representative 

modes (Quinn, 1971; table 5, no. 7) and chemical content 

(table 6, no. 10) indicate that the dominant rock type is 

biotite granite poor in mafic minerals. The plagioclase is 

altered to muscovite and epidote, and the biotite is 

usually chloritized. The REE pattern (fig. 15) is similar 

to that of the Dedham Granite.

GRANT MILLS GRANODIORITE (Zgmgd)

The Grant Mills Granodiorite (Zgmgd) occupies an 

area of 25 km2 in northeastern Rhode Island (fig. 11). 

According to Quinn (1971), it intrudes older quartz 

diorite and is gradational with the Esmond Granite. 0. 

Don Hermes (written commun., 1985; Hermes and Zartman, 1985) considered it to be a porphyritic variety of
118 THE BEDROCK GEOLOGY OF MASSACHUSETTS

72°00' 45'

42°45'

30'

15'

42W

_______NEW_HAMPSHJRE______ / 

MASSACHUSETTS 

70°30'

45'

41°30'

EXPLANATION

Pennsylvanian to Proterozoic Z 

cover rocks

Nongneissic rocks (Dedham batholith and Topsfield Granodiorite)

Gneissic rocks in the Milford antiform 

and New Bedford gneissic terrane

Other rocks

9-n-;" - -Narraga.ns'ett.';: : : .-:: :;.-.'. ' ' '

I- -. .-'. '. : ' basin- '.-.'- ." j- ' ' .Zf9 ro^r5pgr- zgmgd l-\-'.. . . . .' ' -'. . m .: Middle.boro>V 

^ '-' ~-f '. : ': '. '..'.' : '.: ' ' '.: '.- -.' / ^r.'.-.-- - -.-.- - ;. - . ; .--' ;'..'::/

---- Contact Dotted where concealed

 -- Fault Dashed where inferred; 

dotted where concealed

10

20 KILOMETERS

FIGURE 11. Distribution of Praterorozoic 'L batholithic slithic rocks in the 

Milford-Dedham zone. W, Wrentham; C, Canton; A, Acushnet; LP, 

Long Pond; AP, Assawompset Pond; H, Holliston; M, Milford. Ztgd, 

Topsfield Granodiorite; Zdngr, Dedham Granite north of Boston; 

Zdgr, Dedham Granite; Zwgr, Westwood Granite; Zfgr, granite of 

the Fall River pluton; Zpgr, porphyritic granite; Zagr, alaskite; Zgg,

Zgg

granite, gneiss, and schist, undivided; Zgmgd, Grant Mills Granodiorite; Zegr, Esmond Granite; Zsg, Scituate Granite Gneiss; Zpg, Ponaganset Gneiss; Zhg, Hope Valley Alaskite Gneiss; Zmgr, biotite granite 

of the Milford Granite; Zmgd, mafic phase of the Milford Granite; Zgr, 

biotite granite; Zm, Mattapan Volcanic Complex; DZ1, Lynn Volcanic 

Complex.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 119

Quartz

EXPLANATION

 Dedham Granite south of Boston

o Dedham Granite west of Boston

+ Dedham Granite north of Boston

D Topsfield Granodiorite

K-feldspar /\

«0*

Plagioclase

Mafic minerals

FIGURE 12. Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Dedham Granite and Topsfield Granodiorite, from

table 4. Fields of igneous rocks from Streckeisen (1973).
120 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Quartz

K-feldspar

EXPLANATION

 Westwood Granite 

o Westwood Granite in Plymouth 

quarry, Weymouth

 Milford Granite

Plagioclase

Mafic minerals

FIGURE 13. Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Westwood Granite; Westwood Granite in Plymouth 

quarry, Weymouth; and Milford Granite, from table 4. Fields of igneous rocks from Streckeisen (1973).
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 121

Quartz

K-feldspa

EXPLANATION

o Granite of the Fall River pluton 

 Porphyritic granite

Plagioclase

Mafic minerals

FIGURE 14. Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes from the Fall River pluton including the porphyritic

granite, from table 4. Fields of igneous rocks from Streckeisen (1973).
122 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 5. Modes, in percent, of plutonic rocks of the Dedham batholith in northeastern Massachusetts

[Zdgr, Dedham Granite; Zdngr, Dedham Granite north of Boston; Zegr, Esmond Granite; Zwgr, Westwood Granite. Sample localities shown by field number in Wones

and others (1986). a, albite; n, andesine; p, perthitic; tr, trace]

Unit ............

Microcline

Epidote .........

Chlorite. ........

Magnetite .......

Hematite .......

Titanite.........

Allanite .........

Zircon ..........

Calcite. .........

1

.. Zdgr

. . 2009

.. N-3

. . 32.2

. . 27. 6a

. . 37. 8p

. . 1.0*

. . 1.0$

.. 0

.. 0

.. 0

.6

.2$

.. 0

.. 0

.. 0

.. 0

.. 0

.. 0

2

Zdgr

1485

F^O

31.5

36. 6a

30.2p

.8*

.7*

0

0

0

.1

0

0

0

0

0

0

0

3

Zdgr

1451

F 29

25.3

98 Qa

40.0p

3.9

0

.7

fr

0

.2

0

0

0

.4

.6

tr

0

4

Zdgr

1586 W-6*

37.2

41 fia

16.9p

0

0

.4

tr

3.6

tr

0

.1

tr

0

0

0

0

5

Zdgr

1811

W-37

21.6

33. Oa

44. 5p

.2

0

.5

0

3.0

.2

0

tr

0

0

0

0

0

6

1761

BN-1

32.8

33.8n

14.8

.1

0

4.9

tr

12.9

.1

0

.2

.1

0

0

0

0

7

Zegr

2101 P-5*

38.8

25.9

35.4

1.3

.9

1.1

0

.5

0

0

0

0

0

0

0

.1

8

1728

BH-3

29.3

30.4a

37. 9p

.1

.3

1.0

0

.9

.1

0

0

0

tr

0

0

0

* Chemical analysis in table 6.

I Biotite altered to muscovite and hematite.

the Esmond. The dominant rock type is granodiorite 

(Quinn, 1971). It has the pink and green coloration due to 

alteration that is characteristic of the Dedham Granite 

and related rocks. Earlier workers (Warren and Powers, 

1914; Emerson, 1917; Quinn, 1971) considered the Grant 

Mills to be equivalent to the Dedham Granite in both age 

and texture.

WESTWOOD GRANITE (Zwgr)

The Westwood Granite (Zwgr) forms small (less than 

10 km2) masses of light-colored granite (fig. 11) that 

intrude the Dedham Granite and older rocks (Dowse, 

1949; Chute, 1950, 1966). Some masses that resemble 

Westwood Granite, such as those exposed in the Plymouth quarries in Weymouth and in scattered outcrops 

near Scituate, have been included within the Dedham 

Granite on the State bedrock map. The Westwood Granite crops out in the areas dominantly of Dedham Granite, 

and parts of it were included by Kovach and others (1977) 

in an investigation of the age of the Dedham by Rb-Sr 

dating methods. Extensive intrusion breccias occur at 

the contacts of the Westwood with older mafic rocks. The 

contacts with the Dedham Granite commonly are abrupt, 

with dikes of Westwood cutting the Dedham and rare 

inclusions of Dedham within the Westwood (Chute, 

1966). There have been no reports of Westwood cobbles 

within the Roxbury Conglomerate, so it is possible that 

the Westwood is an intrusive equivalent of the extrusive 

Mattapan Volcanic Complex that underlies the Roxbury.

The Westwood was therefore not exposed to erosion at 

the time of deposition of the Roxbury.

The Westwood Granite is fine grained and low in mafic 

minerals (tables 4-6). Much of the plagioclase is altered 

to sericite and the biotite to chlorite. It is usually lighter 

in color than the Dedham Granite but does contain 

microscopic hematite, which gives the rock a pink cast. It 

does not appear to be as deformed as the Dedham 

Granite. The Westwood Granite occupies a very compact 

field when projected onto the quartz-plagioclaseK-feldspar plane (fig. 13). The ranges of modes from this 

study (table 5) and from Chayes (1952) and Chute (1966) 

are 26-35 percent quartz, 29-45 percent K-feldspar, 

17-34 percent plagioclase, and 1-5 percent micas. Common accessories are biotite, magnetite, titanite, and 

apatite.

The mineral assemblage of the Westwood Granite is 

more like that of the Dedham Granite than that of any 

other intrusive unit in the Milford-Dedham zone. The 

Westwood differs from the Dedham Granite in having a 

finer grain size, lower An content of the plagioclase, lack 

of hornblende, and lower color index. It has a slightly 

lower heavy-REE content (fig. 15). It may represent a 

late-stage aplitic differentiate of the Dedham magma or a 

later, separate magma from a similar source material. 

Part of a small elliptical pluton of the Westwood Granite 

in Weymouth (not shown separately on the State bedrock 

map) partly exposed in the Plymouth quarries differs 

from the type locality in having a lower quartz content, 

slightly lower REE content, and higher U and Th 

contents. As mentioned above, the Westwood may be
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 123

the intrusive equivalent of the Mattapan and Lynn 

Volcanic Complexes (Zm and DZ1, fig. 11). A somewhat 

questionable 579±28-Ma age (table 1) obtained from the 

Westwood indicates that it might be younger than the 

Dedham. It may be about the same age as the Mattapan, 

from which a U-Pb age on zircon of 602 ±3 Ma has been 

obtained (Kaye and Zartman, 1980). The granitic intrusion at Pine Hill, Medford, which intrudes the Lynn 

Volcanic Complex (Zarrow, 1978), is probably the Westwood Granite.

FINE-GRAINED GRANITE (fgr)

Two small bodies of fine-grained biotite granite (fgr) 

lie in fault blocks near the southwest end of the Norfolk 

basin near Wrentham (W, fig. 11). Each is less than 5 

km2 in area. They appear to intrude the Dedham Granite, although they may be close to it in age.

GRANITE OF THE FALL RIVER PLUTON (Zfgr)

The granite of the Fall River pluton (Zfgr) occupies an 

area of 300 km2 southeast of the Narragansett basin 

between Middleboro and Buzzards Bay (fig. 11). The 

radiometric age of the granite (table 1) shows the rock to 

be the same age as the Dedham Granite, 630 ±15 Ma, and 

the mineralogy and texture of the granite of the Fall 

River pluton make it most probable that this body is 

equivalent to the Dedham Granite. A minimum age of 

516 Ma has been obtained by Rb-Sr whole-rock methods 

(Galloway, 1973) on rock in the southern part of the 

pluton, south of Fall River, called the Bulgarmarsh 

Granite (Pollock, 1964). We were unable to distinguish 

the Bulgarmarsh from the rest of the Fall River pluton in 

our reconnaissance for the State bedrock map. Skehan 

and others (1978) and Skehan and Murray (1980) suggested that the granite of the Fall River pluton might be 

equivalent to a porphyritic granite (Kay and Chappie, 

1976) on Aquidneck Island, R.I., that was dated at 

592±12 Ma by Rb-Sr methods (Smith, 1978). We think 

this unlikely because the granite of the Fall River pluton 

is not typically porphyritic. The Fall River pluton may 

contain younger intrusions not to date distinguished.

The granite of the Fall River pluton is light gray to 

gray, locally light reddish orange, medium grained, 

equigranular to slightly seriate, and rarely porphyritic. 

Most of the rock is fresh, and it is only locally fractured 

and altered. In the Assonet area, along the edge of the 

Narragansett basin, the granite is fractured and more 

severely altered than elsewhere. Modes and chemical 

compositions of the granite (tables 4, 7, 8) lie within the 

field of the Dedham Granite (figs. 5, 14), and the REE 

patterns (fig. 16) are similar to those of the Dedham 

Granite. Some quartz forms mosaic aggregates. Much of 

the microcline is perthitic. Plagioclase ranges in anorthite content from An10 to An30 but is partly saussuritized. Biotite is less than 5 percent. The muscovite is 

usually next to plagioclase, and we suggest that some, if 

not all, of the muscovite (table 7) may be sericite 

recrystallized to muscovite during a Permian thermal 

high. Accessory minerals are opaque minerals, zircon, 

apatite, sparse titanite, garnet, and allanite. Sericite, 

epidote, and titanite are common alteration phases.

The rock is not gneissic in the north, although local 

cataclastic textures are present, but south of Fall River 

it becomes gneissic, particularly in the Westport area. 

The rock is more leucocratic at Freeport and is coarse 

grained east of Assawompset Pond. No distinct contact 

between the main mass of the Fall River pluton and the 

Bulgarmarsh Granite of Pollock (1964) was observed by 

us. At a roadcut on Route 24 north of Tiverton, in the 

area of the Bulgarmarsh Granite, the rock contains 

granular quartz aggregates similar to those in the 

Milford Granite (table 7, no. 4; table 8, no. 4). Emerson 

(1917), on the earlier State map, showed an area of Quincy 

Granite northeast of Assonet. Although the granite in 

this region is more varied in texture than elsewhere, we 

were unable to locate any Quincy Granite.

PORPHYRITIC GRANITE (Zpgr)

Porphyritic granite (Zpgr) is a distinct rock type in the 

southeastern part of the Dedham batholith (fig. 11). It 

lies mostly east of the Fall River pluton and extends from 

Long Pond (LP, fig. 11) south to Buzzards Bay and east 

and north to Middleboro, where it is exposed in an outlier 

surrounded by Pennsylvanian rocks. Lyons (1969) considered the porphyritic granite to be equivalent to his 

Barefoot Hill quartz monzonite of the Mansfield area (see 

description of Dedham Granite above); its analysis in 

table 4 (DMA-166) shows that it fits within the spread of 

modes of the porphyritic granite (table 7). Chemically, 

however, if no. 1, table 6, and no. 5, table 8, are truly 

representative, the porphyritic granite is somewhat 

more mafic than Lyons' Barefoot Hill.

The porphyritic granite is gray to greenish gray, 

seriate to porphyritic, with microcline phenocrysts 

(locally 2 cm long) set in a finer grained matrix of quartz, 

plagioclase, microcline, and biotite aggregates. It is 

retrogressively sheared or foliated in varying degrees in 

the northern part of its area; near New Bedford it is 

pervasively gneissic but much less altered. Gneissic 

varieties may be seen in gravel pits north of Acushnet, 

and in the Westport shore area (for example, at Gooseberry Neck). An outcrop of gneissic rock north of Marion 

included in the area of granite, gneiss, and schist, 

undivided (Zgg), may be equivalent. Inclusions of dark 

schist and porphyritic mafic rock are oriented parallel to 

the feldspar orientation. In the more foliated rocks, gray
124 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 6. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of Proterozoic Z intrusive rocks, MilfordDedham zone, northeastern Massachusetts

[Major oxides determined by X-ray spectroscopy by Paul Hearn and Susan Wargo; all Fe reported as Fe203. Trace-element abundances determined by instrumental 

neutron activation analyses by L.J. Schwartz except Rb and Sr by X-ray spectroscopy by J. Lindsay, B. McCall, and G.A. Sellars. nd, not determined]

Sample no. .........

Unit. ..............

1

Zdgr

2

Zdgr

3

Zdgr

4 5

Zdgr Zdgr

6

Zdgr

Major-oxide composition, in weight percent,

Si02 ..............

A1203 .............

FeA.............

MnO..............

MgO..............

CaO ..............

Na.,0 .............

KgO ..............

Ti02 ..............

P205 ..............

(Na20+K20)/AlA-

. 71.34

. 14.90

2.21

.08

.92

1.72

4.06

3.94

.36

.11

.54

73.42

13.66

1.54

.04

1.05

1.28

4.11

4.08

.26

.08

.60

74.98

13.18

1.43

.05

.41

.93

3.17

4.99

.16

.04

.62

75.41 76.89

13.63 13.52

.86 .95

.02 .03

.22 .04

.25 .24

4.25 3.70

4.92 4.60

.10 .10

.04 .02

.67 .61

71.48

13.93

2.37

.09

.54

1.66

3.79

3.88

.29

.08

.55

Normative-mineral composition, in weight percent,

Qtz ...............

Crn. ..............

Or................

Ab ...............

An ...............

Wo ...............

En ...............

Fs................

Ilm ...............

Ap...............

DI................

. 26.3

1.1

. 23.4

. 34.5

7.8

0

2.3

3.6

.7

.3

. 84

28.8

.3

24.2

34.9

5.9

0

2.6

2.5

.5

.2

88

34.1

1.0

30.7

27.0

4.4

0

1.0

2.5

.3

.1

92

30.5 36.5

1.0 2.1

29.2 27.2

36.1 31.3

1.0 1.1

0 0

.6 .1

1.5 1.6

.2 .2

.1 0

96 95

29.6

.4

23.0

32.0

8.2

0

1.3

4.0

.5

0

85

7

Zdngr

8

Zdngr

9

Ztgd

10

Zegr

11

Zwgr

12

Zwgr

and alkali-alumina ratio

65.94

15.85

4.63

.10

1.78

2.84

2.66

3.15

.74

.17

.37

72.84

14.49

2.54

.05

.23

1.04

3.88

4.71

.30

.10

.59

68.85

13.98

5.31

.12

1.74

3.16

3.53

.58

.46

.08

.29

75.78

12.50

1.19

.05

.61

.83

3.92

4.42

.04

.02

.67

73.81

14.04

1.00

.05

.17

.69

4.15

4.21

.21

.05

.59

74.11

14.40

1.10

.05

.26

1.29

3.76

3.98

.16

.04

.54

1 and differentiation index (DI)

27.3

3.4

19.0

23.0

13.3

0

4.5

7.6

1.4

.4

69

Trace-element abundances, in parts per million, and

Rb ...............

Cs................

Sr ................

Ba. ...............

Rb/Cs. ............

Rb/Sr.............

Sc ................

Cr................

Co. ...............

Zn. ...............

La. ...............

Ce................

Nd ...............

Sm ...............

Eu ...............

Gd. ...............

Tb. ...............

Ho ...............

Tm ...............

Yb ...............

Lu. ...............

La/Yb ............

Hf ................

Zr................

Th2 ...............

Th. ...............

U2 ................

U ................

Zr/Hf.............

. 130

2.6

. 291

. 1217

. 50

.4

4.2

3.2

3.7

. 44

. 39

. 68

. 27

4

.85

3.7

.51

.7

.33

2.0

.28

. 19

4.5

. 202

. 11.0

11.1

2.4

2.4

45

94

0.7

314

1292

134

.3

2.5

2.3

3.3

22

26

50

18

2

.54

1.4

.24

.6

.24

.8

.13

32

3.4

157

14.3

15.6

4.4

4.4

46

144

1 Q

118

628

76

1.2

2.3

2.4

1.6

27

39

73

28

4

.75

2.0

.50

.7

.26

1.8

.28

22

3.7

178

17.9

16.3

4.1

3.5

48

174 145

14 19

58 54

430 386

124 121

3.0 2.5

4.2 4.2

5.3 5.6

.3 .3

17 18

27 35

55 72

25 34

6 7

.44 .45

4.0 5.0

84 Q7

.9 1

.63 .67

3.4 4.0

.50 .60

8 9

3.6 3.9

210 186

7.2 11.2

12.6 18.5

2.5 3.7

3.5 4.1

58 48

159

3.9

155

522

41

1.0

5.9

nd

2.2

49

35

67

33

7

.77

5.0

1.08

1

.51

4.0

.55

9

5.6

277

20.2

15.9

3.3

3.8

49

114

2.1

284

580

54

.4

13.1

7.6

12.0

79

35

64

28

6

1.18

5.3

.72

.7

.33

2.3

.34

15

5.2

273

11.6

14.2

2.1

2.0

52

28.0

1.4

27.8

32.8

4.5

0

.6

4.3

.6

.7

89

33.5

2.0

3.5

30.5

15.5

0

4.4

9.4

.9

.2

79

32.7

0

26.3

33.4

3.5

.2

1.5

2.2

.1

0

92

31.8

1.5

25.3

35.7

3.1

0

.4

1.6

.4

.1

93

33.4

1.7

23.7

32.1

6.2

0

.7

1.9

.3

.1

89

selected ratios

119

1.5

142

773

79

.8

4.1

2.6

2.6

63

75

133

64

14

1.55

10.0

2.13

2.2

1.0

6.3

.86

12

11.6

383

13.2

13.2

2.5

2.5

33

20

1.6

nd

137

12

nd

23.6

5.4

11.2

85

6

14

7

3

.65

3.0

.77

.8

.47

3.3

.48

2

1.9

510

.7

.9

.5

1.0

nd

130

1.1

94

727

118

1.4

2.7

6.0

.2

31

21

43

19

5

.79

3.5

.64

.5

.46

2.1

.31

10

2.7

54

10.2

11.2

2.5

2.1

20

131

1.4

133

1084

94

1.0

2.1

4.9

.7

42

38

71

24

4

.63

3.0

.43

.3

.28

1.5

.23

25

4.1

153

10.9

12.7

2.1

2.3

37

112

1.1

335

1451

109

.3

11.5

4.2

1.2

12

36

57

18

2

.46

3.3

.18

.2

.28

.6

.12

60

2.8

118

14.2

16.7

4.3

4.2

42

calculated as FeO. Delayed neutron reactivation analyses by H.T. Millard, Jr., C.M. Ellis, and V.C. Smith.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 125

TABLE 6. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of Proterozoic Z intrusive rocks, 

Milford-Dedham zone, northeastern Massachusetts Continued

Description of samples 

Sample localities shown in Wones and others (1986)

1. DMA-166. Inequigranular biotite granite, Dedham Granite; Barefoot Hill quartz monzonite of Lyons (1969). Rockland and Rockland 

Circle, Brockton. UTM grid: N46609-E3295.

2. DMA-123. Dedham Granite; Government Island, Cohasset. UTM 

grid: N46773-E3524.

3. DMA-94. Dedham quartz monzonite of Lyons (1969), Dedham 

Granite; Roadcut, 1-95, 906 m south of Cocasset St., Foxborough. 

UTM grid: N46603-E3158.

4. DMA-105. Aplite dike (35 cm wide), Dedham Granite, intruding 

hornblende-bearing granodiorite; west end of rest area, south side of 

Rte. 3, Weymouth. UTM grid: N46723-E3388.

5. DMA-73. Biotite granite, Dedham Granite; Roadcut, northwest side 

of intersection of Rte. 128 and U.S. Rte. 1A. UTM grid: N46776-E3197.

6. W-6. Slightly altered biotite granite, Dedham Granite; roadcut, 

1^95, 360 m east of U.S. Rte. 1A intersection. UTM grid: 

N46567-E3061.

7. DMA-65. Granodiorite intruding quartzite, Dedham Granite north of 

Boston; SW. corner intersection of Main St. and Middlesex Fells 

Parkway, Saugus. UTM grid: N47038-E3328.

8. DMA-127. Granite intruding amphibolite and diorite, Dedham 

Granite north of Boston; Spring St., Lexington, 100 m north of Rte. 2. 

UTM grid: N46995-E3148.

9. DMA-60. Granodiorite, Topsfield Granodiorite; Topsfield granite of 

Toulmin (1964), Granodiorite of Oxpasture Brook of Bell and others 

(1977). UTM grid: N47334-E3480.

10. P-5. Esmond Granite, slightly altered, locally contains garnet; roadcut, 1-295, 300 m east of Woonsocket Reservoir, Smithfield-Lincoln 

town line, Rhode Island. UTM grid: N46458-E2938.

11. DMA-206. Granite, Westwood Granite (Chute, 1966); roadcut, 

Rtes. 128 and 109, Norwood. UTM grid: N46794-E3182.

12. DMA-110. Granite, Westwood Granite; Plymouth quarries, Weymouth. UTM grid: N46725-E3427.

or bluish quartz occurs in aggregates or granular 

streaks, plagioclase is saussuritized, microcline is white 

to pink, and biotite, titanite, and epidote occur in clusters. Accessory minerals are apatite, opaque minerals, 

allanite, epidote, sericite, and chlorite. Plagioclase and 

microcline show recrystallization at their margins.

Porphyritic granite is well exposed along Route 140 

west of Long Pond and in Fall River (near Quarry Street 

Church and at the Route 24-Brayton Street interchange). Finer grained porphyritic varieties crop out to 

the northeast near Snipatuit Pond. O.D. Hermes and 

D.P. Murray (written commun., 1985) have identified a 

mass of undeformed alkalic granite within the area 

mapped as porphyritic granite on the State bedrock map 

near the small mass of diorite (Zdi) on the southwest side 

of Slocums River.

ALASKITE (Zagr)

Light-colored gneissic alaskite (Zagr) forms phacolithic masses near New Bedford that extend east 

toward Buzzards Bay and Cape Cod (fig. 11). The rock is 

resistant to weathering and is well exposed southwest of 

New Bedford. It has a remarkable resemblance to the 

Hope Valley Alaskite Gneiss (Zhg) of south-central Massachusetts, western Rhode Island, and adjacent Connecticut.

The rock is pale orange to cream colored, fine to 

medium grained, and equigranular and has a weak 

foliation and lineation imparted by preferred orientation 

of micas and flattened quartz aggregates. Coarsegrained phases contain lenses of gray quartz and pinkishorange K-feldspar.

TABLE 7. Modes, in percent, ofplutonic rocks of the Dedham batholith, southeastern Massachusetts

[Zfgr, granite of the Fall River pluton; Zpgr, porphyritic granite; Zagr, alaskite. Sample localities shown by field number in Wones and others (1986). a, albite;

o,oligoclase; p, perthite; tr, trace]

Sample no. ........

Unit .............

Points counted .....

Field no. ..........

Quartz.......

Plagioclase .......

Microcline. .......

Biotite. ..........

Muscovite. .......

Epidote. .........

Chlorite. .........

Magnetite. .......

Titanite..........

Apatite ..........

Allanite. .........

Garnet. ..........

Zircon ...........

Calcite. ..........

