Bed rock Geology of the 

Bristol Quadrangle, Hqrtforct 

litchfield, and New Haven Countie&, 

Connecticut 

U.S. G EO LOG I CAL SURVEY BULLETIN 15 7 3 

Prepared in cooperation with the 

State of Connecticut 

Geological and Natural History Survey 

Bedrock Geology of the 

Bristol Quadrangle, Hartford, 

Litchfield, and New Haven Counties, 

Connecticut 

By HOWARD E. SIMPSON 

Prepared in cooperation with the 

State of Connecticut 

Geological and Natural History Survey 

Metamorphosed sedimentary strata of 

inferred Middle Ordovician to Early 

Devonian age, and sedimentary strata 

of Late Triassic age underlie the 

Bristol quadrangle 

U.S. GEOLOGICAL SURVEY BULLETIN 1573 
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 report is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Geological Survey. 

UNITED STATES GOVERNMENT PRINTING OFFICE [1989] Revised 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Simpson, Howard E. 

Bedrock geology of the Bristol Quadrangle, Hartford, Litchfield, and 

New Haven Counties, Connecticut. 

(U.S. Geological Survey bulletin; 1573) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1573. 

1. Geology, Stratigraphic-Ordovician. 2. Geology, StratigraphicDevonian. 3. Geology, Stratigraphic-Triassic. 4. Geology- Connecticut-Bristol Region. 

I. Title. II. Series. 

QE75.B9 no. 1573 

[QE660] 

557.3 s 

[557.46] 

86-607922 
CONTENTS 

Abstract 1 

Introduction 1 

Rock stratigraphic units and their correlation 1 

Taine Mountain Formation 2 

Bristol Gneiss 3 

Collinsville Formation 6 

The Straits Schist 7 

New Haven Arkose 9 

Intrusive rock units 9 

Metamorphism and deformation 10 

Structural geology 10 

Economic geology 11 

Engineering geology 11 

Slope stability 11 

Excavatability 11 

Drillability 12 

Faults and faulting 12 

References cited 12 

PLATE 

[Plate is in pocket] 

1. Bedrock geologic map of the Bristol quadrangle, Connecticut. 

FIGURES 

1. Index map of southwestern New England 2 

2. Correlation chart for selected metamorphosed stratified rock units in western 

and southern New England 4 

Contents Ill 

Bedrock Geology of the 

Bristol Quadrangle, Hartford, 

Litchfield, and New Haven Counties, Connecticut 

By Howard E. Simpson 

Abstract 

Metamorphosed strata of sedimentary origin underlie the 

upland surface of western Connecticut. The metamorphic rocks 

consist chiefly of micaceous schists and gneisses with small to 

negligible amounts of pegmatite, amphibolite, metaquartzite, and 

serpentinite. These metamorphic rocks represent four formations. 

In order of decreasing age upward they are the Taine Mountain 

Formation, Bristol Gneiss, Collinsville Formation, and The Straits 

Schist. These formations may correlate with the Moretown Formation, Hawley Formation, and Goshen Formation of central 

western Massachusetts, which are inferred to range in geologic 

age from Middle Ordovician to Late Silurian and Early Devonian. 

Regional metamorphism and doming of the strata most probably occurred during the Acadian orqgeny at or near the end 

of Early Devonian time. 

Nonmetamorphosed sedimentary strata underlie the adjacent central lowland of Connecticut, known also as the Hartford Mesozoic basin. These strata are composed mainly of Upper 

Triassic sandstone and conglomerate assigned to the New Haven 

Arkose of the Newark Supergroup of Late Triassic age. These 

sedimentary strata are inclined moderately downward toward 

the east, are commonly massive, and are poorly exposed. 

Gravimeter traverses across the upland-lowland boundary 

suggest the presence of a normal fault of a few hundred feet of 

vertical displacement downthrown on the east. Traces of copper mineralization are found locally in the vicinity of the boundary fault, and at least one mine produced copper ore during the 

1800's. Jointing and foliation planes in the metamorphic rocks 

can affect slope stability and the ease of excavation. 

INTRODUCTION 

The bedrock formations of the Bristol quadrangle, 

Connecticut (fig. 1), are almost wholly mantled by surficial deposits of sand, gravel, and till ("hardpan") that vary 

The present U.S. Geological Survey Bulletin 1573 is a revised and 

updated version of an earlier, smaller format book with the same number. The author thanks Diane C. Schnabel for her editorial and geological contribution to both books. 

in thickness (Simpson, 1961). The bedrock in the western 

two-thirds of the quadrangle consists of metamorphic rocks; 

these underlie part of a broad upland that characterizes 

western Connecticut. Sedimentary rocks that are much 

younger underlie the eastern one-third of the quadrangle, 

which constitutes part of the central lowland of Connecticut. The two areas are separated by a prominent topographic boundary that trends north-south just east of the 

city of Bristol. 

Structurally, the metamorphic rocks form a broad 

dome that is elongate in a south-southwesterly direction. 

The central core of the structure is exposed just north of 

the Bristol quadrangle and is surrounded by a gently dipping sequence of light-colored feldspar-rich gneisses (pl. 1). 

The greater erodibility of one of these units has resulted 

in the formation of an irregular topographic basin on the 

southeastern flank of the dome. This basin, from 2 to 4 

mi (mile) across and as much as 750ft (feet) deep, is now 

partly occupied by the city of Bristol. Geologically, the 

dome has thus been known for many years as the "Bristol 

Dome," and that name is here formally assigned to the 

structure. 

ROCK STRATIGRAPHIC UNITS AND THEIR 

CORRELATION 

Since the late 1940's, numerous geologists involved 

in detailed field mapping in southwestern New England 

have named rock-stratigraphic units and attempted their 

correlation with the work of others. Most of these proposed 

correlations have received little acceptance. Coordinated 

efforts by Hatch and Stanley (1973), based on detailed mapping mainly in western Massachusetts coupled with reconnaissance studies throughout the region, appear to offer 

a reasonable regional solution to past correlation problems. 

The relation of rock units of inferred Ordovician, 

Silurian, and Devonian age in selected quadrangles and 

areas of southern and western New England, in part as 

interpreted by Hatch and Stanley ( 197 3, pl. 2), is shown 

Rock Stratigraphic Units and Their Correlation 
710 43°~----------------~----------~--------~--------------~ I /' 

I J NEW 

\ -~ERMONT \. HAMPSHIRE ) _______ ). ____________ _ 0 North Adams 

NEW 

YORK 42° 

I 

I 

I 

I 

I 

I~ 1 ~ Blandford 0 

I 0. 

