Geology of the Waterford Quadrangle, Virginia and 

Maryland, and the Virginia Part of the 

Point of Rocks Quadrangle 

By William C. Burton, Albert J. Froelich, JohnS. Pomeroy, and K.Y. Lee 

U.S. GEOLOGICAL SURVEY BULLETIN 2095 

A detailed study of the surficial and bedrock geology of part of the Blue 

Ridge province and Culpeper basin in northern Virginia and Maryland 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government. 

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication April 29, 1994. 

Library of Congress Cataloging in Publication Data 

Geology of the Waterford quadrangle, Virginia and Maryland, and the Virginia part of the 

Point of Rocks quadrangle I by William C. Burton ... [et al.]. 

p. em.- (U.S. Geological Survey bulletin; 2095) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2095 

l. Geology-Virginia. 2. Geology-Maryland. I. Burton, William C. (William Chapin), 

1952- . II. Series. 

QE75.B9 no. 2095 

[QE173] 

557.3 s-dc20 

[557 .55] 94-20824 

CIP 
CONTENTS 

Abstract .......................................................................................................................... . 

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

Geology of Proterozoic and Paleozoic Rocks ................................................................ . 2 

Middle Proterozoic Basement ................................................................................ . 2 

Granitic Gneiss .............................................................................................. . 3 

U-Pb Ages of Granitic Gneisses .................................................................... . 5 

Layered and Mafic Gneisses .......................................................................... . 5 

Middle Proterozoic Fabric and Structures ..................................................... . 7 

Middle Proterozoic Metamorphism ............................................................... . 7 

Late Proterozoic Dikes ........................................................................................... . 8 

Dike Lithology .............................................................................................. .. 8 

Dike Density and Orientations ...................................................................... .. 8 

Geochemistry ................................................................................................. . 9 

U-Pb Age of Rhyolite Dike ............................................................................ . 11 

Late Proterozoic Metasedimentary and Metavolcanic Rocks ................................ . 12 

Swift Run Formation ...................................................................................... . 12 

Catoctin Formation ........................................................................................ . 13 

Possible Rift-Related Faults ........................................................................... . 14 

Chilhowee Group Metasedimentary Rocks .......................................................... .. 15 

Loudoun Formation ....................................................................................... . 15 

Weverton Quartzite ....................................................................................... .. 15 

Harpers Formation ......................................................................................... . 16 

Antietam Quartzite ......................................................................................... . 16 

Post-Chilhowee Group Metasedimentary Rocks .................................................. .. 16 

Carbonaceous Phyllite ................................................................................... . 16 

Tomstown Dolomite and Frederick Limestone .............................................. . 17 

Paleozoic Structure and Metamorphism ............................................................... .. 17 

First-Generation Paleozoic Cleavage ............................................................. . 17 

First-Generation Folds ................................................................................... . 19 

Second-Generation Cleavage and Folds ........................................................ . 19 

Late Folds ....................................................................................................... . 19 

The Tectonic Role of Faulting ...................................................................... .. 20 

Late Brittle Structures ................................................................................... .. 20 

Paleozoic Recrystallization and Metamorphic Grade .................................... . 21 40Arf39 Ar Dating of Metamorphic Fabric ....................................................... .. 22 

Geology of Early Mesozoic Rocks ................................................................................ . 22 

Sedimentary Rocks ................................................................................................ . 22 

Manassas Sandstone, Poolesville Member ................................................... .. 22 

Balls Bluff Siltstone and its Leesburg Member ............................................ .. 23 

Igneous and Metamorphic Rocks ........................................................................... . 24 

Diabase ........................................................................................................... . 24 

Thermally Metamorphosed Rocks ................................................................. . 25 

Structural Geology ................................................................................................. . 25 

Bull Run Fault ............................................................................................... .. 25 

Furnace Mountain Fault and Related Faults and Lineament ........................ .. 25 

Inferred Faults ................................................................................................ . 26 

Relative Timing of Mesozoic Faulting .......................................................... . 26 

Joints in the Culpeper Basin .......................................................................... . 26 

Surficial Geology ........................................................................................................... . 26 

Economic Geology ........................................................................................................ .. 27 

Hydrogeology ................................................................................................................ . 27 

References Cited ............................................................................................................ . 28 

III 
IV CONTENTS 

PLATE 

[Plate is in pocket] 

1. Geologic map of the Waterford quadrangle, Virginia and Maryland, and the Virginia part of the Point of Rocks 

quadrangle. 

FIGURES 

1. Map showing the study area, and mapping coverage by authors, of the Waterford quadrangle, Virginia and 

Maryland, an9 the Virginia part of the Point of Rocks quadrangle ................. ................. ........... .. . . . . ..... .. .. ..... ...... .. 3 

2. Lower hemisphere equal-area projections of structures of Middle Proterozoic age................................................ 5 

3. Photomicrograph showing thin section of garnet graphite paragneiss, Waterford quadrangle................................ 6 

4. Photomicrograph showing thin section of hornblende metanorite, Waterford quadrangle . . . .. . . .. .. . . . . . . . . . . . . .. . .. . . . . . . . . . 6 

5. Diagrams showing orientation of Late Proterozoic dikes........................................................................................ 9 

6. AFM diagram showing Late Proterozoic metadiabase and metabasalt................................................................... 9 

7. Variation diagrams of Late Proterozoic metadiabase and metabasalt...................................................................... 12 

8. Chondrite-normalized REE diagram of Late Proterozoic metadiabase and metabasalt.......................................... 12 

9. Photograph showing recumbent, isoclinal F1 folds and more open, upright F2 folds with associated steeply 

dipping crenulation cleavage in interlayered phyllite and quartzite of the Loudoun Formation............................. 15 

10. Photograph of outcrop on western flank of Catoctin Mountain, just east of Furnace Mountain, showing upright, 

post-F2 kink folds in S1-laminated quartzose phyllite of the Harpers Formation.................................................... 16 

11. Photomicrograph showing thin section of well-developed S1 in quartzose phyllite of the Harpers Formation, 

Point of Rocks quadrangle .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

12-15. Lower hemisphere equal-area projections of12. First- and second-generation structures of Paleozoic age.................................................................................. 18 

13. Late folds............................................................................................................................................................ 20 

14. Late brittle structures in pre-Mesozoic rocks..................................................................................................... 21 

15. Contoured poles to joints in the Culpeper basin................................................................................................ 21 

16. Photograph showing outcrop of carbonate conglomerate, Leesburg Member of Balls Bluff Siltstone, just 

west of junction of State Routes 661 and 15....... .. . . . . . . . . . .. . . . . . . . . . . . .. . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

TABLES 

1. Mineral compositions in volume percent of samples of five Middle Proterozoic rock units............................................ 4 

2. Major oxide and trace-element geochemistry of four Middle Proterozoic rocks.............................................................. 4 

3. Major oxide and trace-element abundances and averages of Late Proterozoic metadiabase, metabasalt, and 

metarhyolite...................................................................................................................................................................... 10 

4. Major oxide geochemistry of high-titanium, quartz-normative Jurassic diabase dike, Waterford quadrangle................. 24 

METRIC CONVERSION FACTORS 

Multiply By To obtain 

Length 

millimeter (mm) 0.03937 inch 

centimeter (em) 0.3937 inch 

decimeter (dm) 3.937 inch 

meter (m) 3.281 foot 

kilometer (km) 0.6214 mile 
Geology of the Waterford Quadrangle, Virginia and Maryland, and 

the Virginia Part of the Point of Rocks Quadrangle 

By William C. Burton, Albert J. Froelich1, JohnS. Pomeroy, and K.Y. Leei 

ABSTRACT 

The bedrock geology of the Waterford quadrangle and 

of the Virginia part of the Point of Rocks quadrangle consists of a portion of the Middle Proterozoic basement core 

and its cover sequence on the eastern limb of the Blue Ridge 

anticlinorium and the adjacent early Mesozoic Culpeper 

basin. The three major rock associations in this area are 

(1) Middle Proterozoic gneisses, which have been extensively intruded by Late Proterozoic metadiabase dikes, 

(2) unconformably overlying Late Proterozoic and early 

Paleozoic metavolcanic and metasedimentary rocks, and 

(3) Upper Triassic sedimentary strata intruded by Early 

Jurassic diabase. The Triassic and Jurassic rocks are separated from the older rocks of the anticlinorium to the west 

by a major normal fault, the Bull Run fault. Late Cenozoic 

surficial deposits of three major types unconformably overlie the bedrock: colluvium derived from Catoctin Mountain, 

terrace deposits of the Potomac River, and flood plain alluvium of the Potomac River and its tributaries. 

The Middle Proterozoic rocks of the core of the anticlinorium consist of four types of granitic orthogneiss as well 

as a suite of nongranitic rocks that occur in discontinuous 

lenses and are inferred to predate the orthogneisses. All of 

the gneisses contain high-grade metamorphic assemblages 

and structures of probable Middle Proterozoic (Grenvillian) 

age ( -1 Ga), with both northeast- and northwest-trending 

foliations and a southeast-plunging mineral lineation. 

Intruding these rocks are northeast-striking dikes of metadiabase, which locally constitute more than 50 percent of the 

bedrock, and rare metarhyolite. The metadiabase dikes 

resemble in bulk composition the overlying Catoctin Formation metabasalts for which they were probably feeders. 

Nonconformably overlying the gneiss-dike complex is 

a sequence of variegated clastic metasedimentary and 

metavolcanic rocks of the Swift Run and Catoctin Formations. The Catoctin is overlain by metamorphosed clastic 

rocks of the Chilhowee Group, which consists of the Loudoun Formation, Weverton Quartzite, Harpers Formation, 

and Antietam Quartzite. Locally overlying the Chilhowee 

1

Deceased. 

metasediments is a thin, discontinuous, carbonaceous phyllite that grades laterally into and is overlain by Tomstown 

Dolomite, which is in turn overlain by the Frederick Limestone. The Loudoun Formation and the rocks above the 

Chilhowee Group are limited to the northern part of the map 

area. 

All of the rocks of the Blue Ridge anticlinorium were 

strongly deformed and subjected to greenschist-facies metamorphism, probably in the late Paleozoic. The deformation 

produced a penetrative, well-developed northeast-striking 

schistosity and a later spaced cleavage, each accompanied 

by an episode of folding. The older episode of folding, and 

formation of the penetrative schistosity, is probably associated with the formation of the anticlinorium, whereas the 

second phase is well developed only on the eastern limb, 

particularly at the northern end of Catoctin Mountain in Virginia. Several later, minor fold phases also occur in this 

region. Metamorphism associated with the Paleozoic deformation reached biotite grade and produced retrograde micaceous mineral assemblages in the basement gneisses, 

greenschist-facies minerals in the metadiabase dikes and 

metabasalts of the Catoctin Formation, and prograde muscovite and biotite in the clastic metasediments. Muscovite in 

the cover-sequence metasedimentary rocks that grew during 

formation of the Paleozoic schistosity has been dated by 

the 40 Ar/39 Ar age-spectrum technique as probable late 

Paleozoic. 

The regionally deformed and metamorphosed Proterozoic and Paleozoic rocks of the Blue Ridge anticlinorium in 

the west are separated from the early Mesozoic rocks of the 

Culpeper basin in the eastern part of the map area by the 

north-striking, east-dipping Bull Run normal fault, which 

has a throw of thousands of meters. The Upper Triassic sedimentary rocks of the basin consist of three major units: (1) 

red sandstone and siltstone (Poolesville Member of the 

Manassas Sandstone), which is overlain by (2) red siltstone, 

sandstone, and shale (Balls Bluff Siltstone) that laterally 

interfingers with (3) carbonate conglomerate (Leesburg 

Member of the Balls Bluff Siltstone). These sedimentary 

rocks are locally intruded by Early Jurassic diabase, which 

also intrudes rocks of the anticlinorium to the west. The Triassic sedimentary rocks are gently warped and mostly west 

dipping, are truncated on the west by the Bull Run fault, and 
2 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

are inferred to be cut by a number of unexposed cross-basin 

normal faults. 

The map area contains three types of surficial deposits 

of late Cenozoic age. Colluvium largely derived from 

Catoctin Mountain blankets both the uplands of the Blue 

Ridge anticlinorium and the lowlands of the Culpeper basin. 

Quaternary terrace deposits of the Potomac River occur as 

isolated patches on the Triassic rocks of the Culpeper basin. 

Alluvium occurs in both the Potomac River flood plains and 

the streambeds of its tributaries. Features associated with 

ground-water recharge and discharge, respectively, in the 

Culpeper basin include sinkholes and depressions, springs, 

and tufa deposits. 

INTRODUCTION 

Geologic Setting.-The map area of this report (pl. 1, 

in pocket) includes a portion of the eastern limb of the Blue 

Ridge anticlinorium and a portion of the western part of the 

Culpeper basin south of the Potomac River in Virginia and 

east of the Potomac in Maryland. From west to east the bedrock consists of rocks of the basement core of the anticlinorium, including Middle Proterozoic gneisses that are cut by 

a sheeted dike complex of Late Proterozoic age; a generally 

east dipping sequence of Late Proterozoic to Cambrian lowgrade metasedimentary and metavolcanic rocks that rests 

unconformably on basement; a moderately east dipping 

normal fault and associated faults of early Mesozoic age; 

and generally west dipping sedimentary rocks cut by minor 

intrusive diabase in the early Mesozoic Culpeper basin. 

These rocks bear evidence of four major tectonic events: the 

-1-Ga Grenville orogeny, the -600-Ma Iapetan rifting 

event, the -300-Ma Alleghany orogeny, and the -200-Ma 

early Mesozoic rifting event. Physiographically the rifted 

basement core of the anticlinorium and the Culpeper basin 

occupy a subdued plateau and lowlands, respectively, 

whereas the deformed metasedimentary and metavolcanic 

rocks underlie the north-trending ridge of Catoctin Mountain and its flanking uplands. 

Previous Work in the Map Area.-This area has been 

mapped at smaller scales by several previous authors. Keith 

(1894), Jonas and Stose (1938), and Stose and Stose (1946) 

mapped a large area of the Blue Ridge in Maryland and Virginia, characterizing the local stratigraphy and general style 

of Appalachian deformation. Cloos (1941, 1947), in his 

mapping in Maryland, developed the concept of a large 

west-verging anticlinorium that is cored by basement; the 

Catoctin Mountain represents the eastern, upright limb. 

Whittaker (1955) mapped Catoctin Mountain from Leesburg north to its termination near Emmitsburg, Md. Nunan 

(1979) studied the stratigraphy of the Weverton Formation 

in an area that included Catoctin Mountain. Howard (1991) 

did a reconnaissance of Blue Ridge basement lithologies 

that included some rocks in the map area. Previous studies 

of the Culpeper basin that include the present map area 

include Lee ( 1977, 1979), Lee and Froelich ( 1989), Froelich 

and others (1982), and Lindholm (1979). 

New Work in the Map Area.-Contributions made by 

this report include detailed mapping of the basement core of 

the Blue Ridge anticlinorium, including delineation of individual Iapetan-stage dikes and previously unrecognized 

units in the Middle Proterozoic gneisses; mapping of a 

number of stratigraphic units in the lower cover-sequence 

metasedimentary and metavolcanic rocks (Fauquier and 

Catoctin Formations); a refined interpretation of the structures on the eastern limb of the Blue Ridge anticlinorium; 

and detailed mapping of units and interpretation of block 

faulting in the early Mesozoic Culpeper basin. Isotopic ages 

obtained from various units in this study include U-Pb dating of zircons from a Late Proterozoic metarhyolite dike 

(Aleinikoff and others, 1995) and Middle Proterozoic granite gneiss (Aleinikoff and others, 1993) and 40ArJ39Ar ages 

from cleavage-defining muscovite associated with Paleozoic structures in the cover-sequence rocks (Burton and others, 1992a). 

Map Coverage.-This map and report combines the 

work of four geologists (fig. 1). Burton mapped the basement and most of the cover-sequence rocks west of the 

Culpeper basin at an original scale of 1:12,000, by using 

base maps provided by the Loudoun County Department of 

Natural Resources. The data were compiled later on U.S. 

Geological Survey 1:24,000 topographic base maps. 

Pomeroy mapped some of the cover rocks at 1:12,000 and 

compiled the data at 1:24,000. Lee did much of the original 

mapping of the Culpeper basin in the map area atl :24,000 

(Lee, 1979), and his work was revised by Froelich. The 

final compilation (pl. 1) and most of the text and figures are 

by Burton. 

Acknowledgments.-The authors gratefully acknowledge the support of the Loudoun County Department of 

Natural Resources, which provided funding for mapping 

and research, 1:12,000-scale base maps, and other information used in this report. Peter T. Lyttle (U.S. Geological Survey) provided previously unpublished geochemical data. 

GEOLOGY OF PROTEROZOIC AND 

PALEOZOIC ROCKS 

MIDDLE PROTEROZOIC BASEMENT 

The basement core of the Blue Ridge anticlinorium 

exposed in the map area (pl. 1) contains Middle Proterozoic 

gneisses, and younger, crosscutting Late Proterozoic igneous dikes. The Middle Proterozoic gneisses include two 

main types: relatively homogeneous granitic orthogneisses 

and variegated, well-foliated or layered metasedimentary 

and metaigneous gneisses of nongranitic composition. 