.... 1

. . . . Zfgr

. . . . 1728

FRE-3*

. . . . 33.9

. . . . 28.3a

. . . . 30.8p

. . . . 3.9

1.0

1.5

.1

0

.2

.... 0

.1

.1

0

.... 0

2

Zfgr

1559

FRE-10

32.5

26.5a

39. 3p

.1

.8

0

.3

.1

0

.1

0

0

.1

.4

3

Zfgr

1613

Mn-10

36.1

30.1

26.3

6.9

.1

.1

0

.1

.3

0

tr

0

0

0

4

Zfgr

1870

T-l

40.0

27.5a

28. Ip

2.0

.8

.4

.8

.2

0

tr

tr

.1

0

0

5

Zpgr

1725

Ap-2

37.7

24.8

21.1

2.7

4.5

7.9

1.0

.1

.1

.1

0

0

0

0

6

Zpgr

1967

FR-16

39.7

16.2

34.2

2.5

4.3

2.2

.5

0

.3

.1

tr

0

0

0

7

Zpgr

1632

NBN-22

30.0

27.4

25.4

9.4

3.1

4.2

0

0

.3

.1

0

0

0

0

8

Zpgr

1587

We-2

25.8

20.5a

44.3

5.5

1.1

1.8

.1

.2

.6

.1

.1

0

0

0

9

Zagr

1543

We-14

33.0

34.0a

31.2

1.2

0

0

tr

.3

0

.2

.1

0

0

0

10

Zagr

1680

FRE-6

38.0

34. Oa

25.5

0

1.6

0

0

.1

0

.1

0

.7

0

0

11

Zagr

1500

NBN-3

36.7

33.7o

26.7

1.4

1.3

0

0

.2

0

.1

0

.1

0

0

"Chemical analysis in table 8.
126 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 8. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of Proterozoic Z intrusive rocks, southeastern Massachusetts

[Major-oxide compositions from X-ray spectroscopy by Paul Hearn and Susan

Wargo; all Fe reported as Fe203. Trace-element abundances from instrumental

by X-ray spectroscopy by G. Sellars, B. McCall, and R. Johnson, nd, not 

determined]

Sample no. ................. 1 2 3 4 5

Unit.. .................... Zfgr Zfgr Zfgr Zfgr Zpgr

Major-oxide composition, in weight percent,

Si02 ..................... 75.64 74.73 77.9 73.08 67.48

A1203 .................... 12.46 12.70 12.2 14.47 15.81

Fe203 .................... .87 1.20 1.4 2.13 2.71

MnO..................... .10 .05 .04 .07 .08

MgO..................... .26 .20 .20 .21 .94

CaO ..................... .13 .70 .08 1.08 2.78

NasO .................... 3.71 3.34 2.6 3.35 3.42

KjjO...................... 4.68 4.60 4.58 4.55 3.76

Ti02 ..................... .06 .14 .17 .23 .40

P205 ..................... .01 .04 .01 .07 .13

(NaaO+KgOVAlaOg ........ .67 .62 .59 .55 .45

Normative-mineral composition, in weight percent, 1

and differentiation index (DI)

Qtz ...................... 35.5 35.0 42.2 31.1 24.8

Cm...................... 1.1 .9 1.5 1.9 1.4

Or....................... 28.2 27.2 27.1 27.0 22.8

Ab....................... 32.1 28.3 21.9 32.2 29.7

An....................... .6 3.3 4.0 4.9 13.3

En....................... .7 .5 .5 .5 2.4

Fs....................... 1.7 2.0 2.3 3.7 4.6

Ilm ...................... .1 .2 .3 .4 .8

Ap....................... 0 0 0 .2 .3

DI....................... 96 90 91 90 77

Trace-element abundances, in parts per million, 

and selected ratios

Rb. ...................... 290 143 152 142 135

Cs....................... 3.0 4.0 1.9 1.7 3.0

Sr ....................... 28 48 55 80 302

Ba....................... 93 274 286 465 546

Rb/Cs.................... 97 36 80 84 45

Rb/Sr.................... 10.3 3.0 2.8 1.8 .4

Sc ....................... 4.5 5.9 5.2 7.0 6.9

Cr....................... 3.0 nd nd 4.2 3.0

The alaskite consists of quartz, microcline and microcline perthite, albite to sodic oligoclase, and typically less 

than 2 percent biotite. It contains accessory muscovite, 

garnet, magnetite, apatite, zircon, and rare allanite and 

monazite. Secondary minerals are calcite, chlorite, and 

epidote. Chemically the alaskite is inferred to be similar 

in composition to the Hope Valley Alaskite Gneiss. If the 

albite were plotted as alkali feldspar rather than plagioclase, the location of the alaskite on the ternary plot (fig.

TABLE 8. Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of Proterozoic Z intrusive rocks, southeastern Massachusetts Continued

Sample no. ................. 1 2 3 4 5

Unit ...................... Zfgr Zfgr Zfgr Zfgr Zpgr

Trace-element abundances, in parts per million, 

and selected ratios Continued

Co....................... .3 2.2 1.2 1.5 5.7

Zn....................... 32 30 36 46 41

La....................... 22 24 31 70 29

Po f*1 TO Qfi 1 ^7 t\A.

Nd. ...................... 28 23 44 57 22

Sm ...................... 9.0 4.6 7.9 11 4.0

Eu. ...................... .40 .50 .75 .97 .85

C<A 7Q QQ £ S 1 A Q 9

rpu 1 CO CC QC 1 qo AQ

Ho....................... 2.2 .8 .8 .9 .7

Tm ...................... .67 .25 .41 .80 .41

Vh 79 17 °.°. 44 °ft

Lu....................... 1.0 .25 .47 .62 .30

La/Yb.................... 3 14 9 16 14

m oO >1O A G. G. 1 Ofk

Zr....................... 145 145 nd 255 138

Th2 ...................... 24.8 17.3 19.0 14.4 8.3

Th....................... 30.2 15.9 18.7 14.5 9.9

U2 ....................... 5.6 2.0 4.2 4.0 5.0

U........................ 6.6 2.4 4.0 4.0 4.9

Zr/Hf .................... 38 34 nd 42 46

lFe2Oa calculated as FeO.

2Delayed neutron reactivation analyses by H.T. Millard, Jr., C.M. Ellis, and V.C. Smith.

Description of samples

Sample localities shown in Wones and others (1986)

1. AP-5. Granite of the Fall River pluton; leucocratic granite; roadcut,

Rte. 140, just west of Pickens St. overpass, Lakeville. UTM grid:

N46326-E3352. 

2. FRE-3. Granite of the Fall River pluton; biotite granite; roadcut, 

Rte. 24, 1.1 km south of interchange 40, Fall River. UTM grid:

N46189-E3237.

3. FR-17. Granite of the Fall River pluton; biotite granite, Bulgarmarsh Granite of Pollock (1964); roadcut, Rte. 24, 500 m east of

interchange with Rte. 138, Tiverton. UTM grid: N46123-E3168.

4. BG-72. Granite of the Fall River pluton; biotite granite, Bulgarmarsh Granite; sampled by Galloway (1973); same location as sample 3.

5. DMA-172. Porphyritic granite; inequigranular gneissic biotite granFreetown town line. UTM grid: N46277-E3369.

5) would be shifted to the quartz-K-feldspar side of the 

diagram and would identify it as an alkali-feldspar 

granite.

GRANITE, GNEISS, AND SCHIST, UNDIVIDED (Zgg)

Cape Cod and the adjacent mainland south of Plymouth are covered by Pleistocene glacial materials, 

which conceal the bedrock to such an extent that only
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 127

TABLE 9. Modes, in percent, of plutonic rocks of the Milford antiform

[Zhg, Hope Valley Alaskite Gneiss; Zsg, Scituate Granite Gneiss; Zpg, Ponaganset Gneiss; Zgr, biotite granite; Zmgr, Milford Granite; Zmgd, mafic phase of the 

Milford Granite. Sample localities shown by field number in Wones and others (1986). a, albite; o, oligoclase; p, perthite; tr, trace]

Unit. ............

Points counted. ....

Field no. .........

Quartz ..........

Plagioclase ......

Biotite ..........

Chlorite.........

Magnetite .......

Hematite. .......

Titanite .........

Garnet ..........

Zircon ..........

1

Zhg

1390

M-10*

40.1

28. 5a

29. 9p

0

1.2

0

0

0

.3

0

0

0

0

0

tr

0

3

Zsg

1817

Ge^8

38.7

20. 7a

32.3

6.8

0

1.1

0

.1

.1

0

.1

0

.2

tr

tr

0

4

7qo1619 U-6*

40.3

26.9a

27. 6p

4.5

.3

0

0

0

0

0

.1

.3

tr

0

tr

0

5

7qcr

1819

B-31

33.0

29.2o

33.1

2.1

1.1

.6

0

.4

tr

0

.2

tr

0

tr

0

9

6

Zpg

1525

C-l

39.0

30.7

20.4

7.4

.1

1.0

.3

0

0

0

.7

.2

.2

0

0

0

7

Zgr

1568

B-20

33.8

33.5

23.8

5.7

2.0

.9

0

.2

0

0

.1

.1

tr

0

0

0

8

Zgr

1493

H-6b

33.8

5.0

55. 4p

0

2.7

0

0

0

1.6

1.5

tr

0

0

0

0

0

9

Zmgr

1634

Mi-4

46.6

24.4o

26.7

1.9

.1

.1

0

tr

0

0

tr

tr

tr

.1

tr

0

10

Zmgd

1621

Mi-5*

38.4

42.1

4.9

9.9

.2

3.5

0

.3

tr

0

.5

.2

tr

0

tr

0

'Chemical analysis in table 10.

rare scattered outcrops and a few deep drill holes give 

clues as to the nature of the bedrock (fig. 17). Therefore, 

the units mapped in the Fall River-New Bedford area 

cannot be mapped in this region. The diverse rock types 

encountered in the few exposures and in the drill holes 

are assigned to an undivided unit of granite, gneiss, and 

schist (Zgg). Presumably most of the plutonic and metamorphic rock units in the Fall River-New Bedford area 

project into the concealed area to the east. A marked 

north-south topographic lineament, parallel to faults 

bounding the Assawompset Pond graben, that passes 

through Snipatuit Pond (fig. 11) forms a convenient 

boundary for separating the region on the west, in which 

recognizable rock units can be mapped, from the region 

to the east, in which units cannot be mapped.

The bedrock is known in this region in a few places 

west of Cape Cod. Both gneissic biotite granite and 

gneissic alaskite crop out on Front Street north of 

Marion center. North of Marion, no outcrops are known. 

Goldsmith was unable to locate outcrops indicated by 

Williams and Willey (1973) in the Plympton area southwest of Plymouth. Deep coreholes drilled by Bechtel 

Corporation for the Boston Edison Company at Rocky 

Point near Manomet (fig. 17) encountered a gray, 

medium-grained granodiorite (sample DMA-167) and 

gray aplite. In the Duxbury area, bedrock exposures 

were numerous enough for Chute (1965a) to assign a 

light-colored pinkish-gray granite to the Westwood 

Granite and one outcrop of coarse-grained granite to the 

Dedham Granite and to identify as a separate unit a 

medium-gray foliated biotite granite that intrudes the

Westwood. Most of the outcrops in the Duxbury area are 

light-colored granite. It is not certain that the lightcolored granite in the Duxbury area is actually the same 

Westwood Granite mapped north of the Narragansett 

basin. None of the rocks in the Duxbury area have been 

shown separately on the State bedrock map; all are 

included in the undivided granite, gneiss, and schist unit. 

Chute (1965a) also identified an exposure of light-pink to 

lavender-gray porphyritic rhyolite at Cripple Rocks 

(CR, fig. 17) in Kingston Bay and another of greenish 

rhyolite in Green Harbor cut by aplite dikes. The age of 

these rocks is unknown. Larger areas of rhyolite could 

quite likely be present under the glacial cover.

Even less information is available for Cape Cod (Weed, 

in Goldsmith, this vol., chap. E, table 8, fig. 15). A 300-m 

drill hole in Harwich, Mass. (1, fig. 17), reported by 

Koteff and Cotton (1962) penetrated 171 m of steeply 

dipping, medium-gray, fine-grained, phyllitic schist containing layers of greenish-gray to gray crystalline limestone in the upper 15.2-18.3 m. Two 300-m holes in 

Brewster reported by R.Z. Gore (written commun., 

1978; 2 and 3, fig. 17) penetrated plutonic rocks. One of 

these holes went through 131 m of granite containing 

diorite and quartz-diorite inclusions. The granite is 

flanked above and below by granodiorite. A shear zone or 

foliated zone separates the upper granodiorite from the 

granite, and an alteration zone passes through the lower 

granodiorite near the bottom of the hole. The second hole 

penetrated sheared granodiorite. Another hole near 

Brewster (4, fig. 17) ended in till or granite. Unidentified 

bedrock has been reported from other wells on Cape Cod
128 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 10. Major-oxide, normative-mineral, and trace-element 

compositions, volatiles omitted, of Proterozoic Z intrusive rocks in 

the Milford antiform

Sample no. ................. 1 2 3 4 5

Unit ...................... Zmgr Zmgd Zgr Zsg Zhg

Major-oxide composition, in weight percent,

and alkali-alumina ratio

Si02 ..................... 75.82 71.08 72.59 75.51 76.01

A1203 .................... 12.74 13.36 13.24 13.39 11.72

Fe203 .................... 1.18 3.27 2.64 1.42 1.20

MnO..................... .05 .08 .07 .05 .02

MgO..................... .31 1.18 .45 .21 .18

CaO ..................... .62 3.13 1.88 .97 .22

NifeO .................... 3.58 3.61 3.24 3.35 4.11

KgO. ..................... 4.94 2.58 3.44 4.90 4.02

Ti02 ..................... .09 .48 .36 .17 .07

P206 ..................... .04 .11 .10 .04 .02

(Na20+K20)/Al203 ........ .67 .46 .50 .62 .69

Normative-mineral composition, in weight percent, 1

and differentiation index (DI)

Of? VI 7 Qft t\ ^A Q QQ Q V^ Q

Crn...................... .5 0 1.0 .9 .3

A V\ OA C OA Q OQ A OO O OC C

An....................... 2.8 12.8 8.8 4.6 1.0

Wo ...................... 0 .8 0 0 0

En. ...................... .8 3.0 1.1 .5 .5

Fs....................... 2.1 5.4 4.5 2.4 2.2

Ilm...................... .2 .9 .7 .3 .1

Ap....................... .1 .3 .2 .1 0

DI....................... 94 77 84 91 96

Trace-element abundances, in parts per million, 

and selected ratios

Rb....................... 116 73 77 172 137

Cs....................... 1.1 2.2 .7 2.3 .6

Sr ....................... 66 261 208 63 51

Ba....................... 540 927 1038 297 593

Rb/Cs.................... 105 33 110 75 228

Rb/Sr.................... 1.8 .3 .4 2.7 2.7

Sc ....................... 1.2 11.6 4.1 2.8 5.6

Cr....................... 2.0 3.0 6.8 .2 5.5

(J. Givens and D. LeBlanc, written commun., 1977). A 

hole near Woods Hole (5, fig. 17; Weed, in Goldsmith, 

this vol., chap. E, table 8, fig. 15) encountered gray to 

pink granodiorite at 83 m below sea level. Thus, the 

concealed basement under Cape Cod appears to consist 

of rocks similar to those exposed on the mainland to the 

northwest. As confirmation of this conclusion, seismic 

compressional-wave velocities in the vicinity of the 

Brewster and Woods Hole holes were 18,000-20,000 feet

TABLE 10. Major-oxide, normative-mineral, and trace-element 

compositions, volatiles omitted, of Proterozoic Z intrusive rocks in 

the Milford antiform Continued

Sample no. ................. 1 2 3 4 5

Unit ...................... Zmgr Zmgd Zgr Zsg Zhg

Trace-element abundances, in parts per million,

Co....................... .4 3.6 3.5 1.1 .1

Zn....................... 30 64 59 34 28

La....................... 32 40 50 36 21

Ce....................... 65 84 92 73 72

Nd. ...................... 29 43 47 34 26

Sm ...................... 5.6 8.8 9.7 7.1 6.0

Eu....................... 1.06 2.01 1.93 .62 .71

Gd....................... 2.8 7.1 6.8 5.8 4.6

Tb....................... .49 1.15 1.21 1.06 .92

Ho....................... .5 1.1 1.7 1.1 1.0

Tm ...................... .23 .46 .63 .74 .76

Yb....................... 1.1 3.5 3.5 4.8 4.6

Lu....................... .16 .48 .48 .69 .62

La/Yb.................... 29 11 14 7 5

Hf....................... 3.3 5.8 6.0 4.0 6.9

Zr....................... 109 288 230 175 228

Th2 ...................... 7.1 8.5 9.9 20.3 9.0

Th....................... 8.3 8.8 9.2 21.0 11.2

U2 ....................... 1.3 2.1 1.4 3.7 2.6

U........................ 1.2 1.9 1.5 3.5 2.5

Zr/Hf .................... 33 50 38 44 33

2Delayed neutron determination analysis by H.T. Millard, Jr., and B. McCall.

Description of samples

Sample localities shown in Wones and others (1986)

1. DMA-22. Milford Granite; massive granite; roadcut, southeast side

of intersection of Rtes. 85 and 1^95, Milford. UTM grid:

N46710-E2927. 

2. Mi-5. Milford Granite, mafic phase; gray, seriate granite; roadcut,

northbound lane 1^95 at Haven St. overpass, Milford. UTM grid:

VMCTOr? "CiOfll 1

3. DMA-19. Biotite granite; cataclastic granite; Winch Road, 200 m

southwest of Edmands Rd., Framingham. UTM grid: N46886-E2986.

quarry, Quarry Hill, on Hartford Ave. West, 1.5 km west of North

Uxbridge, Uxbridge. UTM grid: N46623-E2801.

5. M-10. Hope Valley Alaskite Gneiss; leucocratic gneissic granite;

sachusetts Turnpike), Hopkinton. UTM grid: N46812-E2889.

per second (fps), similar to velocities in the areas of 

Dedham Granite on the mainland (Oldale and Tuttle, 

1965). Seismic velocities in the vicinity of the Harwich 

hole had a similar but wider range of 17,800-21,500 fps 

(Oldale and Tuttle, 1964), indicating the greater variability of rock types in this area. The granite, gneiss, and 

schist unit is inferred to underlie upper Cape Cod, the 

Elizabeth Islands, and the sedimentary and volcanic 

rocks of Mesozoic age beneath Nantucket Sound.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 129

PLUTONIC ROCKS OF THE MILFORD ANTIFORM

The plutonic rocks of the Milford antiform (fig. 11) are 

flanked by, and contain belts of, older metamorphic rocks 

of the Plainfield and Westboro Formations and the 

Blackstone Group (included in other rocks of fig. 11). The 

plutonic rocks are granitic in composition, although the 

Milford Granite (Zmgr) has a granodioritic phase 

(Zmgd). The plutonic rocks are similar in composition to 

rocks in the Dedham batholith, ranging from alkali 

feldspar granite to granodiorite (fig. 5); however, most 

tend to have a well-developed to poorly developed 

gneissic fabric, unlike the granitic rocks of the Dedham 

batholith except those in the New Bedford area. The 

Hope Valley Alaskite Gneiss (Zhg) and associated granitic gneisses in the Milford antiform are the northern 

continuation of a belt of Proterozoic Z plutonic rocks that 

extends from the New London anticlinorium in southeastern Connecticut (Goldsmith, 1985; Rodgers, 1985) 

and adjacent Rhode Island along the Connecticut-Rhode 

Island border into southern Massachusetts, where they 

bend around the north side of the Milford antiform in the 

Framingham area. Age relations seen in southeastern 

Connecticut and in the Milford antiform indicate that the 

Hope Valley is the youngest of the gneissic suite. This 

age is also suggested by the 601±5-Ma age obtained from 

the Hope Valley in southern Rhode Island by Hermes 

and Zartman (1985). Its age relative to the Milford 

Granite in the core of the antiform is not known; the two 

may lie in separate terranes (O'Hara and Gromet, 1985). 

The plutonic rocks of the Milford antiform are less 

gneissic to the east than to the west; within the antiform 

are zones of shearing bounding areas of less sheared to 

unsheared rock. O'Hara and Gromet (1985; Gromet and 

O'Hara, 1984) have placed their major terrane boundary 

along one of these.

HOPE VALLEY ALASKITE GNEISS (Zhg)

The Hope Valley Alaskite Gneiss (Zhg) forms tabular 

masses along the west side of the Rhode Island anticlinorium from southern Rhode Island and eastern Connecticut to northwestern Rhode Island; it flanks the west 

side of the Milford antiform and terminates at the north 

end of the anticlinorium in Massachusetts (fig. 11). The 

Hope Valley intrudes the Plainfield Formation in Connecticut and rocks mapped as Blackstone Group in Rhode 

Island (Hermes and others, 1981). The 630-Ma radiometric age (table 1) of this suite of rocks comes from this unit 

in Massachusetts; the 601±5-Ma age comes from the 

Hope Valley in southern Rhode Island.

The rock is light pink to tan. It contains little biotite 

and sparse magnetite (table 9); in some places, magnetite 

is more abundant than biotite. Rodded aggregates of 

quartz give the rock a pronounced lineation in some

places. A foliation is produced in other places by a 

preferred orientation of flat lenses of quartz and flat 

lenses of feldspar as well as by parallel orientation of 

biotite where present. Quinn (1971) gave modal values of 

24-43 percent quartz, 18-4:0 percent K-feldspar, 20-40 

percent plagioclase, and 1-4: percent biotite and magnetite for the Hope Valley in Rhode Island. Muscovite is a 

typical accessory mineral, and some rock contains small 

garnets. Chemically the rock is poor in mafic constituents 

(table 10, no. 5; figs. 7, 8). It shows enrichment in 

rare-earth elements and a negative europium anomaly 

(fig. 18), like most of the other Proterozoic Z rocks.

SCITUATE GRANITE GNEISS (Zsg)

The Scituate Granite Gneiss (Zsg) on the State bedrock map forms elongate masses in northwestern Rhode 

Island and Massachusetts (fig. 11). The name Scituate 

Granite Gneiss is no longer appropriate because the type 

Scituate Granite Gneiss of Quinn (1971) at Scituate, R.I., 

has been shown by Hermes and others (1981) and Hermes and Zartman (1985) to be Middle Devonian (373 ±7 

Ma), on the basis of a radiometric determination of 

zircons from the type Scituate, and is part of a Devonian 

alkalic suite. The Scituate Granite Gneiss of the type 

area in central Rhode Island described by Quinn (1971) is 

here renamed Scituate Granite and assigned a Middle 

Devonian age. The area of the Scituate Granite coincides 

with that shown as the main mass of Scituate by Quinn 

(1971) but with minor adjustments of boundaries (O.D. 

Hermes and Peter Gromet, oral commun., 1984). The 

type Scituate Granite Gneiss of Quinn is henceforth 

referred to as the Scituate Granite. The Scituate Granite 

Gneiss shown on the Massachusetts bedrock map contains roof pendants of Plainfield Formation and is Proterozoic Z in age (table 1). We suggest reviving Emerson's old name Northbridge Granite Gneiss for this unit, 

although detailed mapping in this part of Massachusetts 

may reveal that several distinct types of granite are 

present. The Middle Devonian Scituate Granite is at 

present not known in Massachusetts.

The Scituate Granite Gneiss of the State bedrock map 

is medium to coarse grained, weathers to a pink or tan, 

and has a foliation and pronounced lineation defined by 

aggregates of biotite. Of the modes given in table 9, no. 

4 is probably the most representative. The Scituate 

Granite Gneiss, like the Hope Valley Alaskite Gneiss, 

contains accessory muscovite and garnet; it is gradational with the Hope Valley Alaskite Gneiss. These two 

units may represent products of a large zoned magma 

chamber that has undergone deformation after primary 

crystallization. In chemical composition, the Scituate is 

similar to the Hope Valley in many respects except that 

it contains more mafic constituents (table 10). The REE 

pattern is similar to that of the Hope Valley (fig. 18).
130 THE BEDROCK GEOLOGY OF MASSACHUSETTS

in 

O 

< 

<r in

E Q

z 

o

X

o 

^

o 

o

<r

EXPLANATION

Dedham Granite (Zdgr)

FIGURE 15. Chondrite-normalized plot of rare-earth elements in Proterozoic Z batholithic rocks, northeastern Massachusetts: A, Dedham 

Granite; B, Dedham Granite north of Boston, Westwood and Esmond Granites, and Topsfield Granodiorite. Numbers are sample numbers 

from table 6.

PONAGANSET GNEISS (Zpg)

The Ponaganset Gneiss (Zpg) is largely confined to 

Rhode Island, but masses of porphyritic biotite gneiss 

assigned to the Ponaganset project into Massachusetts 

(fig. 11). The Ponaganset Gneiss intrudes the Blackstone 

Group in Rhode Island (Quinn, 1971). In southern Rhode 

Island, Feininger (1963) demonstrated that the Ponaganset Gneiss is older than the Hope Valley Alaskite Gneiss 

and appears to be the oldest granitic intrusive in the 

Milford-Dedham terrane.

The rock is highly variable in color index, quartz 

content, and ratio of plagioclase to total feldspar, but 

most of it, as mapped in Massachusetts, is a gray,

biotitic, inequigranular, granitoid rock (table 9, no. 6) 

with a gneissic foliation produced by parallel orientation 

of the biotite and elongation of megacrysts. The trace of 

hornblende in sample 6 (table 9) indicates that this rock 

is more mafic than the other plutonic rocks of the 

antiform. In eastern Connecticut, the unit typically 

contains large round to ellipsoidal megacrysts of Kfeldspar and aggregates of K-feldspar and plagioclase 

(Harwood and Goldsmith, 1971). In places, particularly 

in Massachusetts, the megacrysts are flattened, or less 

commonly rodded, to produce a markedly lineated 

gneiss, as in outcrops at the gaging station on the West 

River, north of Route 16 in the town of Uxbridge. The
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 131

200 -

10

9

8

7

6

5

EXPLANATION

Dedham Granite north of Boston

(Zdngr) (samples 7 and 8) 

Topsfield Granodiorite (Ztgd)

(sample 9)

Esmond Granite (Zegr) (sample 10) 

Westwood Granite (Zwgr)

samples 11 and 12)

\ v? v- 11

*X 11

B

La Ce Nd Sm Eu Gd Tb

FIGURE 15. Continued.

Tm Yb Lu

inequigranular texture persists in Massachusetts and is 

one of the bases for mapping the unit there. In places in 

northern Rhode Island, some rock that has been mapped 

as Ponaganset Gneiss is equigranular. The gneissic fabric 

may be due to Paleozoic deformation. As mapped, the 

Ponaganset Gneiss may actually consist of several rock 

types.