~~---- 1 Q) 

..... I (I) 

I~ 

I 

I 

I 

0 

ASSACHUSETTS 

-----~--, 

~ I l 

~ I I 

UJ I 

I 

I 

J 

Long Island Sound 

25 

I 

I 

50 MILES 

I 

50 KILOMETERS 

Figure 1. Index map of southwestern New England showing location of the Bristol quadrangle, 

the central lowland (also known as the Hartford Mesozoic basin), and the eastern and western 

uplands of Connecticut and Massachusetts. 

in the correlation chart (fig. 2) in the present report. In this 

chart the stratigraphic names used are those of the author 

cited, but all units are correlated in the manner used by 

Hatch and Stanley (197 3), and not necessarily in the manner chosen by the authors of the reports cited. To those 

correlation columns is added, as inferred by this author, 

a column containing the names of rock units of pre-Triassic 

age in the Bristol quadrangle, Connecticut, arranged to 

correlate with the chart of Hatch and Stanley (1973). These 

correlations are discussed briefly. 

Taine Mountain Formation 

In 1898, Emerson (p. 156-163) gave the name Savoy 

Schist to metasedimentary strata of inferred (p. 18) Early 

2 Bedrock Geology of the Bristol Quadrangle, Connecticut 

Silurian (now replaced by Ordovician) age in western Massachusetts that underlay his Hawley Schist and that overlay other units named by him. He described the Savoy 

Schist as in part a "chloritic sericite-schist * * *; flat, 

thin-fissile into plates 10-15 mm thick, with the micaceous 

minerals concentrated mostly on the lamination 

planes; * * *."Subsequently, in central Vermont, Richardson (1919, p. 138) gave the name Missisquoi Schists to sericite schist that he considered to be of Late Cambrian age; 

later (1924) he modified the name to Missisquoi Group. 

The name Moretown Formation was introduced by 

Cady (1956) in north-central Vermont for quartz-albitesericite-chlorite granulite characterized by fine, distinct, 

closely spaced stripes he called "pinstriping" -suggestive 

of the "thin-fissile * * * plates" described by Emerson. The 

granulite was correlated by Cady with the Missisquoi 
Group of Richardson (1924) and was inferred to be of Ordovician age. Doll and others (1961) reduced the Missisquoi 

Group to formational status, the Moretown Formation to 

member status, and defined the member as the basal unit 

of three in the Missisquoi Formation. They also concluded 

that the member is of Middle Ordovician age; their reasoning was outlined and evaluated by Hatch and Stanley in 

1973 (p. 16). Chidester and others (1967) subsequently 

returned the Moretown Member to formational status. The 

name Moretown Formation generally has been accepted 

since then for pinstriped metamorphic strata of inferred 

Middle Ordovician age in northern New England. 

The name Hartland Schist was introduced by 

Gregory (1906, p. 96-100) for micaceous schist of inferred 

possible Late Ordovician age in western Connecticut. Since 

its introduction, the name has been used by various 

geologists in western Connecticut with some variation in 

opinion as to the geologic age of the rock. Five subunits 

are now recognized (Gates and Martin, 1976) and designated from the base upward by Roman numerals. They 

(Gates and Martin, p. 316) correlated Hartland unit I 

with the Taine Mountain Formation of Stanley (1964), 

a correlation agreed to by Stanley and Hatch (1976, 

p. 355). 

Stanley (1964) and Simpson (this report), while mapping independently in the Collinsville quadrangle and the 

adjacent Bristol quadrangle, respectively, each identified 

within the Bristol Dome a sequence of three similar but 

distinctly separable stratigraphic units that are pinstriped. 

Logically the units belong to a single formation, but the 

lack of detailed field studies did not permit direct correlation with existing formations in southeastern Vermont 

(Doll and others, 1961) or in south-central Vermont 

(Skehan, 1961) where the name Moretown had been used. 

Stanley (1964, p. 17) therefore designated the three subunits 

as members of his Taine Mountain Formation of possible 

Precambrian(?) to Cambrian age (p. 43-48). The name 

Taine Mountain Formation is adopted formally here. 

Stanley ( 1964, p. 17) and Simpson agreed in the field upon 

names for each of the three members: Wildcat Member 

(basal), Scranton Mountain Member (middle)-whose type 

localities are in the Collinsville quadrangle-and Whigville 

Member (upper), whose type locality is in the Bristol quadrangle. These three names also are adopted here. Because 

of the pinstriping the Taine Mountain Formation is tentatively correlated with the Moretown Formation; thus, the 

inferred age of the Taine Mountain is Middle Ordovician. 

Wildcat Member.-The name Wildcat Member of 

the Taine Mountain Formation, of possible Precambrian 

(Proterozoic) to Cambrian(?) age, originally was assigned 

by Stanley (1964, p. 17-20) to a basal pinstriped gneiss 

subunit in the southern part of the Collinsville quadrangle; it was named for exposures on Wildcat Mountain. The 

member as mapped in the Bristol quadrangle is directly 

traceable into, and is equal to1 Stanley's member. The type 

locality of the member, its boundaries, and its lithologic 

characteristics remain the same as those described by 

Stanley. 

Scranton Mountain Member.-The name Scranton 

Mountain Member of the Taine Mountain Formation, of 

possible Precambrian (Proterozoic) to Cambrian(?) age, 

originally was used by Stanley (1964, p. 17-21) to designate a middle pinstriped gneiss subunit overlying his Wildcat Member of the Taine Mountain Formation in the 

southern part of the Collinsville quadrangle, where this 

member's name is taken from exposures on Scranton 

Mountain. As it is mapped in the Bristol quadrangle, the 

member is directly traceable into, and is equal to, Stanley's 

member. The type locality of the member, its boundaries, 

and its lithologic characteristics remain the same as those 

described by Stanley. 

Whigville Member.-The name Whigville Member 

of the Taine Mountain Formation, of possible Precambrian (Proterozoic) to Cambrian(?) age, was published first by 

Stanley (1964, p. 17, 21 ), for a pinstriped gneiss subunit 

overlying his Scranton Mountain Member of the Taine 

Mountain Formation. He designated no type locality for 

the member, as better exposures are located in the Bristol 

quadrangle. 

The unit is a plagioclase-quartz gneiss marked by distinctive thin layering (pinstriping) of muscovite and biotite. It is well exposed in the type section, located on a 

hillside about 530 ft high and situated 2,000 ft southsouthwest of the road junction in the village of Whigville, 

for which it is named, in the north-central part of the Bristol 

quadrangle. The type focality is identified on the geologic 

map. Apparent variation of thickness of the formation 

probably was caused by intense folding; an average thickness is estimated at 3,000 ft. The Whigville Member, as 

mapped by Stanley in the adjacent Collinsville quadrangle, 

is directly traceable into, and is equal to, the member as 

mapped in the present report. 

Bristol Gneiss 

The name "Bristol Granite-gneiss" was given by Rice 

and Gregory (1906, p. 1 04) to metamorphic rocks that they 

believed to be of granitic origin and that are exposed in 

the vicinity of Bristol, Conn. The name may have been 

modified from Percival's (1842) use of the name "Bristol 

granitic basin" for that area. In 1956, Rodgers and others 

modified the rock name to Bristol Gneiss. 