Granite gneiss is volumetrically the most abundant and 

encloses discontinuous belts and lenses of nongranitic 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 3 

77°37'30" 

39°22'30" 

VIRGINIA 

Point of Rocks 

quadrangle 39°15' ~------+-+--" 

Waterford 

quadrangle 

MARYLAND 

EXPLANATION 

Blue Ridge Late Proterozoic and Middle Proterozoic 

basement-mapped by W.C. Burton 

Lower Cambrian and Late Proterozoic 

cover-sequence rocks-mapped by W.C. Burton 

and J.S. Pomeroy 

Early Mesozoic Culpeper basin-mapped by 

A.J. Froelich and KY. Lee 

MARYLAND 

VIRGINIA 

Figure 1. Study area, and mapping coverage by authors, of the 

Waterford quadrangle, Virginia and Maryland, and the Virginia part 

of the Point of Rocks quadrangle. 

layered and mafic gneiss. The bodies of layered and mafic 

gneiss may represent remnants of a previous sedimentaryvolcanic terrane that was subjected to widespread intrusion 

and (or) partial melting during or before the -1-Ga Middle 

Proterozoic (Grenvillian) high-grade metamorphic event 

(Burton and Southworth, 1993). All of the gneisses contain 

recognizable Middle Proterozoic fabrics and structural 

trends that have been variably overprinted by Paleozoic 

deformation. 

GRANITIC GNEISS 

Four granitic gneiss units have been mapped in this 

area: relatively biotite-rich granite gneiss (Ybgg), biotitepoor leucocratic granite gneiss (Ygg), garnet-bearing leucocratic granite gneiss (Ygt), and biotitic Marshall Metagranite 

(Ymb). The recognition of biotite-poor granite gneiss, 

biotite-rich granite gneiss, and garnetiferous granite gneiss 

parallels Howard's (1991) facies A, B, and C, respectively. 
4 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

Table 1. Mineral compositions in volume percent of samples of 

five Middle Proterozoic rock units, based on point counts in thin 

section (one thin section per sample). 

[-,not found] 

Mineral 

Quartz 

Plagioclase 

Microcline (or 

perthite). 

Biotite 

Garnet 

Graphite 

Orthopyroxene 

Hornblende 

Ilmenite/sphene 

Apatite 

Chlorite (from 

garnet). 

Clinozoisite/ 

epidote (from 

plagioclase). 

Muscovite (from 

plagioclase). 

Rutile 

Total counts 

per sample. 

Map unit 

Description 

(sample number) 

Ygg Ybgg Yqp Yrg Yhp 

Leucocratic Biotite Quartz- Rusty Hornblende 

granite granite plagioclase paragneiss metanorite 

gneiss gneiss gneiss 

(BR- 11) (BR-554) (BR-36) 

35 

22 

39 

3 

<I 

<1 

592 

33 

23 

I9 

14 

<I 

24 

45 

I4 

2 

<I 

2 7 

8 8 

556 571 

(BR- 595) (BR-602A) 

30 

43 58 

I 

5 

3 

15 

<1 

2 

537 

35 

5 

<I 

508 

Modes for the first two units (Ybgg and Ygg) are shown in 

table 1, and major oxide and trace-element geochemistry of 

single samples of both units is shown in table 2. In these 

units the Grenville metamorphic texture is characterized in 

thin section by a flattened granoblastic-elongate fabric 

defined by aligned flakes of biotite and flattened grains of 

quartz, plagioclase, and microcline. The microcline commonly has subordinate amounts of perthite. The biotite-rich 

gneiss (Ybgg) is commonly well foliated and weathers to a 

reddish-orange color, which locally has a rusty tint due to 

disseminated magnetite. The leucocratic gneiss (Ygg) typically weathers white to light gray and has little or no biotite. 

A weak Middle Proterozoic foliation in this rock, where 

present, is defined by flattening of quartz and feldspar 

grains and rare, thin (1-2 em thick) aplite dikes that are concordant to metamorphic foliation. 

The third type, garnet-bearing leucocratic granite 

gneiss (Ygt), is similar in appearance to Ygg but contains 

scattered garnets that compose less than I 0 percent of the 

rock. Garnetiferous leucocratic gneiss is the dominant basement rock type immediately to the west and southwest of 

the map area (Southworth, 1991; Burton and others, 1992b). 

A fourth unit, biotitic Marshall Metagranite (Ymb), occurs 

in the southern part of the map area. It resembles biotitic 

Table 2. Major oxide (weight percent) and trace-element (parts 

per million) geochemistry of four Middle Proterozoic rocks. 

[n.d., not detected. All analyses, including X-ray spectroscopy, by U.S. 

Geological Survey; FeO, H20 , and C02 analysis by H. Smith, J.W. 

Marinenko, and J.R. Gillison; instrumental neutron activation analysis by 

G. Wandless and P. Baedecker] 

Major oxide 

Si02 

Al203 

Fe203 

FeO 

MgO 

CaO 

Na20 

K20 

Ti02 

P205 

MnO 

H2o+ 

H2oco2 

Total 

Trace element 

Scandium 

Chromium 

Cobalt 

Nickel 

Zinc 

Arsenic 

Selenium 

Rubidium 

Strontium 

Zirconium 

Molybdenum 

Antimony 

Cesium 

Barium 

Lanthanum 

Cerium 

Neodymium 

Samarium 

Europium 

Terbium 

Ytterbium 

Lutetium 

Hafnium 

Tantalum 

Gold 

Thorium 

Uranium 

Ygg 

1 

(BR-11) 

75.60 

13.00 

.65 

.44 

.22 

.79 

3.41 

5.11 

.24 

.05 

.02 

.29 

.04 

n.d. 

99.86 

4.71 

8.2 

1.006 

<12 

16.3 

<0.7 

<0.7 

83 

104 

205 

<2 

<0.06 

.049 

659 

41.1 

76.2 

38 

8.4 

.863 

1.109 

2.53 

.329 

5.91 

.347 

<4 

2.58 

.6 

Map unit 

Sample site1 

(sample number) 

Ybgg 

2 

(BR- 554) 

70.20 

13.80 

1.72 

1.80 

.74 

1.79 

2.68 

5.04 

.47 

.18 

.06 

.62 

.11 

n.d. 

99.21 

Yhp 

3 

(BR-602A) 

50.50 

16.40 

1.84 

5.60 

7.55 

1 I .30 

2.48 

.90 

.65 

.07 

.13 

2.50 

.01 

.01 

99.94 

Abundance (parts per million) 

10.13 36.5 

19.6 

6.79 

<31 

68.9 

<0.8 

<0.9 

I74 

I36 

263 

<2 

<0.06 

.91 

778 

38.I 

68.2 

29.1 

6.58 

1.26 

1.18 

3.89 

.5 

7.43 

.538 

<4 

2.89 

.76 

I2I 

46.4 

99 

80 

<0.05 

<2 

17.I 

460 

<70 

<5 

<O.I 

.3 

300 

6.2 

I3.7 

8.7 

2.39 

1.09 

.384 

1.14 

.178 

.9 

.089 

<3 

.38 

.2 

1 Sample site numbers refer to chemistry localities on plate I. 

Yrg 

4 

(BR-660) 

73.2 

11.3 

1.1 

3.4 

1.73 

1.48 

2.72 

1.32 

.67 

.06 

.1 

1.7 

n.d. 

n.d. 

98.78 

I4.25 

76.7 

I3.62 

45 

92 

<0.7 

n.d. 

40.2 

226 

3I 1 

<2 

.14 

<0.2 

357 

46 

90.6 

38.4 

8.17 

1.49 

1.16 

4.31 

.597 

7.73 

.57 

<2 

12.09 

1.15 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 5 

granite gneiss (Ybgg) but is generally less well foliated. This N 

unit has been traced south to the type area in the Marshall 

quadrangle (Burton and others, 1992b; P.T. Lyttle, U.S. 

Geological Survey, oral commun., 1991) as defined by 

Espenshade (1986). A sample of biotitic Marshall Metagranite (Ymb) from the Rectortown quadrangle yields a UPb isotopic age that differs significantly from one obtained 

from biotitic granite gneiss from the Waterford quadrangle, 

as discussed below (Aleinikoff and others, 1993). 

White, coarse-grained, Middle Proterozoic quartzplagioclase-microcline pegmatite of more than one intrusive generation is not shown but locally occurs as bodies 0.5 

to 2 m thick. Folded and concordant bodies probably represent pre- and syntectonic phases, respectively, relative to 

Grenville high-grade metamorphism and deformation. Also 

well exposed along the Potomac River are tabular pegmatite 

dikes, which crosscut Middle Proterozoic foliation (fig. 2A). 

These undeformed dikes clearly represent a post-Grenville 

intrusive phase but are themselves cut by Late Proterozoic 

mafic dikes. They might correlate with the Late Proterozoic 

Robertson River intrusive suite to the south. 

U-Pb AGES OF GRANITIC GNEISSES 

As part of a regional geochronological study of the 

granitic gneiss units in the northern Blue Ridge anticlinorium, zircons were extracted from an exposure of the felsic 

granite gneiss (Ygg) along the Potomac River (sample 1, 

BR-11, in tables 1, 2; symbols 1 and U-1 on pl. 1); they 

yield a U-Pb age of 1060 ± 2 Ma, which is thought to represent the time of intrusion and crystallization (Aleinikoff and 

others, 1993). An exposure of biotite granite gneiss (Ybgg) 

from the Waterford quadrangle (sample 2, BR-554, in 

tables 1, 2; symbols 2 and U-2 on pl. 1) yields a crystallization age of 1055 ± 5 Ma (Aleinikoff and others, 1993). Garnetiferous leucocratic granite gneiss (Ygt) from the Harpers 

Ferry quadrangle yields an age of approximately 1 ,070 Ma 

(Aleinikoff and others, 1993). These three ages belong in 

the youngest group of dates (group 3) of Burton and others 

(1994 ). In contrast, a sample of biotitic Marshall Metagranite (Ym b) from the Rectortown quadrangle, southwest of the 

map area, yields an age of 1110 ± 4 Ma, putting it in the 

older group 2 of Burton and others (1994). 

LAYERED AND MAFIC GNEISSES 

The Middle Proterozoic rocks include units of probable sedimentary and volcanic or mafic intrusive origin in 

association, and contrasting lithologically, with the granitic 

gneisses. These rocks are best exposed in Milltown and 

Catoctin Creeks in the Waterford quadrangle. 

The most abundant unit in this suite of rocks is a 

graphite-bearing, garnet-rich paragneiss (Yrg) (tables 1, 2; 

fig. 3). This rock is distinctively rusty weathering and 

N 

[] 

[] [] 

+ [] [] [] • 

+ 

+ 

++• + [] + 

+ • + 

+ ++ 

++ [] + .. 

Figure 2. Lower hemisphere equal-area projections of structures 

of Middle Proterozoic age. A, Poles to foliation; contour interval is 

2 percent per 1 percent area (n=90); poles to tabular pegmatite 

veins (a, n=12). B, Fold hinges (•, n=5), mineral lineations (+, 

n=15), and poles to axial planes of isoclinal folds (a, n=9). 

contains a well-developed Middle Proterozoic layering 

defined by alternating quartz-plagioclase- and garnetbiotite (chlorite)-rich zones. It is locally migmatitic and has 

thin (1-3 em) sills of aplite that are parallel to the layering. 

Although not always present, graphite occurs as evenly disseminated flakes less than 1 mm across. The original garnet 
6 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

Figure 3. Photomicrograph showing thin section (plane polarized 

light) of garnet graphite paragneiss (Yrg), Waterford quadrangle. 

Minerals are garnet (Gt), graphite (Gr), chloritized biotite (B), 

sericitized plagioclase (P), and quartz (Q). 

content was as much as 30 to 40 percent in some places; 

however, due to Paleozoic deformation, garnet is almost 

completely retrograded to green chlorite and is seen 

commonly only as smeared-out chlorite blebs. Biotite is retrograded commonly also to chlorite. High-grade aluminosilicate minerals such as sillimanite have not been found. 

Accessory minerals are locally common (as much as 15 percent) and include ilmenite, sphene (mantling ilmenite), apatite, rutile, and hematite. The mineralogy of the rusty 

paragneiss suggests that the protolith for this rock was an 

impure sandstone or graywacke containing some organic 

content (Burton and Southworth, 1993). The rusty paragneiss generally is poorly exposed but occurs in several 

quadrangles to the south and west (Burton and others, 

1992b), indicating that it is a widespread unit in the Middle 

Proterozoic basement. This unit resembles the Border 

Gneiss of Hillhouse ( 1960) in the central Virginia Blue 

Ridge province, which is also garnet and graphite bearing, 

as described by Sinha and Bartholomew ( 1984) and Herz 

and Force (1984). 

The second most important member of the suite is a 

hornblende-orthopyroxene-plagioclase gneiss or metanorite (Yhp; table 1; fig. 4 ). Metanorite was first recognized 

by Howard (1991; facies D), who mapped several small 

rounded bodies of it in the Waterford quadrangle. His map 

pattern for the metanorite is revised in this study. This mafic 

gneiss is a medium- to coarse-grained, massive rock that has 

an igneous-appearing texture. Orthopyroxene is the dominant mafic mineral, and hornblende is subordinate. Together 

with plagioclase they form a granoblastic texture typical of 

Middle Proterozoic (Grenvillian) rocks (fig. 4 ). In thin section the pyroxene is extensively uralitized and its original 

optical character almost completely obscured. The hornblende commonly is altered to colorless amphibole but 

where fresh is golden brown. Near contacts with granite 

Figure 4. Photomicrograph showing thin section (crossed 

nicols) of hornblende metanorite (Yhp), Waterford quadrangle. 

Minerals are orthopyroxene (0), hornblende (H), plagioclase (large 

dark grains), and ilmenite (small black grains; opaque in thin 

sections). 

gneiss, the hornblende-pyroxene gneiss locally changes to a 

fine-grained and well-layered charnockite (not mapped separately) with the addition of biotite, quartz, and microcline. 

Ilmenite, sphene, and apatite are local accessory minerals. 

The hornblende-pyroxene gneiss is assumed to have a mafic 

igneous origin. Its major oxide and trace-element geochemistry is shown in table 2. Burton and others (1992b) have 

mapped larger bodies of charnockite and metanorite to the 

west and southwest. Charnockite is much more abundant in 

the central Virginia Blue Ridge than in the northern part 

(Sinha and Bartholomew, 1984; Burton and others, 1992b), 

and the relative abundance of charnockite is one of the main 

differences between the two regions. 

A minor unit within this suite of metasedimentary 

and metaigneous rocks is a gray, well-foliated to layered 

quartz-hornblende-biotite-plagioclase gneiss (Ybg). Where 

amphibole is locally the dominant mafic mineral, the rock is 

a metadiorite or amphibolite. Quartz content ranges from 0 

to 10 percent; ilmenite and sphene (mantling ilmenite) are 

accessory minerals. The hornblende-biotite gneiss is commonly migmatitic, contains thin concordant granitic sills, 

and can grade imperceptibly into surrounding granitic 

gneiss. The protolith for this rock was probably a sediment 

having a volcanic component. 

A fourth unit, quartz-plagioclase gneiss (Yqp), is a 

white- to gray-weathering felsic rock that ranges from massive to well foliated. It strongly resembles either Ybgg or 

Ygg in outcrop depending on biotite content but is characterized by little or no potassium feldspar, and its modal 

composition is that of a tonalite (table 1) or dacite. This unit 

is spatially associated with biotite granite gneiss (Ybgg), 

leucocratic granite gneiss (Ygg), and the layered and mafic 

gneisses. At one locality along Catoctin Creek, white, 

medium-grained, quartz-plagioclase gneiss is in contact 

with fine-grained amphibolite gneiss (Ybg) and contains 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 7 

internal biotite- and amphibolite-rich layers; these layers 

suggest a mafic-felsic volcanic sequence. Quartzplagioclase gneiss (Yqp) decreases in abundance southward 

from the Potomac River within the map area but is found 

northward to the nose of the basement core of the anticlinorium, according to reconnaissance by W.C. Burton (unpublished data, 1992). This rock resembles older (1300 Ma) 

trondhjemitic rocks that have been found in the Adirondacks (McClelland and Chiarenzelli, 1990), Green Mountains (Ratcliffe and others, 1991 ), and Reading Prong 

(Drake, 1984). However, U-Pb dating of zircon from a 

sample of this rock in Maryland puts it in the younger, 

group 3 intrusive suite (J.N. Aleinikoff, written commun., 

1993). 

The nongranitic rocks are interlayered with and enveloped by granite gneiss. Contacts between the two are concordant with respect to the regional Middle Proterozoic 

(Grenvillian) gneissic foliation and are probably pre- or 

syntectonic with respect to the Grenville orogeny; sharp 

crosscutting contacts are observed only for younger 

pegmatites. The present distribution and map pattern of the 

Middle Proterozoic units are thought to reflect partial melting of an older metasedimentary-metavolcanic terrane and 

intrusion by younger granites (Burton and Southworth, 

1993). Mapping to the northwest, west, and southwest 

(Southworth, 1991; Burton and others, 1992b) indicates that 

the older, pregranitic rocks are volumetrically minor but 

widespread. 

MIDDLE PROTEROZOIC FABRIC AND STRUCTURES 

Although the Middle Proterozoic rocks of the Blue 

Ridge have undergone extensive overprinting by subsequent 

Paleozoic deformation and low-grade metamorphism, Middle Proterozoic metamorphic fabrics defined by high-grade 

mineral assemblages and structures are still clearly discernible in many places. At outcrop scale these features, and the 

rock types they are commonly found in, include a foliation 

defined by flattened mafic minerals such as biotite or less 

commonly hornblende (units Ybgg, Ybg) and flattened 

quartz and feldspar grains (Ygg); layering defined by concordant thin (1-10 em) aplite or pegmatite sills (Ygg, Ybgg, 

Yqp, Yrg); biotite mineral lineation (Ybgg); and, rarely, open 

to isoclinal folds in metamorphic layering, the axial planes 

of which are parallel to the regional trend of foliation (Yrg, 

Ybg). 