MILFORD GRANITE (Zmgr, Zmgd)

The Milford Granite (Zmgr, Zmgd) occupies an area of 

about 100 km2 . Its central mass near Milford (fig. 11) is 

elliptical and has been divided into two phases: a lightcolored phase (Zmgr) and a dark-colored phase (Zmgd) 

that defines an irregular border for the largest of the 

light-colored plutons. The Milford intrudes the Black-
132 THE BEDROCK GEOLOGY OF MASSACHUSETTS

200 EXPLANATION

Granite of the Fall River pluton 

(Zfgr) (samples 1 and 2)

 Granite of the Fall River pluton,

Bulgarmarsh Granite phase (Zfgr) 

(samples 3 and 4) 

Porphyritic granite (Zpgr) 

(sample 5)

FIGURE 16. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks in the Milford-Dedham zone, southeastern

Massachusetts. Numbers refer to sample numbers in table 8.

stone Group (Zb) and the Ponaganset Gneiss and has 

been deformed with them at a later undefined time.

The Milford Granite is characterized by a distinctive 

salmon-pink color, bluish quartz on weathered surfaces, 

and a lineation defined by lenticular mosaics of quartz 

and oriented patches of biotite. This lineation contrasts 

distinctly with the brittle fractures and massive texture 

of the Dedham Granite and with the cataclastic fabrics 

commonly found in the biotite granite (Zgr) of the 

Framingham area.

The light-colored phase is granitic in composition, 

whereas the dark-colored phase is granodioritic (tables 

4, 9). Biotite is the most abundant ferromagnesian

mineral. In the dark-colored phase, clots of epidote and 

biotite are suggestive of hornblende as a primary magmatic phase, although no hornblende has been observed 

in these rocks. The light-colored phase of the Milford 

contains garnet, entrained epidote grains, and muscovite 

as accessory minerals, whereas the mafic phase contains 

titanite and opaque minerals. Both phases contain apatite, allanite, and zircon.

The limited chemical data of table 10 (plotted in figs. 7, 

8) illustrate the difference in composition between the 

two phases. Both the granite and granodiorite phases 

have marked light-REE enrichment, but the granite has 

heavy-REE depletion relative to the granodiorite, which
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 133

70°30' 70°00'

42W

Scituate

Green Harbor 

Duxbury B

^-x" Plymoutfe:

' Manomet 

'^Approximate southward 

limit of outcrops

41°30'

10 20 MILES 

I

10 20 KILOMETERS

FIGURE 17. Locations of drill holes encountering bedrock in Cape Cod 

and the adjacent mainland and limit of mappable rock units. CR, 

Cripple Rocks. Numbers refer to drill holes discussed in text. 

DMA 167, granodiorite from a hole near Manomet.

has a fairly flat profile (fig. 18). The granodiorite contains 

more scandium (11.6 ppm) than does the granite (1.2 

ppm). These results are typical, but not definitive, of 

plutons found in compressive tectonic regimes, where 

zoned plutons are common (Bateman and Chappell, 1979; 

Noyes and others, 1983) and where hornblende fractionation may cause the development of peraluminous magmas (Cawthorn and O'Hara, 1976; Guy, 1980).

BIOTITE GRANITE (Zgr)

The biotite granite (Zgr) crops out over 180 km2 south 

of the Bloody Bluff fault zone in the Framingham, 

Holliston, and Franklin areas (fig. 11). It intrudes earlier 

gabbro (Zgb), diorite (Zdi), and Proterozoic Z stratified 

rocks (Zv, Zvf, and Zw) (Nelson, 1975a,b; Volckmann, 

1977). The granite as mapped has different textures in 

different places, and later detailed mapping may show 

that it can be separated into several units or assigned to 

other existing units. Much of the biotite granite in its 

northern part was mapped previously as Milford Granite 

(Nelson, 1975a,b; Volckmann, 1977), and further mapping may delineate areas of rock that should be reassigned to that unit. The biotite granite is deformed by 

mylonitization and cataclasis near fault zones; in places it 

has the brittle fractures characteristic of the Dedham 

terrane, and elsewhere it has a gneissic fabric like the 

other granite gneisses of the Milford terrane.

The biotite granite is gray to light gray and weathers 

to pink or buff. It lacks the conspicuous red and green 

coloration characteristic of the Dedham Granite. The unit 

is mainly granite but is in part granodiorite and tonalite. 

Some of the rock is quite poor in mafic minerals. The 

muscovite reported in the modes (table 9) is probably 

secondary, as are the epidote and chlorite. Some of the 

granite contains phenocrysts of plagioclase and perthitic 

alkali feldspar, although the superposed deformation 

makes the identification of primary igneous textures 

difficult. Biotite is the dominant mafic phase. Adjacent to 

the Bloody Bluff fault zone, allanite is a conspicuous 

accessory that distinguishes the biotite granite from the 

Dedham and Milford Granites, which contain only minor 

amounts of allanite. The biotite granite lacks the euhedral titanites characteristic of the Dedham Granite and 

the garnet characteristic of the Milford Granite and the 

Hope Valley Alaskite Gneiss. The sample analyzed (table 

10, no. 3) comes from near the Bloody Bluff fault zone 

and may not be representative of the larger mass of 

granite south and southwest of Framingham. The biotite 

granite is assigned a Proterozoic Z age because of its 

compositional and textural similarities to the other Proterozoic Z intrusive rocks of the Milford antiform.

PALEOZOIC INTRUSIVE ROCKS

The Paleozoic intrusive rocks of the Milford-Dedham 

zone (fig. 19) comprise a group of relatively discrete 

plutons ranging in age from Ordovician to Devonian 

(table 1) and in composition from gabbro to granite. The 

granitic plutons are alkalic in composition and include the 

quartz-poor phases syenite and monzonite. The dated 

gabbroic and dioritic plutons are Ordovician in age. Some 

might be younger and related to the Ordovician and 

Silurian to Devonian alkalic granites. In addition to the 

gabbro and alkalic granite plutons, micrographic rhyolite 

(DSnr) intrudes the Silurian and Devonian Newbury 

Volcanic Complex, and dikes of diabase and basalt of 

Mesozoic age cut the older rocks of the zone. All these 

rocks in Massachusetts appear to be unmetamorphosed 

but are locally cut by faults of late Paleozoic and Mesozoic 

age.

NAHANT GABBRO (Ongb)

The Nahant Gabbro (Ongb) intrudes the Lower Cambrian Weymouth Formation at Nahant (fig. 19) and thus 

is younger than the gabbroic or dioritic rocks that are
134 THE BEDROCK GEOLOGY OF MASSACHUSETTS

200

cc

LLJ

o

1

CJ 

O 

cc

100

90

80

70

60

50

40

t 30

cc

Q

20

10

9

8

7

6

5

EXPLANATION

 Milford Granite (Zmgr) (sample 1)

 Milford Granite, mafic phase (Zmgd) (sample 2)

 ----- Biotite granite (Zgr) (sample 3)

 Scituate Granite Gneiss (Zsg) (sample 4)

Hope Valley Alaskite Gneiss (Zhg) (sample 5)

La Ce Nd Sm Eu Gd Tb Tm Yb Lu

FIGURE 18. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks, Milford antiform. Numbers refer to sample

numbers in table 10.

intruded by the Dedham Granite and Topsfield Granodiorite. It appears from its aeromagnetic pattern (Harwood and Zietz, 1976) to be a shallow cylindrical plug 

about 0.3 km in diameter. The Nahant Gabbro is projected to lie at depth beneath the Cape Ann Granite, as 

shown in cross section B-B' of the State bedrock map, on 

the basis of the magnetic signature over the pluton. 

Unrecognized equivalents to the Nahant Gabbro at Nahant and at Salem Neck (see p. 136) may be present 

within the gabbros and diorites (Zdigb, Zgb) shown on 

the State bedrock map (see Nelson, 1975b).

Bell (1977) described three facies at Nahant, which are 

included within the Nahant Gabbro on the State bedrock 

map: pyroxene gabbro, olivine gabbro, and quartz diorite. The pyroxene gabbro (table 11, no. 2), which forms 

the main mass at Nahant, is described by Bell (1977) as 

a massive equigranular rock that locally has subophitic

texture. The plagioclase is tabular. In addition to the 

minerals shown in table 11, the rock contains accessory 

zircon. Alteration products include epidote, chlorite, and 

calcite.

The olivine gabbro (table 11, no. 3) crops out on the 

north shore of Nahant and is compositionally banded. It 

has a mottled appearance with white or gray flecks 

within a black matrix and contains accessory sulfide 

minerals. Rb-Sr and K-Ar analyses of biotite from this 

rock were made for the Ordovician age determination 

(table 1).

The quartz diorite crops out on Little Nahant west of 

Nahant proper. The quartz diorite (table 11, no. 4) is fine 

to medium grained. Some samples contain accessory 

apatite and titanite; chlorite, epidote, and calcite are 

alteration products. The quartz diorite has undergone 

more brittle deformation and alteration than have the
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS

72°00' 45' 30' 15' 71°00' 45'

135

70°30'

42°45'

30'

15'

42°00'

45'

41°30'

NEWJjAMPSHIRE 

MASSACHUSETTS"

_MASS_ 

CONN

 .-.' .'.'. Boston'-"; 

.' ' .' ' basin'."-.

EXPLANATION

Mesozoic and Paleozoic 

intrusive rocks

' '. ;.-.' .' Pennsylvanian to Proterozoic Z 

 ' ' ' sedimentary and volcanic rocks

Other rocks

Contact 

Fault

N rr ganseft- '- \i\'.: ' ," .

 .. : .)(- -North Middleboro -'basin . /' j.- . - I - - - 1

10 20 MILES

10 20 KILOMETERS

FIGURE 19. Distribution of Paleozoic intrusive rocks in the Nahant Gabbro; Dpgr, Peabody Granite; Dwm, Wenham MonMilford-Dedham zone. DH, Diamond Hill; AP, Andrews Point. zonite; Jd, Jurassic dike; SOqgr, Quincy Granite; DOgr, alkalic 

SOcgr, alkalic granite and quartz syenite of the Cape Ann granite in Franklin; Drgr, granite of the Rattlesnake Hill pluton. 

Complex; SOcb, Beverly Syenite; SOcsm, Squam Granite; Ongb,
136 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 11. Modes, in percent, of some Paleozoic intrusive rocks of the Milford-Dedham zone

[DOgr, alkalic granite in Franklin; Ongb, Nahant Gabbro; SOqgr, Quincy Granite; SOcgr, granite of the Cape Ann Complex; SOcb, Beverly Syenite; Dpgr, Peabody 

Granite; Dwm, Wenham Monzonite; Drgr, granite of the Rattlesnake Hill pluton. a, albite; L, labradorite; p, perthite; c, orthoclase; u, augite; g, pigeonite; 

r, riebeckite; , not reported]

Sample no. ..........

Unit...............

Quartz ............

Plagioclase ........

Microcline .........

Biotite ............

Muscovite .........

Epidote ...........

Hornblende ........

Other amphibole . . .

Pyroxene ..........

Olivine ............

Chlorite ...........

Opaque minerals . . .

Titanite ...........

Apatite. ...........

Other .............

1

DOgr

. . 34.6

. . 19.4a

.. 36.7p

.. 7.5

.4

.4

tr

.. 0

.4

tr

.2

.1

2*

2

Ongb

0-5

55-65

0-6c

0-2

tr

0-10

30-45g

0-2

tr

3-10

0-1

tr**

3

Ongb

0

5-20

0

2-5

5-15

25-40g

20-50

tr

2-5$

4

Ongb

5-15

25-40

25-40p

3-7

3-5

5-10

tr

2-5

tr

0.2-0.7

0.5-1**

5

Ongb

0

50L

0

15

tr

5

20u

4

tr

5

tr

tr**

6

SOqgr

31

0

60p

0

10*

7

SOcgr

24

3

63p

1

5

0.2-8.1

8

SOcb

0

0

77-97

1-5

1-7

1-9

0-2

9

Dpgr

28

63p

.3

6

.3

.3

.9

 3#

10

Dwm

3

41

43

2

tr

9

1

1

tr

.3

11

Drgr

26^7

0

51-71

1-5

0

0

0

12

Drgr

27-31

0

64-67

0

0

3-4r

0

13

Drgr

23-41

0

53-72

0

0

l-6r

0

*Total amphibole and pyroxene. 

tAntigorite?

*Allanite; also includes trace amounts of zircon and fluorite. 

MNonopaque accessory minerals.

*Calcite.

Description of samples

1. Alkalic granite in Franklin, F^49 (Wones and others, 1986); 1,924 points counted. 

2-4. Pyroxene gabbro from Nahant, olivine gabbro from Nahant, and quartz diorite from Little Nahant (Bell, 1977, p. 20e-25e).

5. Gabbro at Salem Neck (Toulmin, 1964, p. A58).

6. Quincy Granite from Quincy (Dale, 1923; Warren, 1913).

7, 8. Alkalic granite and quartz syenite of Cape Ann Complex and Beverly Syenite (Dennen, 1981).

9, 10. Peabody Granite (average of five samples) and Wenham Monzonite (average of two samples) (Toulmin, 1964, p. A31, A44). 

11-13. Biotite granite, coarse-grained granite, and fine-grained granite (Lyons and Krueger, 1976).

other phases, and quite possibly it lies close to a fault 

zone (Goldsmith, this vol., chap. H). Wones suggested 

that the rock might be a more mafic phase of the Dedham 

Granite.

The gabbro at Salem Neck (Toulmin, 1964; Ongb near 

Salem in fig. 19) is the same general age as the Nahant 

Gabbro (table 1). It is a pyroxene gabbro similar in 

composition to the Nahant Gabbro, but it contains more 

biotite (table 11, no. 5). It is less altered than the diorite 

and gabbro (Zdigb) surrounding it and is intimately 

mixed with syenitic material.

QUINCY GRANITE (SOqgr)

The Quincy Granite (SOqgr) intrudes the Middle Cambrian Braintree Argillite at several localities along its 

eastern margin within the towns of Quincy and Braintree 

(Nellis and Hellier, 1976; fig. 19). The southern contact of 

the body is with the chemically and mineralogically 

similar Blue Hills Granite Porphyry (Chute, 1969). The 

Quincy Granite is bounded on the northwest by a reverse 

fault, the Blue Hills thrust (Billings, 1976; Goldsmith, 

this vol., chap. H), and on the west by the Neponset 

fault. No clasts of the Quincy Granite have been 

observed in the Lower Pennsylvanian Pondville Conglomerate, but clasts of the overlying and probably 

related Blue Hills Granite Porphyry (see below) are 

common in the Pondville Conglomerate (Chute, 1969). 

The combined area of the Quincy Granite and the Blue 

Hills Granite Porphyry is 55 km2. Billings (1982) suggested that the Quincy Granite formed as a product of 

Ordovician cauldron subsidence.

The fresh rock is dark gray to gray green but weathers 

to buff brown or salmon; its texture is hypidiomorphic 

granular. Joint and slickenside surfaces are typically 

coated with riebeckite. The amphiboles in the granite 

(table 11, no. 6) are riebeckitic, and the pyroxene is 

acmitic. The averaged analyses from the type Quincy 

Granite (table 12, no. 4) indicate the peralkalinity of the 

rock, as does the high alkali-to-alumina ratio. The mafic 

minerals are interstitial to subhedral quartz and perthite 

and clearly were late in the order of crystallization. 

Accessory minerals are astrophyllite, aenigmatite, 

ilmenite, magnetite, titanite, fluorite, and, uncommonly, 

biotite. The late crystallization of mafic minerals suggests significant subsolidus reactions. Late-stage quartz 

and calcite have been observed in vugs and fractures, 

along with chlorite, hematite, and limonite.

Textural variants within the Quincy Granite include a 

fine-grained phase, especially abundant near the con-
INTEUSIVE ROCKS OF EASTERN MASSACHUSETTS 137

TABLE 12. Major-oxide, normative-mineral, and trace-element 

compositions, volatiles omitted, of some Paleozoic plutonic rocks, 

Milford-Dedham zone

analyses by O.D. Hermes, L. Kwak, C. Mandeville, and C. Olson; sample 7 by 

O.F. Tuttle and N.L. Bowen. Trace-element abundances from instrumental

X-ray spectroscopy by G. Sellars and B. McCall. No trace-element analyses

available for samples 3-7. nd, not determined]

Sample no. ......... 1 2 3 4 5 6 7

Unit .............. DOgr DSnr Ongb SOqgr SOqgr SOcgr Dpgr

Major-oxide composition, in weight percent,

and alkali-alumina ratio

O4/*k rrc. *}O nC 7 A*) 79 *7A Q 7^ O/l T7 C 7O Oyl

A1203 ............. 15.42 12.3 20.17 11.6 9.80 11.9 13.18

PQ n 1ini1^4939 9Q 717 W 9/1

FeO. .............. nd nd 6.93 1.25 2.50 .87 2.77

MnO .............. .02 .03 0 .02 .14 0 .10

MgO .............. 0 0 3.91 .04 .06 0 .20 

CaO. .............. .42 .5 10.99 .41 .58 .31 1.10

Na-jO. ............. 3.24 3.9 2.42 4.30 5.26 3.80 3.99

K,,0. .............. 4.56 3.83 1.45 4.64 4.24 4.98 5.01

Ti02 .............. .08 .10 4.23 .20 .17 .25 .36

P205 .............. .02 0 .15 0 0 0 .07

Zr02 .............. 0 0 nd 0 .20 0 0

(Na20+K20)/Al203 ... .51 .63 .19 .77 .97 .74 .68

Normative-mineral composition, in weight percent,

and differentiation index (DI)

Qtz ............... 37.1 37.4 0 34.8 25.2 35.5 25.7 

Crn ............... 4.4 .8 0 0 0 0 0

Or ................ 26.9 22.6 8.6 27.5 24.9 29.5 29.6

Ab. ............... 27.4 33.0 20.4 36.3 26.6 32.1 33.7 

An. ............... 2.0 2.5 39.8 1.6 0 .7 3.3

Di ................ 0 0 9.5 .2 0 0 .2

Hd ............... 0 0 1.9 .2 0 .7 1.4

Ac.... ............ 0 0 0 0 15.5 0 0

Wo ............... 0 0 0 0 1.2 0 0

En. ............... 0 0 3.3 .02 .15 0 .4

Fs ................ 2.1 2.0 .7 .01 3.1 .4 3.7

Fo................ 0 0 1.4 0 2.6 0 0

Fa....... ......... 0 0 .7 0 0 0 0

Ilm ............... .2 .2 8.0 .4 .3 .5 .6

Mag. .............. 0 0 6.1 0 0 0 0

Ap. ............... 0 0 .3 0 0 0 .2

DI................ 91 93 29 98 77 97 89

Trace-element abundances, in parts per million, 

and selected ratios

Rb. ............... 358 101

Cs ................ 6.0 .9

Sr ................ 24 48

tacts, that has micrographic textures of quartz and 

perthite. The latter mineral is seriate in this phase of the 

Quincy and, when coarse, gives a porphyritic appearance 

to hand specimens. In other places, pegmatitic segregations contain very coarse crystals of acmitic 

pyroxene and amphibole. There were probably several 

pulses of magma, closely spaced in time and of similar 

composition.

TABLE 12. Major-oxide, normative-mineral, and trace-element 

compositions, volatiles omitted, of some Paleozoic plutonic rocks, 

Milford-Dedham zone Continued

Sample no. ......... 1 2 3 4 5 6 7

Unit .............. DOgr DSnr Ongb SOqgr SOqgr SOcgr Dpgr

Trace-element abundances, in parts per million, 

and selected ratios Continued

Ba. ............... 144 298

Rb/Cs ............. 60 112

Rb/Sr ............. 15.0 2.1

Sc... ............. 1.1 3.5

Cr................ 4.7 nd

Co................ .2 .4

Zn................ 22 22

La................ 46 39

Ce................ 98 83

Nd ............... 41 42

Sm ............... 9.7 9.3

Eu. ............... .28 .42

Gd. ............... 6.9 8.6

Tb.... ............ 1.60 1.78

Ho. ............... 1.5 1.8

Tm ............... 1.98 1.23

Lu. ............... .98 1.16

La/Yb ............. 6 4 

Hf ................ 6.8 6.8

Zr ................ 189 nd

Th2 ............... 37.3 17.3

Th. ............... 38.8 16.8

U2 ................ 9.7 3.3

U ................ 9.0 3.2

Zr/Hf ............. 28 nd

^eaOg calculated as FeO.

Description of samples

1. Alkalic granite in Franklin; massive granite but in this place cut by quartz

veins and mylonite seams; Ledgewood, Franklin Apartments, Unionville, Franklin. Field number DMA-197, locality shown in Wones and others (1986). UTM

grid: N46624-E2999.

2. Micrographic rhyolite from Newbury Volcanic Complex; collected by A.F.

Shride. Field number 957-C.

3. Nahant Gabbro, Nahant; from Emerson (1917, p. 182).

4. Quincy Granite; average of three samples from Quincy, Mass.; from Emerson

(1917, p. 191, no. 4 in table).

319, table 1, no. 1). 

6. Alkalic granite to quartz syenite of Cape Ann Complex; hornblende granite, old

table).

table 11; cited by Toulmin, 1964).

The small mass of "Quincy Granite" (SOqgr) east of 

Woonsocket (fig. 19) near the Massachusetts-Rhode 

Island border is similar in texture to the Quincy Granite 

at Quincy but is more peralkaline (table 12, no. 5). The 

mass at Woonsocket was thought at the time of map 

compilation to be the same general age as the granite at 

Quincy. Its age has more recently been determined to be 

Devonian, or possibly Carboniferous, rather than Late
138 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Ordovician and Early Silurian (Hermes and Zartman, 

1985). It has been most recently described by Rutherford 

and Carroll (1981), who distinguished an equigranular 

variety and a porphyritic variety. The latter contains 

amphibole with less riebeckite component.

BLUE HILLS GRANITE PORPHYRY (SObgr)

Naylor and Sayer (1976) considered the Blue Hills 

Granite Porphyry (SObgr; called Blue Hill Granite Porphyry on the State bedrock map) to be equivalent in 

composition and age to the Quincy Granite. Chute (1969) 

recognized inclusions of Quincy Granite within the Blue 

Hills Granite Porphyry in Quincy and considered it 

comagmatic with, but slightly younger than, the Quincy 

Granite. Warren (1913) observed dikes of Quincy Granite 

in the Blue Hills and suggested that the Quincy Granite 

was the younger unit. Rb-Sr work by Bottino and others 

(1970) suggested a late Paleozoic age for the Blue Hills, 

but Naylor and Sayer (1976) argued that this age was 

reset. The Blue Hills was assigned a Late Ordovician and 

Early Silurian age because of its close similarities, in 

both mineralogy and chemistry, to the Quincy Granite. 

Cobbles of the Blue Hills Granite Porphyry have been 

found in the Pondville Conglomerate of Early Pennsylvanian age (Chute, 1969). The Blue Hills is bounded by 

the Quincy Granite on the north and east and the 

Neponset fault on the west and is overlain unconformably by the Pondville Conglomerate on the south.

The Blue Hills is dark gray to blue gray and weathers 

to buff brown and salmon. Quartz (12 percent) and 

microperthite (40 percent) phenocrysts are set in a 

matrix of quartz, perthite, amphibole, and acmitic pyroxene. Some of the pyroxene occurs as inclusions in the 

perthite, indicating early crystallization of this pyroxene. Accessory minerals are aenigmatite, astrophyllite, 

magnetite, ilmenite, hematite, fluorite, zircon, and 

calcite.

Chemically, the Blue Hills is similar to the Quincy 

Granite and shows some similarities to the Peabody 

Granite and the alkalic granite and quartz syenite of the 

Cape Ann Complex (Buma and others, 1971; Naylor and 

Sayer, 1976). However, both the Quincy and the Blue 

Hills Granite Porphyry are more peralkaline than the 

Peabody Granite and the granite and quartz syenite of 

the Cape Ann Complex. The latter are chemically similar 

to the granite of the Rattlesnake Hill pluton (Lyons and 

Krueger, 1976).

CAPE ANN COMPLEX

Most of the Cape Ann Complex forms the Cape Ann 

peninsula of northeastern Massachusetts (fig. 19), but 

two small stocks lie south of Salem. The Cape Ann 

consists of three rock units: alkalic granite to quartz 

syenite (SOcgr), forming the main phase, the Beverly

Syenite (SOcb), and the Squam Granite (SOcsm). The 

whole forms a pluton covering 385 km2. The Cape Ann 

intrudes greenschist, diorite, and gabbro (Zv, Zdigb) 

that earlier workers (Toulmin, 1964; Dennen, 1981) 

assigned to the Marlboro Formation, Salem GabbroDiorite, or Middlesex Fells Volcanic Complex. Radiometric ages for the unit straddle the Ordovician-Silurian 

boundary (table 1).

Dennen (1975, 1981) considered masses of diorite and 

gabbro mapped as Salem Gabbro-Diorite within and 

adjacent to the Cape Ann Complex to be cogenetic with 

it and to be equivalent in age to the Nahant Gabbro and 

the gabbro at Salem Neck described above. We recommend that the term "Salem Gabbro-Diorite" be 

restricted, in future usage, to these masses of diorite and 

gabbro in and around the Cape Ann pluton that are 

younger than the Dedham Granite and cogenetic with the 

Late Ordovician to Early Silurian Cape Ann Complex. 

The gabbro at Salem Neck is probably representative.

ALKALIC GRANITE AND QUARTZ SYENITE (SOcgr)

The alkalic granite to quartz syenite (table 11, no. 7; 

table 12, no. 6) is medium to coarse grained; it is grayish 

green and weathers to tan and salmon. Quartz and 

feldspar content vary widely; rock lacking quartz is 

mapped as Beverly Syenite (Toulmin, 1964; Dennen, 

1981). Numerous inclusions of anorthositic, dioritic, and 

granitic rocks, as well as segmented mafic dikes, are 

interpreted to have been emplaced contemporaneously 

with the cooling granitic magma. Late-stage aplites and 

pegmatites are common.

Dennen (1981) has studied the Cape Ann Complex 

extensively and has contoured the variations in its quartz 

content. Mineral contents vary on a scale of meters to 

kilometers. In addition to the major constituents, clinopyroxene, fayalite, titanite, zircon, fluorite, allanite, 

magnetite, and ilmenite have been found among the 

accessory minerals. Toulmin (1964) suggested that alkali 

feldspar cumulates are the main cause of the variations. 

Dennen (1981) ascribed the abrupt changes in areal 

extent to later faulting, which displaced the contacts of 

these cumulate masses.