Stanley ( 1964) concluded that the "Collinsville 

Formation consisted of two named members; with the prior 

concurrence of this author he informally called (p. 23) the 

lower subunit the Bristol Member because of its lithologic 

similarity to gneiss exposed in the vicinity of Bristol. Stanley 

described his Bristol Member as marked by "alternating 

layers rich in light or in dark minerals" or by "thin, 

Rock Stratigraphic Units and Their Correlation 3 
AREA AND VERMONT 

REFERENCE MASSACHUSETTS 

Central- No rth- South- SouthWestern Central tral central central SYSTEM cen 

AND Emerson Richardson Currier and Cady Doll and Skehan 

SERIES (1898) (1919, 1924) Jahns (1941) (19 56) others ( 1961) (1961) 

z 

<( 

2 

c ~ > 1.1.1 

c _, ~ 

Goshen Memphramagog Northfield Nort hfield Northfield Northfield 

1----? ?- Schist Slates Formation Slate Formation Slate 

z Upper 

<( 

a: 

:::;) 

_, 

CiS c» Shaw Sh aw Shaw 

:c; 

"'CC Mountain Mou ntain Mountain 

~ Formation Form at ion Formation 

Carbonaceous 

Cram Hill Schist Cram Hill 

Formation Member Formation 

Hawley 

Schist 

c: Barnard Barnard 0 z ·.o:::; Volcanic Gneiss =:!: Missisquoi ca 

E u Member Member c» Schists (1919) C; - :c; u.. > "'CC ·s c ~ or ::I c cr a:: Group ( 1924) ·!I c en 

(unnamed :i 

schists) Q. 

::I 

0 

cE Moretown Moretown Savoy ·s M oretown 

Schist ~ Fo rmation Member Formation -~ en 

en 

~ 

Figure 2. Correlation chart for selected metamorphosed stratified rock units in western and southern New 

England. Formal and descriptive names used are those of the authors cited, but they are arranged in 

discontinuous folia [sic] rich in dark minerals evenly distributed through the rock." He included (p. 23-24) in his 

Bristol Member as its upper part a zone characterized by 

garnet-rich coticule, epidote-bearing amphibolite lenses and 

layers enclosed by hornblende-plagioclase, or by biotiteplagioclase gneiss. He made no reference by name to Moretown-like pinstriping; but from the lithology described the 

unit may be correlated with the Taine Mountain Formation. 

4 Bedrock Geology of the Bristol Quadrangle, Connecticut 

Stanley's name Bristol Member of the Collinsville 

Formation is here raised in rank, revised, and adopted as 

the Bristol Gneiss of inferred Middle Ordovician age. The 

name is restricted to rock composed mainly of foliae of 

plagioclase and quartz that alternate with foliae of 

muscovite, biotite, and chlorite. The garnet- and epidotebearing amphibolite and hornblende gneiss that constitute 

the upper part of Stanley's Bristol member are here 
VERMONT 

AND 

MASSACHUSETIS 

MASSACHUSETIS 

Central 

CONNECTICUT 

Central 

ROWE QUAD. HEATH QUAD. PLAINFIELD QUAD. BLANDFORD AREA COLLINSVILLE QUAD. BRISTOL QUAD. SOUTHINGTON QUAD. 

Chidester and Hatch and 

others (1967) Hartshorn (1968) 

Goshen 

Formation 

Osberg and 

others (1971) 

Goshen 

Formation 

Hatch and 

Stanley ( 1973) 

Goshen 

Formation 

Russell 

Mountain 

Formation 

Stanley 

(1964) 

The 

Straits 

Schist 

This 

report 

Southington 

Mountain 

Member 

The Straits 

Schist 

Fritts 

(1963) 

Southington 

Mountain 

Schist west 

of larsens 

Pond Fault 

Straits Schist 

except a 

large, round 

amphibolite 

rv~~~~~~~~~~~~~~rv~~~~~·~~~~ •• ~.~~·?n.~~~~.hV~~~YV~rv~~ 

Hawley 

Formation 

Moretown 

Formation 

Black 

Schist 

Member 

Volcanic 

unit 

Moretown 

Formation 

Hawley 

Formation 

Moretown 

Formation 

thickbedded 

mbr 

thinbedded 

mbr 

Moretown 

Formation 

Sweatheart c: 

Sweetheart 

Mountain 

Member Mountain .g .g Member ~ t--U-n-n-am_e_d ___ 

~ .., metaquartzite 

s ::! member ... 1---------i 'ii 1-----..J 

::! Ji 

'i 8 

Ji 

"6 

(,) 

Bristol 

Member 

Wildcat 

Member 

Unnamed 

hornblende 

gneiss 

member 

Bristol 

Gneiss 

Whigville 

Member 

Scranton 

Mountain 

Member 

Wildcat 

Member 

Southington 

Mountain 

Schist east 

of larsens 

Pond·Fault 

Prospect 

Gneiss 

the sequence of the present report. Thus, the geologic age correlations shown in this chart may differ from the 

geologic age correlations made by the various authors. Chart mainly from Hatch and Stanley (1973, pl. 2). 

excluded and are reassigned to the overlying unnamed 

hornblende gneiss member of the Collinsville 

Formation. 

The Bristol Gneiss as stratigraphically restricted here 

is composed of plagioclase-quartz gneiss characterized by 

foliae of muscovite or biotite, or locally chlorite, and contains scattered small garnets and large pods of amphibolite. It is exposed in and near the city of Bristol and is 

particularly well displayed in the type locality, which is the 

Penn Central Railroad (formerly the New York, New 

Haven, and Hartford R.R.) cut on the south side of St. 

Thomas Cemetery near the center of the quadrangle. This 

locality is identified on the geologic map. Apparent variation in the thickness of the member is thought to be due 

to locally intense folding; available data suggest that the 

average thickness may be about 2,000 ft. The lower part 

Rock Stratigraphic Units and Their Correlation 5 
of the unit as mapped by Stanley in the adjacent Collinsville quadrangle is directly traceable into, and is equal to, 

the member as it is mapped in the Bristol quadrangle. 

The Bristol Gneiss locally is marked strikingly by pinstriping that generally consists of laminae of biotite, muscovite, and some chlorite interlayered with granitoid 

plagioclase and quartz. The micaceous foliae are as much 

as 2 mm (millimeter) thick, and the plagioclase-quartz 

laminae are as much as 5 mm thick. Elsewhere, as on the 

west edge of urban Bristol, granitoid rock of plagioclase 

and quartz is more massive and laminae are fewer, thinner, and discontinuous. Because of the laminae the rockstratigraphic unit may be correlated with the Moretown 

Formation of inferred Middle Ordovician age in central 

Massachusetts. If the correlation is correct, the Bristol 

Gneiss would constitute a fourth member of Stanley's 

Taine Mountain Formation. However, the Bristol Gneiss 

is so different lithologically and texturally that its use as 

a key bed in mapping the metamorphic sequence in the 

Bristol area warrants its formational status. 