Petrographically, Middle Proterozoic textures are distinguished by the presence of a granoblastic elongate fabric, 

which has triple-junction grain boundaries, that is defined 

by mineral assemblages that are stable at granulite-facies 

metamorphic grade (figs. 3, 4), as well as the absence of a 

dominant penetrative fabric defined by low-grade minerals 

(muscovite, chlorite, epidote, actinolite). However, even in 

areas having a well-defined older fabric, static retrograde 

alteration is ubiquitous due to Paleozoic and possibly Late 

Proterozoic low-grade (greenschist-facies) metamorphism. 

In thin section, plagioclase is commonly cloudy and has 

overgrowths of sericite or clinoizoisite, garnet grains are 

reduced to fragmented cores that have chlorite rims, biotite 

may have chlorite intergrowths or opaque exsolution lamellae, hornblende has retrograded to colorless amphibole, and 

pyroxene is extensively uralitized. 

Middle Proterozoic foliation in the map area has two 

main trends: a northwest trend with steep to moderate dips 

from the Potomac River to south of the Point of RocksWaterford quadrangle boundary and a more north-northeast 

trend with gentler dips in the rest of the Waterford quadrangle (fig. 2A). Rare, steeply plunging isoclinal folds are associated with and are axial planar to both foliation trends, and 

biotite granite gneiss (Ybgg) has a locally well-developed 

biotite lineation in both northeast and northwest trends. The 

biotite lineation, which is commonly parallel to rodded 

quartz and feldspar grains, suggests a stretching lineation. 

Significantly, the fold hinges, mineral lineations, and 

rodding associated with both foliation trends plunge southeast (fig. 2B). The poles to foliation (fig. 2A) appear to form 

a girdle around a pole approximately parallel to the mean of 

the fold axes and lineations (fig. 2B), which suggests that all 

these structures were affected or formed by a common tectonic event. 

MIDDLE PROTEROZOIC METAMORPHISM 

Mesoscopic and microscopic features in Middle Proterozoic rocks in the Blue Ridge basement, such as migmatitic layering and granoblastic mineral textures, clearly 

indicate high-grade metamorphic conditions during formation, and granulite-facies mineral assemblages are found in 

some lithologies in the map area. The presence of relict 

orthopyroxene and brown hornblende in textural equilibrium in hornblende-pyroxene gneiss (Yhp) (fig. 4) indicates 

hornblende granulite grade. A marginal facies of this 

gneiss that contains potassium feldspar is technically a 

charnockite-also diagnostic of granulite facies. Charnockitic rocks also are found elsewhere in the northern Blue 

Ridge basement (Howard, 1991; Southworth, 1991; Burton 

and others, 1992b). Well-developed Middle Proterozoic fabrics occur in both of the granitic gneiss samples from the 

map area dated by Aleinikoff and others (1993). Since these 

dates are presumably crystallization ages, it is reasonable to 

assume that at lease some of the metamorphism and formation of associated metamorphic fabrics and structures is 

younger than about 1060 to 1055 Ma. Cooling ages from 

hornblende from Blue Ridge basement in northern Virginia 

are about 1000 to 900 Ma and represent minimum ages of 

metamorphism (Kunk and others, 1993). The bracketed age 

of metamorphism and deformation thus corresponds to the 
8 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

late (Ottawan) phase of the Grenville orogeny of Moore and 

Thompson (1980). 

LATE PROTEROZOIC DIKES 

The Middle Proterozoic gneisses in the core of the 

Blue Ridge anticlinorium are intruded by a dense swarm of 

northeast-trending, steeply southeast dipping Late Proterozoic dikes that are basaltic in composition (table 3), with the 

exception of two dikes that are rhyolitic in composition (Zr, 

pl. 1 and table 3). These dikes have long been recognized 

(for example, Furcron, 1939; Reed, 1955; Bartholomew, 

1977), but a dike swarm of the density seen in this map area 

has not previously been mapped in such detail. The chemical composition of most of the dikes has led previous workers to consider them as feeders to the overlying metabasalts 

of the Catoctin Formation (for example, Reed, 1955; Espenshade, 1986), a conclusion that is corroborated by this 

study. 

DIKE LITHOLOGY 

In the map area, two main types of mafic dikes were 

mapped on the basis of grain size and texture: fine- to 

medium-grained (grains up to 2 mm) metadiabase dikes 

(Zd) and medium- to coarse-grained (2-8 mm grain size) 

actinolitic metadiabase dikes (Zda). The mineral characterizing this difference in grain size is pale-green to colorless 

actinolite. In outcrop, the coarse-grained dikes (Zda) have a 

distinctive nubby surface texture due to the weathering out 

of stubby amphibole grains. In the map area, the actinolitic 

dikes (Zda) are confined largely to the eastern margin of the 

exposed basement and decrease in abundance southward. 

Fine- to medium-grained actinolitic metadiabase dikes in 

the basement have been mapped to the south and west by 

previous workers (Espenshade, 1986; Allen, 1963; Furcron, 

1939), although some of the dikes are described as 

"amphibolites." Those dikes have a maximum grain size of 

2 mm (Lukert and Nuckols, 1976) and, therefore, probably 

correspond to the finer grained dikes (Zd) of this study. 

Truly coarse grained, gabbroic textured metabasic dikes, 

possibly correlative with Zda, have been reported only 

within cover-sequence metasedimentary and metavolcanic 

rocks to the south, including the "ultramafic rocks" of Allen 

(1963) and the "metagabbro" ofFurcron (1939). Reconnaissance by Burton (unpub. data, 1992), however, has shown 

coarse-grained dikes to be present as well near the western 

margin of the basement core of the anticlinorium. 

The original igneous texture and mineralogy of the 

mafic dikes Zd and Zda have been largely altered by a Paleozoic low-grade metamorphic event and deformation. In 

thin section the dominant minerals are those expected for a 

rock of basaltic composition metamorphosed under greenschist-facies conditions, namely chlorite, epidote, actinolite, 

and altered plagioclase. The pale-green to colorless actinolite is the most abundant mineral in the coarse-grained 

dikes, making up 50 to 70 percent of the rock. Its stubby, 

equant character suggests that it is derived from primary, 

igneous pyroxene. Relict ophitic and subophitic texture is 

preserved locally in the form of thin elongate laths of 

altered plagioclase enclosed in the amphibole. The dominant fabric in both coarse- and fine-grained types is a 

closely spaced, anastomosing metamorphic foliation primarily composed of fine-grained chlorite, epidote, and muscovite. This fabric deforms and clearly postdates formation of 

the amphibole crystals. Greenish-brown biotite, stilpnomelane, and needle actinolite are also found. The significance of the metamorphic minerals and fabric is discussed 

further in the section on Paleozoic recrystallization and 

metamorphic grade. 

A third dike lithology, Zr, is rhyolitic. Two such dikes 

were found; each was 1 to 3 m thick and could be traced 

along strike only a few meters. The best exposed rhyolite 

dike, along the Potomac River, is cream colored, has a 

dense, fine-grained, flinty texture, sharp planar margins, 

and an internal foliation parallel to the contacts, which originally may have been a primary flow foliation. The rhyolite 

dike intruded a metadiabase dike along one margin and 

thereby postdates it. In thin section it has a fine-grained, 

felted texture composed of microcrystalline quartz and 

potassium feldspar and scattered larger rounded grains of 

plagioclase. The foliation is marked by inclusion trains of 

fine-grained epidote, biotite, and hematite. Rhyolite dikes 

have also been reported elsewhere in the northern Virginia 

Blue Ridge basement (Southworth, 1991; Lukert and 

Nuckols, 1976). 

A fourth type of dike, Zi, is porphyritic metadiabase 

containing abundant white to pink subhedral phenocrysts of 

plagioclase (now heavily saussuritized) 1 to 5 mm in length 

in a fine-grained groundmass of actinolite and chlorite. It 

appears more felsic in composition than the coarse- or finegrained metadiabase dikes. A single body of this type was 

mapped north of Waterford, and its map pattern indicates 

that it cuts across the northeast-trending metadiabase dikes. 

Similar porphyritic dikes occur to the south and west (Klein 

and others, 1990; J.S. Schindler, U.S. Geological Survey, 

oral commun., 1991), but their petrogenetic significance is 

unknown. 

DIKE DENSITY AND ORIENTATIONS 

A fairly accurate rendering of the true distribution and 

density of the Late Proterozoic dikes can be seen best in 

cross section A-A' and the western part of cross section 

C-C' (pl. 1). In such areas of near-continuous exposure the 

dikes can be seen to vary in width from a few meters to a 

few tens of meters. Although a few narrow dikes are not 

shown, in these areas the map resolution is limited more by 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 9 

N 

• • 

• • 

• • • • • 

•• • ... l • • • ., .. •• • 

• • • + • 

N 

Figure 5. Orientation of Late Proterozoic dikes. A, Lower 

hemisphere equal-area projection of poles to exposed dike contacts 

(•, n=36). B, Rose diagram of contact azimuths. 

gaps in exposure than by scale, and the dike pattern shown 

is generally correct. Elsewhere on the map there is much 

approximation and the dike-basement pattern is certainly 

far more complex than shown. In the best exposed section 

(A-A') along the Potomac bluffs, the metadiabase dikes 

make up about 60 percent of the exposed rock as measured 

across strike. Assuming that dike emplacement was by 

forceful intrusion and dilation rather than by stoping and 

assimilation, this locally implies a crustal extension of 150 

percent. 

The dikes were intruded into the crust in a northeast 

orientation, in accordance with a northwest extension direction during the opening of the Iapetus Ocean (Rankin, 

1975). Figure 5 shows stereonet plots of the orientations of 

exposed dike margins in the field (fig. SA) and azimuthal 

+ Zc 

o Zd 

x Zda 

F 

EXPLANATION 

• Low Ti02 , high-MgO metabasalt 

D Zi 

Figure 6. AFM diagram showing Late Proterozoic metadiabase 

and metabasalt. A, Na20 + K20; F. total iron as FeO; M, MgO. 

Line separates tholeiitic (TH) from calc~alkaline (CA) field. Zc, 

metabasalt of the Catoctin Formation; Zd, fine-grained metadiabase dikes; Zda, coarse-grained metadiabase dikes; Zi, porphyritic 

metadiabase. Data from Espenshade (1986), Southworth (1991), 

P.T. Lyttle (U.S. Geological Survey, unpub. data), and table 3, this 

report. 

trends of dikes (fig. 5B). Dike contacts, whether exposed or 

inferred on the map, are generally parallel to the dominant 

Paleozoic schistosity, suggesting either transposition or 

rotation of the dikes into parallelism with regional foliation 

during its development (Southworth, 1991), exploitation by 

the developing foliation of the previously existing dikebasement anisotropy, or both. 

GEOCHEMISTRY 

Major-oxide geochemical analyses of metadiabase 

dikes (Zd) and actinolite dikes (Zda) indicate that both types 

are of tholeiitic basalt composition (table 3, fig. 6). These 

rocks probably were relatively closed (isochemical) systems during metamorphism, as evidenced by the lack of features such as extensive veining and wall-rock alteration. 

The chemical composition of the porphyritic metadiabase 

(Zi) is on the boundary between the calc-alkaline and tholeiitic fields, according to the AFM diagram (fig. 6). Some 

chemical abundances in this rock, such as Al20 3 (in weight 

percent) and Sr (in parts per million) (table 3), indicate that 
Table 3. Major oxide (weight percent) and trace-element (parts per million) abundances and averages of Late Proterozoic metadiabase, metabasalt, and metarhyolite. 

[n.d., not detected. All analyses, including X-ray spectroscopy, by U.S. Geological Survey; FeO, H20, and C02 analysis by H. Smith, J.W. Marinenko, and J.R. Gillison; instrumental neutron activation 

analysis by G. Wandless and J. Mee] 

Major oxide 

Si02 

Ti02 

Al203 

Fe203 

FeO 

MnO 

MgO 

CaO 

Na20 

K20 

H2o+ 

H2oP20s 

C02 

Total 

Trace element 

Scandium 

Chromium 

Cobalt 

Nickel 

Zinc 

Arsenic 

Selenium 

Rubidium 

Strontium 

Zirconium 

Molybdenum 

Antimony 

(BR-2) 

5 

45.40 

1.98 

14.50 

3.80 

9.10 

.20 

7.67 

11.10 

1.81 

.78 

2.80 

.20 

.18 

.01 

99.53 

39.1 

163 

51.1 

104 

77 

<1 

<2 

20 

446 

190 

<4 

<0.1 

Zda 

Actinolitic metadiabase 

(BR-67) (BR-82) (BR-452A) (BR-738) 

6 7 8 9 

44.60 

2.31 

14.00 

3.74 

10.50 

.24 

6.97 

10.30 

2.54 

.20 

2.90 

.03 

.32 

.01 

98.66 

38.2 

72.1 

51 

92 

93 

<0.9 

<3 

<7 

330 

160 

<7 

<0.1 

46.40 

2.18 

14.00 

3.76 

9.40 

.22 

6.92 

11.20 

2.47 

.12 

2.50 

.06 

.23 

n.d. 

99.46 

40.8 

182 

42.9 

89 

78.8 

4.5 

<1 

<7 

320 

<200 

<6 

<0.1 

46.80 

2.42 

13.70 

3.61 

9.90 

.22 

6.47 

10.40 

2.76 

.36 

2.10 

.01 

.36 

n.d. 

99.11 

43.6 

207 

44.5 

78 

85 

<1 

<2 

16.7 

330 

151 

<4 

<0.1 

46.90 

1.97 

15.10 

3.42 

8.90 

.20 

6.55 

11.00 

2.69 

.26 

3.20 

.02 

.24 

.01 

100.46 

37.6 

159 

39.3 

71 

105 

<0.7 

<5 

7.5 

370 

<500 

<7 

.12 

Map unit 

Description 

(sample number) 

Sample site 

(BR-452B) 

10 

46.70 

2.53 

13.50 

3.83 

9.70 

.22 

6.11 

10.70 

2.76 

.45 

2.10 

.06 

.34 

n.d. 

99.00 

Zd 

Meta diabase 

(BR-226) 

11 

46.10 

2.18 

13.90 

2.53 

10.60 

.22 

6.94 

11.20 

2.54 

.12 

2.20 

.09 

.24 

n.d. 

98.86 

(BR-610A) 

12 

48.60 

2.62 

13.50 

2.51 

11.70 

.22 

5.56 

10.50 

2.39 

.71 

1.40 

.04 

.30 

.01 

100.06 

Abundance (parts per million) 

41.9 

153 

42.2 

62 

90 

<2 

<2 

17 

350 

<260 

<5 

<0.1 

38.9 35.9 

139 37.7 

43.9 50 

89 56 

84 125 

<1 1.04 

<2 <4 

14.4 26 

300 330 

125 <500 

<5 5.4 

<0.1 <0.08 

Zi 

Porphyritic 

meta diabase 

(BR-681) 

13 

48.70 

1.34 

17.40 

2.97 

6.20 

.15 

5.61 

11.30 

3.01 

.47 

2.80 

.03 

.15 

.01 

100.14 

29.3 

154 

31.4 

35 

78.7 

<0.7 

<0.7 

12 

430 

140 

<5 

<0.09 

Zc 

Metabasalt 

(BR-117) 

14 

47.20 

2.28 

14.00 

5.08 

7.40 

.19 

6.12 

10.80 

1.88 

.05 

3.00 

.11 

.21 

1.00 

99.32 

34.9 

134 

47.4 

104 

71 

<1 

<2 

<7 

280 

200 

<4 

.16 

(BR-807) 

15 

45.90 

2.66 

13.20 

5.38 

9.20 

.24 

6.46 

11.20 

2.13 

.28 

3.30 

.04 

.39 

.01 

100.39 

42.5 

131 

45 

69 

126 

<0.6 

<2 

11.2 

291 

200 

<6 

<0.1 

Zr Zda 1 

Actinolitic 

Averages 

(no. of samples) 

zd2 zc3 

Metarhyolite metadiabase Metadiabase Metabasalt 

(BR-38) 

16 

74.20 

.19 

12.60 

1.32 

.76 

.02 

.25 

1.32 

3.71 

4.33 

.27 

.02 

.05 

n.d. 

99.04 

1.8 

10.2 

1.3 

9.9 

62.4 

<1 

<1 

81.1 

180 

447 

2.9 

.51 

(5) 

46.02 

2.17 

14.26 

3.67 

9.56 

.22 

6.92 

10.80 

2.45 

.34 

2.70 

.06 

.27 

.01 

99.44 

(14) 

48.24 

2.48 

13.82 

3.60 

9.45 

.20 

5.63 

10.34 

2.46 

.64 

2.23 

.18 

.31 

n.d. 

99.58 

(13) 

48.02 

2.57 

14.08 

6.49 

7.36 

.22 

6.41 

7.46 

2.74 

.53 

3.01 

.20 

.29 

.08 

99.45 

-

0 

a 

gs 

r 

8 

--< 

0 

"T1 

~ 

i 

~ 0 

0 

c::: 

> p 

< 

> 

> z 

0 

3:: 

p 

> z 

0 

< 

sa 

a 

z 

> 

~ 

~ 

~ 

C3 

~ 

~ :A' 

g 

~ 

en 

0 

c::: 

> 0 
00 

....... V) M \0 

""""; \0 -.::1: M ~ ~ -.::1: 0 \Ci 0 r-- ...t M M M 0\ 0\ 0\ N r-- N 

\0 ""'" M N M N 0: ~ -.::1: 0 00 <"'i 00 0 V) <"'i ....... M 

v 00 - N N 

""'" ""'" N N V) V) M 0\ ~ ~ N 

0 0 <"'i 00 00 ...t <"'i 

v N - N ....... 

v 

- N V) r-- N M V) C"': ~ ~ ~ 0 

0 0\ 0 <"'i M M <"'i 

v ~ - N 

r-- r-- \0 r-- ~ r-- C"': N ~ 0: 

8 r-- r-: M \Ci M ....... M N N 

M N 00 V) 

N 

""'" ~ 0: 0: ~ -.::1: 0 00 ...t 0\ V) 

- ...... M 

v ;:::: - M -

0\ N r--

""'" N r-- V) ~ 0 ~ 0 r-- ...t <"'i 0 V) <"'i - M 

v ~ ....... M N 

""'" 0\ M M M N ""'" 10 ~ 00 

0 M 00 0 ...t ...t <"'i 

v 0\ N 

\0 r-- ""'" V) r-- 0\ N 0 

""'" r-- ~ 0: 0: """"; 

0 00 0 ...t \Ci V) M 

v N ...... N 

V) N 

""'" N ~ \0 ~ ""'" ~ ~ r--

0 r-- 00 o\ M ...t - <"'i 

v - ....... 