An outcrop at Andrews Point (fig. 20) illustrates the 

complicated relationships among the textural variations 

(Martin, 1977). Here a foundered block of fine-grained 

granite contains disjointed diabase dikes. The diabase 

contains miarolitic cavities with terminated quartz and 

alkali feldspar. This block is intruded on the north side by 

the main phase of the Cape Ann Complex. On the south 

side, the block and the main phase of the Cape Ann are 

intruded by an aplitic dike that contains pegmatite pods 

with coarse crystals of quartz, feldspar, amphibole, and 

fayalite. Much of the fayalite has been altered to grunerite and magnetite and is rimmed by a titaniferous annite.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 139

EXPLANATION

Holocene

Solvsbergite

Pegmatite vein

Pegmatite

Aplite

Cape Ann Granite

Basaltic dikes

"Nordmarkite"

Mafic inclusions

Contact

Dip of contact 

Sample locality

0 10 20 30 40 50 60 FEET

I i i i I.I

FIGURE 20. Geology of Andrews Point, Cape Ann (see fig. 19 for location). Mapped by D.R. Wones and P.L. Pelke in 1971.
140 THE BEDROCK GEOLOGY OF MASSACHUSETTS

The youngest rock at Andrews Point is a metasomatic 

mass of solvsbergite, formed in a late fracture where 

solutions replaced the host rocks (Martin, 1977).

Numerous light-colored dikes southwest of the Cape 

Ann Complex have been labeled Quincy or Cape Ann. 

Such dikes may be related not only to the Cape Ann 

Complex but also to the Peabody Granite or even to 

Dedham Granite. Ross (1984) has suggested a wide range 

of ages for lamprophyre and dolerite dikes in the Boston 

area.

BEVERLY SYENITE (SOcb)

The Beverly Syenite (SOcb) phase of the Cape Ann 

Complex was described by Toulmin (1964) and Dennen 

(1981). It forms subordinate elongate masses within the 

main phase of the Cape Ann (fig. 19), suggesting that it 

is most probably a cumulate. However, dikes of syenite 

do intrude the gabbro at Salem Neck, and other dikes of 

syenite are common in the Salem area (Toulmin, 1964), 

suggesting that the Beverly is in part, at least, a 

differentiate.

The syenite is medium to coarse grained, cream colored, and rich in alkali feldspar (table 11, no. 8). Accessory minerals are apatite, zircon, titanite, magnetite, 

sulfides, allanite, and astrophyllite.

SQUAM GRANITE (SOcsm)

The Squam Granite (SOcsm) forms a modally variable 

5-km2 mass within the Cape Ann Complex (fig. 19). 

Dennen (1981) recognized smaller masses of this rock 

elsewhere in the Cape Ann. Contact relations suggest 

that the Squam is an inclusion of perhaps an earlier 

cognate phase; however, it could be a synchronous 

textural variant of the alkalic granite.

The Squam Granite is fine to medium grained, is gray, 

and weathers to brown. The texture ranges from aplitic 

to porphyritic. The plagioclase content is highly variable 

(5^10 percent), and the anorthite content lies between 30 

and 55 percent. The rock contains both microcline and 

orthoclase microperthite. Quartz ranges from 15 to 30 

percent; hornblende and biotite, from 5 to 20 percent. 

Rare pigeonite remains as unreacted cores in hornblende. Accessory minerals include opaque minerals, 

apatite, zircon, monazite, allanite, and titanite.

MICROGRAPHIC RHYOLITE OF THE NEWBURY VOLCANIC 

COMPLEX (DSnr)

Lenticular sills of micrographic rhyolite (DSnr) 

100-600 m thick intrude the Newbury Volcanic Complex 

of the northern Newbury basin. These hypabyssal intrusions are contemporaneous with deposition of the volcaniclastic strata in the basin. They are brownish gray to 

orange pink and aphanitic to sugary-textured massive 

felsite characterized by micrographic and spherulitic

intergrowths (Shride, 1976). The composition of one 

sample of rhyolite is shown in table 12. The sample is 

included in the ternary diagram for chemically analyzed 

Paleozoic intrusive rocks in the Milford-Dedham zone 

(see fig. 23).

ALKALIC GRANITE IN FRANKLIN (DOgr)

The alkalic granite in Franklin (DOgr), mislabeled 

SOqgr on the State bedrock map (DOgr in the explanation), forms a north-trending pluton of about 70 km2 

north of Franklin (fig. 19). Volckmann (1977) mapped 

this rock as Milford Granite. The pluton is fault bounded 

on the north, south, and west. The eastern contact with 

the Dedham Granite may be intrusive, but mylonitization 

seen in places along this contact suggests that it too 

might be a fault. The rock is considerably more aluminous than the other alkalic units (table 12, no. 1), but the 

traces of hornblende, the ubiquitous presence of fluorite 

(table 11, no. 1), and hypersolvus texture suggested to 

Wones that the rock belongs with the Ordovician to 

Devonian alkalic granite suite.

The light-gray rock weathers to buff or brown. The 

hypidiomorphic granular texture is dominated by subhedral to anhedral perthite and quartz. Plagioclase, biotite, 

and fluorite are all anhedral. The zircons are euhedral 

and are confined to regions of recrystallized biotite. The 

biotite crystals may be pseudomorphs after amphibole, a 

trace of which is present in sample 1, table 11. The 

zircons are all within biotite aggregates, which themselves are interstitial to the perthite and quartz. The lack 

of inclusions of mafic minerals or zircon within the quartz 

or the perthite, implying a late-stage crystallization of 

the mafic minerals and zircon, is characteristic of peralkaline rocks (Watson, 1979).

Mylonitic fabrics are common and tend to concentrate 

quartz, sericite, and fluorite. Mylonitic zones contain 

lenticles of equigranular (0.1-mm) quartz and also, in 

places, biotite. Quartz on the margins of the lenticles 

shows undulose extinction, and the enclosing perthite 

and plagioclase are highly fractured. A partly crushed 

rock, not listed in table 11, consists of 41 percent 

perthite, 35 percent quartz, 14 percent plagioclase, 5 

percent biotite, and 5 percent sericite. Sericite is confined to plagioclase and zones of mylonite.

PEABODY GRANITE (Dpgr)

The Peabody Granite (Dpgr) occurs in two plutons (fig. 

19) that total approximately 50 km2 in area. The large 

mass at Peabody (Peabody pluton) is the main body 

(Toulmin, 1964). The smaller mass near Reading (Reading pluton) is lithologically similar to the Peabody Granite in the Peabody pluton. Both masses intrude the rocks 

of the Proterozoic Z mafic volcanic-plutonic complex (Zv, 

Zdigb).
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 141

The Peabody is a creamy- to tan-weathering graygreen rock (Toulmin, 1964), consisting of alkali feldspar, 

quartz, ferrohornblende (table 11, no. 9), and accessory 

pyroxene, biotite, magnetite, ilmenite, zircon, titanite, 

allanite, and sulfide minerals. Xenoliths are locally abundant. The paucity of aplite and pegmatite is in striking 

contrast to the Cape Ann Complex.

Chemical analyses of the Peabody (Toulmin, 1964; 

table 12, no. 7) show values of (Na20+K20)/Al203 less 

than 1, indicating a subalkaline magma. Buma and others 

(1971) observed an early saturation of zircon in the 

Peabody magma in contrast to the Quincy Granite in 

which zircon crystallized late in the sequence. However, 

the rock still tends toward the alkaline field, as do the 

Quincy and Cape Ann. These observations fit well with 

the experimental work of Watson (1979), who suggested 

that peralkaline magmas have higher Zr solubilities than 

subalkaline magmas. Although the major minerals and 

bulk compositions of the Quincy Granite, the Cape Ann 

Complex, and the Peabody Granite are similar, their 

petrography, fabrics, and ages of intrusion indicate that 

they resulted from distinct magmatic events.

WENHAM MONZONITE (Dwm)

The Wenham Monzonite (Dwm) (Toulmin, 1964) is a 

body of 10-km2 area north of Peabody (fig. 19) that 

intrudes the Proterozoic Z mafic volcanic-plutonic complex (Zv, Zdigb). The rock is gray, weathering to a cream 

color, is medium grained, and is made up of alkali 

feldspar, plagioclase, amphibole (table 11, no. 10), and 

accessory biotite, quartz, opaque minerals, apatite, zircon, and titanite (Toulmin, 1964). The presence of plagioclase and biotite and the interstitial nature of the quartz 

distinguish this unit from the rocks of the Cape Ann 

Complex.

CHERRY HILL GRANITE (Dcygr)

The Cherry Hill Granite (Dcygr) is a 3-km2 body of 

coarse-grained leucocratic granite that intrudes the 

Wenham Monzonite. It contains 1 percent or less magnetite and sparse zircon (Toulmin, 1964). The feldspar is 

microperthitic microcline that weathers to pink.

GRANITE OF THE RATTLESNAKE HILL PLUTON (Drgr)

The granite of the Rattlesnake Hill pluton (Drgr) 

(Lyons and Krueger, 1976) has an area of 22 km2 east of 

Wrentham (fig. 19) and intrudes the Dedham Granite. 

The granite is made up of three separate phases. The 

coarse biotite granite (table 11, no. 11) is light to medium 

gray, weathers yellowish brown, and consists of quartz, 

alkali feldspar, biotite, and accessory magnetite, zircon, 

apatite and fluorite. The coarse riebeckite granite (table 

11, no. 12) is a gray, tan- to orange-weathering rock that 

consists of quartz, alkali feldspar, riebeckite, and accessory biotite, magnetite, zircon, and fluorite. The finegrained riebeckite granite (table 11, no. 13) is light gray, 

weathers to pink to orange, and consists of quartz, alkali 

feldspar, riebeckite, and accessory magnetite, zircon, 

and apatite. The western margin of the fine-grained 

granite contains irregular masses of pegmatite containing riebeckite (Lyons and Krueger, 1976).

MESOZOIC INTRUSIVE AND SILICIFIED ROCKS

DIABASE DIKES (Jd)

Many mafic dikes cut the Proterozoic Z and Paleozoic 

rocks of the Milford-Dedham zone, but only those that 

can be traced any distance are shown on the State 

bedrock map. Most are considered, or are known, to be 

Triassic or Jurassic in age, but some in northeastern 

Massachusetts range in age from Devonian to Carboniferous (Ross, 1984). Some dikes seem to be lamprophyres 

related to the Paleozoic alkalic granites. LaForge (1932) 

distinguished at least four sets of diabase dikes in the 

Milford-Dedham zone. He recognized that an east-west 

set that was highly altered intruded the Dedham Granite 

but was cut by the Quincy Granite. A set of northwestsoutheast dikes may be related to these older east-west 

dikes, because they are also highly altered and their age 

relationships are obscure. A younger east-west set is not 

as severely altered. A set of north-trending unaltered 

dikes may be correlative with the Medford Diabase dike 

(see below). These observations agree with those made 

by Ross (1984).

Ross (1981) observed that dikes of Triassic or Jurassic 

age in this region tend to strike north or close to north. 

The large north-trending Medford Diabase dike (fig. 19) 

of Jurassic age (table 1) cuts the Boston Bay Group and 

crosses the northern border fault of the Boston basin into 

the Melrose block containing the Lynn Volcanic Complex. An undated north-trending dike cuts rocks of the 

Bellingham basin at Woonsocket. Lyons (1977) found a 

small north-trending trap dike cutting Pennsylvania!! 

strata at North Middleboro. Basalt dikes and sills cut the 

Boston Bay Group on islands in the Boston Harbor (C. A. 

Kaye, written commun., 1979). All these dikes have been 

assigned to the Jurassic on the State bedrock map on the 

basis of the age of the Medford Diabase dike. Other 

narrow north-trending dikes not shown on the State 

bedrock map have been observed in roadcuts in the area, 

such as at several places on 1-90 between Natick and 

Marlborough and on 1-495 in Wrentham, Franklin, and 

Milford.

The Medford dike is the largest diabase dike in the 

Milford-Dedham zone. This particular dike trends northnortheast and, as mentioned above, crosses without 

offset the northern border fault of the Boston basin 

(Goldsmith, this vol., chap. H), but centimeter-scale
142 THE BEDROCK GEOLOGY OF MASSACHUSETTS

left-lateral displacements along east-west faults are 

present west of Spot Pond, Stoneham. The Medford dike 

weathers deeply, and immature soil profiles have developed on the glaciated surfaces. The fresh diabase is made 

up of plagioclase and augite with accessory orthoclase, 

biotite, magnetite, and ilmenite. It contains 1.7 percent 

K20 (Emerson, 1917, p. 77) and is an alkaline rock. 

McHone (1981) and Hermes and others (1978) have 

shown that alkaline mafic dikes of Mesozoic age are 

common in New England, and the Medford dike appears 

to be part of that group.

MASSIVE QUARTZ AND SILICIFIED ROCK (q)

Masses of quartz and silicified rock (q) are common 

near many of the faults in eastern Massachusetts (Goldsmith, this vol., chap. H). The only one large enough to 

show on the State bedrock map is the large mass at 

Diamond Hill (DH, fig. 19) east of Woonsocket (Quinn, 

1971). The host rock that was silicified was a felsic 

volcanic rock of presumed Pennsylvanian age. The silicification lies along a north-trending fault, which cuts the 

Lower and Middle Pennsylvanian Wamsutta Formation. Terminated quartz crystals suggest open voids and 

a brittle fracture regime. The faulting is postPennsylvanian and from its style is presumed to be 

Mesozoic in age.

DISCUSSION OF THE INTRUSIVE ROCKS OF THE 

MILFORD-DEDHAM ZONE

The oldest dated intrusive rock in this zone is the 

diorite at Rowley (Zrdi) (table 1). The diorite at Rowley 

and gabbroic dikes intrude the metavolcanic rocks (Zv) 

northwest of the Boston basin. These rocks and the 

extensive diorite and gabbro (Zdigb) may be penecontemporaneous with the volcanic material and form part of 

a volcanic-plutonic complex in which the mafic plutons 

have intruded the volcanic cover. The diorite at Rowley 

appears to be undeformed relative to the older rocks and 

is perhaps the youngest of the intrusive suite. On the 

other hand, it may be the earliest recognized intrusion 

that is comagmatic with the Milford and Dedham Granites, as is suggested by the apparent fit of the other 

Proterozoic Z diorites (Zdi) of table 3 to the lines through 

the batholithic rocks shown on the Peacock diagram (fig. 

6) and by the Th/U ratios plotted on figure 10. If so, the 

sequence is similar to mafic to felsic intrusive sequences 

recognized from the circum-Pacific Mesozoic batholiths 

(Silver and others, 1979; Bateman, 1983). The polarity of 

such a sequence in eastern Massachusetts is not discernible because the exposed zone is narrow and disrupted. 

The greater abundance of mafic rocks north of Boston is 

attributed more to the level of erosion across a northQuartz

Plagioclase

EXPLANATION

 Dedham Granite as used in this report

 - -- Dedham Granite of Dowse (1949)

 -- Dedham Granite of Lyons (1969)

 ----------- Milford Granite

 Westwood Granite

FIGURE 21. Ternary quartz-plagioclase-K-feldspar plot of modal 

composition of Dedham Granite as compared with fields of composition of Dedham Granite of Dowse (1949), Dedham Granite of Lyons 

(1969), Milford Granite, and Westwood Granite.

plunging structure than to a lateral change in composition of intrusions across a belt.

Most of the batholithic rocks fall in the granite field 

(figs. 5, 12-14). A few of the batholithic rocks, like some 

of the Dedham Granite north of Boston (Zdngr) and the 

Topsfield Granodiorite (Ztgd), fall in the granodiorite 

field (fig. 12). The mafic phase of the Milford Granite 

approaches tonalite (fig. 13). The Westwood Granite 

samples (fig. 13) cluster closely in the granite field. 

Porphyritic phases, such as the porphyritic granite of the 

New Bedford area and the Ponaganset Gneiss, range 

from granite to granodiorite (fig. 14), the same range as 

the other batholithic rocks. The field of Dedham Granite 

established in our study for the State bedrock map is 

broader than the fields established in the more areally 

limited studies of Dowse (1949) and Lyons (1969) (fig. 

21). The REE patterns of the Dedham Granite and 

granite of the Fall River pluton are closer to the REE 

patterns of the Hope Valley Alaskite Gneiss and the 

Scituate Granite Gneiss than to those of the Milford 

Granite and the biotite granite (Zgr) (figs. 15, 16, 18), 

although the small number of samples of all these rocks 

diminishes the significance of this.

The peraluminous Milford Granite and the metaluminous Dedham Granite are contemporary, according to 

the available radiometric ages, but they differ in their 

mineralogy, composition, and deformational history. The
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 143

Al,03

50% AI 2O3

Na2O

EXPLANATION

o Paleozoic intrusive rocks 

(table 12)

 Proterozoic Z batholithic 

rocks (tables 6, 8, and 10)

 Proterozoic Z mafic plutonic 

rocks (table 3)

FIGURE 22. Plot of a part of the ternary system NagO-I^O-A^Og in 

molecular percent for intrusive rocks of the Milford-Dedham zone. 

Sample 1 is alkalic granite in Franklin; see table 12 for further data.

Milford contains garnet, muscovite, and lensoid quartz 

that has been annealed. The Dedham contains titanite 

and magnetite, and hornblende is commonly found adjaFIGURE 23. Ternary plot of normative albite (Ab), anorthite (An), 

and orthoclase (Or) for Paleozoic intrusive rocks of the MilfordDedham zone, from table 12. Fields of silica-saturated rocks from 

O'Connor (1965) as modified by Barker (1979).

cent to the contacts with older mafic rocks. The Dedham 

is brittlely deformed and altered by hydrolysis and 

oxidation to a characteristic red color with green veins 

and patches of epidote and chlorite. The Dedham is 

characteristic of plutonic rocks that form in subduction 

regimes, with its great volume relative to enclosed and 

adjacent metasedimentary and metavolcanic material, 

its intrinsically high oxidation state, and its chemistry. 

The high but variable percentage of K-feldspar in the 

Dedham and Milford (tables 4, 5, 7, 9; figs. 5, 12-14) is 

similar to that found in subduction zone batholiths farthest "inboard" from the continental margin (Brown and 

others, 1984).

Gromet and O'Hara (1984; O'Hara and Gromet, 1984, 

1985) have proposed a major tectonic boundary within 

the Milford-Dedham zone between the terrane containing the Hope Valley Alaskite Gneiss and the terrane 

containing the Milford Granite in the Milford antiform. 

Such a boundary could have formed before intrusion of 

the Middle Devonian Scituate Granite and the granite in 

Franklin, but these rocks must have been emplaced 

before the major motion on the boundary because they 

are deformed along it.

Our data on the Paleozoic rocks are limited, but it is 

clear that they tend to be alkalic. They plot in the part of 

the Na20-K20-Al203 diagram (fig. 22) in which the 

amount of A1203 is only slightly greater than the sum of 

the alkalis and towards the alkalic side of figures 23 and 

24. The granite in Franklin (DOgr) has a higher A1203 

content than the other alkalic granites of table 12. An

FeO

MgO

FIGURE 24. Ternary AFM (alkalis-FeO-MgO) plot of Paleozoic intrusive rocks of the Milford-Dedham zone, from table 12, showing field 

of Proterozoic Z intrusive rocks of the Milford-Dedham zone (dashed 

line) from figure 8.
144 THE BEDROCK GEOLOGY OF MASSACHUSETTS

200 I l l I l l

EXPLANATION

Alkalic granite in Franklin (DOgr) (sample 1) 

Porphyritic rhyolite, Newbury Volcanic 

Complex (DSnr) (sample 2)

 Granodiorite of the Indian Head pluton (igd)

Indian Head pluton \

FIGURE 25. Chondrite-normalized plot of rare-earth elements of Paleozoic intrusive rocks. Numbers refer to sample numbers in table 12; data

for granodiorite of the Indian Head pluton from table 15.

REE pattern of a sample from this unit (fig. 25, no. 1) 

shows marked REE enrichment and a pronounced negative europium anomaly, remarkably similar to the pattern of the rhyolite (DSnr) from the Newbury Volcanic 

Complex (fig. 25, no. 2). Possibly they are cogenetic.

RELATION TO EXTRUSIVE ROCKS

Some or all of the Proterozoic Z volcanic rocks around 

the Boston basin and at a few places elsewhere in the 

Milford-Dedham zone may be related petrogenetically to 

the batholithic rocks. These rocks are described in the 

chapter in this volume on the stratigraphy of the MilfordDedham zone (Goldsmith, this vol., chap. E). In this 

chapter, the principal extrusive units discussed that are

equivalent to or younger than the southeastern Massachusetts batholith are the Lynn and Mattapan Volcanic 

Complexes (DZ1, Zm) and the Brighton Melaphyre 

(ftZrb) of the Boston Bay Group. Felsic metavolcanic 

rocks (Zvf) in the Framingham area are probably cogenetic with the batholithic rocks.

The Mattapan and Lynn Volcanic Complexes consist 

largely of rhyolite and rhyodacite flows, in part porphyritic, but andesitic rocks have been reported (Nelson, 

1974). Chute (1966) reported that the rocks tend to be 

sodic rather than potassic. The volcanic rocks in the Blue 

Hills, the aporhyolite of Emerson (1917), are devitrified 

rhyolitic flows assigned to the Mattapan Volcanic Complex (Zm). The evidence for the stratigraphic positions of
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 145

the Mattapan and Lynn is conflicting. LaForge (1932) 

and Bell (1976) cited evidence for nonconformable relations between the Lynn and the Dedham Granite. On the 

other hand, Zarrow (1978) described inclusions of Lynn 

in the Dedham and noted that the Lynn and the Dedham 

have similar trace-element contents. Kaye and Zartman 

(1980) described places where the Dedham Granite 

grades upwards texturally into fine-grained rhyolitic 

rock. Chute (1966) noted that fine-grained phases of the 

Westwood Granite resemble the Mattapan. The U-Th-Pb 

isotopic age of 602 ±3 Ma for the Mattapan (Kaye and 

Zartman, 1980) indicates that the Mattapan is younger 

than the Dedham, although possibly close in age to the 

Westwood. Volcanic rocks cogenetic with the Dedham 

and Westwood would differ in time of extrusion but 

might be difficult to sort out in the field. The felsic 

metavolcanic rocks (Zvf) in the Framingham area are 

considered to be part of the prebatholithic, largely mafic 

volcanic complex (Zv). Felsic metavolcanic rocks in this 

suite have been described and mapped by Nelson 

(1975a,b) and Volckmann (1977). The affiliation with the 

batholithic rocks is suggested by the fact that some of the 

felsic rocks were mapped as Milford Granite. In outcrop 

these are not unlike fine-grained phases of the Hope 

Valley Alaskite Gneiss of southeastern and eastern Connecticut and adjacent Rhode Island. Both felsic and mafic 

metavolcanic rocks of pre-Dedham age have been identified in the Middlesex Fells area. The Brighton Melaphyre in the Brookline Member of the Roxbury Conglomerate interfingers with and overlies the Mattapan, and 

dikes of Brighton cut the Mattapan. The Brighton consists primarily of quartz keratophyre, keratophyre, and 

spilite. Nelson (1975a) described basaltic and andesitic 

flows and tuffs. Bouchard (1979) described basaltic 

extrusive rocks and subordinate andesitic to rhyolitic 

lavas and tuffs. These volcanic rocks are a suite clearly 

younger than the batholithic rocks; they record a separate igneous event that is probably related to the deposition of the Proterozoic Z turbiditic material forming the 

Boston Bay Group in the Boston basin.

Little volcanic rock that can be associated with the 

suite of Paleozoic alkalic granite intrusions is preserved 

in Massachusetts. The Blue Hills Granite Porphyry is 

clearly associated with the Quincy Granite, and volcanic 

rocks of the Newbury Volcanic Complex may be cogenetic with the alkalic plutons. In Rhode Island, the 

Spencer Hill Volcanics (Quinn, 1971) represent extrusive 

equivalents of mid-Paleozoic intrusions.

INTERPRETATION OF IGNEOUS EVENTS

The age relations and the composition and character of 

the plutonic rocks indicate the following history of events 

in the Milford-Dedham zone. A volcanic-plutonic arc 

developed probably in Proterozoic Z time. The early

plutonic rocks in this arc were mafic; the later were 

primarily granitoid batholithic rocks such as the Dedham 

Granite. These later rocks may have been consanguineous with the mafic rocks. The sparseness of tonalite and 

granodiorite in the batholithic rocks suggests that they 

were intruded within a continental craton, which in turn 

suggests that the volcanic-plutonic arc formed at the 

edge of a continental mass. After the intrusion of the 

Dedham batholith, a small graben (rift basin?), the 

Boston basin, formed, siliceous lavas and pyroclastic 

rocks (Mattapan and Lynn Volcanic Complexes) were 

extruded, and the basin was filled by a sedimentary 

sequence (Boston Bay Group) that included basaltic lavas 

(Brighton Melaphyre) in Proterozoic Z and Early Cambrian time (Kaye and Zartman, 1980; Goldsmith, this 

vol., chap. E). The Westwood Granite may be the 

intrusive equivalent of the Mattapan and Lynn, whereas 

the swarms of older east-west-trending mafic dikes (parallel to the faults defining the basin) may be the intrusive 

equivalent of the basaltic lava flows of the Brighton.

The next period of activity was during the Ordovician, 

when the Nahant Gabbro was intruded into the Boston 

basin at Nahant and into the Proterozoic Z basement at 

Salem Neck. This gabbro is 30 m.y. older than alkalic 

feldspar granites like those of the Cape Ann Complex 

and the Quincy Granite that followed it (table 1). The 

compositions of these alkalic granites indicate that the 

crust had matured sufficiently that no H20-rich material 

melted to make more typical granites. The Cape Ann 

Complex and Quincy Granite are similar to rocks usually 

associated with intracratonic plutonism or a continental 

rift zone. Similar rocks extend well into the Gulf of Maine 

to the north (Hermes and others, 1978) and into Rhode 

Island to the south (Hermes and Zartman, 1985).

A period of apparent quiescence, between about 450 

and 400 Ma, was followed during the Devonian by 

intrusion into stable crust of another group of alkaline 

rocks such as the Wenham Monzonite, Peabody Granite, 

granite of the Rattlesnake Hill pluton, and the Scituate 

Granite of central Rhode Island. Zartman (1977) 

believed, however, that the intrusions may be sequential 

rather than falling into two distinct groups. The composition of these Devonian granites is distinctly different 

from that of the widespread Devonian plutons west of the 

Milford-Dedham and Nashoba zones.