Collinsville Formation 

In 1898 Emerson (p. 163-175) assigned the name 

Hawley Schist to metamorphic rock of sedimentary and 

volcanic origin and inferred a middle Ordovician age for 

it in western Massachusetts. Emerson distinguished this 

unit by its "general distribution of hornblende" from his 

Savoy Schist below and ftom his Goshen Schist above. In 

central Vermont, Richardson (1919, p. 138) gave the name 

Missisquoi Schists to certain sericite schists (then a possible correlative of the Savoy Schist of Emerson, 1898); in 

1924 Richardson modified that name to Missisquoi Group, 

and in 1927 (p. 143) identified an unnamed hornblende 

schist as the uppermost formation of the group. Subsequently, Currier and Jahns (1941), also in central 

Vermont, defined the Cram Hill Formation of probable 

Middle Ordovician age as the equivalent of the upper part 

of Richardson's "Missisquoi." A middle unit of that group 

later was identified by Doll and others (1961) as their 

"Barnard volcanic member" of Richardson's "Missisquoi." 

Since then, the "Barnard volcanic member" and the 

overlying Cram Hill unit or their equivalents have 

constituted, either individually or together, the Hawley 

Formation of probable Middle Ordovician age for most 

authors (Chidester and others, 1967; Hatch, 1967; Hatch 

and Hartshorn, 1968; Osberg and others, 1971). Hatch 

(1967) redefined the Hawley as all rocks between the 

Moretown Formation and the Goshen Formation. 

Stanley in 1964 (p. 22), with the concurrence of 

Simpson (Stanley, 1964, p. 23), took the name Collinsville 

Granite-gneiss as used by Rice and Gregory (1906, p. 

105-107) and modified it to the Collinsville Formation for 

certain rocks of inferred Cambrian age (Stanley, 1964, 

6 Bedrock Geology of the Bristol Quadrangle, Connecticut 

fig. 6) "cropping out on the southern face of Sweetheart 

Mountain" (p. 22) in the Collinsville quadrangle. These 

rocks constitute metamorphic strata that are younger than 

those of Stanley's Taine Mountain Formation and older 

than those of The Straits Schist. The rocks are exposed in 

both the Bristol and Collinsville Domes. Stanley used the 

name of Collinsville in part because the lack of detailed 

field studies did not permit direct correlation of these strata 

with existing mapping in northern New England, and in 

part because the formation contains subunits that lithologically differ markedly from the rocks commonly associated with the older name of Bristol Granite-gneiss of Rice 

and Gregory (1906, p. 104). 

In 1967 Stanley, working in the Blandford area of 

west -central Massachusetts, mapped a sequence of two 

units to which he assigned the temporary field names 

"4TB" and "5." The units subsequently were designated 

by Hatch and Stanley (1973, p. 9) as the unnamed lower, 

or thin-bedded member and the unnamed upper, or thickbedded member, respectively, of their new Cobble Mountain Formation of inferred Middle Ordovician age. They 

concluded (p. 15) that the Cobble Mountain Formation "is, 

at least in large part, a facies equivalent of the Hawley Formation." They also stated (p. 25) that the unnamed upper 

member of the Cobble Mountain Formation is correlated 

with the uppermost unit, the Sweetheart Mountain Schist 

Member, of the Collinsville Formation. 

Metamorphic strata in the Bristol quadrangle that 

lie between the Bristol Gneiss and The Straits Schist of this 

report are assigned to the Collinsville Formation as agreed 

upon earlier with Stanley (1964, p. 23). However, because 

of the work of Stanley (1967) and of Hatch and Stanley 

(1973), the formation is no longer considered to be of 

Precambrian or of pre-Triassic age but is now inferred to 

be of Middle Ordovician age and, at least in part, a correlative of the Hawley Formation in Massachusetts. 

Assignment to the Collinsville Formation is based in part 

on the similarity of lithologies and in part on the sequence 

of those lithologies. Together the strata constitute three 

members of distinctive character. The lowermost member 

is an unnamed amphibolite and hornblende gneiss overlain locally by an unnamed metaquartzite and by the . 

Sweetheart Mountain Member. 

Unnamed hornblende gneiss member.-Garnet-rich 

coticule, epidote-bearing lenses and layers of amphibolite, 

and hornblende gneiss formerly constituted the uppermost 

part of Stanley's (1964, p. 23) Bristol Member of the Collinsville Formation, of possible Precambrian or Cambrian(?) 

age. In the present report the amphibolite and hornblende 

gneiss are excluded from the Bristol Gneiss because they 

differ markedly in mineral composition and lack the characteristic foliae. The strata are reassigned here to constitute 

an unnamed hornblende gneiss member that directly overlies the Bristol Gneiss of this report. The member is best 

exposed on the ridge south of Hurley Hill at the east end 
of a roadcut just south of and approximately parallel to 

the Pequabuck River. The thickness of the member near 

the roadcut may be about 400 ft. The upper part of 

Stanley's (1964) Bristol Member of the Collinsville 

Formation is directly traceable into, and is equal to, this 

member of the Collinsville Formation in the Bristol 

quadrangle. 

Unnamed metaquartzite member.-Locally, a thin 

metaquartzite unit directly overlies the unnamed hornblende gneiss member of the Collinsville Formation of inferred Middle Ordovician age. The unit is best exposed in 

the same roadcut as the hornblende gneiss member (see 

preceding discussion) and is mappable for a distance of 

about 2 mi along strike; its thickness probably averages 

about 100ft. This informal member may be of little value 

in regional correlation because similar local metaquartzite 

units are found at various localities and in scattered 

horizons in the metamorphic sequence of southern New 

England. In the Bristol quadrangle it is tentatively assigned 

to constitute an unnamed middle member of the Collinsville Formation of inferred Middle Ordovician age. 

Sweetheart Mountain Member.-The name 

Sweetheart Mountain Member of the Collinsville Formation, of possible Precambrian to Cambrian(?) age, was given 

by Stanley (1964, p. 27) to metamorphic strata that overlie his Bristol Member of the Collinsville Formation. The 

name of the unit was taken from a type locality on 

Sweetheart Mountain, in central Collinsville quadrangle. 

Stanley's name is adopted here as Sweetheart Mountain 

Member of the Collinsville Formation of inferred Middle 

Ordovician age. The stratigraphic unit to which the name 

is assigned constitutes the uppermost member of three belonging to that formation in the Bristol quadrangle and 

overlies the unnamed hornblende gneiss member or the unnamed metaquartzite member, whichever is locally present. 

The type locality and its lithologic characteristics remain 

the same as those described by Stanley (1964). 