- 0\ M 0 

""'" N 0\ 

""'" C"': 10 0: ~ 0 0 o\ <"'i r-- V) 

- M 

v V) 

v N 

M \0 00 M 00 N t; t; N -.::1: C"': ~ ""'" N 

0 \0 V) M 0 M - <"'i 

v 0\ 

M -

8 8 

;:l .2 8 8 8 c:: 8 .§ ;:l 8 ;:l 8 8 ro 8 ·- ;:l :0 ;:l ;:l ..c ;:l :>..ro 8-:o 1-o 

·;;; ·c c ·c 15 8 2 

~~ Q) ro ro Q) Q) ro u co ;>:: ..J u ZCZl 

V) V) 

~ ....... V) 

....... 

0\ ""'" r-- -.::1: ~ 

M 

0\ ""'" V) 

~ ~ 

M 

00 

\0 r-- ~ \0 

<"'i 

N 

M \0 

-.::1: ...t ""'" 

00 

\0 \0 

-.::1: 00 

M 

....... 

\0 00 

-.::1: t; 

M 

00 

0\ \0 

C"': 0\ 

<"'i 

00 

N r-- -.::1: ~ 

M 

00 

C"': ""'" 00 

<"'i 

0\ 

""'" \0 

-.::1: -.::1: 

M 

\0 

C"': ""'" 0 

<"'i 

8 8 .2 ;:l 

Q) ;.§ "5 ro ..J ::r: 

GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 11 

00 r-- \0 00 00 

tri V) 0 <"'i 

v -

M V) 

t; ~ t; ""'" 0 

v 

M V) r-- ~ ~ 

- 0\ 

v -

""'" V) 

~ 0: ~ N 

v 

r-- V) 

~ 00 10 -.::1: r-: 

\0 M 

~ 0: C"': N 

v 

N N 0\ 

~ ~ ~ N 

v -

M V) 

10 10 N M v 0 

v 

M 

~ 10 N N v 0 

v 

M N 

10 10 00 N 

v t; 

V) 

N 0\ 

10 C"': \0 

""'" v 0 

v 

""'" 

-.::1: ""'" -.::1: 

- N v 0 

v 

8 8 8 ;:l ;:l ;:l "@ ·c ·a c "C) 

0 0 ro 

~ ..c 1-o 

0 E- ::::J 

~~ 

~ ~ ;>, ;>, 

~"';j 

c: c: (<:j (<:j 

0\ "' 0\ 0\ 0\0\ 

..c: ..c: t: t: 0 0 

~ ~ ..c: ..c: 5 5 0 0 

Cll Cll 

"0 "0 c: c: ro ro 

~ ~ ;>, ;>, 

:;:; 

c: c: ro ro N '0 

-a-a 

00 00 0\ 0\ 

"0 "' "0 "' ro ro -s; -s; c: c: 

"' "' 0. 0. 

~ ~ ..; .s .s ::0::0 ::0 ;3 ;3 ;3 

V> Vi V> 

:§ :§ :§ 

E E E 

0 0 0 

t!:=~ <.!::: (<:j ro ;3 c;a c;a Cl Cl 8 - N ""' 

it is a high-aluminum basalt (McB irney, 1984 ). Comparison 

of finer grained, metadiabase dikes (Zd) and Catoctin Formation metabasalt (Zc) from the field area (table 3) and 

from published data from adjacent areas (Espenshade, 

1986; Southworth, 1991) indicates that the fine-grained 

dikes and metabasalt are basically identical with respect to 

major elements (table 3; fig. 7) and belong to the high-Ti02> 

low-MgO suite of Espenshade (1986). The coarser grained 

dikes (Zda), in comparison to the finer grained dikes (Zd), 

have a narrower range of abundance in the major oxides 

(fig. 7) and tend to be lower in silica, higher in MgO, and 

lower in Ti02 (table 3). 

In the rare-earth-element (REE) pattern, the actinolite 

dikes (Zda) are generally depleted in light REE compared to 

the fine-grained dikes and Catoctin Formation metabasalt, 

which have similar REE patterns (fig. 8). In this respect the 

actinolitic dikes resemble the rocks of the low-Ti02, highMgO suite, which have flat REE patterns (fig. 8). 

Espenshade (1986) considered the rocks of this suite, which 

are flows or flow breccias confined to the base of the Catoctin Formation, to represent products of relatively undifferentiated early magmas. The actinolite dikes also may 

represent earlier intrusions but perhaps more likely represent a cumulate phase resulting from dike differentiation 

during intrusion. This suggests that the coarse-grained dikes 

and some of the fine-grained dikes and Catoctin flows are 

complementary differentiates. The unequal distribution of 

the coarse-grained dikes and their greater abundance along 

the eastern exposed margin of the rifted basement in the 

map area may be a function of proximity to the axis of 

rifting. 

The chemistry of the rhyolite dike (Zr) along the Potomac River, (sample BR- 38, table 3), indicates that it is 

compositionally a granite and has elevated REE abundances 

and a negative europium anomaly (fig. 8). This REE pattern 

indicates that the rhyolitic magma was a highly evolved 

product of magma differentiation that included plagioclase 

fractionation. The dike is probably a feeder dike for felsic 

flows in the Catoctin. 

U-Pb AGE OF RHYOLITE DIKE 

Zircons from the rhyolite dike BR-38 (sample 16 on 

table 3 and pl. 1; symbol U-3, pl. 1) were analyzed by the 

U-Pb isotopic method and yield an age of 572 ± 5 Ma 

(Aleinikoff and others, 1995). The fact that the dike cuts an 

adjacent metadiabase dike establishes a minimum age for 

the latter, as well as a reasonable age for rhyolite flows in 

the overlying Catoctin Formation. A flow from South 

Mountain, Pa., was dated during the same study at 564 ± 9 

Ma (Aleinikoff and others, 1995). In this map area (pl. 1), 

the nearest known possible extrusive counterparts for the 

rhyolite dike are four thin tuffaceous units (Zct) mapped in 

the lower part of the Catoctin Formation 3 km away. If one 
12 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

'E 

Q) 

(.) 

14 

A 

12- - .. 'E 

~ 10- -

Q; 

c.. 

.E 8- X 

Cl 

'Qj 

3: 6-

·= 0 4- Cl 

:!: 

2-

0 

42 44 46 48 50 52 54 

SiC 2 in weight percent 

4.50.------,-----,-----,.-----:---.-----..-----. 

B 

3.75-

Q; 3.00- c.. 

.E 0 + oJ 

.~ 2.25- Q) 

+ 9c•+~ 0 

+ .. 

)( 

3: ::-:: 

·= 1.50 - ON D 

i= 

0.751-- -

0.00 L__ __ __L_ ___ ...J...._ __ ____L ___ __t___ ___ L__ __ _j 

0.00 0.10 0.20 0.30 0.40 0.50 0.60 

P20 5 in weight percent 

4.50 ,----,------,------,,---,----.--+---.--------.---,--~ 

4.00r- c 

'E 3.50r- Q) 

~ 3.001-- + 0 

Q. 0 0 

.E 2.50r-- Cl 

-~ 2.00r--

c: 

·- 1.50-

0N 

j:: 1.00-

0.50-

0 ~ .. : 

D 

. .. . . o.oo 4;-------':5,.----=-6 ------:7:-----=8--9L_ _ __J1_o __ 1..L1 __ 1L2 _ __j13 

MgO in weight percent 

Figure 7. Variation diagrams of Late EXPLANATION 

Proterozoic metadiabase and metabasalt.A, + zc 

MgO versus Si02. B, Ti02 versus P20 5. C, o Zd 

Ti02 versus MgO. Zc, metabasalt of the x Zda • Low-TiO.,, highCatoctin Formation; Zd, fine-grained MgO metabasalt 

metadiabase dikes; Zda, coarse-grained Lo_Z=-i ____ _J 

metadiabase dikes; Zi, porphyritic metadiabase. Data from 

Espenshade (1986), Southworth (1991), P.T. Lyttle (U.S. 

Geological Survey, unpub. data), and table 3, this report. 

or more of 

~ 100 

~ c 

0 

..c 

(.) 

(i3 

0.. 

E 

ctl 

({) 1 0.0 

1.00L__ __________________ ___j 

La Ce Nd Sm Eu Tb Yb Lu 

Rare earth elements 

Figure 8. Chondrite-normalized REE diagram of Late Proterozoic metadiabase and metabasalt. 1, low-Ti02, high-MgO metabasalt; 2, coarse-grained actinolitic metadiabase dikes (Zda); 3, finegrained metadiabase dikes (Zd); 4, metabasalt of the Catoctin Formation (Zc); and 5, metarhyolite dike (Zr). La, lanthanum; Ce, 

cerium; Nd, neodymium; Sm, samarium; Eu, europium; Tb, terbium; Yb, ytterbium; Lu, lutetium. Data from Espenshade ( 1986), 

Southworth (1991), P.T. Lyttle (U.S. Geological Survey, unpub. 

data), and table 3, this report. 

these is indeed the subaerial product of the dated rhyolite 

dike then the maximum age for Catoctin rocks stratigraphically above these tuffs is about 572 Ma. 

LATE PROTEROZOIC METASEDIMENTARY 

AND METAVOLCANIC ROCKS 

SWIFT RUN FORMATION 

The Swift Run Formation is a thin sequence of clastic 

and tuffaceous metasediments that nonconformably overlies 

the Middle Proterozoic gneisses of the basement along most 

of the eastern margin of the basement core of the Blue 

Ridge anticlinorium in the map area. The Swift Run, 

defined in this area as the cover sequence below the lowermost metabasalt, is thickest at the southern end of the map 

area (about 305m; cross section D-D', pl. 1) and thins gradually to the north where it pinches out in places beneath the 

Catoctin Formation. Along the Potomac bluffs, the Swift 

Run Formation appears to have been tectonically removed 

(see "Possible Rift-Related Faults" below). Just to the north 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 13 

in Maryland, it is restricted to scattered lenses (Farth and 

Brezinski, 1994; Jonas and Stose, 1938) and reaches its 

maximum thickness on the western limb of the Blue Ridge 

anticlinorium in Virginia (Southworth, 1991 ). 

The basal cover-sequence metasedimentary unit farther south along the eastern limb of the Blue Ridge anticlinorium is known as the Fauquier Formation, which is 

characterized mainly by a sequence of crossbedded arkoses, 

interbedded siltstone and sandstone, and rhythmically bedded siltstones (Espenshade, 1986; Klein and others, 1990). 

The boundary between the Fauquier and Swift Run Formations is placed at two east-trending, synsedimentary(?) 

faults in the Lincoln quadrangle; one marks the southern 

termination of the Swift Run Formation, and the other 

marks the northern extent of the Fauquier Formation. The 

Catoctin Formation rests on basement in between (Burton 

and others, 1992b). 

The Swift Run Formation in the map area has two 

principle members: a basal member composed of quartzpebble-bearing metasandstone, metaconglomerate, and pebbly quartz-muscovite phyllite (Zsq) and an overlying finegrained quartz-muscovite phyllite (Zst). The pebbly member (Zsq) is discontinuous and is only a few meters thick at 

its maximum. It is characterized by the presence of scattered blue-quartz pebbles several millimeters in diameter in 

a finer grained matrix. An exposure in the southern Waterford quadrangle just north of Paeonian Springs contains a 

1-m thick bed, which contains white quartz pebbles as 

much as 3 em in diameter and which is the only coarsegrained Swift Run lithology exposed in the map area. 

Overlying this basal member is the fine-grained 

quartz-muscovite phyllite, which lacks quartz pebbles. This 

unit is thicker and more widespread and is distinctive in 

outcrop and float as a conspicuous, creamy-white lustrous 

schist. Its fine grain size and felsic composition suggest a 

felsic tuff and (or) tuffaceous sediment as protolith. 

About 30 to 40 m above the basal unconformity is a 

discontinuous layer as much as several meters thick of 

buff- to brown-weathering, white calcite marble. This marble occurs within the tuffaceous member of the Swift Run 

Formation in the southern part of the map area (Zsm ), and a 

similar marble (Zcm) occurs within metabasalts of the 

Catoctin Formation to the north. If these marbles represent a 

single lithostratigraphic horizon then the tuffaceous rocks to 

the south and lowermost metabasalts to the north are timestratigraphic equivalents. The section of tuffaceous Swift 

Run Formation (Zst) would thus grade laterally into the 

sequence of intercalated metabasalt (Zc) and felsic metatuff 

or tuffaceous metasediment (Zct) in the lower part of the 

Catoctin Formation near the Potomac River. To the south of 

the map area, marble occurs somewhat higher in the section, just above or below the contact between the Swift Run 

or Fauquier Formations and the Catoctin Formation (Espenshade, 1986; Klein and others, 1990; Burton and others, 

1992b ). The apparent change in stratigraphic position of the 

marble (assuming a single marble horizon) and the change 

from Swift Run to Fauquier lithologies suggest significant 

changes in sedimentation from north to south before deposition of the Catoctin. 

The depositional environment for the Swift Run Formation in this region was clearly one of low energy and 

small topographic gradients, as evidenced by the absence of 

thick clastic sediments, by the prevalence of fine-grained 

sediments and (or) volcanic rocks, and by the presence of a 

marble horizon. The tuffaceous upper member of the Swift 

Run Formation possibly represents early, distal ash-fall 

deposits, which mostly preceded the voluminous outpouring 

of basalt during formation of the Catoctin. 

CATOCTIN FORMATION 

The Catoctin Formation in the Waterford and Point of 

Rocks quadrangles is characterized primarily by metabasalt 

(greenstone), as it is regionally (Espenshade, 1986; Klein 

and others, 1990), but a number of other lithologies occur as 

well, particularly in the lower part. The lower part of the 

Catoctin is best exposed in lower Catoctin Creek, just 

before it enters the Potomac River. Here the formation contains dark-green, fine-grained metabasalt (Zc) and coarsegrained, actinolitic metabasalt or metadiabase (Zca), 

white lustrous fine-grained quartz-muscovite phyllite, 

interpreted as metamorphosed tuff or tuffaceous metasediment (Zct), buff-weathering, fine- to medium-grained, 

greenish- or bluish-white calcite marble (Zcm), and fine- to 

medium-grained yellow- to white-weathering, crossbedded 

plagioclase-quartz metasandstone (Zcs). Farther south only 

the marble and discontinuous horizons of metasandstone 

were mapped in the lower Catoctin, whereas the tuffaceous 

horizons may have their lateral equivalent in the Swift Run 

Formation (Zst), as stated above. Scattered throughout the 

Catoctin are submeter-wide zones of alternating light- and 

dark-colored, centimeter-scale quartz-rich and mafic layers, 

testifying to local episodes of sedimentation during eruption 

of the Catoctin basalts. 

An enigmatic rock type in the lower part of the Catoctin is a coarse-grained, actinolitic metamafic rock, which 

strongly resembles the coarse-grained dikes in the basement 

(Zda). Pale-green to colorless actinolite is the dominant 

mineral in thin section, forms porphyroblasts as much as 1 

to 2 em in diameter, and is accompanied by lesser chlorite 

and epidote. The coarse-grained Catoctin is interpreted here 

to be of two origins. Near the base of the Catoctin, where 

coarse-grained actinolitic rock is mapped as crosscutting 

other units, it is indeed considered intrusive (Zda). Higher in 

the section, coarse-grained zones seem to follow the 

regional trend of the layering as defined by the metasandstone and are interpreted as coarse-grained flow interiors 

(Zca). A problem with the latter interpretation is that such 

coarse-grained rocks are absent from the upper part of the 
14 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

Catoctin where metabasalt is more predominant and such 

features should be wore abundant. 

In the southern part of the map area, coarser grained 

mafic rocks mostly are absent, and the continuum of 

metabasalt in the lower part of the Catoctin is broken only 

by discontinuous thin lenses of metasandstone. The upper 

part of the Catoctin, however, contains another metasedimentary unit, Zcp, which is quartzose phyllite. It is composed of fine-grained quartz, biotite, muscovite, epidote, 

and magnetite and, locally, graphite. Zcp has a slabby habit 

in outcrop, and its relative lack of green minerals such as 

chlorite distinguishes it from the surrounding metabasalt. 

The phyllite unit is best exposed in the upper Limestone 

Branch and pinches out to the north and south. 

The Catoctin in the southern part of the map area is 

more than twice as thick (nearly 7,000 ft; section D-D', 

pl. 1) as that near the Potomac River (2,700 ft; section B-B', 

pl. 1 ). This may be due in part to the presence of metasediments such as phyllite (Zcp) but more likely reflects thickening of metabasalt to the south, a trend which continues 

south into the Lincoln quadrangle (Burton and others, 

1992b). A later tectonic influence on thickness in the form 

of large-scale isoclinal folds is unlikely, however, due to the 

compositional asymmetry of the lower and upper part of the 

Catoctin. However, unrecognized north-trending Mesozoic 

normal faults, which would increase the apparent thickness, 

cannot be ruled out. Along with the change in thickness, 

differences between the Catoctin as mapped in the northern 

part and the southern part include (1) lack of felsic tuffaceous units above the lowest metabasalt in the south, as 

opposed to the north, (2) apparent lack of any coarsegrained mafic units in the lower part of the Catoctin in the 

south, and (3) a mappable phyllitic layer in the upper part of 

the Catoctin in the south. These along-strike variations 

testify to lateral changes in conditions during deposition of 

the Catoctin protolith. 