The final intrusions in the Milford-Dedham zone were 

of Permian age, the Westerly and Narragansett Pier 

Granites in Rhode Island. These granites are H20 rich, 

are meta- to peraluminous, and from their composition 

and contact effects (Grew and Day, 1972; Day and others, 

1980; Hermes and others, 1981) must have intruded 

Pennsylvanian-age sediments from depths of about 12-15 

km. Their mineralogy suggests that they were derived 

from a crustal source different from the source of the
146 THE BEDROCK GEOLOGY OF MASSACHUSETTS

72°00' 45' 71°00' 70°45'

42°45' -

EXPLANATION

Orange-pink granite

Sharpners Pond Diorite and granodiorite of Indian Head pluton

Straw Hollow Diorite and 

Assabet Quartz Diorite

__NEW_HAMPSHIRE / MASSACHUSETTS" / > .Georgetown.'. 

.. . --. -- . 

Massachusetts 

Bay

(' '. '. OZmg/.

 ' ' : ///.Grafton

MASSACHUSETTS^ 

42°00' -

-RHODE" ISLAND

FIGURE 26. Distribution of intrusive rocks in the Nashoba zone. OZmg, feldspathic gneiss in the Marlboro Formation; mgr, light-gray

muscovite granite.

earlier Paleozoic intrusions. This new source or modification of an older source for the siliceous magma formed 

between the Devonian and Permian probably is best 

explained by thrusting of the Milford-Dedham zone over 

a thick sequence of sediments before Permian time, or 

possibly by underplating of entirely different material as 

indicated by Zartman and Hermes (1984). Thus, the

plutonic record documents a period of Precambrian subduction, a long period through the early and middle 

Paleozoic during which the crust had matured so that 

alkaline magmas were generated as a result of lower 

crustal heating, and finally the transport of the MilfordDedham rocks over sedimentary rocks during the late 

Paleozoic.
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 147

TABLE 13. Ages and age relations of intrusive rocks of the Nashoba zone and the eastern part of the Merrimack belt

[Radiometric data from Zartman and Marvin, this vol., chap. J, table 1, except where noted. Ages are interpreted to be within 5 percent of the primary age of intrusion.

wr, whole rock; , no data]

Map unit Name Intrudes Age (Ma), method

Dfgr Fitchburg Complex

Dmgr Granite at Millstone Hill 

Dcgr Chelmsford Granite

igd Granodiorite of Indian Head pluton

Sgr Orange-pink granite

Ssqd Sharpners Pond Diorite

Ssaqd Straw Hollow Diorite and Assabet

Quartz Diorite.

Sacgr Ayer Granite, Clinton fades 

SOad Ayer Granite, Devens-Long Pond

fades. 

SOagr Andover Granite:

younger phase

older, gneissic phase

Sngr Newburyport Complex granite 

SOngd Newburyport Complex granodiorite

Sp, Paxton Formation; Dl, Littleton 

Formation; DSw, Worcester 

Formation.

So, Oakdale Formation

Sb, Berwick Formation; Sagr; DSdi, 

diorite and tonalite.

OZm, Marlboro Formation

Ssqd

OZn, Nashoba Formation; OZm, Marlboro Formation; SOagr, older phase.

OZn

So, SOrh, Reubens Hill Formation 

Sb, Berwick Formation

OZn, OZm, SOagr, older phase 

OZn, OZm

Se, Eliot Formation; SOk, Kittery 

Formation.

390±15, U-Pb/zircon, 402±11, RbSr/wr.

372±7, Rb-Sr/wr. 

389±5, Pb-Pb/zircon. 1

402±5, Rb-Sr/wr. 2 

430±5, U-Pb/zircon.

433±5, U-Pb/zircon. 

433 ±5, U-Pb/zircon.

408±22, Rb-Sr/wr, 1 415, Rb-Sr/wr. 2 

446±32, Rb-Sr/wr, 1 450±23, Rb-Sr/wr, 

460, Rb-Sr/wr.

455±15, U-Pb/zircon.

'Data from Zartman and Naylor (1984). 

2Data from Hill and others (1984).

INTRUSIVE ROCKS OF THE NASHOBA ZONE

The intrusive rocks of the Nashoba zone (fig. 26) range 

in age from Ordovician to Devonian (table 13) and are 

dominated by the Ordovician and Silurian, peraluminous, 

gneissic Andover Granite and a Silurian calc-alkaline 

suite consisting of quartz diorite, granodiorite, and granite. The largest masses of intrusive rocks are in the 

northern part of the Nashoba zone. The area of the 

Andover Granite shown on the State bedrock map 

includes granite of several ages. Most of it is Ordovician 

in age, but it contains some Devonian granite and 

pegmatite. A feldspathic gneiss (OZmg) near Grafton in 

the southern part may be an intrusive rock (Hepburn, 

1978) or may be a metavolcanic unit in the Nashoba 

Formation. This gneiss is discussed in the description of 

the stratigraphic units in the Nashoba zone (Goldsmith, 

this vol., chap. F).

ANDOVER GRANITE (SOagr)

The Andover Granite (SOagr) is a light-colored 

muscovite- and garnet-bearing, mostly gneissic granite 

(table 14, no. 1) that intrudes the Nashoba Formation 

(OZn) at all scales. Within the Andover are nongneissic 

granitic rocks that are apparently younger than the main 

mass of the unit (Hansen, 1956; Castle, 1964). Dikes and 

apophyses of aplite and pegmatite infiltrate much of the 

Nashoba terrane and contribute to the migmatitic aspect

TABLE 14. Modes, in percent, of intrusive rocks of the Nashoba zone 

[Modes 1-6 from Castle (1964); a, albite; o, oligoclase; n, andesine; tr, trace]

Sample no. . . 

Unit .......

Plagioclase . . Microcline . . . 

Muscovite . . .

Hornblende. .

Titanite.

Opaque 

minerals . . 

Others1

1 

. SOagr

.24-49

, . 20-30a,o 

. 1&41 

. 2-14 

. 0-6

. 0 

. 0-1

. 0

, . tr 

. tr

2 

SOagr

27-38

26-16 

10-44 

0-11 

0-11

0 

0-0.6

tr

0-0.2 

0-6.5

3 

SOagr

25^39

32-49 

0-33 

0.2-17 

0-4

0 

0-3

0

tr 

tr

4 

Ssqd

0-7

35-52n 

0 

0 

0-13

19-38 

0

1

2-7 

1-3

5

Ssqd

13-18

44-49n 

0 

0 

10-23

11-22 

0

2

.2 

1

6

Ssqd

25-40

28-57o,n 

0-33 

0 

2-24

0-4 

tr

0-1.6

.4 

0-10

7 

igd

32.5

35.9o 

22.8 

tr 

7.4

0 

0

0

0 

1.4

Others = sphene, zircon, and alteration minerals sericite, chlorite, epidote, and carbonate 

minerals.

Description of samples

1. Andover Granite; biotite granite gneiss facies, range of 14 samples.

2. Andover Granite; binary granite facies, range of 24 samples.

3. Andover Granite; pegmatitic granite facies, range of 9 samples.

4. Sharpners Pond Diorite; hornblende-diorite facies, range of 5 samples.

5. Sharpners Pond Diorite; biotite-hornblende tonalite facies, range of 

4 samples.

6. Sharpners Pond Diorite; biotite-tonalite facies, range of 9 samples.

7. Granodiorite of the Indian Head pluton; roadcut on W95 at 

Southboro-Marlborough town line. Field no. M-6; sample locality given 

in Wones and others (1986); 1,152 points counted.

of much of the Andover. The Acton Granite of Hansen 

(1956) probably belongs to the older material. The preferred orientation of muscovite defines a pronounced 

foliation conformable to the regional trend. Mortar tex-
148 THE BEDROCK GEOLOGY OF MASSACHUSETTS

tures and recrystallized quartz are common. The gneissic 

Andover Granite is about 445-450 Ma, and the younger 

phase of granite and pegmatite is 410-415 Ma (table 13).

A crosscutting muscovite granite of the younger phase 

(table 14, no. 2), termed "binary granite" by Castle 

(1964), is white to gray and weathers to chalk white. The 

texture is seriate at most localities. Alteration minerals 

are chlorite (0-3 percent), epidote (0-2 percent), and 

carbonate minerals (0-3 percent). Myrmekite is well 

developed in this younger facies.

Pegmatitic granite of the younger phase (table 14, no. 

3) is abundant in the southern parts of the Lawrence and 

South Groveland areas (fig. 26). It intrudes the gneissic 

facies of the Andover and the Sharpners Pond Diorite 

and is therefore younger than the rest of the Andover. It 

is gradational with the younger muscovite granite. Crystal shape and grain size vary randomly among the 

outcrops of this unit. Rutile and chlorite are alteration 

minerals from biotite. All three facies contain traces of 

zircon and apatite, in addition to the ubiquitous small 

amounts of garnet.

SHARPNERS POND DIORITE (Ssqd)

The Sharpners Pond Diorite (Ssqd) covers about 150 

km2 and includes diorite, tonalite, and small amounts of 

granodiorite. It intrudes the Nashoba (OZn) and Marlboro (OZm) Formations and the older gneissic phase of 

the Andover Granite. It is intruded by pegmatites, the 

younger phase of the Andover, and the orange-pink, 

rusty-weathering biotite granite (Sgr). The Sharpners 

Pond has a reliable age of 430 Ma (table 13).

Castle (1964) recognized three separate facies within 

the Sharpners Pond: hornblende diorite, biotitehornblende tonalite, and biotite tonalite. These three 

facies are gradational in composition and were intruded 

penecontemporaneously. Some of the hornblende diorite, 

where gneissic, may be recrystallized Marlboro Formation, although much of the gneissic diorite clearly is 

intrusive.

The hornblende diorite (table 14, no. 4) is equigranular, medium grained, and hypidiomorphic to allotriomorphic. Some rocks are quite altered and contain appreciable chlorite (2-10 percent), white mica (0-20 percent), 

epidote (0.5-3 percent), and carbonate minerals (less 

than 0.6 percent). Plagioclase ranges in composition from 

An28 to An55 , averages An40-An45 , and displays weak 

zoning. Some hornblende encloses clinopyroxene.

The biotite-hornblende tonalite (table 14, no. 5) is 

slightly younger than the hornblende diorite. The higher 

biotite content gives this facies a more foliate appearance. Alteration minerals include chlorite (1-2 percent), 

white mica (less than 3 percent), epidote (0.3-2 percent),

and traces of carbonate minerals. The plagioclase compositions are An30-An40.

The biotite tonalite (table 14, no. 6) is limited to about 

10 km2 in the Reading area, and its boundaries with both 

the Andover Granite and the other facies of the Sharpners Pond Diorite are ill defined (Castle, 1964). The rocks 

are fine to medium grained and variably foliated. Rocks 

with little K-feldspar are hypidiomorphic, whereas those 

with more K-feldspar are allotriomorphic. Alteration 

minerals are chlorite (0-5 percent), epidote (0.1-2.4 

percent), white mica (0-0.3 percent), and carbonate 

minerals. Plagioclase compositions are An15-An33 . Secondary K-feldspar has formed along early fractures.

STRAW HOLLOW DIORITE AND ASSABET QUARTZ 

DIORITE (Ssaqd)

The Straw Hollow Diorite and Assabet Quartz Diorite 

(fig. 26) are shown as one unit (Ssaqd) on the State 

bedrock map because of their similar occurrence and 

general lithology and their small size. The Straw Hollow 

Diorite, only 3 km2 in area, intrudes the Nashoba Formation and is in fault contact with the Marlboro Formation. The rock is light gray to dark green, medium 

grained, and weakly foliated and contains hornblende, 

biotite, and accessory magnetite, titanite, garnet, and 

pyrite. It is considered to be Silurian in age because of its 

lithologic similarity to the Sharpners Pond Diorite.

The Assabet Quartz Diorite, 23 km2 in area, is believed 

to intrude the Marlboro Formation (although Hansen 

(1956) did not observe any intrusive contact) and the 

Andover Granite. The rock is weakly foliated and is 

composed of andesine, hornblende, quartz, and biotite 

with accessory apatite, titanite, and hematite. The 

gneissosity is defined by parallel orientation of hornblende. Hansen (1956) reported that the Assabet is cut 

by dikes of aplite and pegmatite, which may be equivalent to the granite (Sgr) that intrudes the Sharpners 

Pond Diorite.

GRANODIORITE OF THE INDIAN HEAD PLUTON (igd)

The granodiorite of the Indian Head pluton (igd) (fig. 

26) consists of a normal and a coarse-grained facies 

(Hepburn and DiNitto, 1978), which intrude the Marlboro Formation. It ranges from light gray where fresh to 

pink on weathering, is massive, and ranges in composition from granodiorite to monzogranite. A mode given 

in table 14 (no. 7) indicates a granitic composition. A 

chemical composition of another sample given in table 15 

also indicates a granitic composition verging towards 

granodiorite. Accessory minerals are muscovite, epidote, and hornblende. This granitic sample may correspond to a two-mica granite phase of the Indian Head
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 149

TABLE 15. Major-oxide, normative-mineral, and trace-element 

compositions of granodiorite of the Indian Head pluton

[Major-oxide composition from X-ray spectroscopy by Paul Hearn and Susan 

Wargo; all Fe reported as Fe203. Trace-element abundances from instrumental 

neutron activation analyses by L.J. Schwartz except Rb and Sr determined by 

X-ray spectroscopy by G. Sellars and B. McCall. Sample locality (field no. 

DMA-191) given in Wones and others (1986)]

Major-oxide composition,

in weight percent, and

alkali-alumina ratio

Trace-element abundances,

in parts per million, and

selected ratios

Si02 ...........

A1203 ..........

Fe203 ..........

MnO. ...........

MgO. ...........

CaO ............

Na^O ...........

K20 ............

Ti02 ............

P205 ..... .......

(Na20+K20)/Al20

. . . . . 69.34

. . . . . 16.02

. .... 1.10

. . . . . .06

. . . . . .92

..... 2.60

. .... 3.58

. .... 3.81

. . . . . .39

..... .11

3 ... .46

Normative-mineral composition, 

in weight percent,1 and differentiation index (DI)

Qtz.............

Crn ............

Or. .............

Ab .............

An .............

En .............

Fs. .............

Ilm.............

Ap... ..........

DI .............

..... 25.4

..... 1.5

..... 22.6

..... 30.4

..... 12.2

. . . . . 2.3

. . . . . 4.5

..... .7

..... .3

..... 78

Rb ..........

Cs ..........

Sr ..........

Ba ..........

Rb/Cs .......

Rb/Sr .......

Sc ..........

Cr ..........

Co ..........

Zn ..........

La .......... 

Ce ..........

Nd .........

Sm .........

Eu ..........

Gd ..........

Tb ..........

Ho ..........

Tm .........

Yb ..........

Lu ..........

La/Yb .......

Hf ..........

Zr ..........

Th2 .........

U2 ..........

Zr/Hf .......

........ 108

........ 2.6

........ 404

........ 1078

........ 41

........ .3

........ 6.7

........ 7.4

........ 5.3

........ 57

........ 52

........ 93

........ 35

........ 5.5

........ 1.01

........ 4.8

........ .66

........ .80

........ .44

........ 1.7

........ .25

........ 31

........ 5.2

........230

........ 16.7

........ 2.5

........ 44

^e^ calculated as FeO.

2Delayed neutron reactivation analysis (by H.T. Millard, Jr., C. McFee, and C. Bliss) gave 

same result as did instrumental neutron activation analysis.

Description of sample

Granodiorite, Farm Road, 300 m southwest of Cook Lane, Marlborough. UTM grid: N46902-E2918.

pluton that cuts an older, more mafic gneissic phase 

described by Hill and others (1984). They dated the older 

phase at 402 Ma. Our sample has an REE pattern (fig. 

25) that falls between the patterns of the two phases of 

Hill and others. This granite has a relatively high Th/U 

ratio compared with those of the Proterozoic rocks shown 

on figure 10. The Indian Head is the granodiorite 

described by Nelson (1975a) in the Framingham area. 

The entire mass in the Framingham area is about 70 km2 

in area. The Indian Head may be equivalent in age to 

small (less than 1 km2) masses of monzogranite near 

South Groveland and Georgetown reported by Castle 

(1964) and to the orange-pink rusty-weathering granite 

(Sgr) described next.

ORANGE-PINK RUSTY-WEATHERING GRANITE (Sgr)

The orange-pink rusty-weathering granite (Sgr) forms 

three masses, near Byfield, near Reading, and near 

Sudbury (fig. 26). The distribution of the granite shown 

on the State bedrock map is derived from reconnaissance 

mapping by Shride (written commun., 1979). The 

orange-pink granite consists of both medium-grained and 

coarse-grained facies. The granite at Byfield is a 

medium-grained, equigranular to porphyritic biotite 

granite that contains about equal amounts of white 

oligoclase and gray quartz, lesser amounts of perthitic 

microcline, and about 5 percent biotite (Shride, 1971). 

The rock is only altered near contacts and where faulted. 

It contains many inclusions and is contaminated by wallrock material in contact zones; in these places it is 

difficult to tell from Sharpners Pond Diorite. It does, 

however, intrude the Sharpners Pond Diorite (Shride, 

1976). Hansen (1956) mapped the mass at Sudbury as 

Dedham granodiorite, but part of the rock Hansen 

mapped as Dedham is granodioritic to tonalitic in composition and was subsequently mapped by Hepburn and 

DiNitto (1978) as Indian Head Hill granodiorite.

The orange-pink granite may be equivalent in age to 

the granodiorite of the Indian Head pluton or to the 

younger muscovite granite and pegmatite phase of the 

Andover Granite.

LIGHT-GRAY MUSCOVITE GRANITE (mgr)

The light-gray muscovite granite (mgr) forms a pluton 

in the isolated fault block of the Nashoba Formation in 

the Clinton-Newbury fault zone near Shrewsbury (fig. 

26). This mass was called the Rattlesnake Hill pluton by 

Skehan and Abu-Moustafa (1976) (not the Rattlesnake 

Hill pluton of the Milford-Dedham zone). It is uncertain 

whether this muscovite granite is equivalent to the 

Devonian muscovite granite at Millstone Hill (Dmgr) or 

the Chelmsford Granite (Dcgr) of the Merrimack belt or 

is equivalent to the older gneissic phase of the Andover 

Granite.

DISCUSSION OF THE INTRUSIVE ROCKS OF THE 

NASHOBA ZONE

The sequence of intrusion in the Nashoba zone began 

with pre- or synkinematic intrusion of muscovite-bearing 

granite. This was followed by postkinematic intrusion of 

diorite to tonalite, succeeded by less mafic tonalite, 

granodiorite, and monzogranite, and finally intrusion of 

muscovite-bearing pegmatite and aplite at a late stage to 

produce migmatite in many areas.

Confusion exists concerning the age of these 

sequences, as both the oldest and the youngest intrusive
150 THE BEDROCK GEOLOGY OF MASSACHUSETTS

rocks have been included in the Andover Granite map 

unit and both are peraluminous. The early Andover 

Granite is foliate but also has the fabric of a migmatite in 

places. Resolving the age of this material will require 

precise geochronology after careful mapping of local 

areas. Zartman and Marvin (this vol., chap. J) consider 

the 450-Ma age reliable for the gneissic Andover. The 

415-Ma age of Hill and others (1984) can be considered to 

represent the age of the younger pegmatite and granite 

in the Andover. Both the older gneissic and the younger 

nongneissic Andover have the muscovite and garnet 

(table 14) typical of S-type granites derived from deepseated sedimentary material in a high-temperature 

regime. This compositional similarity suggests regeneration of magma derived from the original source or 

generation of magma from a newly arrived but similar 

source. The age of metamorphism and intrusion of the 

gneissic phase is considered to be 450 Ma (Zartman and 

Naylor, 1984).

The Sharpners Pond Diorite and related intrusions 

form a suite that has mineralogies characteristic of lower 

pressure I-type intrusives. We have chemical analyses 

from only the granodiorite of the Indian Head pluton 

from this suite (table 15). This rock is relatively granitic 

and is probably the younger phase of the Indian Head 

Hill pluton of Hill and others (1984). Their older gneissic 

mafic phase dated at 402 Ma (table 13) would fit into a 

Late Silurian and Early Devonian plutonic-volcanic 

sequence of which the Newbury Volcanic Complex may 

be a part. The younger phase of the Indian Head 

granodiorite and the orange-pink granite could be a late 

granitic phase of a plutonic-volcanic suite having a calcalkaline trend, as illustrated by Hill and others (1984); 

the dioritic intrusions, the Sharpners Pond Diorite and 

the Assabet Quartz Diorite, were an earlier mafic phase 

of this plutonic-volcanic suite. The Assabet Quartz Diorite and the Sharpners Pond Diorite intrusions are very 

suggestive of the formation of a short-lived continental 

arc. Hill and others (1984) suggested that the suite of 

rocks is the product of processes at convergent plate 

boundaries and contains assimilated crustal material of 

Proterozoic Z to early Paleozoic age, a suggestion with 

which we concur.

The exposed Nashoba zone is too narrow to develop a 

sense of polarity for the plutons within it, so that 

determining the direction of subduction is difficult, if not 

impossible. Zartman and Naylor (1984) have dated the 

younger intrusions at 408±22 Ma (table 13). If this age is 

correct, then the precursor event at 446±32 Ma may 

represent the initial melting of trench sediments and may 

be analogous to the event that produced the Cretaceous 

granitoids of the Ryoke-Sanba region in Japan (Czamanske and others, 1981). Thus, we may be observing a 

record of about 30 m.y. of subduction. At a rate of

1 cm/yr, 300 km of oceanic crust could have been subducted during this time. However, uncertainty in field 

identification of the different phases of the Andover and 

the scatter of the radiometric ages make such an assessment speculative.

The intrusion of the younger muscovitic granites, the 

young phases of the Andover Granite and the Indian 

Head pluton, and possibly the orange-pink granite may 

represent another major event rather than part of a 

continuum. Wones (1974, 1976, 1985) identified a similar 

sequence of intrusive events in the Orrington-Liberty 

anticlinorium, Passagassawakeag terrane, eastern 

Maine. Zircon dates on material collected by Wones 

yielded an age of 410 Ma for the formation of muscoviteand garnet-rich migmatite in that terrane (Marvin and 

Dobson, 1979, p. 18-19). Rb-Sr whole-rock ages from the 

same terrane include an older age of 1,360 ±68 Ma and 

younger ages of 494 ±25 Ma and 426 ±27 Ma, the latter 

ages being not unlike those from the Andover. The 

similarities between the Passagassawakeag (including 

the Casco Bay Group) and Nashoba terranes have been 

recognized earlier (Hussey, 1968). If the ages of 408 to 

415 Ma for the Andover (table 13) are correct, then the 

younger Andover Granite is analogous to the Manaslu 

granite of Nepal (Le Fort, 1981) and may reflect thrusting followed by decoupling of the Avalon terrane as it 

moved westward under the eastern part of the Merrimack belt. These ages roughly correspond to the age of 

nappe formation in central Massachusetts and New 

Hampshire. It is important to note that the apparently 

younger diorites of the western and eastern Merrimack 

terrane have no counterparts in the Nashoba terrane.

INTRUSIVE ROCKS OF THE EASTERN PART OF 

THE MERRIMACK BELT

The intrusive rocks of the Merrimack belt east of the 

Wekepeke fault are dioritic to granitic intrusions of Late 

Ordovician to Early Devonian age (table 13) that lie 

primarily along the east side of the belt west of the 

Clinton-Newbury fault (fig. 27). Most of these intrusions 

are clustered in a zone from Clinton to the Pepperell 

area, where they consist primarily of the Ayer Granite 

and its facies. The Ayer Granite continues south of 

Worcester into Connecticut, where it is called the Canterbury (and formerly Eastford) Gneiss. In northeastern 

Massachusetts, the Newburyport Complex forms an 

isolated pluton. The Ordovician and Silurian granites 

tend to be calc-alkaline, and the Devonian granites tend 

to be at least in part peraluminous. We have undertaken 

no chemical studies of the rocks in the eastern part of the 

Merrimack belt, and our descriptions are all from the 

observations of others. Masses of Ayer Granite and its 

equivalents in Connecticut lie on both sides of the
72°00'

42°45' -

INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS

30' 15' 71W

151

70°45'

EXPLANATION

Pennsylvanian granite (Pgr

Devonian and Silurian granite Chelmsford Granite (Dcgr), Fitchburg 

Complex (Dfgr), and muscovite-biotite granite (Dmgr)

= « \ Devonian and Silurian diorite (DSdi) I = \\ » I

'^Vj'] Clinton facies of Ayer Granite (Sacgr) and granite of the 

Newburyport Complex (Sngr)

£££ Devens-Long Pond facies of Ayer Granite (SOad)

Ayer Granite (Sagr) and granodiorite of the Newburyport 

Complex (SOngd)

Metamorphic and sedimentary rocks

CONN " L_MASS_ACHUSETTS_ 

42°00' - ISLAND

FIGURE 27. Distribution of intrusive rocks in the eastern part of the Merrimack belt.

Wekepeke fault as it is projected south from the Worcester area, but all the Ayer and Canterbury granitic 

intrusions lie west of the Clinton-Newbury fault and its 

possible projection into Connecticut (see Pease, 1982, p. 

264, for example), indicating that the Clinton-Newbury 

fault is a more significant terrane boundary than the

Wekepeke. East of the Wekepeke fault, west of Clinton, 

in the part of the Merrimack belt we are describing, are 

a few small granite stocks of the Devonian Fitchburg 

Complex, whose main mass lies west of the Wekepeke 

fault. These stocks are the only intrusions in Massachusetts in the western part of the belt we are describing.
152 THE BEDROCK GEOLOGY OF MASSACHUSETTS

TABLE 16. Modes, in percent, of intrusive rocks of the Merrimack

belt east of the Wekepeke fault

[o, oligoclase; m, microcline]

Unit...................

Plagioclase ............

K-feldspar .............

. ... . 1

...... Sngr

...... 33

...... 35

...... 27

..... 5

2

SOngd

6-33

30-48

3-35

12-33

3

27

36o

281

8

4

SOad

26

40

29m

4

^0 percent as phenocrysts, 8.5 percent in groundmass.

2Mafic minerals unidentified in samples 1-2; includes biotite, chlorite, and opaque minerals in 

samples 3-4.

Description of samples

1. Granite of Newburyport Complex. Average of two point-counts of 

stained slabs.

2. Tonalite and granodiorite of the Newburyport Complex. Range of 

modes from point-counts of stained slabs.

3. Clinton facies of Ayer Granite. Average modal composition from 

Gore (1976).