The correlation of the Sweetheart Mountain Member 

with the Hawley Formation of Hatch and Hartshorn 

(1968) depends upon (1) direct tracing of the member from 

the Bristol quadrangle into Stanley's member in the adjacent Collinsville quadrangle, (2) correlation of Stanley's 

member with the unnamed thick-bedded (upper) member 

of the Cobble Mountain Formation of Hatch and Stanley 

(1973, p. 25), and (3) whether or not that thick-bedded part 

of the Cobble Mountain Formation is a time-equivalent 

of the Hawley Formation in northern and central Massachusetts or at least that part of the Hawley presently exposed near Blandford, Mass. If the thick -bedded member 

is not time-equivalent to the Hawley, the member is 

younger and so is a correlative of the Goshen Formation. 

The two possibilities were proposed by Hatch and Stanley 

(1973, p. 15). 

Perhaps the best time marker so far identified in 

rocks of the Hawley and Goshen Formations of western 

Massachusetts is an unconformity described by Hatch 

(1967, p. D14) and by Hatch and Stanley (1973, p. 13-14) 

and recognized at least as far south as the Blandford quadrangle. From their description, one can infer that it might 

mark a widespread erosion surface and that evidence of 

it might be found elsewhere in southern New England. 

In the Bristol quadrangle no unconformity between 

the Sweetheart Mountain Member and the overlying strata of The Straits Schist has been identified. However, a 

basal facies of The Straits Schist that was observed locally 

suggests the possibility of such an erosion surface. The facies is described as part of that unit. Stanley ( 1964) did not 

recognize an unconformity at the same stratigraphic position in the Collinsville quadrangle, but did map "calc-silicate 

gneiss or granulite" in a position comparable to the basal 

facies in the Bristol quadrangle. Hatch and Stanley (1973, 

p. 24-25) considered it "lithically identical with and at the 

same stratigraphic horizon as the Russell Mountain Formation in the Blandford" area. In the Southington quadrangle Fritts (1963) drew an unconformity at the base of 

his Straits Schist. 

If the inference of an erosion surface at the top of 

the Sweetheart Mountain Member in the Bristol quadrangle is valid, the surface could be presumed to correlate 

with the unconformity described by Hatch (1967) and 

Hatch and Stanley (1973). In that event, the unnamed 

thick-bedded (upper) member of the Cobble Mountain Formation is logically correlative in age with the upper part 

of the Hawley Formation and with the Sweetheart Mountain Member of the Collinsville Formation. If the presumption of correlation is not valid, then the thick-bedded 

member of the Cobble Mountain Formation must be 

younger than the Hawley or at least that part of the 

Hawley exposed near Blandford, Mass. 

The Straits Schist 

In 1898, Emerson (p. 177) gave the name Goshen 

Schist to metamorphic strata of inferred Late Silurian age 

in central western Massachusetts. In northern Vermont, 

Richardson (1908, p. 276-279) applied the name "Memphramagog slates" to a loosely constituted unit of inferred 

Ordovician age that included slate bodies known commercially as Montpelier and Northfield slates. In 1941, Currier 

and Jahns redefined Richardson's unit, renamed it the 

Northfield Slate, and tentatively considered it to be of 

Middle Ordovician age. Cady, in 1956, believed the Northfield Slate to be of possible Silurian age; Skehan, however, 

in 1961 considered the unit to be Silurian or Devonian in 

age from the work of Boucot and others (1958), but Doll 

and others (1961), although they accepted Skehan's age 

assignment, used the name Northfield Formation. 

Subsequently, geologists (Hatch, 1967; Hatch and 

Hartshorn, 1968; Osberg and others, 1971; Hatch and 

Rock Stratigraphic Units and Their Correlation 7 
Stanley, 197 3) working in and near Emerson's original area 

in Massachusetts readopted Emerson's original name, 

modifying it to Goshen Formation (Hatch, 1967) of 

Silurian and Devonian age. The formation is described 

(Hatch and Stanley, 1973, p. 16) as a mainly "thinly bedded, 

graded bedded unit of carbonaceous pelitic schist and 

micaceous quartzite." An unconformity marks the base of 

the formation (Hatch, 1967, p. D 14; Hatch and Stanley, 

1973, p. 13-14). 

The work in northern New England summarized 

here was in progress when the metamorphic stratigraphy 

of western Connecticut began to attract significant interest 

among geologists in the late 1940's and early 1950's. The 

work in Connecticut was hampered by the lack of detailed 

studies in the area between Connecticut and northern New 

England, for dependable correlations between those 

localities were not feasible. Correlations mainly were 

attempted with the work of others nearby, and a 

proliferation of proposals resulted. Commonly the proposals 

were tentative as the stratigraphy was complicated, the 

structure complex, and the geologic age of field units was 

consistently indefinite and at best inferred. Undoubtedly 

the most significant advance in unraveling the geology of 

western Connecticut was the recognition by Rodgers and 

others (1959) of a lithologically distinctive stratigraphic unit 

of wide lateral extent. 

The name "The Straits Schist Member of the 

Hartland Formation" and an age of possible Middle 

Ordovician or older was given by Rodgers and others (1959, 

p. 40) to metamorphic strata that are well exposed in a 

gorge that was formerly known as The Straits, and that 

is on Connecticut Route 63 just north of Bethany, Conn. 

They characterized the rock as a coarse to very coarse, 

commonly crumpled, muscovite schist with various minor 

minerals; "locally quartz (and feldspar) and mica dominate 

alternate layers" causing a more gneissic character in them. 

The Straits Schist was recognized by various geologists in 

western Connecticut, but inferences as to its correlation 

and age have differed greatly. Rodgers and others (1959, 

p. 33) correlated the Hartland Formation with Emerson's 

(1898) Hoosac Schist that Emerson considered to be of 

Early Silurian age. Subsequent authors (Cady, 1960; Doll 

and others, 1961) considered the Hoosac Formation to be 

of Early Cambrian(?) age. The Hartland Formation has 

been divided recently by various geologists (see Gates and 

Martin, 1976) into five Hartland units designated by 

Roman numerals from the basal unit upward. Gates and 

Martin (1976, p. 322) correlated Hartland unit II with The 

Straits Schist, but this is not agreed to by Stanley and Hatch 

(1976, p. 355). 

Fritts (1963) raised The Straits Schist Member to 

formational rank, modified the name to "Straits Schist," 

and thought it to be of Cambrian(?) age. Stanley (1964, 

p. 28) preferred the original form of the name, "The Straits 

8 Bedrock Geology of the Bristol Quadrangle, Connecticut 

Schist," retained the formational rank, and reassigned it 

to the Hartland Group, a name both revived and raised 

in rank. He inferred the age of the group to be Cambrian 

following the reasoning of Rodgers and others (1959). In 

1968, however, Stanley concluded, that "The Straits Schist" 

was in part equivalent to the Hawley Formation and in 

part to the Goshen Formation in central western 

Massachusetts. That conclusion implies his acceptance of 

a Late Silurian and Early Devonian age (Hatch, 1967; 

Hatch and Hartshorn, 1968). 