Epidosite, a fine-grained, massive, yellowish-green 

rock consisting of epidote and quartz, is widespread but 

restricted to the fine-grained metabasalt of the Catoctin Formation (Zc). It typically occurs as discontinuous layers or 

boudins as much as a few meters in length that are enclosed 

in greenstone, and it is considered to be an alteration product of the metabasalt (Reed and Morgan, 1971). Due to a 

resistance to erosion, it is abundant as float boulders. The 

epidosite commonly has relict amygdules filled with quartz 

or less commonly calcite, albite, or biotite. The amygdules 

appear to be relict primary features, suggesting that the epidosite preferentially formed from scoriaceous lava, perhaps 

at the tops and bottoms of flows, where fluids could more 

easily circulate. The pinched and boudinaged nature of the 

epidosite indicates that the alteration occurred prior to Paleozoic deformation, but whether the alteration itself is Paleozoic or older is unknown. 

As stated above, the four thin felsic metatuff layers 

(Zct) near the base of the Catoctin Formation represent the 

nearest known extrusive equivalents to a rhyolite dike in the 

basement (Zr) that has been dated at 572 ± 5 Ma. This suggests that the upper part of the Catoctin Formation is 

younger than that age. 

POSSIBLE RIFT-RELATED FAULTS 

The dike complex intruding Middle Proterozoic basement rocks and the Catoctin Formation metabasalts overlying the basement are the most visible products of the Late 

Proterozoic crustal rifting event that occurred along the 

eastern margin of the craton during the opening of the Iapetus Ocean (Rankin, 1975). Normal faults associated with 

the widespread crustal extension that is presumed to have 

accompanied this event, however, have been difficult to pinpoint in this region, probably due to subsequent reactivation 

and obliteration during the compressional event(s) of the 

Paleozoic. Espenshade (1986) and Klein and others (1990, 

1991) have mapped cross-strike and along-strike faults in 

the Marshall quadrangle to the south, which they consider 

to be rift related. 

In this map area (pl. 1), two east-dipping mylonite 

zones were mapped that in thin section show kinematic 

indicators of a normal, down-to-the-east sense of movement 

(sections A-A', C-C, pl. 1). In terms of mineralogy and 

field aspect these mylonites are indistinguishable from 

reverse-sense (up to the west) mylonites also mapped 

(section C-C'). The chlorite- to biotite-grade mineral 

assemblages seen in thin sections of these apparent normal 

fault-zone mylonites also resemble those associated with 

Paleozoic compressional deformation. Therefore, despite 

their normal movement sense, these faults are considered to 

be coeval with the Paleozoic reverse faults. 

Perhaps more convincing evidence for normal faulting 

associated with rifting is found just above the basal unconformity between metatuff of the Catoctin Formation (Zct) 

and an underlying basement dike (Zd) along the Potomac 

River. Although no outcrop occurs, a narrow fault zone is 

evidenced by float blocks of dark-green, mylonitic greenstone of the Catoctin (Zc) containing angular clasts, one to 

several centimeters in diameter, of white fine-grained marble. The marble clasts are probably derived from marble of 

the Catoctin (Zcm). The missing section implied by the 

proximity of the marble to basement (as much as 100 m) 

suggests that this fault zone reflects either west-over-east 

reverse faulting or east-side-down normal faulting. The latter interpretation is adopted on the map. This mylonite zone 

cannot be traced any distance southward due to exposure, 

but Jonas and Stose (1938) found a similar zone along strike 

north of the Potomac River. Significantly, the mylonite contains tight kink folds that resemble folds in the Paleozoic 

cover-sequence rocks, suggesting that Paleozoic compression occurred subsequent to faulting. 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 15 

CHILHOWEE GROUP METASEDIMENTARY 

ROCKS 

LOUDOUN FORMATION 

There has been discussion over the years about the 

possible presence or absence of a mappable metasedimentary unit between the Catoctin Formation metabasalts and 

the clean Weverton Quartzite. Keith (1894) and Jonas and 

Stose (1938) recognized such a unit but also mistakenly 

mapped some metasedimentary rocks in the Catoctin and 

the basal cover-sequence metasediments of the Fauquier 

and Swift Run Formations as infolds of the Loudoun Formation. Whittaker ( 1955) correctly mapped the Loudoun 

Formation metasediments between the Catoctin and Weverton and found it pinching out south of the Potomac River. 

Nickelsen (1956) likewise mapped the Loudoun between 

the Catoctin and Weverton on the western limb of the Blue 

Ridge anticlinorium. Nunan (1979), in contrast, concluded 

that the Loudoun Formation was not a mappable unit and 

included the metasediments above the metabasalts within 

the basal Weverton. Reed ( 1955) on the other hand thought 

that phyllites in this stratigraphic position represented metamorphosed saprolite developed on the Catoctin. The present 

study agrees with Whittaker as to both the validity of the 

Loudoun Formation and its pinching out to the south. On 

this map (pl. 1 ), the formation pinches out just south of the 

Waterford-Point of Rocks quadrangle boundary, about 3 km 

north of where Whittaker terminated it. The Loudoun Formation is here considered to be the basal unit, at least 

locally, of the Chilhowee Group and has a maximum thickness in the map area of about 30 m. 

The Loudoun Formation (£1) is best exposed on the 

east-facing slope of Catoctin Mountain west of U.S. Route 

15 and just south of the Point of Rocks bridge. The contact 

of the Loudoun with the underlying Catoctin Formation is a 

gradational zone marked by interlayered greenstone and 

distinctive, bluish-black phyllite layers from 10 em to 1 to 2 

m thick (the "purple phyllite" of Whittaker, 1955). These 

fine-grained black phyllites also are distinctive compositionally and consist mostly of muscovite and graphite and 

lesser amounts of chlorite and quartz. The Loudoun Formation proper, overlying the uppermost metabasalt, is an 

extremely variegated unit containing phyllite, phyllitic 

metaconglomerate and pebbly metasandstone, and minor 

vitreous quartzite (fig. 9). Loudoun Formation phyllite in 

contact with Catoctin Formation metabasalt is exposed in 

the roadcut on U.S. Route 15, where it is a chlorite-quartzmuscovite phyllite, containing minor amounts of magnetite 

and graphite, and contains quartzite layers as much as 1 m 

thick. More quartz-rich lithologies, including phyllitic 

metasandstone and metaconglomerate, and a 3- to 4-m thick 

vitreous quartzite layer are found upslope and upsection 

(west). These units apparently pinch out southward along 

with the formation as a whole; the southernmost exposures 

Figure 9. Recumbent, isoclinal F1 folds (arrow) and more 

open, upright F2 folds with associated steeply dipping 

crenulation cleavage in interlayered phyllite and quartzite of the 

Loudoun Formation, at the northern end of Catoctin Mountain 

(Virginia portion), just south of U.S. Route 15 bridge over the 

Potomac River, Point of Rocks quadrangle. View looking south 

approximately parallel to azimuths of fold hinges; lens cap 

diameter is 5 em. 

of Loudoun Formation consist solely of graphite-quartzmuscovite phyllite. An outcrop on the northern face of 

Catoctin Mountain, interpreted as the exposed contact 

between the Loudoun Formation and Weverton Quartzite, 

contains thickly bedded, white vitreous quartzite in contact 

with and overlying black phyllite. This study could not corroborate Whittaker's (1955) subdivision of the Loudoun 

Formation into an upper conglomeratic member and a lower 

phyllitic member. 

WEVERTON QUARTZITE 

The Weverton Quartzite (£w) in the map area is a 

grayish-white- to cream-weathering, massive to thickly 

bedded vitreous orthoquartzite about 30 to 46 m thick. The 

thinness and uniformity of the Weverton Quartzite in the 

map area contrasts with the formation as mapped on the 
16 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

Figure 10. Outcrop on the western flank of Catoctin Mountain, 

just east of Furnace Mountain, showing upright, post-F2 kink folds 

in S1-laminated quartzose phyllite of the Harpers Formation. View 

looking north approximately parallel to strike of axial planes of 

kink folds; area is about 1 m across. 

western limb of the anticlinorium (Nickelsen, 1956) and to 

the north in Maryland (Whittaker, 1955), where three members are recognized. Lithologic variation in the Weverton 

Quartzite here consists of minor thin (less than 10 em) 

interbeds of dark phyllite and sugary-white-weathering 

pebbly layers 1 to 2 m thick. Bedding is typically marked 

by very fine dark laminae, which commonly outline crossbeds and scour and fill structures; these yield the topping 

directions shown on the map. The contact of the Weverton 

with the underlying Loudoun or Catoctin Formations is 

apparently sharp; the contact with the overlying Harpers 

Formation is gradational. No evidence for an angular 

unconformity beneath the Weverton could be found; however, the apparent pinching out of the Loudoun Formation 

under the Weverton Quartzite may be a result of erosion 

before or in the earliest stages of deposition of the 

Weverton. 

HARPERS FORMATION 

The Harpers Formation (£h) is a fine-grained, wellfoliated chlorite-biotite-muscovite-quartz phyllite (figs. 10, 

11 ). Its thickness ranges from about 500 ft in the Point of 

Rocks quadrangle (section B-B', pl. 1) to more than 2,000 

ft in the Waterford quadrangle (section C-C', pl. 1 ). Harpers Formation phyllite is distinguished from phyllite of the 

Loudoun Formation by the abundance of quartz, the presence of biotite, and the absence of graphite. Magnetite is 

locally abundant. The phyllite commonly contains conspicuous, millimeter-scale, alternating light and dark laminae, 

which are quartz rich and mica rich, respectively. A meterthick quartzose layer sampled in the Harpers Formation is 

composed of quartz, potassium feldspar, and plagioclase 

Figure 11. Photomicrograph showing thin section (plane 

polarized light) of well-developed s) in quartzose phyllite of the 

Harpers Formation, Point of Rocks quadrangle. Minerals are 

biotite (8), muscovite (M), quartz (0), plagioclase (P), and 

magnetite (Mt). 

and is similar in compositiOn to the overlying Antietam 

Quartzite. No mappable subdivisions could be distinguished 

within the Harpers Formation; lithologic variation consists 

of gradual, slight variations in quartz and mica content. 

More quartz-rich phyllite occurs just above the Weverton 

Quartzite and immediately below the Antietam Quartzite, 

with which the Harpers has gradational contacts. 

ANTIETAM QUARTZITE 

The Antietam Quartzite (£a) is a poorly exposed, 

gray- to buff-weathering, massive, fine-grained meta-arkose 

about 100 ft thick. It is composed of roughly 45 percent 

quartz, 30 percent potassium feldspar, and 25 percent 

plagioclase. Physiographically the Antietam forms a 

topographic high in contrast to the phyllite of the Harpers 

Formation (£h) and carbonate of the Tomstown Dolomite (£t) between which it occurs. At Furnace Mountain, 

the Antietam Quartzite is the host rock for deposits of limonite that were mined for iron ore in the 19th century 

(Holden, 1907) and have an irregular distribution, as indicated by the old mine pits of Furnace Mountain (limonitequarry and limonite-prospect symbols on pl. 1). 

POST-CHILHOWEE GROUP 

METASEDIMENTARY ROCKS 

CARBONACEOUS PHYLLITE 

Overlying the Antietam Quartzite in places, but poorly 

exposed, is a very fine grained, sooty gray to black carbonaceous phyllite (£cp). The phyllite locally contains light 

and dark laminae suggesting bedding. In thin section 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 17 

quartz-rich zones alternate with feathery micaceous zones 

consisting of fine-grained sericite and finely disseminated 

carbonaceous material. Fine-grained sulfides are also 

present. The rock is notable for the acidic ground water it 

generates (James Athey, oral commun., 1990). The 

maximum thickness for the carbonaceous phyllite is probably less than 61 m. 

The carbonaceous phyllite occurs in the same stratigraphic position as the lower part of tlie Tomstown Dolomite and apparently interfingers with it. The map pattern 

indicates that this lateral change from dolomite to phyllite 

occurs on a scale of a few tens of meters, perhaps suggesting extremely localized euxinic basins in which a black mud 

was deposited that later formed the phyllite. The unit may 

be equivalent to a thin black phyllite, 5 to 10m thick, recognized in the uppermost part of the Araby Formation 

(equivalent to the Antietam) in Maryland, just below the 

lowermost carbonate (Reinhardt, 1974 ). 

TOMSTOWN DOLOMITE AND FREDERICK LIMESTONE 

Overlying the Antietam Quartzite and perhaps laterally 

equivalent to the carbonaceous phyllite (£cp) is the Tomstown Dolomite (£t), a gray- to buff-weathering finegrained dolomite that has a white to bluish-gray fresh surface. Bedding, where visible, ranges in style and thickness 

from coarse beds 10 to 20 em thick to fine millimeter-scale 

light and dark laminae. Sandy layers are locally present. 

The dolomite is well exposed in fields near the Potomac 

River, where it is in contact with the underlying Antietam 

Quartzite and the overlying Frederick Limestone. The contact with the quartzite is sharp and well defined, whereas the 

contact with the limestone is gradational (J. Reinhardt, oral 

commun., 1991). Another exposure of Tomstown Dolomite 

is in a creek southwest of Furnace Mountain, where it is 

separated from the Harpers Formation by a normal fault 

(Furnace Mountain fault). Southward, the dolomite grades 

laterally into carbonaceous phyllite (£cp), which, along 

with the dolomite and the underlying Antietam Quartzite, is 

truncated by the early Mesozoic Bull Run fault. The maximum thickness of the Toms town is the same as that for the 

carbonaceous phyllite. 

The Frederick Limestone (£f) is exposed only in 

fields just south of the Potomac River in the map area. 

Strongly resembling the Tomstown Dolomite in appearance, 

it is gray weathering, bluish gray, well bedded to finely laminated, and in these exposures highly folded. Due to this 

deformation, a stratigraphic thickness cannot be estimated. 

PALEOZOIC STRUCTURE AND METAMORPHISM 

The Proterozoic and early Paleozoic rocks of the map 

area make up a portion of the core and eastern, upright limb 

of the west-verging Blue Ridge anticlinorium, a structure 

formed during one or more compressional tectonic events in 

the middle to late Paleozoic. The formation of this large 

structure was accompanied by the development of at least 

two sets of cleavage, several generations of folds, minor 

local thrust and normal faults, and perhaps major underlying 

thrust faults not exposed in the map area. Deformation 

occurred under greenschist-facies metamorphic conditions, 

at chlorite or biotite grade, which has resulted in substantial 

recrystallization of these rocks and formation of Paleozoic 

mineral assemblages. 40 Arf39 Ar dating of metamorphic 

muscovite from early-developed cleavage in these rocks has 

set a probable age for deformation in this region as late 

Paleozoic (Burton and others, 1992a; Kunk and others, 

1993). 

FIRST-GENERATION PALEOZOIC CLEAVAGE 

The most pervasive manifestation of Paleozoic deformation in the Blue Ridge anticlinorium is the development 

of a closely spaced cleavage or schistosity in both basement 

and cover-sequence rocks that accompanied a weak to 

strong metamorphic recrystallization. The first-generation 

schistosity, or SI> is northeast striking and moderately to 

gently east dipping (fig. 12A), roughly parallel to the axial 

plane of the anticlinorium. The small scatter of data in 

figure 12A illustrates the uniformity of the S1 cleavage in all 

rock units. 

In the Middle Proterozoic basement gneisses, S 1 is 

commonly the dominant fabric in the rock, overprinting and 

obscuring the Middle Proterozoic foliation and layering. 

Formation of S1 was accompanied by the breakdown of 

older high-grade minerals and the growth of muscovite, 

chlorite, and (or) biotite along foliation planes, along with 

epidote and other greenschist-facies minerals. A steeply 

southeast plunging mineral lineation, typically composed of 

biotite or chlorite, is locally visible on the S1 surface. The 

lineation may in some cases represent the intersection of 

Middle Proterozoic foliation (or lineation) and Paleozoic 

schistosity or alternatively may indicate slip direction during noncoaxial compression and shear along the S1 plane. 

"Ductile deformation zones" or DDZ's, which consist 

of outcrop-scale, complex anastomosing networks of small 

faults and shear zones in flat "pavement" outcrops of 

basement rock, were described by Mitra (1979) and Boyer 

and Mitra (1988) in areas farther south in the Blue Ridge 

anticlinorium. In these areas the regional cleavage in the 

Blue Ridge anticlinorium ("Blue Ridge-South Mountain 

cleavage") is, according to them, represented by the acute 

bisectrix of these anastomosing shear zones. DDZ's are rare 

in the field area, perhaps partly due to the paucity of pavement outcrop. In any case, S1 in the map area has a much 

more planar fabric than that described for the DDZ's. 

In the metadiabase dikes of the basement and the lithologically similar metabasalts of the Catoctin Formation, 
18 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

A 

N 

Mean S1 foliation 

N7° E, 36° SE 

N 

Figure 12. Lower hemisphere equal-area projections of first- and 

second-generation structures of Paleozoic age; contour interval is 2 

percent per 1 percent area. A, Poles to S1 foliation and mean S1 

foliation (n=l017). B, Poles to axial planes of F1 folds (o, n=5) 

and F2 folds ( +, n=22). C, Poles to S2 foliation and mean S2 

foliation (n=71). 