4. Porphyroblastic variety of Devens-Long Pond facies of Ayer Granite. From Gore (1976).

NEWBURYPORT COMPLEX

Shride (1971) divided the Newburyport Complex into 

two facies: tonalitic granodiorite and granite. The tonalitic facies was originally termed the Newburyport 

Quartz Diorite, which also included dioritic rocks south of 

the Clinton-Newbury fault zone that are now called 

Sharpners Pond Diorite in the Nashoba zone and Topsfield Granodiorite in the Milford-Dedham zone. These 

correlations are not tenable on grounds of both age and 

composition. For this reason, the name Newburyport is 

restricted to the two facies in the Newburyport area. 

The complex forms a large mass near Newburyport (fig. 

27) and a small mass to the west of it. Both are truncated 

by the Clinton-Newbury fault.

TONALITE AND GRANODIORITE (SOngd)

The tonalite and granodiorite facies (SOngd) occupies 

the core of the Newburyport Complex at Newburyport 

and is intruded on the northwest by the granite facies. 

Medium- to dark-gray in fresh rock, the tonalite weathers to both green and red. Rock along joints has been 

altered to brick red. An age of 455 ±15 Ma was determined on zircon of the tonalite (table 13).

The rock is fine to medium grained and is highly 

variable in mineralogy (table 16, no. 2). The mafic 

minerals include hornblende, biotite, titanite, pyrite, and 

other opaque minerals. The euhedral biotite is usually 

completely chloritized. Ovoid inclusions are common.

GRANITE (Sngr)

The granite of the Newburyport Complex (Sngr) 

intrudes both the Kittery Formation and the tonalite and

granodiorite facies; it is at least 45 km2 in area. The 

granite (table 16, no. 1) is porphyritic and is light gray to 

dark gray, weathering to buff. Phenocrysts of K-feldspar 

(0.5-0.9 cm) are set in a medium-grained matrix. Kfeldspar is uncommon in the groundmass (Shride, 1976). 

Dioritic inclusions are common and, where oriented, 

define a foliation. No radiometric ages are available for 

this facies, so it is conceivable that the tonalite and 

granodiorite and the granite facies are distinctly different in age.

AYER GRANITE (Sagr)

Gore (1976) divided the Ayer Granite (Sagr) in its type 

locality into two facies, the Clinton facies (Sacgr) and the 

Devens-Long Pond facies (SOad). In addition, there are 

masses of granite to tonalite (Sagr), not assigned to 

either of these facies, that intrude the Berwick Formation (Sb) west and northwest of Lawrence (fig. 27) and 

masses that intrude the Paxton and Oakdale Formations 

(Sp, So) south of Worcester and west of the probable 

southern continuation of the Wekepeke fault. Radiometric ages obtained on the facies of the Ayer pose a problem 

in assigning ages to the unfossiliferous metasedimentary 

rocks that they intrude. The Clinton facies has a 

well-defined Early Silurian age and the Devens-Long 

Pond facies a similar age (table 13), which greatly 

compresses the time available for the deposition, burial, 

deformation, and metamorphism of the Berwick and 

Paxton Formations, if these units are truly Silurian. 

Some of the bodies labeled Sagr on the map might have 

been more properly correlated with the Lower Devonian 

Chelmsford Granite and muscovite-biotite granite at 

Millstone Hill. The bodies south of Worcester that 

intrude the Paxton Formation might more properly be 

correlated with the Canterbury Gneiss of Connecticut, 

which lies on strike and has an Early Devonian age of 

392 ±9 Ma (Zartman and Naylor, 1984). Zartman and 

Naylor (1984) believed that the Ayer Granite is of the 

same age range as the Newburyport Complex.

CLINTON FACIES (Sacgr)

The Clinton facies of the Ayer Granite (Sacgr) occupies 

35 km2 northwest of, and within, the Clinton-Newbury 

fault zone (fig. 27). The Clinton intrudes the Oakdale and 

Berwick Formations and, from Gore's (1976) map pattern, may intrude the Devens-Long Pond facies.

The Clinton facies of the Ayer Granite is a foliated, 

porphyritic, coarse-grained, light- to dark-gray granite 

(table 16, no. 3) that weathers to a buff color. Apatite, 

allanite, zircon, opaque minerals, and muscovite are 

present as primary accessory minerals. Biotite is partly 

altered to chlorite. Epidote, sericite, chlorite, titanite,
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 153

and carbonate minerals are alteration products. The 

mean composition of the K-feldspar megacrysts is Or80, 

and the Or-rich lamellae are maximum microclines (Gore, 

1976). The Clinton facies is different in texture and 

mineralogy from the Devens-Long Pond facies and could 

represent a distinct and different magmatic event.

DEVENS-LONG POND FACIES (SOad)

The Devens-Long Pond facies of the Ayer Granite 

(SOad) forms an elongate pluton of 45 km2 southeast of 

Pepperell (fig. 27), entirely west of bodies of the Clinton 

facies. The facies consists of both equigranular and 

porphyroblastic varieties. Both varieties are gneissic, 

whereas the Clinton facies is gneissic only in the ClintonNewbury fault zone. The contact relations between the 

Devens-Long Pond facies and the Oakdale and Berwick 

Formations are indeterminate. Gore (1976) thought the 

contact could be an unconformity. Goldsmith, following 

mapping by Robinson (1978, and written commun., 1978) 

in the Pepperell-Ayer area, made the contact with the 

Oakdale a fault on the State bedrock map. However, the 

contact relations of the Devens-Long Pond facies with 

the Berwick Formation are not determined. The map 

pattern suggests that the Devens-Long Pond facies is 

intruded by the Clinton facies, but no diagnostic outcrops 

have been observed. The Chelmsford Granite clearly 

intrudes the Devens-Long Pond facies (Gore, 1976).

The porphyroblastic variety (table 16, no. 4) typically 

is light to medium gray and exhibits granoblastic textures. Quartz and plagioclase grains are 3-8 mm long and 

porphyroblasts of microcline are 1-2 cm long. Accessory 

minerals are allanite, tourmaline, zircon, apatite, opaque 

minerals, and muscovite. Alteration minerals are chlorite, epidote, sericite, titanite, and carbonate minerals. 

The alkali-feldspar porphyroblasts have the bulk composition of Or95 (Gore, 1976). The equigranular variety has 

a similar mineralogy but ranges in composition from 

quartz monzonite (granite) to quartz diorite and in texture from homogeneous to inhomogeneous. Some of the 

rock contains schlierenlike bands (Gore, 1976). The 

gneissic texture of the rock gives it the appearance of 

being older than the radiometric age (433 ±5 Ma; see 

table 13) would indicate. Wones thought it resembles the 

Passagassawakeag Gneiss of Bickel (1976) in Maine. 

Gore's studies of the megacrysts of the Clinton facies and 

the porphyroblasts of the Devens-Long Pond facies 

demonstrated that the rocks have undergone different 

histories. Possibly the Devens-Long Pond facies should 

be a separate unit from the rest of the Ayer Granite.

DIORITE AND TONALITE (DSdi)

The diorite and tonalite designation (DSdi) includes all 

dioritic or gabbroic rocks that intrude the Berwick

Formation and the Ayer Granite and are intruded by 

dikes of two-mica granite that may be correlative with 

the Chelmsford. Two main masses one in Dracut 

(Dracut Diorite) and one east of Pepperell (fig. 27) are 

assumed to be correlative with the Exeter Diorite in 

New Hampshire that intrudes the Eliot Formation. 

Other smaller masses intrude the Berwick Formation 

(SOb) in northern Massachusetts and southeastern New 

Hampshire. The Exeter had been dated at 408 Ma 

(Bothner, 1974), but since the State bedrock map was 

prepared the Exeter has been found to be 473±37 Ma 

(Bothner and others, 1984; Gaudette and others, 1984). 

Dennen (1943) described the stock at Dracut as a 

norite, as it contains hypersthene, augite, hornblende, 

olivine, plagioclase (An37-An71), and opaque minerals. 

Plagioclase is interstitial or poikilitic to hypersthene and 

augite. Hornblende occurs as a late-stage magmatic 

mineral and as an alteration product (uralite) with biotite, sericite, chlorite, talc, serpentine, kaolin, and hematite. Opaque minerals include ilmenite, magnetite, pyrrhotite, chalcopyrite, and pentlandite. The Dracut 

Diorite has been mined for nickel from a pyrrhotitepentlandite-chalcopyrite assemblage.

CHELMSFORD GRANITE (Dcgr)

The Chelmsford Granite (Dcgr) intrudes the Berwick 

Formation, the Ayer Granite, and the Dracut Diorite 

northwest of the Clinton-Newbury fault. It occurs in two 

elongate bodies that strike parallel to the regional northeast trend and occupy an area of 60 km2 (fig. 27). 

Zartman and Naylor (1984) could not obtain a definitive 

age on the Chelmsford but believed it to be in the range 

of the Ayer Granite because the age would lie within the 

margin of error of their age determinations. The Devonian designation for the age of the Chelmsford given on 

the State bedrock map is based on a 207Pb/206Pb age of 

389 ±5 Ma (Zartman and Naylor, 1984). The difficulties in 

obtaining acceptable age determinations from the 

Chelmsford may be due to the pervasive ductile deformation in this granite.

The Chelmsford is light colored and gneissic; it consists 

of quartz, microcline, plagioclase (An5-An13), muscovite, 

and biotite. Zircon and garnet are accessory minerals, 

and epidote, chlorite, and titanite are alteration products. Much of the foliation in parts of the Chelmsford is 

produced by elongate quartz grains aligned parallel to 

the preferred orientation of muscovite and biotite; this 

foliation indicates the pervasive ductile deformation of 

the unit.

MUSCOVITE-BIOTITE GRANITE AT MILLSTONE HILL

(Dmgr)

A small stock of muscovite biotite granite (Dmgr) 

intrudes the Oakdale Formation at Millstone Hill in
154 THE BEDROCK GEOLOGY OF MASSACHUSETTS

Worcester (fig. 27). An Rb-Sr age of 372±7 Ma has been 

obtained from this rock (table 13).

FITCHBURG COMPLEX (Dfgr)

Three small stocks of muscovite granite that intrude 

the Worcester Formation (DSw) east of the Wekepeke 

fault and west of Clinton were mapped as Fitchburg 

granite by Peck (1975). The granite in these stocks 

contains muscovite, and most samples contain biotite. 

Tourmaline is a characteristic accessory mineral, according to Peck, and garnet, magnetite, apatite, and zircon 

are other accessory minerals. These stocks are shown as 

Fitchburg Complex (Dfgr) on the State bedrock map.

DISCUSSION OF THE INTRUSIVE ROCKS OF THE EASTERN 

PART OF THE MERRIMACK BELT

The oldest intrusion recognized in the eastern part of 

the Merrimack belt is the tonalite and granodiorite of the 

Newburyport Complex at 455±15 Ma. This calc-alkaline 

unit is older and richer in K-feldspar than the tonalite of 

the Sharpners Pond pluton in the Nashoba zone. The two 

facies of the Ayer Granite have an age similar to that of 

the Sharpners Pond Diorite in the Nashoba zone (table 

13). In New England, these early Silurian ages are 

unique to the eastern Merrimack belt. The diorites, 

gabbros, and norites of the Merrimack belt in Massachusetts are undated but may be equivalent to the Exeter 

pluton, which has now been dated as Early Ordovician. 

The youngest unit in this belt of plutons is the granite at 

Millstone Hill at 372 Ma (table 13). The Chelmsford 

Granite is a muscovitic granite thought to be equivalent 

in age to the granite at Millstone Hill because of its 

muscovite content, but the radiometric data are equivocal (Zartman and Marvin, this vol., chap. J). Zircons in 

the Chelmsford are dated at 430 Ma, but an Rb-Sr 

whole-rock age is 356 ±71 Ma. The margin of error of the 

Rb-Sr age permits the Chelmsford to be the same age as 

the Ayer. On the other hand, each of the ages of the 

Chelmsford may be approximately correct, reflecting 

both the original intrusion and the later deformation. The 

data of Zartman and Marvin make the equivalence in age 

of the Chelmsford to the granite at Millstone Hill uncertain but possible.

The igneous rocks of the eastern part of the Merrimack 

belt do not appear to correspond to those of the western 

part of the belt, nor to those in any other part of New 

England. The nearest possible equivalent group is in the 

Nashoba zone, where the Newburyport tonalite may be 

approximately equivalent to the Sharpners Pond tonalite, and the younger phase of the Andover Granite to the 

granite at Millstone Hill. However, the metamorphic

grade and lithologies of the host rocks make such a 

correlation highly unlikely. The host-rock lithologies 

(Merrimack Group, SOk, Se, Sb, and Oakdale Formation, So) of the Ayer and Newburyport intrusions in the 

eastern part of the Merrimack belt were considered by 

Goldsmith and Robinson to be similar enough to the 

Paxton Formation (Sp) and associated rocks in the region 

to the west to combine the two regions into a single 

Merrimack belt on the State bedrock map. Wones felt 

that the ages and nature of the plutonic rocks argue 

against a straightforward correlation, no matter how 

similar the lithologies of the nonfossiliferous host rocks. 

The ages of the Merrimack Group have been recently 

suggested to be Proterozoic Z (Lyons and others, 1982; 

Bothner and others, 1984) on the basis of conformity of 

the Berwick Formation of the Merrimack Group with the 

Proterozoic Z to Ordovician Massabesic Gneiss Complex 

(OZma) of southern New Hampshire and adjacent Massachusetts. If these relations are true, an older age for 

the Merrimack Group solves the problem of Ordovician 

rocks like the Newburyport Complex intruding a supposedly Silurian section. It may be that Gore's (1976) 

suggestion of an unconformity between the Oakdale 

Formation and the Devens-Long Pond facies of the Ayer 

Granite is correct and that the Merrimack belt east of the 

Wekepeke fault lies unconformably above the Nashoba 

terrane as Skehan and Murray (1980) suggested (see 

Robinson and Goldsmith, this vol., chap. G) and is 

somewhat older than Silurian. The relationship is complicated because east-directed thrusts in the ClintonNewbury fault system place Merrimack-belt rocks over 

the Nashoba-zone rocks (Skehan and Murray, 1980). 

Goldsmith believes that the Wekepeke fault is not a 

major terrane boundary but juxtaposes similar regions of 

a single terrane that were at different crustal levels. 

Both the Ayer Granite and the Fitchburg Complex lie on 

both sides of the Wekepeke fault.

The mineral compositions of the Ordovician and Silurian intrusions in the eastern part of the Merrimack belt 

resemble those derived from processes along convergent 

plate margins (Hepburn and others, 1987); these rocks 

may represent part of a volcanic-plutonic arc developed 

near or on a continental margin. The sequence of intrusion appears to be from mafic rocks and granodiorite to 

muscovite granite. The Devonian plutons, because they 

tend to be two-mica granites, were derived from more 

sedimentary source material than the older rocks, and 

their ages indicate that they were at least in part 

emplaced during Acadian deformation and metamorphism (Robinson and Hall, 1980). The source for the 

Devonian plutons, like that for the intrusions of the 

Nashoba zone, was probably a westward-subducting 

Avalonian plate (Wones, 1984).
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 155

INTRUSIVE ROCKS OF EASTERN

MASSACHUSETTS AND PLATE-TECTONIC

MODELS

The contrast in both the age and the style of the 

intrusive rocks of the Milford-Dedham and Nashoba 

zones and eastern part of the Merrimack belt that form 

eastern Massachusetts indicates that eastern Massachusetts was consolidated over a period of time from at least 

three different terranes (Hatch and others, 1984; Hill and 

others, 1984). The ages of the intrusive rocks permit 

reconstruction of events in time, and the compositions of 

the rocks permit an estimation of the tectonic setting. 

Some of these matters have been discussed elsewhere in 

this chapter; here we consider them in reference to 

plate-tectonic models. A number of analyses of the 

structural framework of New England with reference to 

lithotectonic belts and accretionary terranes have been 

published in recent years (Osberg, 1978; Robinson and 

Hall, 1980; Hall and Robinson, 1982; Williams and 

Hatcher, 1983; Zen, 1983). We do not present an exhaustive discussion of these matters here but only point out 

how the intrusive rocks of eastern Massachusetts constrain the formation of eastern Massachusetts.

The Proterozoic Z mafic volcanic-plutonic rocks represented by the diorite and gabbro (Zdigb, Zdi, Zgb) and 

the mafic and felsic volcanic rocks (Zv, Zvf) in the 

Milford-Dedham zone represent part of a volcanicplutonic arc that was subsequently intruded by calcalkaline granitic rocks of batholithic dimensions, the 

Dedham Granite (Zdgr) and similar rocks. Some evidence exists that the rocks of the mafic complex were 

metamorphosed before intrusion by the 630-Ma granites 

(Goldsmith, this vol., chap. H). Mosher (1983) suggested 

a back-arc setting for the Blackstone Group of northern 

Rhode Island, which we consider to be part of the 

prebatholithic complex. The sparsity of tonalite and 

granodiorite in the Proterozoic Z batholithic rocks suggests that they were intruded into a sialic continental 

craton.

After the intrusion of the batholiths, a graben developed in the Boston area accompanied by bimodal volcanism, which formed the Mattapan and Lynn Volcanic 

Complexes (Zm, DZ1) and Brighton Melaphyre (ftZrb), 

and further intrusion of granite, the Westwood (Zwgr). 

During the early and middle Paleozoic, the MilfordDedham terrane acted as a stable platform intruded by 

alkaline and peralkaline granite and gabbro under conditions of crustal extension. These intrusions correspond to 

continental intraplate activity and could be construed as 

being associated with a failed rift. Following intrusion of 

Devonian plutons, intrusive activity ceased in eastern 

Massachusetts although some bimodal volcanism (Pwv) 

accompanied deposition in the Narragansett basin in the 

Pennsylvanian. However, Permian granite was intruded

in southern Rhode Island and in the Massabesic Gneiss 

Complex of southern New Hampshire and adjacent Massachusetts following compressive deformation of the 

Pennsylvanian strata (Mosher, 1983). A high thermal 

regime (Zartman and others, 1970) and compressive 

deformation during the Permian may have produced the 

gneissosity in the Proterozoic Z rocks of the New Bedford area.

The Nashoba zone shares none of this plutonic history. 

The peraluminous Andover Granite (SOagr) intruded 

metamorphosed slope-facies sediments and off-arc volcanic rocks of the Marlboro and Nashoba Formations that 

are for the most part unlike the rocks of the MilfordDedham zone. The Ordovician, gneissic phase of the 

Andover, an S-type granite (Chappell and White, 1974), 

was emplaced at considerable depth, and the Nashoba 

Formation is at high metamorphic grade. No Proterozoic 

intrusive rocks are known in the Nashoba zone in eastern 

Massachusetts. The Silurian intrusive rocks of the Nashoba zone are I-type quartz-diorite to granite and are 

quite different from the middle Paleozoic alkaline granites of the Milford-Dedham zone. Hill and others (1984) 

have shown that the Silurian intrusive rocks of the 

Nashoba zone are isotopically heterogeneous; they interpreted this as indicating that the intrusions developed at 

a convergent plate boundary and assimilated varying 

proportions of Proterozoic Z to early Paleozoic crustal 

rock. There is some evidence, discussed above in this 

chapter, that the granite and pegmatite that intruded the 

rocks of the Nashoba zone in Devonian time were peraluminous like the Ordovician gneissic phase of the 

Andover Granite, suggesting a recurrence of the crustal 

conditions that produced the earlier granite. The difference in the Paleozoic intrusive rocks in the two zones 

indicates that they were derived from different crust 

under different conditions at least until after most of the 

Devonian and accordingly must have become joined after 

that time.

The intrusions in the eastern part of the Merrimack 

belt bear a greater similarity to the intrusions in the 

Nashoba zone than the intrusions of the Nashoba zone do 

to those of the Milford-Dedham zone, except that there is 

no equivalent to the Ordovician peraluminous Andover 

Granite. The early intrusions, the Newburyport Complex and part of the Ayer Granite, are calc-alkaline. The 

peraluminous muscovitic granites in the Merrimack belt 

are Devonian in Massachusetts, although there is some 

question as to the age of the Chelmsford Granite. Carboniferous peraluminous granite does form plutons in 

southern Maine and New Hampshire, however (Gaudette and others, 1982; Hayward and Gaudette, 1984). The 

Devonian granites of the eastern part of the Merrimack 

belt, such as the Ayer and its equivalent the Canterbury 

Gneiss of Connecticut, tend to be sheetlike, gneissic,
156 THE BEDROCK GEOLOGY OF MASSACHUSETTS

and involved in the regional dynamothermal metamorphism; they did not produce recognizable hornfels aureoles in the host rocks. The rocks of the eastern part of the 

Merrimack belt in Massachusetts were affected by the 

Acadian metamorphism that predominates in central 

Massachusetts (Hall and Robinson, 1982). In contrast, 

the Milford-Dedham zone and most of the Nashoba zone, 

except possibly the west flank, show no indication of an 

Acadian dynamothermal metamorphic event. The 

recently recognized Devonian peraluminous granite and 

pegmatite associated with the Late Ordovician and Silurian Andover Granite may reflect an Acadian event, 

however, and suggest that at this time a source for the 

magmas of the eastern part of the Merrimack belt may 

have been Nashoba-zone material at depth and at high 

grade during the Silurian. Later faulting along the 

Clinton-Newbury fault has separated the Nashoba-type 

rocks beneath the Merrimack-belt rocks from the rocks 

now forming the Nashoba zone at the surface.

The age and nature of the intrusive rocks, then, 

indicate that the eastern part of the Merrimack belt and 

the Nashoba belt may have been joined together by the 

end of the Devonian but that the Milford-Dedham zone 

may not have been accreted to the Nashoba until after 

the Devonian. Continental crust was the major source for 

the Paleozoic intrusive rocks of the Milford-Dedham 

zone, the Nashoba zone, and the east part of the Merrimack belt, but the crust differed in kind and level at 

different times within and between each zone. The 

Nashoba and Milford-Dedham zones represented separate crustal blocks, if not microcontinents, early in the 

Paleozoic although at one time, elsewhere, they might 

have been part of a single plate before being split and 

migrating differentially to arrive at their present positions. We suggest that rocks similar to those in the 

Nashoba zone are the basement on which the Merrimackbelt rocks were deposited. The Massabesic Gneiss Complex within this belt resembles the rocks of the Nashoba 

zone. However, if the strata in the east flank of the 

Merrimack belt are as old as the intrusions indicate, they 

may be a more basinward facies of Nashoba-zone rock 

and may represent the dislocated basin-fill of a progenitor basin to the subsequent Silurian and Devonian trough 

of the Merrimack synclinorium.

The intrusive rocks of Massachusetts have a complex 

history because they occupy different lithotectonic belts, 

whose relations have changed through time in response 

to plate-tectonic processes. The earlier Proterozoic Z 

mafic intrusive rocks and the later Proterozoic Z granitic 

batholithic rocks of the Milford-Dedham zone are the 

products of a subduction-related cycle of magma generation. The latest Proterozoic Z volcanic rocks, such as the 

Mattapan and Lynn, could be related to a rifting center. 

The anhydrous peralkaline magmas that formed the

Paleozoic plutons of the Milford-Dedham zone are the 

result of subsequent heating of crust under the influence 

of underplating of crust in the subduction process during 

the Paleozoic.

The Nashoba zone has undergone a more repetitious 

history, during which there are renewed cycles of activity. The older and younger phases of the Andover 

Granite were probably formed from magmas generated 

from added new sedimentary crustal material during the 

subduction process, whereas the intermediate-in-time 

Assabet Quartz Diorite, the Sharpners Pond Diorite, and 

the orange-weathering granite represent a cycle of magmas derived from mantle material.

The origin and source for the calc-alkaline Paleozoic 

plutons of the eastern part of the Merrimack belt are 

somewhat more enigmatic, but there seems to be a mafic 

to felsic trend with time. This is interpreted as meaning 

that early, predominantly mantle-derived plutons were 

succeeded by plutons derived from continental crust. The 

sequence of plutons probably formed near a convergent 

plate boundary during subduction, although Wones 

(1984) ascribed the Devonian plutons farther west and 

north in Vermont, New Hampshire, Massachusetts, central Maine, and coastal Maine to underplating of crustal 

material. The Carboniferous and late Paleozoic peraluminous intrusions of adjacent New Hampshire may be 

derived from overridden crust during westward thrusting of material during late Paleozoic plate collision.

REGIONAL RELATIONS

The Milford-Dedham zone is one of the Appalachian 

terranes considered to be Avalonian by Williams and 

Hatcher (1983). It does, however contain much more 

intrusive rock and much less extensive, and thinner, 

Proterozoic Z and Paleozoic supracrustal strata than the 

Avalonian terrane in Newfoundland. Other Avalonian 

terranes in the Appalachians have been recognized in 

New Brunswick and Nova Scotia (East and others, 

1976b; Skehan and others, 1978; East, 1980; East and 

Skehan, 1981) and in the southeastern Piedmont of the 

United States (St. Jean, 1973; Seiders, 1978).

The intrusive rocks of the Milford-Dedham zone are in 

some respects similar in lithology, age relations, and 

setting to those of the Charlotte belt of North Carolina 

and South Carolina (Goldsmith and others, 1988). In the 

Charlotte belt of North Carolina, Proterozoic Z calcalkaline plutonic rocks of batholithic dimensions intrude 

metavolcanic and metasedimentary rocks, which are 

present as roof pendants, screens, and mantling 

sequences. These granitoids consist primarily of quartz 

diorite to granodiorite, with lesser amounts of gabbro 

and diorite, an assemblage less mafic than the prebath-
INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 157

olithic, Proterozoic Z mafic plutonic rocks of the MilfordDedham zone but more mafic than the granitoids of the 

Dedham batholith and Milford antiform. In the Charlotte 

belt, discrete plutons of early to middle Paleozoic age 

include syenite and gabbro, an assemblage found also in 

the Milford-Dedham zone in Massachusetts. However, 

other early to mid-Paleozoic granites in the Charlotte 

belt are not particularly alkaline (Butler and Ragland, 

1969; Butler and Fullagar, 1978), in contrast to the early 

to mid-Paleozoic granites of the Milford-Dedham zone. In 

addition, the Charlotte belt contains large masses of late 

Paleozoic muscovite-bearing granite (Speer and others, 

1980) not found in the Milford-Dedham zone except in 

southern Rhode Island. The Paleozoic intrusions in the 

Charlotte belt of North Carolina are types that are 

present over a wide area of central and eastern New 

England and not specifically confined to the MilfordDedham zone. Although the Charlotte belt does not have 

the Proterozoic Z and Paleozoic sedimentary basins that 

are found in the Milford-Dedham zone, Proterozoic Z to 

Cambrian rocks, both sedimentary and volcanic, which in 

many respects are similar to those in and around the 

Boston basin and which truly correlate with rocks of the 

Avalon Peninsula, Newfoundland, are present in the 

adjacent Carolina slate belt.