In the Bristol quadrangle, a distinctive sequence of 

metamorphic strata younger than the Sweetheart 

Mountain Member of the Collinsville Formation is 

correlated with the Straits Schist of Fritts (1963) in the 

adjacent Southington quadrangle. The strata in the Bristol 

quadrangle consist mainly of rusty-weathering, mediumto coarse-grained, commonly crenulated, garnet-bearing, 

graphitic, muscovite-rich, plagioclase-quartz schist 

characterized by graded bedding. The unit is differentiated 

from the underlying Sweetheart Mountain Member of the 

Collinsville Formation by the smaller size of the garnets 

present, the presence of graded bedding, the lesser amounts 

of both biotite and plagioclase, the common presence of 

crenulation, and, most obviously, the presence of graphite 

dusting the muscovite flakes in The Straits Schist. The 

correlation with Fritts' (1963) unit is based on those same 

characteristics. His name, Straits Schist, is modified here 

to include the capitalized article "The" originally used by 

Rodgers and others (1959). Fritts (1963) inferred a 

Cambrian(?) age for his unit, but here the age is tentatively 

inferred to be Silurian or Devonian, and the unit is 

tentatively correlated with the Goshen Formation of Hatch 

and Hartshorn (1968) and of Hatch and Stanley (1973). 

This correlation is based on the lithologic similarity and 

the relation to a possible unconformity at the base of the 

two units. 

The basal contact of The Straits Schist in that part 

of the Bristol quadrangle south of the Pequabuck River 

and just west of Lake A venue is marked by a lithologic 

facies that has a greater feldspar content than does the 

remainder of the unit. Much of the feldspar appears as 

rounded clots. This basal part may represent a coarser, 

probably more feldspar-rich, facies of the original sediment; 

the clots may represent feldspathic pebbles. Such a facies 

may represent a basal sandstone on an unconformity; if 

so, it supports the correlation of The Straits Schist as a 

correlative of the Goshen Formation, for Hatch (1967, 

p. D 14) referred to "a major regional unconformity" at the 

base of the Goshen, and Hatch and Stanley (1973, p. 13) 

stated that in the Blandford quadrangle, central western 

Massachusetts, Goshen strata "rest on successively older 

units." In the Collinsville quadrangle Stanley locally 

mapped ( 1964) calc-silicate-quartzite lenses in the upper part 

of the Collinsville Formation (1968); they have been 
correlated (Hatch and Stanley, 1973) with the Russell 

Mountain Formation in western Massachusetts (Hatch and 

others 1970; Hatch and Stanley, 1973). Fritts (1963) 

showed an unconformity at the base of the Straits Schist 

in the Southington quadrangle. 

Southington Mountain Member.-In 1963, Fritts 

gave the name Southington Mountain Schist to 

metamorphosed strata of Cambrian(?) and Ordovician(?) 

age in the northern part of the adjacent Southington 

quadrangle, where they are well exposed on Southington 

Mountain. The strata, which were described as the upper 

part of The Straits Schist Member of the Hartland 

Formation by Rodgers and others (1959), consist of 

alternating bands of quartz-feldspar granulite and of 

graphitic, muscovite-biotite schist that are characterized by 

distinct, widespread, graded bedding and locally by much 

staurolite. 

These rocks are directly traceable into the southern 

part of the Bristol quadrangle, where strata that are 

lithologically the same are widely exposed. These rocks are 

therefore assigned to the same unit, which is here revised 

as the Southington Mountain Member of The Straits Schist 

and may correlate with the Goshen Formation (Hatch and 

Hartshorn, 1968) of the Heath quadrangle, MassachusettsVermont, of Middle Silurian to Early Devonian age. The 

correlation is based on the graded bedding and the 

stratigraphic sequence, including the possible unconformity 

at the base of the underlying Straits Schist. Gates and 

Martin (1976, p. 325) correlated the Southington Mountain 

Schist with their Hartland unit III, but this was not agreed 

to by Stanley and Hatch (1976, p. 355). 

New Haven Arkose 

The eastern one-third of the Bristol quadrangle lies 

within the central lowland of Connecticut, known 

geologically as the Hartford Mesozoic basin (see fig. 1). 

Bedrock consists of the New Haven Arkose of the Newark 

Supergroup (Upper Triassic), which was named and defined 

by Krynine in 1950 for exposures in New Haven County, 

Conn. In the Bristol quadrangle the strata are poorly 

exposed. They probably consist mainly of massive- to wellbedded, coarse- to fine-grained reddish-brown arkosic 

sandstone and conglomerate with iron-oxide cement. The 

beds dip downward to the east and southeast at angles as 

great as 20 degrees. 

The western margin of the lowland is marked by a 

prominent fault-line scarp, with the fault downthrown on 

the east. In places along the fault the normal rock color 

has been altered by ground water leaching the iron-oxide 

cement. This alteration turns the rock to a distinctive pink 

to light-orange. Leaching is best exposed on the west 

side of Jerome Avenue about 0.5 mi north-northeast of 

Hurley Hill. 

INTRUSIVE ROCK UNITS 

Minor amounts of six intrusive rock units are present 

in the Bristol quadrangle. Two kinds of pegmatites are exposed at many localities in the metamorphic rocks of the 

upland; serpentinite and granite are each found at two 

places, and an amphibolite at one. In addition basalt, which 

may be intrusive, is present at one locality within the Triassic rocks of the lowland. 

Pegmatite is widely distributed and is present in all 

metamorphic rock units except the unnamed metaquartzite. However, pegmatite is noted predominantly in rocks 

of inferred Middle Ordovician age. Most pegmatite bodies 

constitute pods or dikes. The pods are as much as 20 ft 

across and 7 5 ft long; the dikes are generally less than 12 

ft across and are of indeterminate length. Pegmatite pods 

are composed mainly of quartz and twinned, white 

plagioclase feldspar. Several small pegmatite dikes are composed chiefly of quartz and pink orthoclase feldspar. At 

one locality, a tabular orthoclase pegmatite about 8 inches 

thick was observed to crosscut diagonally another dike of 

similar composition about 1.5 feet thick. 

Crystals of black tourmaline as much as 2 mm in 

diameter and about 1.2 em (centimeter) long are present 

in a few pods. In rare cases the number of crystals increases 

as the pod margin is approached. 

Two serpentinite bodies in the Bristol quadrangle are 

enclosed by the Sweetheart Mountain Member of the Collinsville Formation. The body near the northwest corner 

of the map area (pl. 1) must postdate the original sedimentary structures that enclose it and may have been intruded 

no later than the metamorphism that altered those sediments. If the internal gneissic structure of the body was 

caused by flowage during emplacement, then it may be 

younger than the metamorphism. No definitive information was observed in the other serpentinite. 

Amphibolite constitutes a single dike about 500 ft 

east of Bristol Reservoir No. 1. The rock may represent 

a mafic intrusive nearly as old as the enclosing sediments 

that became the Bristol Gneiss, or as young as pre-late 

metamorphic age. 