N 

which had little or no preexisting fabric, sl is closely spaced 

and moderately to strongly developed and commonly 

imparts a schistose or slabby habit to the rock. Chlorite is 

the main mineral producing the foliation, and locally developed down-dip chlorite streaking on the foliation surface 

may indicate approximate slip direction. sl generally parallels the present margins of the metadiabase dikes and compositional layering in the Catoctin Formation. S1 in the 

dikes and S1 in the metabasalts of the Catoctin also are basically of the same orientation, indicating that the dikes and 

flows, once presumably at high angles to each other, have 

been rotated into near parallelism (sections B-B', C-C', 

pl. 1). 

S1 is more thoroughly developed in the fine-grained 

metasedimentary and metatuffaceous rocks of the cover 

sequence. In the Swift Run Formation and lower part of the 

Catoctin Formation these felsic lithologies have a schistose 

habit produced by abundant planar muscovite. In the phyllites of the upper part of the Catoctin Formation and in the 

Loudoun and Harpers Formations, S1 is composed of muscovite, biotite, chlorite, and (or) graphite and commonly 

parallels a fine compositional layering (figs. 10, 11 ). 

In the more quartzofeldspathic metasediments, particularly those of the Loudoun Formation and Weverton Quartzite, S1 can be seen distinct from and at an angle to bedding. 

It most commonly strikes parallel to bedding and has a 

steeper east dip, as would be expected on the normal limb of 

a major west-verging fold. Cleavage density and dip angles 

change with lithologic transition from quartzitic (steeper, 

widely spaced) to phyllitic (shallower, closely spaced) 

layers. 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 19 

FIRST-GENERATION FOLDS 

The structures identified as S 1 on the map (pl. 1) are 

axial planar to rare isoclinal first-generation folds (F1) (fig. 

12B). Outcrop-scale F1 folds are best seen in the Loudoun 

Formation and Weverton Quartzite at the northern end of 

Catoctin Mountain, along with their axial planar (SJ) 

cleavage (fig. 9). Recumbent, isoclinal F1 folds are also well 

displayed in the lower part of the Catoctin Formation in thin 

metasedimentary layers intercalated with the metabasalt. 

Cryptic, larger isoclinal F1 folds in the Weverton Quartzite 

are implied by the presence of overturned cross beds in some 

of the quartzite layers. F1 fold hinges are subhorizontal and 

generally strike northeast-southwest, parallel to the long 

axis of the Blue Ridge anticlinorium. The anticlinorium 

may itself be an F1 megafold and S1 its associated axial planar schistosity (Mitra and Elliott, 1980). S1 is the "Blue 

Ridge-South Mountain cleavage" of Mitra and Elliott 

(1980). 

SECOND-GENERATION CLEAVAGE AND FOLDS 

Second-generation cleavage and schistosity (S2) is 

widespread but only locally developed in both basement 

and cover rocks. It is typically a spaced ( 1 em or more) 

cleavage that cuts sl schistosity, is approximately parallel to 

it in strike, and dips more steeply eastward (fig. 12C). In the 

map area, S2 is generally not accompanied by extensive 

recrystallization except at the northern end of Catoctin 

Mountain, where second-generation cleavage and folds are 

intensely developed. The roadcut of phyllite of the Loudoun 

Formation on U.S. Route 15 features a well-developed, penetrative schistosity of s2 generation. 

s2 cleavage is typically axial planar to second-generation (F2) folds. F2 folds generally have subhorizontal, northeast-striking hinges and gently to moderately east dipping 

axial surfaces (fig. 12B). F2 folds are rarely seen in the basement as mesoscopic folds of S1 schistosity in Middle Proterozoic gneiss or Late Proterozoic intrusive dikes. They also 

are found locally in greenstones in the lower part of the 

Catoctin Formation as closely spaced similar folds deforming S1• 

F2 folds are most strongly developed and best exposed 

along the eastern limb of the anticlinorium in the uppermost 

part of the Catoctin Formation and lowermost part of the 

Chilhowee Group rocks (fig. 9), and they are a dominant 

factor in the map pattern of the northern end of Catoctin 

Mountain in Virginia. Here occurs an isolated inlier of 

Weverton Quartzite, which contrasts structurally with the 

continuous east-dipping section found along most of the 

limb. Whittaker (1955) inferred a fault to explain the disconnected quartzite; it is in fact the hinge of a large, shallow 

F2 syncline (Catoctin Mountain syncline), which has probably overprinted smaller F1 structures (section B-B', pl. 1). 

The inlier of Tomstown Dolomite ECt) and carbonaceous 

phyllite (-£cp) west of Furnace Mountain is also interpreted 

as the hinge of an F2 syncline that was later cut by an early 

Mesozoic normal fault (Furnace Mountain fault) and that 

lies west of a complex (later overprinted) F2 anticline (Furnace Mountain anticline). Outcrop-scale, large-amplitude F2 

folds are well displayed in well-foliated (S 1) greenstone of 

the Catoctin Formation just west of U.S. Route 15, and are 

sufficiently developed to generate a locally pervasive s2 

schistosity in phyllite of the Loudoun Formation, as mentioned above. From west to east in this small area, the axial 

surfaces of these folds appear to rotate from steeply to moderately east dipping, perhaps in response to later folding or 

as a result of fanning cleavage. F2 folds in thin phyllitic layers in the Weverton Quartzite just above its contact with the 

Loudoun Formation produce a closely spaced crenulation 

cleavage axial planar to the folds. Farther south along 

Catoctin Mountain, F2 folding is less pervasive but probably responsible for local dip reversals in S1 in the Catoctin 

Formation and Weverton Quartzite. 

Second-generation folds also are found locally on the 

western limb of the anticlinorium, such as at Purcell Knob 

(Nickel sen, 1956; Southworth, 1991 ). In the anticlinorium 

as a whole, second-generation F2 folds probably occur as 

minor, local modifications of the major F1 structure. 

LATE FOLDS 

Folds that postdate F 1 and F2 folds are locally well 

developed in the cover sequence and rarely occur in basement rocks. These can be divided into two types based on 

fold style: low-amplitude and short-wavelength (few millimeters to centimeters) crenulate or kink folds (figs. 10, 

13A) and larger (up to 1-2 m in amplitude and wavelength) 

open to isoclinal cylindrical folds (fig. 13B). 

Planar kink folds and minor crenulations are found in 

phyllitic cover-sequence rocks and schistose rocks of the 

basement. They all have subhorizontal hinges but can be 

divided into three sets based on axial surface orientation: a 

north-trending set having steep east or west dips; a northwest-trending set, also having steep dips; and a minor set 

having gentle to moderate dips (and one steep dip) and 

widely varying strikes (fig. 13A). These minor folds are particularly prominent along the northeastern side of Catoctin 

Mountain in the phyllites of the Loudoun and Harpers Formations (fig. 10). Steeply to moderately west dipping 

crenulations are locally associated with minor east-verging 

back thrusts that have as much as 1 em of offset. These kink 

or crenulation folds may represent the last stages of compression in the stress field that produced the structure of the 

Blue Ridge anticlinorium; the back thrusts represent a final 

strain-induced response to the buttressing effect of the 

already formed anticlinorium. 

Rare late cross folds generally have shallow- to 

moderate-plunging hinges and northeast or northwest axial 
20 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

N 

N 

Figure 13. Lower hemisphere equal-area projections of late folds. 

A, Contoured poles to steeply dipping late crenulate cleavage or 

steeply dipping axial planes of crenulate folds; contour interval is 2 

percent per 1 percent area (n=35); poles to gently dipping cleavage 

or axial planes of crenulate folds (•, n=9). B, Poles to axial planes 

of late cross folds ( o, n=7) and hinges of late cross folds, where 

measurable(+, n=4). 1, meter-scale, east-verging recumbent fold 

in Frederick Limestone (£f); 2, 3, open folds in Weverton 

Quartzite (£w); 4, meter-scale recumbent fold in Catoctin 

Formation (Zc); 5, 6, 7, open folds jn Tomstown Dolomite. 

planar trends (fig. 13B). They have a different geometry 

from the kink folds, as mentioned above. These cross folds 

were found only in Catoctin Formation, Weverton Quartzite, and Tomstown Dolomite on or east of Catoctin Mountain (fig. 13B). The tectonic significance of these folds is 

poorly understood; one observed fold in Frederick Limestone is east verging (no. 1 in fig. 13B). In the carbonate 

rocks, interaction between cross folds of several orientations has produced complex interference patterns in outcrop. On a larger scale, the complex map pattern produced 

by the Harpers Formation, Antietam Quartzite, and the carbonates around Furnace Mountain is interpreted to be the 

product of these interfering cross folds superimposed on 

older, larger amplitude folds such as the Furnace Mountain 

(F2) anticline. The cross folds are perhaps a product of late 

Alleghanian dextral transcurrent motion that has been documented elsewhere (Gates and others, 1986). 

THE TECTONIC ROLE OF FAULTING 

On a large scale, the structures of the Blue Ridge are 

considered by many workers to be ultimately derived from 

low-angle, east-over-west thrusting (Harris, 1979; Mitra 

and Lukert, 1982; Evans, 1989). In this study, convincing 

evidence for or against large-scale thrusting was not found. 

A few mylonite zones were mapped in the basement of 

which two, as discussed above, have kinematic indicators 

showing a normal (east side down) sense of movement (sections A-A', C-C', pl. 1). A third has features showing an 

east-over-west sense of movement (section C-C') and is 

probably coeval with Blue Ridge compressional events. All 

mapped faults, however, are minor in extent and probable 

degree of offset; strain accommodation during formation of 

the Blue Ridge anticlinorium, in this small area at least, was 

by the more ductile processes of folding and cleavage 

development. The model developed by Mitra and Lukert 

(1982) of a single, fanning generation of cleavage (Blue 

Ridge-South Mountain cleavage) linked to an episode of 

deep-seated thrusting may apply to sl but does not explain 

the two overlapping generations of cleavage here (S 1 and 

S2) nor the several late generations of folds. 

LATE BRITTLE STRUCTURES 

The youngest tectonic structures of possible Paleozoic 

age in the Blue Ridge anticlinorium are joints and brittle 

faults. The joints typically have steep dips but no strong 

preferred azimuth (fig. 14A). A Mesozoic or younger age 

for these structures also cannot be ruled out, as there is significant overlap in orientation between joints in pre-Mesozoic and Mesozoic rocks (figs. 14A, 15). However, the 

north-northeast and south-southwest pole concentrations in 

the pre-Mesozoic rocks (fig. 14A) are absent from the plot 

of Mesozoic joints_ (fig. 15); these pole concentrations, at 
GEOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS 21 

N 

N 

Figure 14. Lower hemisphere equal-area projections of late 

brittle structures in pre-Mesozoic rocks. A, Poles to joints; contour 

interval is I percent per 1 percent area, (n=80). B, Poles to faults 

with outcrop scale offset or slickenlines; letter indicates apparent 

movement sense: N, normal; R, reverse; RL, right lateral; D, dip 

slip; 0, oblique slip. 

least, can be more confidently assigned a Paleozoic age. A 

number of brittle faults measured in outcrop have offsets of 

a few centimeters or slickenlines indicating slip direction 

(fig. 14B). These show no consistent preferred orientation. 

N 

Figure 15. Lower hemisphere equal-area projection showing 

contoured poles to joints in the Culpeper basin; contour interval is 

I percent per 1 percent area (n=94 ). 

PALEOZOIC RECRYSTALLIZATION AND 

METAMORPHIC GRADE 

The Paleozoic deformation of the rocks in the northern 

Blue Ridge was accompanied by recrystallization under 

greenschist-facies conditions. In pelitic rocks, such as those 

of the Harpers Formation, this metamorphism produced 

widespread biotite but no garnet (fig. 11 ). In some rocks, 

metamorphic recrystallization began before principal deformation (S1, F 1) as well as accompanying and following it: 

Recrystallization in the Middle Proterozoic gneisses 

produced low-grade mineral assemblages that are in sharp 

contrast to the original high-grade assemblages of the 

gneisses. In the granitic rocks, Paleozoic muscovite, chlorite, epidote, and secondary greenish biotite grew at the 

expense of feldspar and primary brown biotite, primarily 

during development of the S1 schistosity. Garnet in the rusty 

paragneiss (Yrg) is largely chloritized, and the mafic minerals in the hornblende-pyroxene gneiss are uralitized or 

extensively altered to actinolite and chlorite. In all of these 

rocks, plagioclase has cloudy overgrowths of sericite and 

saussurite and is now albite in composition. Quartz commonly has undulatory extinction or has recrystallized into a 

fine-grained mosaic. Biotite and muscovite grew in the 

mylonitic foliation of shear zones in the basement, indicating that the metamorphic age of these zones is compatible 

with Paleozoic deformation. 

The greenstones of the Late Proterozoic mafic dikes 

and the metabasalts of the Catoctin Formation have typical 
22 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

mafic greenschist-facies mineral assemblages, including 

actinolite, chlorite, and epidote. Muscovite, derived from 

plagioclase, is also common, and minor amounts of greenish-brown biotite may be present. Metamorphic recrystallization appears to have begun before and continued during 

development of foliation. In the coarser grained, actinolitebearing diabase dikes (Zda) and actinolitic greenstones of 

the Catoctin (Zca), prekinematic amphibole growth is 

shown by stubby crystals of pale-green actinolite, presumably pseudomorphs developed from igneous clinopyroxene 

during the early stages of metamorphism, that have rims 

that have broken down into foliation-parallel chlorite. In the 

rine-grained greenstones, particularly the metabasalts of the 

Catoctin, small blue-green actinolite laths have grown in the 

plane of the S1 foliation, indicating amphibole growth that 

is synkinematic with respect to S1• 

In the pelitic rocks such as the Harpers Formation 

phyllite, muscovite, biotite, and to a lesser extent chlorite 

are well developed and define the foliation. No garnet has 

been found, even in the easternmost rocks, where metamorphic grade might be higher. F 1 and F2 folding both occurred 

under peak biotite-zone conditions, whereas in the noses of 

the later crenulations and cross folds biotite is partially retrograded to chlorite. Only the late brittle structures such as 

joints and microfaults, some of which may be younger than 

Paleozoic, appear to be postmetamorphic in age. 

40Arf39Ar DATING OF METAMORPHIC FABRIC 

Muscovite that defines the dominant foliation in several units in the cover-sequence rocks has been sampled, 

and three of these samples were analyzed by the 40 Arf39 Ar 

age-spectrum technique to determine the age of the foliation 

and timing of Paleozoic deformation. The rock units sampled (from west to east) were fine-grained tuffaceous 

quartz-muscovite phyllite in the lower part of the Catoctin 

Formation (Zct), strongly cleaved Weverton Quartzite (£w) 

on the western (overturned) limb of the Catoctin syncline, 

and Loudoun Formation phyllite (£1) from the roadcut on 

U.S. Route 15 (black diamond symbol, pl. 1). The schistosity in the tuffaceous schist and Weverton Quartzite is 

mapped as S~o and the muscovitic cleavage in the quartzite 

is axial planar in outcrop to a mesoscopic F1 fold. The dominant schistosity in the phyllite on U.S. Route 15 is mapped 

as S2, with earlier, S~o fabric being completely transposed. 

These samples yield complex spectra that do not have 

plateaus and do not yield precise estimates of the timing of 

muscovite growth (Burton and others, 1992a; Kunk and 

others, 1993). This suggests that the muscovite is of multiple generations, although petrographic evidence indicates a 

single, dominant generation of cleavage-producing mica; 

apparently there was some fine-grained muscovite growth 

or recrystallization at temperatures below the argon closure 

temperature for muscovite ( -350°C) (Kunk and others, 

1993). Despite the difficulty of interpretation, these data 

and other data to the south suggest a late Paleozoic age 

(350--300 Ma) for the development of the regional (S1) 

cleavage and seem to rule out a pervasive early Paleozoic 

(Taconian) metamorphic event (Burton and others, 1992a). 

GEOLOGY OF EARLY MESOZOIC ROCKS 

The eastern half of the map area is underlain largely by 

Upper Triassic sedimentary rocks of the Culpeper Group of 

the Newark Supergroup and by minor areas of Early Jurassic diabase in the early Mesozoic Culpeper basin. The sedimentary rocks, from base to top, consist of pink to gray 

arkosic sandstone and red-brown siltstone of the Poolesville 

Member of the Manassas Sandstone, conformably overlain 

by dark-red Balls Bluff Siltstone, which grades laterally 

into and is locally overlain by light-gray limestone conglomerate lentils of its Leesburg Member. No diagnostic 

fossils have been found in these rocks within the map area; 

however, correlative beds in the Balls Bluff Siltstone nearby 

have been dated as Late Triassic on the basis of palynomorphs (Cornet, 1977). These strata are locally intruded by 

dark-gray diabase dikes and sheets that have been dated 

nearby as Early Jurassic (Sutter, 1985, 1988). Similar diabase dikes also intrude the pre-Mesozoic crystalline rocks 

west of the basin, one of which also yields an Early Jurassic 

age (Kunk and others, 1992). The Triassic sedimentary 

rocks regionally dip westward at moderate to gentle angles, 

are inferred to be cut by poorly exposed, northeast- and 

northwest-striking normal faults, and are separated from the 

Lower Paleozoic rocks to the west by a major normal border fault, the Bull Run fault of Roberts (1923). 