The intrusive rocks of the Nashoba zone can less 

clearly be correlated with the intrusive rocks of similar 

lithotectonic terranes in the Appalachians. As pointed 

out above, the younger peraluminous intrusive rocks of 

the Nashoba zone are similar in composition and field 

relations to intrusive rocks in the Passagassawakeag 

block in southeastern Maine (Bickel, 1976; Osberg and 

others, 1984) as described by Stewart and Wones (1974, 

p. 231) and Kaszuba and Wones (1985); an older synmetamorphic phase, the Winterport Granite (Stewart and 

Wones, 1974), similar to the gneissic phase of the 

Andover Granite, is also present. Rocks like the Silurian 

Sharpners Pond Diorite, however, seem to be absent in 

this block in Maine. The host rocks are similar, however, 

in lithology and metamorphic grade to those in the 

Nashoba zone. The two terranes are not on strike with 

each other and are inferred to be separate slices or blocks 

of what once was a single lithotectonic terrane. They 

apparently did not share all aspects of early Paleozoic 

intrusive history. Terranes similar to the Nashoba are 

present in the Maritime Provinces in Canada (Rast and 

others, 1976a), and some observers (for example, 

Williams and Hatcher, 1983) have placed the Nashoba 

zone within the Gander zone as defined in Newfoundland. 

Similar terranes have not been identified in the southern 

Appalachians.

The intrusions into the eastern flank of the Merrimack 

belt in Massachusetts are only in part similar to those in 

the eastern part of the Merrimack belt of Maine. The

older Ordovician to Silurian intrusions are not found 

there, and only Devonian granite forms plutons (Osberg 

and others, 1984). The synkinematic fabric and sheetlike 

nature of some of the Devonian intrusions in Massachusetts and Connecticut (Dixon and Pessl, 1966; Snyder, 

1967; Pease, 1972; Tucker, 1977; Maczuga, 1981) correspond to the style of emplacement of Devonian plutons in 

the Merrimack belt farther north in New England (Hayward and Gaudette, 1984), but the metamorphic evidence indicates that the southern plutons were emplaced 

at a slightly deeper structural level than the northern 

ones.

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Radiometric Ages of Rocks in 

Massachusetts

By ROBERT E. ZARTMAN and RICHARD F. MARVIN 

THE BEDROCK GEOLOGY OF MASSACHUSETTS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-J

CONTENTS

Page

Abstract..................................................................................................................... Jl

Introduction................................................................................................................ 1

Published studies......................................................................................................... 1

Critical evaluation........................................................................................................ 2

Tables of abbreviated RADB records.............................................................................. 3

References cited.......................................................................................................... 4

Appendix: Radiometric Age Data Bank ........................................................................... 18

ILLUSTRATION

FIGURE 1. Map of Massachusetts showing locations of radiometrically dated samples ........... J2

TABLES

[Tables follow References Cited]

Page

TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts........... J6

2. Published zircon Pb-alpha and apatite fission-track ages of rocks from

Massachusetts........................................................................................ 17

THE BEDROCK GEOLOGY OF MASSACHUSETTS

RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS

By ROBERT E. ZARTMAN and RICHARD F. MARVIN

ABSTRACT

In this chapter, K-Ar, Rb-Sr, U-Pb, Pb-alpha, and fission-track 

analyses published through 1986 are tabulated and critically evaluated 

for use in assigning geologic ages to the rock units of Massachusetts. 

The compilation was facilitated by the use of the U.S. Geological 

Survey's Radiometric Age Data Bank (RADB), a computer-based data 

storage and retrieval system for geochronological information. Approximately 400 individual radiometric ages are contained herein in a 

synoptic table derived from the RADB. Each entry is identified 

according to lithotectonic zone, town or village, latitude and longitude, 

rock unit, analyzed mineral(s), method of dating, an interpretive 

coding, and reference.

A complete RADB record is described in the appendix, which 

illustrates how the various information fields are arranged and can be 

interrogated. A printout of all RADB records for Massachusetts is 

available as U.S. Geological Survey Open-File Report 87-170.

INTRODUCTION

Geologic interpretation requires that stratigraphic 

units and structural elements be placed into a time 

framework of sufficient accuracy to resolve the sequence 

of events affecting the rocks. Age assignments are made 

both in a relative sense, by determining the order of 

interrelated features, and in an absolute sense, by paleontologic and radiometric dating. In a State as geologically complex as Massachusetts, many problems arise 

when we attempt to construct such a time framework. 

Long-range correlation with fossil localities has sometimes been made over distances of hundreds of kilometers, even where mapping is inadequate and stratigraphic continuity is, at best, uncertain. The juxtaposing 

of distinct tectonic blocks across major faults further 

inhibits the extrapolation of meager stratigraphic control. Indeed, entire rock sequences are now known to 

abruptly terminate at such boundaries.

The purpose of this chapter is to provide a general 

documentation for the radiometric ages from MassachuManuscript approved for publication November 16, 1987.

setts that bear on the time framework of primary stratigraphic units and superimposed metamorphic fabrics. 

To this end, a tabulation (table 1) has been made of all 

published ages that were used for compilation of the 

State bedrock map (Zen and others, 1983). This table 

thus serves as (1) an abbreviated summary of the radiometric ages and (2) a link to the original literature. The 

specific application of these data in establishing a relative 

and absolute chronology within each compilation area is 

incorporated into the appropriate chapters elsewhere in 

this professional paper.

Although modern isotopic geochronology spans barely 

three decades, a vast body of analytical data has already 

accumulated. To take proper advantage of the radiometric ages pertinent to this study, it was necessary to 

catalogue the published literature bearing on the isotopic 

dating in Massachusetts and to critically evaluate these 

data. Particular attention was given to providing time 

control on the primary stratigraphic ages of rock units 

and to recognizing patterns of metamorphic overprinting 

that are recorded by disturbed or recrystallized mineral 

systems. A comprehensive treatment of the rock chronology, of course, cannot be carried out independent of 

firm paleontologic and stratigraphic correlations, and the 

age assignments accompanying the geologic map represent an attempt to accommodate all evidence. Generally, 

if the responses of the various dating methods, the 

regional pattern of metamorphic overprinting, and other 

geologic constraints are carefully examined, even very 

complex isotopic systematics can contribute important 

age control.

PUBLISHED STUDIES

Approximately 400 individual radiometric ages have 

been reported in the literature for rocks of Massachusetts. As of the end of 1986, these data included 140 

K-Ar, 174 Rb-Sr, 49 U-Pb, 32 Pb-alpha, and 4 fissiontrack analyses determined on 300 separate samples. A

Ji
J2

43°

74°

THE BEDROCK GEOLOGY OF MASSACHUSETTS

72° 71° 70°

42°

FRANKLIN

o °°v

~^r

dp °»,L«

L_. y._-o f WORCESTER o r

\ fl ©o Co MIDDLESEX 

HAMPSHIRE \Jl_ >-"%. 0(o~""V

ji n J 9 ( ' °^ o ° °| ^^ /L-^^^-/ ( I

o o en ^ 8 ^ ^-"1 O\ v*" nvyni'vyi-n. ^> j

HAMPDEN ° '

0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS

"s \ NANTUCKET

FIGURE 1. Locations of radiometrically dated samples, Massachusetts.

similar number of ages from adjacent States also bear 

directly on a regional time framework; they were evaluated during compilation of the map, but only Massachusetts localities are contained in the tabulation of this 

chapter. Taken at face value, the radiometric ages are a 

complex mixture of chronometric systems, some yielding 

the time of primary deposition of strata or emplacement 

of plutons, others recording various episodes of metamorphic overprinting, and a substantial number providing ambiguous or minimally useful information. They 

represent the measurements of many different rock 

types, tectonic environments, and grades of metamorphism found within this statewide transect of the Appalachian orogen (fig. 1). Only after careful evaluation of 

interrelated sets of data, often containing apparent contradictions, could we hope to construct a significantly 

precise chronology of events. Although this goal has 

certainly not yet everywhere been reached, many 

revised or confirmed age assignments incorporated into 

the new State map arise either entirely or in large part 

from modern radiometric dating.

Some years ago it was realized that the manual recovery of all data bearing on a specific age assignment was 

becoming increasingly difficult because of the rapidly

growing quantity of radiometric age information. The 

ever-expanding literature on the subject, which includes 

easily overlooked articles in obscure publications and 

geochronologic data buried in broader studies, demanded 

that abstracted records of radiometric ages be systematically stored by a computer for more ready identification and retrieval. The Radiometric Age Data Bank 

(RADB) was established by the U.S. Geological Survey 

(Zartman and others, 1976) to fulfill this objective. Its 

application in compiling the pertinent information for the 

geologic map of Massachusetts represents the first 

attempt to use the RADB on a comprehensive statewide 

basis. Additional information about the RADB is given in 

an appendix to this chapter, in which a record, the basic 

unit of the data storage and retrieval system, is 

described in some detail. Tables 1 and 2 have been 

abstracted and compiled from the inventory of all Massachusetts records. A printout of all RADB records for 

Massachusetts was prepared by Zartman and Marvin 

(1987).

CRITICAL EVALUATION

If the observed parent and daughter isotopic contents 

of a whole-rock or mineral sample were determined
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS J3

solely by radioactive transformation subsequent to its 

formation, a calculated "age" would always relate 

straightforwardly to the origin of that rock or mineral. 

However, because physicochemical conditions also influence the behavior of these isotopic species, some amount 

of interpretation is usually necessary in order to translate calculated "ages" into a meaningful time framework. 

Although for a rock having a simple history, such as a 

rapidly cooled granitic rock that encounters no subsequent metamorphism after its initial crystallization, all 

the radiogenic systems would ideally yield the correct 

crystallization age, this situation rarely prevails for the 

rocks of a polymetamorphic terrane. In fact, many of the 

geochronologic studies in southern New England make 

sense only after one recognizes the different responses to 

subsequent metamorphism of isotopes used in the various dating methods. Success in deciphering the complex 

age patterns found even within a single rock unit relies 

heavily on an understanding of both the relative stability 

of the radiogenic systems during later thermal and 

hydrothermal events and the constraints placed on each 

sample by the local and regional geologic setting.

What, then, are the distorting factors that can cause a 

calculated "age" of, say, an igneous rock to deviate from 

the true age of crystallization? Basically, two types of 

problem may beset any radiometric system: (1) the initial 

presence of daughter isotopes in the crystallizing rock or 

mineral and (2) movement of parent or daughter isotopes 

into or out of the rock or mineral after its original 

crystallization. Examples of the former are radiogenic 

argon trapped into a newly forming pyroxene and an 

overgrowth of zircon nucleating on an inherited core of 

the same mineral. Examples of the latter are the diffusive loss of radiogenic argon from a biotite during 

thermal metamorphism and the metasomatic removal of 

radiogenic strontium from a retrogressively altered 

rock. All of these complicating processes were operative, 

at least locally, during the geologic development of 

Massachusetts and have affected the radiometric systems to varying degrees. It is not surprising, therefore, 

that early attempts to establish the ages of rocks were 

sometimes unsuccessful because neither the stratigraphic and structural complexities nor the vagaries of 

the chronometers were fully appreciated. Only as modern field mapping revealed the true nature of the geology 

and as the stability ranges of the various dating methods 

were determined have studies been designed to produce 

a time framework of necessary accuracy.

Particularly valuable for deciphering the temporal 

relations within erogenic terranes, such as the Appalachians, is the intercomparison of "ages" obtained from 

several radiometric systems with quite different 

responses to superimposed physicochemical conditions. 

Although it has proven difficult to quantify precisely the

temperature, pressure, and chemical environment necessary to reset a given radiometric system, a relative 

ranking and semiquantitative calibration for each system 

have evolved from field and laboratory attempts to 

determine the activation energies associated with diffusion and recrystallization. In practice, however, there 

are rarely enough geochronologic data to permit a thorough intercomparison of radiometric systems involving 

numerous dating methods and many sample localities for 

a rock unit. Rather, the recognition of isotopic age 

patterns has often been haphazard. The synthesis of a 

number of separate studies may be required to sort out 

the effects of primary crystallization and later metamorphic overprintings. Eventually, the accumulating body of 

data allows one to identify and delineate areas in which, 

say, all K-Ar analyses of biotite give a Permian thermal 

event age even for host rocks formed at a much earlier 

time. Likewise, certain granitic rocks may contain 

ancient inherited zircon in addition to newly crystallizing 

zircon where magmas have penetrated a considerably 

older sialic basement. When brought to light, these 

patterns not only contribute to an increased understanding of the time dimension of geologic processes but also 

provide information about the physical and chemical 

conditions surrounding these processes.

TABLES OF ABBREVIATED RADB RECORDS

All pertinent radiometric age data for rock units of 

Massachusetts and relevant parts of adjacent States 

were reviewed in conjunction with the preparation of the 

State bedrock map. A judgment based on this information together with paleontologic and stratigraphic evidence has resulted in the age assignments appearing on 

the map and in this report. Documenting this effort are 

two tables giving the abbreviated RADB records of 

K-Ar, Rb-Sr, and U-Pb ages (table 1) and Pb-alpha 

and fission-track ages (table 2) for the Massachusetts 

localities.

The records in each table are arranged by increasing 

geologic age within lithotectonic zones (Hatch and others, 1984) in a west-to-east progression across the State. 

Record locality is identified geographically by town or 

village, latitude, and longitude. Rock units are named 

according to the terminology of the State bedrock map, 

which may be revised from the original author's usage. 

Map symbols of rock units are also included except where 

the dated rock is not given map status (for example, 

pegmatite, dike, and minor intrusion). Each record contains one or more radiometric ages listed under the 

appropriate dating method by mineral analyzed, numerical value and uncertainty of calculated or interpreted 

age, and an explanatory symbol. This latter explanatory 

symbol serves the very important function of providing
J4 THE BEDROCK GEOLOGY OF MASSACHUSETTS

some critical evaluation of the radiometric age data. 

Generally, the critical evaluation was made by the original authors, but sometimes subsequent information 

available at the time that the RADB record was being 

compiled has resulted in a reevaluated interpretation. 

Accordingly, one can recognize at a glance those ages 

that closely constrain the primary stratigraphic time 

framework or that record some superimposed disturbing 

event.

The Pb-alpha ages in table 2 are not well defined 

analytically or geologically, and they are presented here 

mainly for historical purposes and completeness of the 

compilation. The four fission-track ages in table 2 record 

times of final uplift and cooling when the terrane experienced broad upwarping long after igneous and metamorphic activity of the Paleozoic orogenies had ceased.

The last column of tables 1 and 2 gives reference to the 

publication from which the RADB record was compiled. 

The reader should go to this primary literature for more

information about the dated sample and for a detailed 

explanation of its geologic interpretation.

REFERENCES CITED

Hatch, N.L., Jr., Zen, E-an, Goldsmith, Richard, Ratcliffe, N.M., 

Robinson, Peter, Stanley, R.S., and Wones, D.R., 1984, Lithotectonk assemblages as portrayed on the new bedrock geologic map 

of Massachusetts: American Journal of Science, v. 284, p. 

1026-1034.

Steiger, R.H., and Jager, E., 1977, Convention on the use of decay 

constants in geo- and cosmochronology: Earth and Planetary 

Science Letters, v. 36, p. 359-362.

Zartman, R.E., Cole, J.C., and Marvin, R.F., 1976, User's guide to the 

Radiometric Age Data Bank (RADB): U.S. Geological Survey 

Open-File Report 76-674, 77 p.

Zartman, R.E., and Marvin, R.F., 1987, Radiometric ages on file in the 

Radiometric Age Data Bank (RADB) of rocks from Massachusetts: 

U.S. Geological Survey Open-File Report 87-170, 421 p.

Zen, E-an, editor, and Goldsmith, Richard, Ratcliffe, N.M., Robinson, 

Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 

of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 

scale 1:250,000.
TABLES 1 AND 2 AND APPENDIX
TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts

[All radiometric ages are given in terms of the decay constants recommended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger a

symbol indicates that the radiometric system records an age interpreted to be as follows: x, within 5 percent of the primary age of intrusion or volcanism; xx, slightly dist

probably within 10 percent of the primary age of intrusion or volcanism; xxx, significantly disturbed by later metamorphism, weathering, or other chemical open-system 

too old due to the presence of an inherited component of daughter isotope. Ref., reference from list at end of table. , not determined; Do., do., ditto]

No. Town or village Lat, N. Long, W. Rock unit

Mineral

K-Ar Rb-Sr 

. Explanatory ,,. . . Explanatory 

Age ii Mineral Age . . 

symbol symbol 

Taconic-Berkshire zone

1 

2 

3

4

5

6

7

8

9

10 

11

12

13

14

15 

16 

17

18

19

20

21

22

23

Becket 

Otis 

Do. 

Becket

Granville

Tolland

Hancock

Great Barrington

Tyringham

East Lee 

Savoy Center

Do.

Do.

Windsor

Williamstown 

Dalton 

Great Barrington

Lee

East Lee

Otis

West Otis

Great Barrington

Tolland

42°15'10" 

42°14'03" 

42°13'55" 

42°14'42"

42°03'20"

42°05'00"

42°31'18"

42°15'05"

42°14'30"

42°21'30" 

42°37'15"

42°36'50"

42°36'45"

42°28'45"

42°42'48" 

42°27'30"

42°18'00"

42°16'15"

42°11'15"

42°11'30"

42°12'54"

42°07'22"

73°00'31" 

73°04'15" 

73°01'22" 

73°00'59"

72°57'47"

72°59'00"

73°19'12"

73°17'06"

73°12'45"

73°12'30" 

73°05'00"

73°04'30"

73°03'15"

73°07'40"

73°13'06" 

73°10'00" 

73°18'50"

73°13'30"

73°08'30"

73°03'45"

73°09'30"

73°17'32"

72°59'41"

Granite (Ogr) 

do. 

do. 

do.

do.

Gabbroic pegmatite

Walloomsac (Ow)

Tyringham Gneiss

(Ytg). 

do.

Aplite in Tyringham 

Gneiss (Ytg). 

Stanford Granite

Gneiss (Ysg). 

do.

do.

do.

do. 

Granite gneiss 

Gneiss

Lee Gneiss (Yl)

do.

do.

do.

Washington Gneiss

(Yw).

do.

-

Biotite 

Hornblende

Muscovite

Biotite

Hornblende

Biotite 

do.

do.

do.

do.

do. 

do. 

do. 

Hornblende

do.

do.

do.

do.

-

_

363±11 

381 + 12

375±13

402±4

404 ±5

414+15 

380+4

363±4

351+3

357±4

395 ±14 

422±5 

398±3 

415±5

467+5

447±6

364+4

406+5

- -

 

xxx 

XX 

xxx 

 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

 

 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 1

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 

- - 1

 1

1 Rowe

2 Do.

3 Do.

4 Do.

5 Chester

42°41'00" 72°52'00"

42°41'00" 

42°41'00" 

42°41'00" 

42°16'40"

72°52'00" 

72°52'00" 

72°53'00"

72°58'55"

Hawley Formation

(Oh), 

do. 

do. 

do. 

Moretown Formation

(Om).

Rowe-Hawley zone

Muscovite 351 ±10

Biotite 

Hornblende 

Biotite 

Muscovite

344±12 

318±9 

507+15 

338±10

xxx 

xxx 

xxx

XX
TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W. Eock unit

Mineral

K-Ar Eb-Sr 

. Explanatory .,. . , Explanatory 

Age ^ u i Mineral Age ^ J 

symbol symbol 

Rowe-Hawley zone Continued

6

7

Do.

Do.

42°16'30"

42°16'55"

73°00'00"

72°58'45"

Rowe Schist (0 r)

do.

Biotite

Margarite

Chlorite

321±10 xxx - - 

306±30 xxx - - 

494 xxx 

Connecticut Valley Mesozoic basin

1

2

3

South Hadley

Holyoke

Deerfield

42°18'06"

42°13'00"

42°32'00"

72°31'35"

72°40'00"

72°35'00"

Holyoke Basalt (Jhb)

do.

Deerfield Basalt 

(Jdb).

Plagioclase

Whole-rock

do.

186±7 xx - 

197±6 x - - - 

195+6 x - - - 

Connecticut Valley synclinorium

1

2

3

4 

5

6

Middlefield

Belchertown

Do.

Do.

New Salem

Greenfield

42°20'57"

42°11'13"

42°11'13"

42°10'33" 

42°25'00"

42°36'00"

72°57'58"

72°22'43"

72°22'43"

72°22'55" 

72°22'00"

72°39'00"

Middlefield Granite 

(Dmg).

Belchertown Complex (Dbmd).

Belchertown Complex (Dbmdg).

do. 

Prescott Complex

(Dpgg).

Gile Mountain Formation (Dgm).

-

Hornblende

do.

Whole-rock

 Biotite 358±15 xx 

- - - - - 

325±6 xxx - 

361±6 xxx - 

 Whole-rock 377±20 x 

327±9 xxx - 

Bronson Hill zone

1

2

3

4

5

6

7

8

9

10

11

12

13

Ashburnham

Palmer

Do.

Hampden

Erving

Orange

Do.

Do.

Do.

Do.

Do.

Do.

Do.

42°42'00"

42°09'30"

42°09'30"

42°02'00"

42°37'08"

42°38'25"

42°38'25"

42°38'25"

42°38'25"

42°38'25"

42°35'35"

42°35'35"

42°35'35"

71°56'00"

72°22'00"

72°22'00"

72°24'00"

72°21'54"

72°19'45"

72°19'45"

72°19'45"

72°19'45"

72°19'45"

72°16'43"

72°16'43"

72°16'43"

Partridge Formation

(Ops). 

Glastonbury Gneiss 

(Ogl).

do.

do.

Pauchaug Gneiss

(Ope).

Ammonoosuc Volcanics (Oa).

do.

do.

do.

do.

do.

do.

do.

Biotite

Muscovite

258±8 xxx - - 

238+8 xxx - 

- - - - - 

 - Whole-rock 375+80 xxx 

 - do. 375±80 xxx 

 

- do. 450±15 x 

- - do. 450±15 x 

 do. 450±15 x 

 do. 450±15 x 

- do. 450±15 x 

- - do. 450±15 x 

- - do. 450±15 x 

- - do. 450±15 x 
J8 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W.

K-Ar

Rock unit Explanatory . , Mineral Age , . Mineral 

symbol

Rb-Sr 

. Explanatory 

symbol 

Merrimack synclinorium Continued

23

24

25

26

27

28

29

Ayer

Boylston

Bolton

Do.

West Chelmsford

Salisbury

Leominster

42°33'48"

42°21'12"

42°25'04"

42°25'00"

42°37'00"

42°51'21"

42°31'08"

71°34'30"

71°43'18"

71°39'23"

71°39'00"

71°24'00"

70°57'24"

71°44'17"

Ayer Granite (SOad) 

Ayer Granite (Sacgr) 

do. - - -

do. Biotite 256+13 xxx

Ayer Granite (Sagr) Muscovite 334 ±17 xxx

Newburyport Complex (SOngd).

Oakdale Formation 

(So).

- 

- 

- 

_ 

_ 

- 

 

Nashoba zone

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Georgetown

Newbury

Middleton

Wilmington

North Reading

Andover

Wilmington

Do.

Do.

Do.

Tewksbury

Do.

Lincoln

Billerica

North Reading

Lawrence

Do.

Do.

Do.

Andover

Do.

Tewksbury

Billerica

Marlboro

Burlington

Andover

42°43'35"

42°45'48"

42°37'00"

42°35'00"

42°37'00"

42°37'35"

42°34'41"

42°34'41"

42°34'41"

42°34'41"

42°37'12"

42°37'16"

42°26'49"

42°33'00"

42°35'42"

42°40'48"

42°41'10"

42°41'08"

42°41'08"

42°40'44"

42°40'10"

42°38'38"

42°32'14"

42°21'00"

42°32'00"

42°38'00"

70°56'57"

70°54'58"

71°01'00"

71°09'00"

71°06'00"

71°10'47"

71°08'35"

71°08'35"

71°08'35"

71°08'35"

71°13'50"

71°13'42"

71°18'45"

71°17'23"

71°05'12"

71°08'13"

71°08'34"

71°08'36"

71°08'37"

71°09'11"

71°11'12"

71°13'57"

71°16'45"

71°34'00"

71°12'00"

71°11'00"

Sharpners Pond Diorite (Ssqd).

do. - - -

do. Hornblende 313±15 xxx

do. do. 336±18 xxx

do. Biotite 340+9 xxx

Andover Granite 

(SOagr). -

do. - - -

do. -

_ _ _

_ _ _

_ _ _

do. _ _ _

do. - - -

do. -

do. -

do. -

do. -

do. - - -

do. - - -

do. -

do. -

do. - - -

do. - - -

do. -

do. - - -

do. -

do. Muscovite 244 ±8 xxx

do. do. 284±10 xxx

do. do. 312+9 xxx

Microcline

Muscovite

Whole-rock

Microcline

Muscovite

do.

Biotite

Whole-rock

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

390±8

228+6

408 ±22

414±10

218+6

331+7

305 ±6

408±22

408±22

446±32

446±32

446+32

446±32

446±32

446±32

450+23

450 ±23

450±23

450±23

450±23

450±23

450 ±23

- 

- 

_ 

_ 

_ 

xxx 

xxx 

XX 

xxx 

xxx 

xxx 

xxx 

XX 

XX 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

_ 

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J10 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W. Rock unit 

Mineral

K-Ar

. Explanatory 

Age , . Mineral 

symbol

Rb-Sr 

Explanatory 

symbol 

Milford-Dedham zone Continued

22 

23

24

25 

26

27 

28 

29 

30 

31 

32 

33 

34 

35

36 

37 

38 

39 

40 

41 

42 

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

Do. 

Lynnfield

Wenham

Do. 

Sharon

Do. 

Do. 

Do. 

Do. 

Do. 

Stoughton 

Do. 

Do. 

Rowley

Do. 

Do. 

Do. 

Do. 

Salem 

Salem Neck 

Salem 

Rockport

Do.

Do.

Do.

Do.

Gloucester

Do.

Do.

Manchester

Beverly

Rockport

Do.

Beverly

Annisquam

Do.