Gray granite forms two narrow dikes. The rock in both 

bears a striking resemblance to the intrusive Thomaston 

Granite (Cassie, 1965), exposed in the adjacent Thomaston 

quadrangle. Both dikes are nonmetamorphosed and thus are 

presumed to postdate the regional metamorphism. The maximum observed thickness of the granite dikes is 90 em. 

Three small closely spaced exposures of basalt at a 

single locality on the southwest flank of Campground Hill 

in the east-central part of the Bristol quadrangle lack visible contacts with the surrounding New Haven Arkose. The 

rock is composed of massive basalt and might represent 

a large boulder nearly covered by glacial deposits; it also 

might represent a basalt dike. If the rock is a dike, it may 

correlate with rock mapped as Buttress Diabase in the 

lntrrusive Rock Units 9 
adjacent Southington quadrangle (Fritts, 1963), as dolerite 

in the adjacent Collinsville quadrangle (Stanley, 1964), as 

Buttress Diabase in the contiguous Avon quadrangle 

(Schnabel, 1960), and as Buttress Dolerite on the bedrock 

geological map of Connecticut (Rodgers, 1985). The age 

of the Buttress Diabase is Early Jurassic based on 

potassium-argon age deterninations by Sutter (1985). 

METAMORPHISM AND DEFORMATION 

Most rocks of the Bristol Dome owe their present character to metamorphism that altered original rocks, minerals, 

and textures to those now apparent. Because the existing 

rocks contain evidence of both graded bedding and crossbedding and constitute a sequence of stratigraphic units, the 

original rocks probably were mainly of sedimentary origin, 

perhaps derived in part from reworked volcanic material and 

possibly in part from some interlayered volcanic deposits. 

Little evidence has been obtained from the rocks of 

the Bristol quadrangle regarding the geologic age or ages 

of the metamorphism and crustal deformation that affected 

them. The deformation and metamorphism must postdate 

deposition of the youngest domed and metamorphosed 

unit, the Southington Mountain Member of The Straits 

Schist. If the inferred correlation of that schist with the 

Goshen Formation of Silurian and Devonian age is correct, then the doming and perhaps the regional metamorphism could have occurred during the Acadian orogeny 

at or near the end of Early Devonian time. Evidence of 

deformation or metamorphism during the Taconic orogeny 

in Late Ordovician time was not observed. 

The grade of metamorphism attained by rocks of the 

Bristol quadrangle is indicated by the presence of kyanite 

and staurolite, mainly in the Southington Mountain Member of The Straits Schist and locally in the Whigville 

Member of the Taine Mountain Formation. 

In Late Triassic and Jurassic time, normal faulting 

broke the younger sedimentary rocks into great blocks that 

were tilted downward to the east. 

STRUCTURAL GEOLOGY 

Faults.-The prominent topographic boundary between the metamorphic rocks of the Bristol Dome and the 

sedimentary New Haven Arkose of the adjacent lowland 

is a faultline scarp that reflects a normal fault. The fault 

formerly was exposed in a long-abandoned copper mine 

at the southern end of Mine Mountain. This exposure was 

described by early geologists, whose works were summarized by Bateman (1923), as a normal fault dipping 70 ° east, 

with a vertical displacement of 360 ft; adjacent to the fault 

the sedimentary strata show drag. No other exposure of 

the fault is known in the Bristol quadrangle. 

10 Bedrock Geology of the Bristol Quadrangle, Connecticut 

In 1960, Martin F. Kane (U.S. Geological Survey, 

written commun.) made two east-west reconnaissance 

traverses with a gravimeter across the quadrangle-one 

along and at the latitude of Shrub and Stevens Streets, and 

another adjacent to the Pequabuck River. Stations were 

spaced at about 1,000- to 3,000-ft intervals on both 

traverses. On the basis of assumed average rock densities 

of 2. 70 for crystalline rocks and 2.55 for arkosic strata, and 

a profile difference of about 1 milligal where each of the 

traverses crosses the inferred location of the boundary fault, 

Kane calculated vertical displacement along the boundary 

fault to be about 500 ft. 

The position of the boundary fault on the geologic 

map is inferred mainly from the topography, from the two 

gravimeter profiles, and from its inferred position in the adjacent Southington and Collinsville quadrangles. Southward 

from the Pequabuck River the fault may follow the foot 

of the topographic scarp more closely than is shown. 

Two other faults of significant observable displacement are known or inferred in the quadrangle. One trends 

north-northeast between Hurley Hill and Federal Hill and 

is clearly indicated by the stratigraphic displacement it 

caused. The other trends north from Cedar Swamp Pond; 

it is inferred from the pattern of lithologies in available exposures, but the evidence is not clear. 

Several faults of indeterminate throw and without 

topographic expression were observed in manmade exposures. The faults are marked by characteristics that include slickensides, clayey fault gouge, drag of foliation, and 

both fine and coarse brecciation. Together, they suggest two 

sets of faults-one trending northwestward, the other northnortheastward. On the west flank of Hurley Hill, brecciated rock recemented by quartz veinlets and containing flecks 

of copper minerals is additional evidence of faulting. 

Bristol Dome.-Martin F. Kane (written commun., 

1960) measured a drop of 3.5 milligals on the gravimeter 

traverse he made in Bristol along the Pequabuck River. On 

the basis of an assumed average rock density of 2. 70 for 

the metamorphic rocks, he concluded that a mass of lower 

density rock underlies the structural dome at a depth of 

1,500-2,000 ft. 

Lineaments.-Northeast-trending topographic lineaments along the east side of the quadrangle are in part extrapolated from faults mapped in the adjacent New Britain 

quadrangle (Simpson, 1966) and in part inferred from the 

topography of the Bristol quadrangle. The lineaments may 

extend southwestward to the north-trending boundary fault, 

but, if so, that extension is hidden beneath the thick glacial deposits that lie west of Redstone Hill. The lineaments 

shown may represent faults of small displacement; others 

of the set that are not recognized may lie to the north. 

The offset of the amphibolite rock unit mapped at 

the south end of Mine Mountain may be due to faulting. 

Extension of an inferred fault to the northwest is suggested 

by the topographic lineament of the northwest branch of 
Wildcat Brook; this valley extends into the adjacent 

Collinsville quadrangle, where it is buried beneath thick 

surficial deposits. 

Distinct topographic lineaments that constitute three 

major sets plus a minor set are apparent on aerial photographs of the southwest part of the quadrangle, and are 

almost as distinct on the topographic base map. From their 

orientation relative to the mapped syncline separating the 

Bristol Dome from the Waterbury Dome to the southsouthwest (Fritts, 1963), and from foliation attitudes, these 

lineaments are thought to represent faults of small displacement, or joints. Three major sets of lineaments are also 

distinct in the northwest part of the quadrangle, but they 

are less clear on the topographic base. 