SEDIMENTARY ROCKS 

MANASSAS SANDSTONE, POOLESVILLE MEMBER 

The stratigraphically lowest Mesozoic unit within the 

map area is the Poolesville Member of the Manassas Sandstone ("Amp), which consists of pinkish-gray to reddishbrown sandstone and reddish-brown siltstone that occur in 

upward-fining sequences 3 to 6 m thick. The sandstone is 

fine- to coarse-grained, locally pebbly, arkosic, micaceous, 

ferruginous, and locally silty and calcareous. It consists 

chiefly of subrounded and subangular quartz and feldspar 

grains in a ferruginous, micaceous, silty clay matrix 

cemented by clay, iron oxide, silica, and calcite. It is thin- to 

thick bedded and massive and is both planar and crossbedded, especially in pebbly channel-fill sequences where 

angular red shale chips are locally common. Dusky-red to 

reddish-brown siltstone and silty and sandy shale form the 

top of the upward-fining units. Only the uppermost 105 m 

of the thick-bedded sandstone crops out along the northeastern margin of the quadrangle; it is poorly exposed and 
GEOLOGY OF EARLY MESOZOIC ROCKS 23 

deeply weathered on upland surfaces. The Poolesville 

Member is conformably and gradationally overlain by the 

Balls Bluff Siltstone. Although the base of the formation is 

not exposed in the map area, in adjacent areas it overlies a 

basal carbonate conglomerate member that is unconformable on Paleozoic carbonates (Lee and Froelich, 1989). 

BALLS BLUFF SILTSTONE AND ITS LEESBURG MEMBER 

This unit ("'Rb) consists of dusky-red to reddish-brown 

siltstone, sandstone, mudstone, and shale, which are interbedded in upward-fining sequences. The siltstone is mainly 

dusky red, reddish brown, dark red, and grayish red, micromicaceous, feldspathic, ferruginous, clayey, sandy, and calcareous; carbonate nodules (caliche) and calcite veinlets are 

locally common. It is thin bedded to massive and commonly 

bioturbated (mainly by rootlets) and has irregular bedding 

and hackly fractures. Reddish-brown, fine-grained sandstone that grades upward into climbing-ripple, cross-laminated, sandy siltstone is common near the base of the unit, 

and lenticular layers of dusky-red, silty and sandy calcareous shale are common throughout the sequence. Where the 

poorly exposed sandy siltstone intertongues with the carbonate conglomerates of the Leesburg Member (libl), the 

latter commonly forms low strike ridges that are prominent 

to the southeast but generally die out to the northwest. The 

lower fluvial siltstones of the Balls Bluff overlie the 

Poolesville Member of the Manassas Sandstone conformably and gradationally along the eastern margin of the 

quadrangle. Siltstones of the main mass of Balls Bluff intertongue extensively with the alluvial fan conglomerates of 

the Leesburg Member to the northwest; therefore much of 

the estimated thickness of 1 ,200 m is applicable to the 

conglomerates as well. The Balls Bluff Siltstone is well 

exposed at the type locality along the bluffs of the Potomac 

River near Balls Bluff National Cemetery where Lee (1977) 

described his partially measured section 6. Farther north 

and west, the siltstone is deeply weathered and mantled on 

upland surfaces by leached, micaceous residuum as much as 

6 m thick. Although no diagnostic fossils have been found 

in these beds in the map area, the upper part of the Balls 

Bluff Siltstone in nearby areas (lacustrine member) has 

been dated as Late Triassic (probably Norian) on the basis 

of palynomorphs in gray siltstone (Lee and Froelich, 1989). 

The Leesburg Member (lib I) of the Balls Bluff Siltstone is chiefly limestone and dolostone conglomerate that 

contains minor partings and lenticular intercalations of calcareous sandstone and siltstone and a friable, coarsegrained, pebbly, arkosic sandstone at the base in the northern part of the area. In weathered outcrops the conglomerate 

is light gray, and the varicolored clasts give it a conspicuous 

mottled appearance. On fresh surfaces it consists mainly of 

subangular to subrounded boulders, cobbles, and pebbles of 

light-gray to grayish-black and pinkish-red clasts of 

Figure 16. Outcrop of carbonate conglomerate, Leesburg 

Member of Balls Bluff Siltstone, just west of junction of State 

Routes 661 and 15. Outcrop is about 4 m across. 

Cambrian and Ordovician limestone and dolomite embedded in a variable matrix of calcareous reddish-brown gravel, 

sand, or clayey silt. The conglomerate contains minor clasts 

of quartzite, vein quartz, chert, marble, phyllite, slate, and 

greenstone, but more than 95 percent of the clasts are laminated to massive carbonate (Hazlett, 1978; Lindholm and 

others, 1979). 

The conglomerate is thick bedded to massive; beds 

average 3 m in thickness (fig. 16). These beds are commonly lenticular where defined by alternations of red 

muddy matrix-supported conglomerates and clast-supported 

conglomerates having well-rounded pebble layers and 

matrix- and calcite-cement-filled interstices. Measured section 7 of Lee (1977) includes the type section of the Leesburg Member, which is exposed in road cuts southeast of the 

junction of U.S. Route 15 bypass and the entrance road to 

Balls Bluff National Cemetery. A more detailed description 

of the variety of conglomerate fabrics near the type section 

and interpretations of their possible origin as debris flow 

and shallow sheetflood or flashflood deposits on an alluvial 

fan can be found in field trip no. 3, stop 4, in the Geologic 

Society of America Guidebook (p. 67-69 in Froelich and 

others, 1982). Rock fragments in the conglomerate generally increase in size northwesterly; cobbles that average 

about 6.35 em in diameter near the type section increase to 

as large as 1.2 m near Limestone Branch about 3.2 km to 

the north. In places the conglomerate layers are interbedded 

with 0.3- to 3.0-m-thick layers or lenses of red calcareous 

pebbly sandstone, calcareous sandy siltstone, and redbrown massive mudstone that are progressively thicker to 

the south and east where they tongue into the main body of 

Balls Bluff Siltstone ("Rb). 

The conglomerate apparently occurs in lobate deposits 

that overlap stratigraphically, although some conglomerate 

lentils are separated by red-brown sandstone and siltstone 
24 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

sequences. Hazlett (1978) has shown that the stratigraphically lowest lobes (most northerly and easterly) predominantly are composed of dolostone clasts, whereas the 

stratigraphically highest lobes (most southwesterly) mainly 

are composed of limestone clasts. In the northern part of the 

map area, the lowest dolostone conglomerate is underlain 

by a distinctive, friable, pebbly arkose that appears to be 

disconformable on the underlying siltstone. The Leesburg 

Member is fairly well to poorly exposed; commonly low, 

rounded strike ridges of bare, pitted conglomerate can be 

traced continuously for about 100 m and discontinuously 

for about 0.5 km (fig. 16). Such low ridges are separated by 

subdued parallel valleys and are underlain in places by 

deeply and irregularly weathered limestone conglomerate 

pinnacles mantled by as much as 10 m of noncalcareous, 

dark-red, clay- and silt-rich residuum (terra rosa); elsewhere 

the strike valleys are developed on intercalations of friable, 

calcareous, red-brown sandstone and siltstone. In many 

places on the carbonate conglomerate landscape, characteristics typical of karst or karren country are developed, such 

as sinkholes, caves, overhangs, solution pits, disappearing 

streams, tufa deposits, and springs (discussed below). The 

sinkholes, solution pits, tufa deposits, and springs are 

shown on plate 1. 

The Leesburg Member conformably overlies the lower 

fluvial siltstones of the Balls Bluff Siltstone ("Rb) and 

tongues into and is laterally gradational to the main body of 

siltstone to the southeast. Lee ( 1977) has calculated the 

thickness of the Leesburg Member in this area as 3,510 ft. 

However the carbonate conglomerate lentils overlap stratigraphically, are cut by faults, thin and pinch out into Balls 

Bluff Siltstone to the southeast, and are truncated by the 

border fault on the west; therefore it is unlikely that this 

composite thickness is representative. 

IGNEOUS AND METAMORPHIC ROCKS 

DIABASE 

The Upper Triassic sedimentary rocks of the Culpeper 

basin and the pre-Mesozoic rocks to the west are intruded 

by diabase dikes and plugs of Early Jurassic age. In most 

cases the dikes are poorly exposed and are known only from 

linear arrays of spheroidal boulders embedded in residual 

saprolite and soils, which weather a distinctive orange. 

Where fresh the diabase is medium to dark gray and chiefly 

medium grained and equigranular, but it is fine grained and 

aphanitic near chilled margins and in narrow dikes. Compositionally, the diabase is a quartz-normative tholeiite of at 

least two magma types: (1) low-Ti02 quartz normative (Jdl) 

and (2) high-Ti02 quartz normative (Jdh) (LTQ and HTQ 

magmas, respectively, of Weigand and Ragland, 1970). 

Minor diabase dikes of undetermined compositions are 

shown on plate 1 as Jd, including most of the dikes mapped 

in the pre-Mesozoic rocks. As no direct age determinations 

Table 4. Major oxide geochemistry (weight percent) 

of high-titanium, quartz-normative Jurassic diabase dike 

(Jdh), Waterford quadrangle. 

[Ten-element ICP analysis by Hiram Smith; total iron as Fe20 3] 

Major oxide 

Si02 

Al20 3 

Fe20 3 

MgO 

CaO 

Na20 

K20 

Ti02 

P20s 

MnO 

Loss on ignition 

Total 

Sample 

Map unit 

Sample site 

AJF-91-Wat-1 

Jdh 

17 

51.3 

13.8 

11.3 

7.6 

10.4 

1.9 

.46 

1.1 

.16 

.2 

1.72 

99.94 

were made, the inferred Early Jurassic age is based on 40Ar/ 

39 Ar age spectrum dates from similar diabase elsewhere in 

the Culpeper basin (Kunk and others, 1992; Sutter, 1985, 

1988). 

Texturally, LTQ diabase (Jdl, pl. 1) is characterized by 

widely scattered centimeter-sized phenocrystic clusters of 

calcic plagioclase (chiefly labradorite) embedded in generally aphyric diabase. The diabase is composed of an interlocking crystalline mosaic of fine, equigranular pyroxene 

(mainly augite), euhedral laths of fine-grained groundmass 

plagioclase, and minor amounts of interstitial oxides 

(mostly magnetite) and sulfides. In the map area, LTQ diabase occurs as a northeast-striking dike about 25 m wide 

intruding siltstone near the Potomac River about 4 km 

southeast of Lucketts, as a northwest-striking narrow dike 

intruding sandstone about 3.8 km northeast of Lucketts, and 

as a narrow, north-striking dike cutting the crystalline rocks 

of Catoctin Mountain about 4 km northwest of Lucketts. 

Elsewhere, minor spheroidal boulder debris may indicate 

the presence of other narrow dikes, as near State Route 658 

about 4.3 km north-northeast of Lucketts. 

HTQ diabase (Jdh, pl. 1) consists mainly of mediumgrained, interlocking crystals of pyroxene (mainly augite) 

and euhedral laths of plagioclase and minor amounts of 

interstitial oxides (magnetite and ilmenite). It is generally 

aphyric but locally aphanitic at chilled margins and where 

dikes are thin. HTQ diabase occurs as an irregularly shaped 

(inverted V) sheet or plug, about 900 m wide and possibly 

as much as 150 m thick, that intrudes siltstone and limestone conglomerate about 1.6 km northeast of Lucketts. 

Major oxide geochemistry from a sample of this body (no. 

17 on pl. 1) indicates that it is a quartz-normative tholeiite 

(table 4). 
GEOLOGY OF EARLY MESOZOIC ROCKS 25 

THERMALLY METAMORPHOSED ROCKS 

Hornfels, meta-arkose, and marble occur in zoned contact aureoles (Jtm) adjacent to diabase bodies. The hornfels 

is gray, dark gray, bluish gray, and mauve, and the metaarkose is commonly pink or light gray. Cordierite pseudomorphs and biotite crystals are common in the inner aureole, and epidote and chlorite occur in the outer aureole. The 

marble, derived from clasts in the carbonate conglomerate 

("Rbl ), is gray to light gray and contains coarse calcite and 

minor amounts of grossularite, diopside, and wollastonite 

locally developed (Lee, 1982; Lee and Froelich, 1989). The 

thermal aureole surrounding the thickest part of the HTQ 

diabase sheet or plug northeast of Lucketts may be as much 

as 30 m thick, but in most areas the aureole is less than 6 m 

thick. The hornfels and meta-arkose are hard, brittle, and 

fractured but do not weather readily; bedrock is thus generally shallow; soil and saprolite are less than 1 m thick. The 

fractured marble, however, weathers readily and is rarely 

exposed. It is generally only seen as angular blocks and 

fragments in soil and as leached and weathered material 

more than 6 m thick. 

STRUCTURAL GEOLOGY 

The structure of the Culpeper basin in the map area can 

be characterized generally as consisting of sedimentary 

beds dipping homoclinally westward towards a north- to 

northeast-striking, east-dipping, normal border fault, the 

Bull Run fault of Roberts (1923). The Triassic sedimentary 

rocks generally strike northerly and dip moderately to gently westward at 10 to 35 degrees. The orientation of the 

strata combined with the strike of the fault result in progressively lower Triassic strata being truncated by the fault in a 

northward direction. The Triassic strata are warped gently 

into broad, probably faulted flexures, most notably the 

south-plunging Morven Park syncline (pl. 1 and cross 

section D-D') and a west-plunging anticlinal nose near 

Limestone Branch, and are also offset by inferred crossbasin faults that apparently define a series of graben and 

horsts. 

BULL RUN FAULT 

The Bull Run fault (Roberts, 1923) is the normal border fault that marks the western margin of the Culpeper 

basin in the map area. This fault extends north-northeasterly 

along the eastern flank of Catoctin and Bull Run Mountain. 

In the southern part of the map it borders the Weverton 

Quartzite, whereas to the north the fault truncates progressively younger pre-Mesozoic units. North of U.S. Route 15 

near the Potomac River, a splay of the border fault extends 

sharply eastward and then northward again, separating the 

highly deformed Cambrian Frederick Limestone (£f) from 

the Triassic limestone conglomerate of the Leesburg Member (li bl). Another splay of the fault is inferred to continue 

straight northward parallel to the contact of Tomstown 

Dolomite (£t) and Frederick Limestone. The Bull Run fault 

is also responsible for a small wedge of downdropped 

Antietam Quartzite (£a) near the center of the Waterford 

quadrangle. 

The border fault in this area is inferred to dip about 50 

degrees eastward at the surface (sections B-B', C-C', D-D', 

pl. 1 ), based on sparse excavation and well data from, and 

former exposures in, the Waterford quadrangle and the adjacent Leesburg quadrangle (Roberts, 1923). However, in 

Limestone Branch where the stream intersects the trace of 

the fault, the evidence suggests a locally shallower dip. 

Here the fault is at least partially exposed at the eastern end 

of a long continuous outcrop of Weverton Quartzite. From 

west to east, the fault zone exposure contains about a meter 

of greenish-gray to rusty-weathered, shallowly east-dipping 

foliated cataclasite, which is derived from the Weverton 

Quartzite (the footwall). The cataclasite is overlain to the 

east by about 3 m of massive, saprolitized, orange fault 

gouge containing irregular to planar dark-red hematite-rich 

zones near the contact with the cataclasite, followed eastward by bright red saprolite apparently derived from Balls 

Bluff Siltstone (hanging wall). The dip in the foliated cataclasite was measured as approximately 25 degrees. To the 

west this fault fabric disappears over a short distance (a few 

meters or so) within the main exposure of Weverton 

Quartzite. 

An outcrop of cataclastic Weverton Quartzite about 50 

m west of gently dipping Balls Bluff Siltstone can also be 

seen in a creek immediately north of State Route 698 at the 

northwestern corner of the town of Leesburg. Weak fluxion 

structure in the cataclastic Weverton at one spot dips eastward at 45 degrees. 

Most of the observable border-fault-related structures 

in the footwall of the Bull Run fault are complex and 

consist of a variably oriented, crisscrossing network of fine 

fractures, microfaults, and thin zones of microbreccia. 

These features disappear a few tens of meters from the fault. 

Mineralization associated with the fractures and microbreccia includes fine veins of quartz and calcite, hematite and 

carbonaceous material in the microbreccia zones, and minor 

sulfides and euhedral hematite crystals. Discontinuous, 

dark-red hematite-filled microfractures in white Weverton 

Quartzite adjacent to the border fault constitute the most 

commonly seen form of mineralization associated with the 

Mesozoic border fault in this area. 

FURNACE MOUNTAIN FAULT AND RELATED FAULTS 

AND LINEAMENT 

Two small faults extend northwest from the Bull Run 

fault north of State Route 663 and converge into the 

Furnace Mountain fault, which cuts north and crosses the 

Potomac River near the Point of Rocks bridge. The 
26 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

northwest-striking faults downdrop a portion of the Antietam Quartzite, and one is continuous with a northwesttrending lineament that crosses the Culpeper basin (pl. 1). 

The latter feature is a conspicuous tonal lineament on aerial 

photos (Leavy, 1984) and radar imagery. 

The best evidence for normal offset on the Furnace 

Mountain fault can be found in a creek just southwest of 

Furnace Mountain. Here an exposure of Tomstown Dolomite occurs only about 20 m east of an outcrop that is compositionally transitional between the Harpers Formation and 

the Antietam Quartzite and that passes westward (downward) into continuously exposed phyllite of the Harpers. 

The minimum fault offset here is thus inferred to be equal to 

the thickness of the Antietam, or about 100 ft; on section 

B-B' (pl. 1 ), about 450 m to the north, it is shown as about 

250ft. Farther north, near the river, the offset is greater, and 

the fault juxtaposes Catoctin Formation (Zc) and carbonaceous phyllite (fcp ). The Furnace Mountain fault crosses 

the river to Point of Rocks, Md., where it may account for 

the juxtaposition of the Harpers Formation and Tomstown 

Dolomite as mapped by Whittaker (1955). 