42°31'28" 

42°32'00"

42°35'56"

42°36'00" 

42°05'30"

42°05'30" 

42°05'12" 

42°05'00" 

42°05'01" 

42°06'43" 

42°05'30" 

42°05'30" 

42°05'26" 

42°40'00"

42°40'00" 

42°40'00" 

42°40'00" 

42°40'00" 

42°32'00" 

42°31'44" 

42°32'00" 

42°41'15"

42°41'12"

42°40'03"

42°40'03"

42°40'03"

42°37'24"

42°37'30"

42°36'52"

42°35'19'

42°34'38"

42°39'52"

42°41'15"

42°34'27"

42°36'49"

42°36'49"

70°58'10" 

70°58'00"

70°54'40"

70°55'00" 

71°08'30"

71°08'30" 

71°08'54" 

71°09'42" 

71°09'29" 

71°09'53" 

71°08'00" 

71°08'00" 

71°08'14" 

70°53'00"

70°53'00" 

70°53'00" 

70°53'00" 

70°53'00" 

70°52'00" 

70°52'29" 

70°52'00" 

70°38'14"

70°38'08"

70°37'35"

70°37'35"

70°37'35"

70°38'42"

70°39'44"

70°43'53"

70°48'45"

70°50'03"

70°38'25"

70°38'14"

70°50'24"

70°43'46"

70°43'46"

do. - 

do. Biotite 

Hornblende 

Wenham Monzonite 

(Dwm). 

do. Hornblende 

Granite of Rattle- Riebeckite 

snake Hill 

pluton (Drgr). 

do. do. 

do. do. 

do. Biotite 

do. do. 

do. Riebeckite 

do. do. 

do. do. 

do. Whole-rock 

Newbury Volcanic 

Complex (Dsn). 

do. 

do. 

do. - 

do. - 

Essexite dike Biotite 

do. do. 

Trachytic syenite Whole-rock 

Cape Ann Complex 

(SOcgr).

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

Hornblende

Biotite

Riebeckite

Hornblende

Hornblende

do.

266±8 xxx 

366±10 xx

358±8 xxx 

378±14 x

382 ±14 x 

359±11 xx 

315±10 xxx 

260±14 xxx 

350 ±14 xx 

370+15 x 

377+16 x 

234±11 xxx

403 ±12 xx 

418+12 xx 

415 ±12 xx

388±11 xxx

 

 

359±11 xxx

401 ±12 xxx

 

405±12 xxx

 

381 ±11 xxx

390+12 xxx

 

 

 

 

Whole-rock

do. 

do. 

do. 

do.

Whole-rock

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

Whole-rock

do.

338+10

338±10 

338+10 

338±10 

338±10

426+6

426±6

426 ±6

426±6

426+6

426±6

426±6

426±6

426±6

426±6

426+6

371

369

- 

- 

xxx 

xxx 

xxx 

xxx 

xxx 

X 

X 

x 

X 

X 

X 

X 

X 

X 

X 

xx 

- 

- 

xxx 

xxx 
TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W. Rock unit

Mineral

K-Ar

Explanatory 

symbol

Mineral

Rb-Sr 

Explanatory 

symbol 

Milford-Dedham zone Continued

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

Do.

Do.

Beverly

Do.

Gloucester

Rockport

Do.

Braintree

Quincy

Do.

West Quincy

Do.

Do.

Do.

Do.

Quincy

Do.

Do.

Do.

Do.

West Quincy

Quincy

West Quincy

Quincy

Do.

Do.

Do.

Randolph

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Milton

Canton

Quincy

42°37'20"

42°37'20"

42°34'00"

42°35'00"

42°37'00"

42°40'00"

42°39'00"

42°13'38"

42°14'49"

42°14'51"

42°14'38"

42°14'22"

42°14'33"

42°14'43"

42°14'26"

42°14'37"

42°14'37"

42°14'49"

42°13'56"

42°14'08"

42°14'45"

42°14'51"

42°14'33"

42°15'18"

42°15'00"

42°12'35"

42°14'00"

42°12'19"

42°12'19"

42°12'25"

42°12'25"

42°12'24"

42°12'24"

42°12'40"

42°12'40"

42°12'40"

42°12'33"

42°12'33"

42°13'25"

70°39'11"

70°39'11"

70°53'00"

70°50'00"

70°39'00"

70°38'00"

70°37'00"

71°01'06"

71°00'27"

71°01'00"

71°02'10"

71°01'52"

71°02'46"

71°03'03"

71°03'09"

71°02'00"

71°02'00"

71°00'50"

71°02'19"

71°01'28"

71°02'40"

71°01'00"

71°02'46"

71°01'48"

71°01'00"

71°02'58"

71°02'11"

71°03'55"

71°03'55"

71°03'52"

71°03'52"

71°03'51"

71°03'50"

71°02'46"

71°02'46"

71°02'47"

71°05'55"

71°07'06"

71°03'42"

do.

do.

do.

do.

do.

do.

do.

Quincy Granite

(SOqgr).

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

Blue Hills Granite

Porphyry (SObgr).

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

Hornblende

do.

do.

Biotite

Hornblende

Riebeckite

Riebeckite

do.

do.

do.

Pyroxene

Whole-rock

Aegirine

Riebeckite

-

 

 

363±10 xxx

360±10 xxx

418±16 xxx

364±10 xxx

317±10 xxx

 

466±14 x

 

438±13 x

452±13 x

445±13 x

445±13 x

944±28 xxxx

 

 

 

 

 

 

 

 

 

286±15 xxx

1015±50 xxxx

 

307±9 xxx

_ _

 

 

 

 

 

 

 

 

 

 

 

do.

do.

Whole-rock

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

Whole-rock

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

350

364

306±22

306±22

306±22

306±22

306±22

306±22

306±22

306±22

333±7

357±7

357±7

357±7

357±7

357±7

306±22

306+22

276±8

276±8

276±8

276±8

276±8

276±8

276±8

276±8

276±8

276±8

276±8

276±8

xxx 

xxx 

_ 

_ 

_ 

_ 

_ 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 4

_ 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

- 4

- 4

_ 

_ 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 
TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W.

K-Ar

Rock unit Explanatory 

Mineral Age . , 

symbol

Mineral

Rb-Sr 

Explanatory 

symbol 

Milford-Dedham zone Continued

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

Salem

Do.

Nahant

Do.

Marblehead

Saugus

Lexington

Saugus

Do.

Natick

Boston

Foxborough

North Abington

West Roxbury

Do.

Sharon

Do.

Foxborough

Do.

Do.

West Roxbury

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Sharon

Foxborough

Do.

Wellesley

Do.

Braintree

42°29'57"

42°30'00"

42°25'38"

42°25'38"

42°29'26"

42°28'08"

42°25'27"

42°28'45"

42°28'10"

42°18'50"

42°16'06"

42°04'18"

42°07'32"

42°16'05"

42°16'05"

42°05'25"

42°05'25"

42°04'15"

42°04'15"

42°04'15"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°16'05"

42°05'25"

42°04'15"

42°04'15"

42°17'30"

42°17'30"

42°12'05"

70°55'33"

70°53'00"

70°54'44"

70°54'44"

70°50'43"

71°01'27"

71°14'58"

71°01'20"

71°01'30"

71°20'05"

71°09'29"

71°13'36"

71°56'55"

71°09'30"

71°09'30"

71°13'30"

71°13'30"

71°13'30"

71°13'30"

71°13'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71°09'30"

71W30"

71°09'30"

71°09'30"

71°13'30"

71°13'30"

71°13'30"

71°17'30"

71°17'30"

70°58'30"

do. Biotite 452+12 x

Gabbro diorite do. 363±8 xxx

Nahant Gabbro do. 498+15 x

(Ongb).

do. Pyroxene 1782±55 xxxx

Dedham Granite Hornblende 640+14 x

north of Boston

(Zdngr).

do. do. 622+12 x

do. do. 384±6 xxx

do. - - -

do. -

Dedham Granite 

(Zdgr).

do. -

do. - - -

do. -

do. -

do. - - -

do. -

do. -

do. - - -

do. -

do. -

do. - - -

do. -

do. -

do. - - -

do. -

do. -

do. -

do. -

do. - - -

do. -

do. _ _ _

do. - - -

do. -

do. -

do. - - -

do. -

do. -

do. -

do. -

Biotite

Biotite

Whole-rock

do.

Whole-rock

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

do.

450±12

451+23

595 ±17

778

579+28

595+17

595 ±17

595+17

595±17

595+17

595+17

595±17

511

550

455

530

569

534

465

579

564

445

523

543

506

431

588

538

398

457

330

x 

_ 

X 

_ 

_ 

_ 

_ 

X 

xxxx 

- 

- 

- 

XX 

x 

x 

X 

x 

x 

x 

x 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

x 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

XX 

xxx 

xxx 

xxx 

xxx 
J14 THE BEDROCK GEOLOGY OF MASSACHUSETTS

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l> t- l> t-t-t-t-t-t-b-l> t>t-t- t-t-t-t-t-t- t-t-t-t-t-t>t-t-t>t- t>t-t-t-t-tj 4 ^* CO ^^ CTi GO LO tO '^ ^^ '^ ^J1 CTi t^~ t^~ t^~ t^~ t>* t^~ ^^ ^H CTi ^^ 0} ^^ C^ ^H 0i O^i GO 00 "*^ CO OJ "*^ rH

1 

3 &

111 iiillili 11 } i 1 f i

^"oo (uS^ :So'a3 1^a} oRo dododo dSdddddddo dxidddo 

^OW p3cCP3<JhJP3SP QSZ QQQQQH) QSOQQQQPQIz; PfPPPP

^O t^~ GO 05 ^^ ^"^ OJ CO '^ LO ^O t** GO 05 ^^ ^^ OJ CO ^^ *-O ^O t^~ GO 05 ^^ ^^ OJ CO ^J LO ^O t^~ GO 05 ^^ ^^
TABLE 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts Continued

No. Town or village Lat, N. Long, W. Rock unit

Mineral

K-Ar Rb-Sr

. Explanatory ,.. , . Age 11 Mineral Age 

symbol

Explanatory 

symbol 

Milford-Dedham zone Continued

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

Do. 

Freetown

Brewster

Do.

Do.

Braintree

Do.

Quincy

Do.

Do.

Braintree

Quincy

Lincoln

Belmont

Lexington

Do.

Do.

Stoneham

Do.

42°02'16" 

41°45'45"

41°44'50"

41°44'50"

41°44'50"

42°13'35"

42°13'35"

42°13'49"

42°13'49"

42°13'48"

42°13'39"

42°13'36"

42°25'31"

42°24'30"

42°28'00"

42°27'00"

42°26'53"

42°27'20"

42°27'20"

71°45'40" 

71°05'23"

70°05'11"

70°05'11"

70°05'11"

71°01'53"

71°01'52"

71°02'40"

71°02'40"

71°02'37"

71°01'41"

71°02'55"

71°18'14"

71°11'00"

71°12'00"

71°13'15"

71°13'54"

71°06'05"

71°06'05"

Scituate Granite 

Gneiss (Zsg). 

Granite of the Fall 

River pluton

(Zfgr).

Granodiorite

do.

Mafic xenolith

Mattapan Volcanic

Complex (Zm).

do.

do.

do.

do.

do.

do.

Metavolcanic rocks

(Zv).

do.

do.

do.

do.

Westboro Formation

(Zw).

do.

-

Biotite

do.

Hornblende

Biotite

Hornblende

Biotite

Hornblende

Biotite

Hornblende

Biotite

Hornblende

Biotite

Hornblende

Biotite

Hornblende

-

380±19

348 ±12

518±30

400±14

566±22

-

330

394

366

388

353

349

321

343

349

407

-

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

xxx

- -

 

 

 

 

 

Whole-rock 229

do. 253

do. 229

do. 266

do. 346

do. 411

do. 358

 

 

 

 

 

 

 

 

 _

 

 

 

- 

- 

- 

- 

- 

- 

- 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

xxx 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

- 

 

1. Aleinikoff, J.N., Zartman, R.E., and Lyons, J.B., 1979, U-Th-Pb geochronology of 

the Massabesic gneiss and the granite near Milford, south-central New Hampshire: New evidence for Avalonian basement and Taconic and Alleghenian 

disturbances in eastern New England: Contributions to Mineralogy and Petrology, v. 7, p. 1-11.

2. Ashwal, L.D., Leo, G.W., Robinson, Peter, Zartman, R.E., and Hall, D.J., 

1979, The Belchertown quartz monzodiorite pluton, west-central Massachusetts: A syntectonic Acadian intrusion: American Journal of Science, v. 279, 

p. 936-969.

3. Bottino, M.L., and Fullagar, P.D., 1968, The effects of weathering on whole-rock 

Rb-Sr ages of granitic rocks: American Journal of Science, v. 266, p. 661-670.

4. Bottino, M.L., Fullagar, P.D., Fairbairn, H.W., Pi

P.M., 1970, The Blue Hills igneous complex, Ma

open systems: Geological Society of America Bu

5. Brookins, D.G., 1967, Rb-Sr age evidence for Pe

Monson gneiss, west central Massachusetts: Geo

v. 31, p. 281-283.

6. 1968, Rb-Sr age of the Ammonoosuc volcan

Journal of Science, v. 266, p. 605-608.

7. DeBoer, Jelle, 1968, Paleomagnetic differentiation 

Triassic volcanic rocks in central Appalachians 

Connecticut Valley): Geological Society of Amer
J16 THE BEDROCK GEOLOGY OF MASSACHUSETTS

3 

_C

c 

o

u I

42

^,

1

^

a, 

^>

~« 

1

op 

-i 

D^

C

^

^

1

1

a

» 

<u

h

gj 83 t^CO
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS J17

TABLE 2. Published zircon Pb-alpha and apatite fission-track ages of rocks from Massachusetts

[Lead-alpha ages calculated by the equation, t (m.y.) = K x (p/a), where p is lead content in ppm, a is alpha activity in alphas/mg-hr, and K = 2580. Fission-track 

ages are given in terms of the decay constant, \f = 8.46 xlO~17 yr" 1 . The explanatory symbol (x) indicates that the radiometric system records an age interpreted 

to be time of uplift and cooling below 110 °C; (xx) indicates that the age is of questionable analytical or geological meaning. Ref., reference from list below. , 

not determined; Do., do., ditto]

Pb-alpha Fission-track

No. Town or village Lat, N. Long, W. Rock unit

. Explanatory Age , . Age symbol

Explanatory Refi 

symbol

Bronson Hill zone

1 

2 

3

Erving 

Orange 

Erving

42°37'18" 

42°33'30" 

42°36'00"

72°22'06" 

72°18'12" 

72°22'00"

Pauchaug Gneiss (Ope) 

Monson Gneiss (OZmo) 

Fourmile Gneiss (OZfm)

-

- 97±5 

- 82+3 

- 94±5

x 4 

x 4 

x 4

Merrimack synclinorium

1 

2 

3 

4

Leominster 

West Chelmsford 

Bolton 

Clinton

42°31'45" 

42°37'57" 

42°25'00" 

42°23'50"

71°48'00" 

71°25'05" 

71°39'00" 

71°41'15"

Fitchburg Complex (Dfgd) 

Chelmsford Granite (Dcgr) 

Ayer Granite (Sacgr) 

do.

230±25 

480±51 

410±50 

420 ±50

XX 

XX 

XX 

XX 

- 2 

- 2 

- 3 

- 3

Nashoba zone

1 Berlin 42°23'25" 71°40'40" Muscovite granite (mgr) 520±60 XX - 3

Milford-Dedham zone

1

2

3

4

5 

6

7

8

9

10

11

12

13

Peabody

Do.

Rockport

Do.

Beverly 

Norfolk 

Foxborough

Do.

Wrentham

Milford

Do.

Whitinsville

Uxbridge

42°31'00"

42°30'05"

42°40'50"

42°40'41"

42°33'18" 

42°07'00" 

42°03'18"

42°02'30"

42°02'30"

42°09'45"

42°09'45"

42°07'00"

42°04'36"

70°58'00"

70°57'55"

70°38'04"

70°39'00"

70°52'12" 

71°20'00" 

71°17'32"

71°17'30"

71°20'00"

71°30'00"

71°30'15"

71°40'00"

71°38'54"

Peabody Granite (Dpgr)

do.

Cape Ann Complex (SOcgr)

do.

do. 

Dedham Granite (Zdgr) 

do.

do.

do.

Milford Granite (Zmgr)

do.

Ponaganset Gneiss (Zpg)

do.

275 ±28 

273±27

260±21

290 ±23

260 ±21

290±33 

230 ±18

265±25

235±15

265 ±22

225 ±25 

390 ±30 

335±31

380±33

365±35

220 ±22

385±38 

365±33

310±28

360±33

355±32

270 ±30 

270±29

265±31

215±17 

245 ±26

620±53

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

- 64±4

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

XX 

- 1 

- 1

- 2

- 2

- 2

- 2 

- 2

- 2

- 2

- 2

x 4 

- 2 

- 2 

- 2

- 2

- 2

- 2

- 2 

- 2

- 2

_ 2

- 2

- 2 

- 2

- 2

- 2 

- 2

- 2

1. Quinn, A.W., Jaffe, H.W., Smith, W.L., and Waring, C.L., 1957, 

Lead-alpha ages of Rhode Island granitic rocks compared to 

their geologic ages: American Journal of Science, v. 255, p. 

547-560.

2. Webber, G.R., Hurley, P.M., and Fairbairn, H.W., 1956, Relative 

ages of eastern Massachusetts granites by total lead ratios in 

zircon: American Journal of Science, v. 254, p. 574-583.

3. Zartman, R.E., Snyder, G.L., Stern, T.W., Marvin, R.F., and 

Bucknam, R.C., 1965, Implications of new radiometric ages in 

eastern Connecticut and Massachusetts: U.S. Geological Survey Professional Paper 525-D, p. D1-D10.

4. Zimmerman, R.A., Reimer, G.M., Foland, K.A., and Faul, H., 

1975, Cretaceous fission-track dates of apatites from northern 

New England: Earth and Planetary Science Letters, v. 28, p. 

181-188.
J18 THE BEDROCK GEOLOGY OF MASSACHUSETTS

APPENDIX: RADIOMETRIC AGE DATA BANK

The Radiometric Age Data Bank (RADB) is a means 

for collecting and organizing the approximately 100,000 

radiometric ages presently published for the United 

States. Although the goal of providing complete coverage for the entire country has not yet been reached, a 

concentrated effort directed toward the six New England States (Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut) has resulted in 

the incorporation of at least 95 percent of the ages 

published through 1986 for this area into our data file. 

The RADB is constructed such that sample location, 

petrography, analytical data, calculated age(s) together 

with interpretive remarks, and literature citation are 

linked to form an independent record for each sample. 

The record can accommodate, singly or in combination, 

all results on a sample pertinent to the K-Ar, Rb-Sr, 

U-Th-Pb, Pb-alpha, and fission-track methods.

Data are processed by using the General Information 

Processing System (GIPSY) developed at the University 

of Oklahoma, which maintains the data file and builds, 

updates, searches, and prints the records through simple 

yet versatile command statements. The retrieval of 

records is accomplished by specifying the presence, 

absence, or numeric or alphabetic value of any element of 

information in the data bank. For example, searches can 

be made for all records relating to the Quincy Granite, all 

records containing Rb-Sr biotite ages between 400 Ma 

and 600 Ma, or all records derived from a particular 

literature citation. Output is available in the form of 

complete or abbreviated records, listings of specified 

record elements, columnar tabulations, or input data for 

certain ancillary programs, such as map or histogram 

plots. A typical RADB record is discussed below to 

illustrate the sort of information available from the data 

bank. The reader with further interest in the operational 

aspects of RADB is referred to the user's guide to the 

Radiometric Age Data Bank (Zartman and others, 1976) 

and additional references contained therein.

As an example, a slightly abridged RADB record (a 

few coded elements, which, for our purpose, would be 

redundant, have been deleted from the original record) 

for a sample of granite from the Cape Ann Complex from 

Rockport, Mass., is shown in figure Al. Many of the 

individual elements of the record are self-explanatory, 

but certain entries that are given in code or may be 

otherwise enigmatic require further definition. The italicized letters next to some of the elements are keyed to

the following comments. A, General information about 

each sample, including a unique record number, a reference code (a complete reference is given at the end of the 

record), and a code for the laboratory or laboratories that 

performed the analytical determinations. B, Name of 

person who compiled this record and the date (month and 

year) of compilation. C, A one-digit code indicating 

accuracy of sample location given by latitude and longitude coordinates. D, A one-digit code indicating the 

source of the sample; that is, outcrop, core, quarry, 

mine, and so forth. E, A four-letter code of a formalized 

geologic name conforming to the Standard Stratigraphic 

Code of the U.S. Geological Survey. F, The known or 

most probable stratigraphic age of the unit (which may or 

may not coincide with the measured radiometric age) 

given in three-digit code conforming to the Standard 

Stratigraphic Code of the U.S. Geological Survey. G, A 

four-character petrographic code for the rock from which 

the sample was taken; this code is used in searches for 

general rock groupings. H, The laboratory sample number as given by the author(s) of the published article. /, 

A three-character mineralogic code for the analyzed 

mineral; this code is used in searches for general mineral 

family groupings. /, A two-digit code identifying the 

method of analysis. K, Interpretive comments about the 

radiometric age and its geologic significance. L, A suite 

number. If a calculated age is related to the analyses of 

other samples, as for a Rb-Sr whole-rock isochron or 

U-Pb zircon discordia array, it is designated as being 

part of a suite. A unique number ties together all 

analyses belonging to the suite and can be used to 

retrieve the other members of the suite.

Each sample locality is represented by a record similar 

to the one shown in figure Al, and each record contains 

1-6 individual ages (our example gives a K-Ar hornblende, an Rb-Sr whole-rock, and a U-Th-Pb zircon age). 

This pool of information provides the data base from 

which tables 1 and 2 have been compiled. The entire set 

of unabridged records is not reproduced here, but a copy 

of it is available as U.S. Geological Survey Open-File 

Report 87-170 (Zartman and Marvin, 1987). Of course, 

the data bank must be recognized as only a convenient 

method of processing information originating elsewhere, 

and primary references remain those of the published 

articles. The list of primary references from which the 

Massachusetts RADB records were compiled is included 

in tables 1 and 2.
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS J19

RADIOMETRIC AGE DATA BANK - U.S.G.S. BRANCH OF ISOTOPE GEOLOGY

LOCATION 

COUNTRYSTATE 

COUNTYQUAD SCALE

Is 24000

LATITUDE 

LONGITUDE--

UNITED STATES 

MASSACHUSETTS 

ESSEX

QUAD NO. / NAME

GLOUCESTER 

42-39-52 N 

70-3B-25 W

SAMPLE IDENTIFICATION

RECORD NUMBER 0001708 

REFERENCE NUMBER 71-00005 

LABORATORY(IES) SO

COMPILED BY

NAMEs WILLIAMS. 

DATE: 77 06

B. R.

B

COMMENT ROCKPORT (FORMERLY LEONARD JOHNSON) QUARRY. 4200' 

SOUTHWEST OF PIGEON HILL. ROCKPORT

PRECISION OF LOCATION: 2 

SOURCE OF SAMPLE: 3

SAMPLE DESCRIPTION

GEOLOGIC NAME: CAPE ANN GRANITE 

ROCK TYPE: ALKALIC GRANITE

DESCRIPTION: LIGHT BLUISH-GRAY.

GEOLOGIC NAME CODE: CPAN 

LEXICON AGE(S): 361 357 340 

PETROGRAPHIC CODE: B240 

MEDIUM-GRAINED GRANITE

POTASSIUM-ARGON

LABORATORY SAMPLE NUMBER: 

ROCK/MINERAL CODE: PCS

ANALYTICAL DATA:

K20 (%) - 

40AR-RAD (MOLES/GM) - 

t RADIOGENIC - 

CALCULATED AGE - 

TYPE OF ANALYSIS -

260 H 

I HORNBLENDE

1.240 

7.93 X E-10

97 

390 +/- 12 MILLION YEARS

10

COMMENTS: MINIMUM AGE OF INTRUSION; DISTURBED BY LATER METAMORPHISM; FOR « 

ADDITIONAL INFORMATION. SEE REFERENCE(S) 77-00005

RUBIDIUM-STRONTIUM

LABORATORY SAMPLE NUMBER: 

ROCK/MINERAL CODE: AAO

ANALYTICAL DATA:

RB (PPM) 

SR (PPM) 

87RB/86SR 

87SR/86SR 

CALCULATED AGE 

INITIAL 87SR/86SR 

TYPE OF ANALYSIS 

260

WHOLE-ROCK

N

MILLION YEARS

 154.4

 16.3

 27.36

 0.871

 435 +/- 6

 0.703

 22 J 

ANALYTICAL COMMENT: 11-POINT WHOLE-ROCK ISOCHRON

SAMPLE SUITE: 0008B L

COMMENTS: AGE OF INTRUSION; FOR ADDITIONAL INFORMATION. SEE REFERENCE 

77-00005

URANIUM-THORIUM-LEAD

260 H 

I ZIRCON

LABORATORY SAMPLE NUMBER: 

ROCK/MINERAL CODE: MCI

ANALYTICAL DATA:

U (PPM) 1330 

TH (PPM) 599.7 

PB (PPM) 92.9 

ISOTOPIC COMPOSITION OF LEAD (ATOM PERCENT) 

204PB 206PB 207PB 

0.042 81.83 5.168 

206PB/238U AGE 417 

207PB/235U AGE 421 

207PB/206PB AGE 452 

208PB/232TH AGE 

CONCORDIA INTERCEPT AGE 

208PB 

12.96 

f- 8 

t- 9 

f- 10 

450 +/- 8 

450 +/- 25

MILLION YEARS 

MILLION YEARS 

MILLION YEARS 

MILLION YEARS 

MILLION YEARS

ANALYTICAL COMMENT: 4-POINT ZIRCON DISCORDIA LINE

SAMPLE SUITE: 00087 L

COMMENTS: AGE OF INTRUSION; FOR ADDITIONAL INFORMATION. SEE REFERENCE Jf 

77-00005

REFERENCE: ZARTMAN. R.E.. AND MARVIN. R.F.. 1971. RADIOMETRIC AGE (LATE 

ORDOVICIAN) OF THE QUINCY, CAPE ANN. AND PEABODY GRANITES FROM EASTERN 

MASSACHUSETTS: GEOL. SOC. AMER. BULL.. V. 82. P. 937-958

FIGURE Al. A Radiometric Age Data Bank (RADB) record for a sample of granite from the Cape Ann 

Complex from Rockport, Mass. For explanation of abbreviations and italicized letters, see facing page.