ECONOMIC GEOLOGY 

An abandoned copper mine is at the south end of 

Mine Mountain. Reportedly (Norton, 1907), this mine was 

opened sometime in the early 1800's and closed in 1895, 

having yielded a profit as great as $2,000 a month at the 

time of a temporary closing in 1857. The orebody (Bateman, 1923) followed a normal-fault contact between the 

metamorphic rocks and the younger sedimentary strata. 

The principal ore minerals were chalcocite and bornite, 

with subordinate chalcopyrite and small amounts of covellite, pyrite, sphalerite, galena, and barite; quartz and calcite were also present in considemble amounts. The ore 

minerals occurred in veins as much as 8 in. thick and as 

disseminated veinlets and blebs in the sedimentary strata. 

Bateman stated that the minerals were deposited by solutions at slightly less than 91 oc that bleached the strata 

by leaching from them the ferrous oxide that causes their 

red color. Bateman also inferred that the solutions were 

derived from basaltic magmas that formed the nearby dikes 

and lava flows. An alternative source may have been the 

granitic magma that yielded either the gray granite or the 

pink pegmatite in the quadrangle. 

Two other apparently unsuccessful attempts to mine 

copper ore were located farther south. One (Peck, 1932, 

p. 140-141) included the construction of one or more 

shafts and some lateral drifts about 1850 "between Bradley 

Street and Belridge Road, near Brewster Road." The other 

(W. R. Roseen, oral commun., June 1961) was located 

across the Pequabuck River from the south end of Hurley 

Hill but on the north side of Mountain Road and about 

500 ft west of King Street, behind the Roseen home. He 

described "a shaft, now filled," and showed ore samples 

containing wolframite, pyromorphite, galena, sphalerite, 

and wulfenite. 

Pegmatite.-No pegmatite has been quarried for 

production of feldspar, quartz, or mica. The size of the pegmatite pods and lenses, none more than about 400 ft long, 

suggests that such an operation is not presently economic. 

Cut stone.-Widely scattered small quarries have 

been opened in each of the principal types of metamorphic 

rock exposed in the Bristol quadrangle; generally, only one 

quarry has been opened in each type. Some of the rock 

quarried appears to have been used as foundation stone 

for farm buildings in the vicinity. Most of the quarries, 

however, apparently were opened to supply trimmed foundation blocks used in the construction of the former Fuller 

Barnes residence. 

Other resources.-No other rock or mineral 

resources of present economic character were observed in 

the bedrock. 

ENGINEERING GEOLOGY 

All bedrock units in the Bristol quadrangle are composed of strong rocks capable of in-place support of great 

loads. Their support can fail by rocksliding (landsliding) in 

some circumstances. The potential for such failure is relative, and varies from moderate to negligible. It depends 

mainly on the orientation of faults, fractures, mineral layering, and the land surface in relation to the orientation and 

design of excavations. 

Slope Stability 

Excavations with vertical faces or slopes cut more 

or less perpendicular to the trend (strike) of the layering 

(foliation) of minerals will be most stable and probably will 

present only minor raveling of freshly blasted faces. Those 

faces or slopes cut nearly parallel to the strike of mineral 

layering can be subject to rocksliding along fractures or 

mineral-layering planes that slope downward into an excavation. If the fractures or mineral layering are oriented 

downward, away from· a cut face or slope, there is no hazard other than minor raveling. 

Fractures and mineral layering are most likely to be 

a cause of rocksliding in The Straits Schist and its Southington Mountain Member, less so in the Sweetheart Mountain Member of the Collinsville Formation and the Wildcat 

and Scranton Mountain Members of the Taine Mountain 

Formation-and even less so in the Bristol Gneiss, and least 

so in the hornblende gneiss and metaquartzite member of 

the Collinsville. 

Excavatabi I ity 

New Haven Arkose.-Thin beds of sandstone and, 

rarely, of conglomerate may be rippable locally with heavy 

equipment, especially where near the ground surface. Rare 

Engineering Geology 11 
thin beds of siltstone and shale are rippable but do not constitute thick-enough parts of the geologic sequence, or occur sufficiently frequently in interlayered sequences, to be 

of much benefit. Massive beds of sandstone and conglomerate undoubtedly will require blasting and should be shielded 

with blasting mats. Debris will tend to be irregular and angular in form. Bleached sandstone and conglomerate like 

that exposed in Page Park may occur elsewhere along the 

boundary fault. This sandstone is somewhat weaker than 

rock of normal red color and so may be more easily ripped 

and may yield slightly larger quantities of debris per blast. 

Wildcat, Scranton Mountain, and Whigville Members of the Taine Mountain Formation and the Sweetheart 

Mountain Member of the Collinsville Formation.-These 

rock units probably will require blasting and will yield 

somewhat slabby debris; the use of blasting mats is advised. 

Bristol Gneiss, and the hornblende gneiss and metaquartzite members of the Collinsville Formation.-These 

rocks are relatively massive as compared with the other 

rock units and cannot be ripped. Blasting will be necessary, 

and the debris will be somewhat irregular in form and angular; the use of blasting mats is advised. Some unstemmed 

shotholes in the hornblende gneiss member are reported 

to have "rifled out" without rock displacement. 

Dri llabi I ity 

No major problem should be presented by any of the 

bedrock units; however, The Straits Schist and its Southington Mountain Member tend to clog drill bits. 

A fault may constitute a construction problem if it 

slopes downward into an excavation in such a manner that 

the rock above the fault can slide along the fault surface 

into the excavation. 

No seismic activity along any of the mapped faults 

is know to have occurred in so long a time that all faults 

in the quadrangle are considered to be inactive. 

Zones of broken and pulverized rock along a fault 

may yield small to moderate flows of ground water when 

the fault is intersected by an excavation. The presence of 

ground water increases the potential for rocksliding into 

an excavation. 

Faults and Faulting 

Faults are mapped on the basis of visible geologic evidence of relative displacement of rock on the two sides of 

the fault trace; other faults may be present for which geologic evidence was not visible at the ground surface. 

Unknown faults will be similar in character to known faults. 

The attitudes of significant faults are unknown, with 

the exception of the fault along the east side of Mine Mountain, which is known from subsurface information to dip 

70 o east in the vicinity of the abandoned copper mine. 

12 Bedrock Geology of the Bristol Quadrangle, Connecticut 

That fault is represented by a wide zone of broken 

and pulverized rock that is known to be more or less 

cemented by subsequent mineralization at that locality. 

Other mapped faults may be marked by similar zones that 

are 1-5 ft thick, but they are not known to be mineralized. Minor faults of indeterminate displacement visible in 

some roadcuts are marked by similar zones less than 4 in. 

wide that lack mineralization. 

None of the faults is known to show evidence at or 

near the ground surface of movement at least once within 

the past 35,000 years or movement of recurring nature 

within the past 5,000,000 years. 

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