INFERRED FAULTS 

The Triassic sedimentary rocks of the Culpeper basin 

in the Waterford and Point of Rocks quadrangles are here 

interpreted to be cut into fault blocks by a series of inferred 

regional north-northeast-trending faults. Previously the 

mapped distribution of basin lithofacies (carbonate 

conglomerate, lib I, and sandy siltstone and shale, lib were 

interpreted to be the product of lateral facies changes (Lee 

and Froelich, 1989). However these facies changes are too 

abrupt when viewed in the context of feasible alluvial-fan 

depositional models (Smoot, 1991; J.P. Smoot, U.S. Geological Survey, written commun., 1991). The sharp lateral 

transition between thick sequences of conglomerate and 

adjacent sandy siltstone and shale are better explained by a 

faulted, graben and horst geometry. The inferred faults on 

plate 1 are projected from better exposed faults mapped to 

the northeast that cut and offset the pre-Mesozoic rocks 

along the scalloped northeastern margin of the Culpeper 

basin. Radar imagery and air photos of the map area show 

truncated features and subtle tonal changes across linear 

features coincident with some of the inferred faults. The 

inferred faults at and northeast of the junction of U.S. Route 

15 and Business Route 15 are associated with a series of 

springs and tufa deposits (pl. 1). 

RELATIVE TIMING OF MESOZOIC FAULTING 

The timing of movement on the Mesozoic faults is not 

precisely known. No Jurassic diabase dikes have been 

observed cutting the faults, and no faulted dikes are known. 

Continued border fault movement in the Triassic is implied 

by the presence of the thick conglomerates ('Rbl) in the 

basin, but whether these originated at the present -day 

border fault or one farther west is unknown. The fact that 

the conglomerates are tilted and faulted means that movement on the present border fault continued after their deposition and lithification; to the south, tilted and faulted 

Jurassic formations indicate that faulting continued after all 

sedimentation had ceased in the basin (Froelich and others, 

1982). 

JOINTS IN THE CULPEPER BASIN 

Lee (1979 and this report) made extensive measurements of joint surfaces in the Triassic sedimentary rocks of 

the Culpeper basin as shown on plate 1. A stereonet plot 

shows them to be mostly steeply dipping but having a wide 

range of azimuths (fig. 15). Despite the apparent randomness of plotted joint orientations, however, consistent northnortheast and west-northwest trends in joints in carbonate 

conglomerate (libl) are indirectly evidenced by aligned 

sinkholes (pl. 1 ), which are probably largely joint 

controlled. 

As discussed above, there is much overlap and scatter 

in orientation between joints in the Culpeper basin and 

those in the Blue Ridge anticlinorium (figs. 14A, 15). A 

study by Hardcastle (1993) of more than 4,800 joints in the 

basin and adjacent crystalline rocks found a number of distinct trends shared by both regions (N. 13° W., N. 12° E., N. 

77° E.) and one trend (N. 67° W.) largely restricted to Blue 

Ridge rocks. This joint pattern was not observed in the 

present report, perhaps due to the comparatively small number of joints measured. 

SURFICIAL GEOLOGY 

The post-Mesozoic geology of the map area is shown 

on plate 1. A detailed discussion of the processes active 

during this period and their chronology is beyond the scope 

of this report, but a regional map and discussion of 

engineering properties was provided by Froelich (1985). 

The materials shown include mountain slope colluvium 

(OTc), river terrace deposits (OTt), and flood-plain alluvium 

(Oal). The principle source of the colluvium is Catoctin 

Mountain, and the chief source for the terrace deposits and 

Potomac River alluvium is the upstream drainage area of 

the Potomac River. Alluvium derived from local sources is 

also found along most streams. Although the alluvium is 

Holocene in age, some of the colluvium and terrace deposits 

may be Pleistocene or older. The surficial deposits are 

unconformable on older units. In addition to the mappable 

surficial deposits, thick residual soils and saprolite mantle 

most of the upland bedrock surfaces, such as the areas 

shown as undifferentiated basement (Middle Proterozoic 

gneiss and late Proterozoic dikes) on plate 1. Some of the 

saprolite and residuum and their regional distribution in the 

Culpeper basin are shown in Froelich (1985). 
HYDROGEOLOGY 27 

Colluvium (OTc) occurs as bedrock clasts embedded in 

gravel, sand, and clayey silt in sheetlike to lensoid aprons 

flanking Catoctin Mountain. It is composed chiefly of 

unsorted angular to subangular boulders, cobbles, and pebbles of quartzite, phyllite, greenstone, and vein quartz in a 

loose matrix of yellowish-orange to reddish-brown micaceous sand and silt. Debris flows consisting of large jumbled blocks of Weverton Quartzite are locally conspicuous 

on the flanks of Catoctin Mountain. Thickness of colluvium 

ranges from a veneer at the eroded margins of the deposits 

to greater than 15 mat the foot of the mountains. 

The terrace deposits (OTt) are gravel, sand, silt, and 

clay in fairly well bedded, gently sloping, graded deposits 

flanking the Potomac River. Terrace deposits have two 

modes of occurrence: (1) as incised but nearly continuous 

lowland benches 3 to 9 m above the adjacent modern flood 

plain and (2) as erosional upland remnants capping Manassas Sandstone and Balls Bluff Siltstone at elevations of 100 

min Maryland and 85 to 91 m and 100 to 110m in Virginia. 

The gravel is composed mainly of rounded clasts of quartzite, vein quartz, and greenstone (including epidosite) and 

has rare clasts of chert, phyllite, and diabase; the matrix is 

chiefly yellowish-orange to yellowish-brown silty sand. 

Thickness ranges from a feather edge to about 10 m. As no 

direct age determinations were made, the lowermost of the 

highest terraces could be Pliocene or older. 

The alluvium (Oal) is composed of sand, silt, gravel, 

and clay in layers underlying flood plains along most 

streams, as well as the broad flood plain of the Potomac 

River. The sediments are typically yellowish-brown, fairly 

well bedded, poorly to moderately well sorted, graded 

deposits; gravel commonly fills channels at the base of 

upward-fining sequences. Most gravel clasts in streams 

draining the Catoctin Mountain are subrounded quartzite, 

vein quartz, greenstone, and phyllite. Streams draining the 

Culpeper basin contain pebbles and cobbles of limestone, 

sandstone, siltstone, and, locally, diabase and hornfels. 

Alluvium is probably as much as 30 ft thick along the Potomac River and averages 20 ft in broad areas of the river 

flood plain at elevations from 190 to 220 ft; in tributary 

streams the deposits are at most 6 to 10 ft thick. 

ECONOMIC GEOLOGY 

Marble from the Swift Run and basal Catoctin Formations and limonite from the Antietam Quartzite (quarry 

symbols on pl. 1) in the past have been quarried or mined in 

the map area, mostly in the 1800's. No mining or quarrying 

has occurred in these pre-Mesozoic crystalline rocks for 

many decades, and the resource potential is considered low. 

In the Culpeper basin, potential resources include 

those for construction materials, agricultural lime, brick, 

and tile. Ground water is discussed in "Hydrogeology" 

below. No active quarries are present, but several 

abandoned quarries were operated in the limestone conglomerate of the Leesburg Member of the Balls Bluff Siltstone northeast of Lucketts near old lime kilns along State 

Route 658. Early Mesozoic diabase is rare in this quadrangle, and only one small body northeast of Lucketts is potentially large enough to sustain an open-pit quarry operation. 

Much larger bodies of diabase are quarried nearby, and 

large volumes of better quality limestone are available and 

quarried in adjacent areas. 

Although extensive deposits of alluvium, terrace 

gravel, and colluvium, largely composed of resistant quartzite clasts, are common in this area, none have been commercially exploited, although borrow pits for local usage and 

fill are known. The deposits have irregular thickness, are 

poorly sorted, and commonly are in designated parkland 

and thus are not of economic interest. 

Three samples of red-brown Triassic shale and sandy 

siltstone in the Waterford quadrangle have been tested for 

firing properties (Calver and others, 1961; samples R-237, 

R-238, and R-240). Only sample R-238 from the lower 

fluvial part of the Balls Bluff Siltstone showed potential use 

for brick. Commercial clay deposits are currently being 

exploited for brick and tile from much more extensive Triassic deposits elsewhere in the Culpeper basin and apparently fill the present needs. A regional discussion of mineral 

resources in the Culpeper basin is provided by Froelich and 

Leavy (1981 ), and engineering and geotechnical characteristics of the bedrock in the Culpeper basin are discussed in 

Leavy and others (1983). 

HYDROGEOLOGY 

In terms of bedrock control of surface flow and 

ground-water permeability, the map area consists of three 

regimes, each having characteristic rock properties and 

structural fabrics that influence local hydrology: (1) the dissected plateau west of Catoctin Mountain, (2) Catoctin 

Mountain and its flanks, and (3) the lowlands of the 

Culpeper basin. In the western plateau the obvious structural features, which may exert a considerable influence on 

ground-water flow, are the closely spaced, northeaststriking, moderately to steeply dipping sheetlike metadiabase dikes intruding gneiss, together with the subparallel 

and generally pervasive Paleozoic schistosity developed in 

both lithologies. This overall fabric contrasts with that of 

the Catoctin Mountain and the uplands along its flanks, 

which are underlain by relatively massive metavolcanic 

rocks and finely layered metasedimentary rocks, which 

have a generally gently dipping and broadly warped finegrained internal fabric. In the Culpeper basin, the dominant 

hydrogeologic factors are the west-dipping sedimentary 

strata, which contain carbonate conglomerates having high 

dissolution potential. Superimposed on the gross structural 

features are local joints and faults of widely varying orien-
28 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

tations that act in concert with the regional structures to 

control the local hydrology. In the Culpeper basin, joint and 

fault trends in the carbonate conglomerates have apparently 

influenced the locations of aligned areas of both recharge 

(sinkholes) and discharge (springs or seeps). In terms of 

ground-water flow, the Culpeper basin is likely to be the 

most permeable, the Catoctin Mountain uplands the least, 

and the western plateau intermediate. The pre-Mesozoic 

crystalline rocks produce water-well yields ranging from 

highest to lowest in the following order: basement gneissdike complex, Swift Run Formation, Weverton Quartzite, 

Harpers Formation, and Catoctin Formation (Brutus Cooper, Loudoun County Department of Natural Resources, 

written commun., 1992). 

In the Culpeper basin portion of the map area, ground 

water of good quality and in volumes more than adequate 

for domestic use is available at depths less than 150 m. 

Areas of potential ground-water recharge and discharge in 

the Culpeper basin are shown (pl. 1) by sinkholes and 

depressions (asterisks) and springs and seeps (spring symbols), respectively. The carbonate conglomerates have the 

highest yields of any aquifer in the Culpeper basin. In areas 

underlain by the Leesburg Member of the Balls Bluff Siltstone, numerous springs discharge at elevations from 240 to 

260ft (pl. 1). Hydrology of the Culpeper basin as a whole ,is 

discussed in Laczniak and Zenone (1985) and Froelich 

(1989). 

Sinkholes and subsidence pits (asterisks, pl. 1), 

mapped in 1991 by Alex Blackburn (Loudoun County 

Department of Natural Resources, written commun., 1991), 

are subtle surface depressions having local relief of about 

0.6 to 0.9 m. Sinkholes are developed in areas underlain by 

limestone conglomerate of the Leesburg Member of the 

Balls Bluff Siltstone or by residuum developed on it, 

whereas subsidence pits are developed in areas underlain 

mainly by colluvium, and locally terrace deposits that overlie limestone conglomerate of the Leesburg Member, 

assumed to be dissolved at depth. The presence of sinkholes 

or subsidence pits are prima facie evidence of past or current solution, subsidence, or collapse of soluble carbonate 

bedrock, mainly in areas of active migration of ground 

water or of recharge from infiltrating surface water. As 

mentioned above, the sinkholes are commonly aligned 

along consistent trends (north-northeast and west-northwest) that are parallel to joints mapped on surface outcrops 

of limestone conglomerate. Joint intersections along these 

trends are likely sites for fractured bedrock and zones of 

weakness capable of solution and enlargement by infiltrating ground water. 

In contrast, springs (spring symbol, pl. 1) are zones of 

upward hydraulic gradient and ground-water discharge 

from the Leesburg Member. Other sites of discharge (spring 

symbol D, pl. 1) are persistent swampy or boggy areas at 

comparable elevations. Ground-water discharge occurs at 

sites where the regional potentiometric surface is locally 

above ground surface, commonly in stream valleys. Tufa 

deposits (half-filled square, pl. 1), dry valleys, and caves 

and enlarged cavities (triangles, pl. 1), lie at regionally low 

elevations and may be surface exposures of formerly subterranean solution features. 

REFERENCES CITED 

Aleinikoff, J.N., Walter, Marianne, Lyttle, P.T., Burton, W.C., Leo, 

G.W., Nelson, A.E., Schindler, J.S., and Southworth, C.S., 

1993, U-Pb zircon and monazite ages of Middle Proterozoic 

rocks, northern Blue Ridge, Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 2, p. 2. 

Aleinikoff, J.N., Zartman, R.E., Walter, Marianne, Rankin, D.W., 

Lyttle, P.T., and Burton, W.C., 1995, U-Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central 

and southern Appalachians-Evidence for two pulses of 

Iapetan rifting: American Journal of Science, v. 295, no. 4, 

p. 428-454. 

Allen, R.M., 1963, Geology and mineral resources of Greene and 

Madison Counties: Virginia Division of Mineral Resources 

Bulletin 78, 102 p. 

Bartholomew, M.J., 1977, Geology of the Greenfield and Sherando quadrangles, Virginia: Virginia Division of Mineral 

Resources Publication 4, 43 p. 

Boyer, S.E., and Mitra, Gautam, 1988, Relations between deformation of crystalline basement and sedimentary cover at the 

basement/cover transition zone of the Appalachian Blue 

Ridge province, in Mitra, G., and Wojtal, S., eds., Geometry 

and mechanisms of Appalachian thrusting, with special reference to the Appalachians: Geological Society ·of America 

Special Paper 222, p. 119-136. 

Burton, W.C., Aleinikoff, J.N., Southworth, C.S., and Lyttle, P.T., 

1994, Grenvillian history of intrusion and deformation, northern Blue Ridge, Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 26, no. 4, p. 5. 

Burton, W.C., Froelich, A.J., Schindler, J.S., and Southworth, C.S., 

1992b, Geologic map of Loudoun County, Virginia: U.S. 

Geological Survey Open-File Map 92-716, scale 1:100,000. 

Burton, W.C., Kunk, M.J., and Lyttle, P.T., 1992a, Age constraints 

on the timing of regional cleavage formation in the Blue 

Ridge anticlinorium, northernmost Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 2, 

p. 5. 

Burton, W.C., and Southworth, C.S., 1993, Garnet-graphite paragneiss and other country rocks in granitic Grenvillian 

basement, Blue Ridge anticlinorium, northern Virginia and 

Maryland [abs.]: Geological Society of America Abstracts 

with Programs, v. 25, no. 4, p. 6. 

Calver, J.L., Hamlin, H.P., and Wood, R.S., 1961, Analyses of 

clay, shale, and related materials-Northern counties: Virginia Division of Mineral Resources, Mineral Resources 

Report 2, 194 p. 

Cloos, Ernst, 1941, Geologic map of Washington County: Maryland Geological Survey, scale 1 :62,500. 

---1947, Oolite deformation in the South Mountain fold, 

Maryland: Geological Society of America Bulletin, v. 58, 

no. 9, p. 843-918. 
REFERENCES CITED 29 

Comet, Bruce, 1977, The palynostratigraphy and age of the Newark Supergroup: University Park, Pennsylvania State University, Ph.D. thesis, 504 p. 

Drake, A.A., 1984, The Reading Prong of New Jersey and eastern 

Pennsylvania-An appraisal of rock relations and chemistry 

of a major Proterozoic terrane in the Appalachians, in Bartholomew, M.J., ed., The Grenville event in the Appalachians 

and related topics: Geological Society of America Special 

Paper 194, p. 75-108. 

Espenshade, G.H., 1986, Geology of the Marshall quadrangle, 

Fauquier County, Virginia: U.S. Geological Survey Bulletin 

1560,60 p. 

Evans, M.A., 1989, The structural geometry and evolution of foreland thrust systems, northern Virginia: Geological Society of 

America Bulletin, v. 101, p. 339-354. 

Fauth, J.L., and Brezinski, D.K., 1994, Geologic map of the Middletown quadrangle, Frederick and Washington Counties, 

Maryland: Baltimore, Maryland Geological Survey, scale 

1 :24,000, 1 sheet. 

Froelich, A.J., 1985, Map and geotechnical properties of surface 

materials of the Culpeper basin and vicinity, Virginia and 

Maryland: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1313-E, scale 1 : 125,000. 

---1989, Maps showing geologic and hydrologic factors 

affecting land-use planning in the Culpeper basin, Virginia 

and Maryland: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1313-J, scale 1:125,000. 

Froelich, A.J., and Leavy, B.D., 1981, Mineral resources map of 

the Culpeper basin, Virginia and Maryland: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-1313-B, 

scale 1:125,000. 

Froelich, A.J., Leavy, B.D., and Lindholm, R.C., 1982, Geologic 

traverse across the Culpeper basin (Triassic-Jurassic) of 

northern Virginia, in Lyttle, P.T., ed., Central Appalachian 

geology, Northeast-Southeast Section of the Geological Society of America Field Trip Guidebooks, field trip 3: Falls 

Church, Va., American Geological Institute, p. 55-81. 

Furcron, A.S., 1939, Geology and mineral resources of the Warrenton quadrangle, Virginia: Virginia Geological Survey Bulletin 54, 94 p. 

Gates, A.E., Simpson, Carol, and Glover, Lynn, III, 1986, Appalachian Carboniferous strike-slip faults-An example from 

Brookneal, Virginia: Tectonics, v. 5, p. 119-133. 

Hardcastle, Ken, 1993, Fracture fabric of the Mesozoic Culpeper 

basin and adjacent, pre-Triassic crystalline basement rocks 

[abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 2, p. 21. 

Harris, L.D., 1979, Similarities between the thick-skinned Blue 

Ridge anticlinorium and the thin-skinned Powell Valley anticline: Geological Society of America Bulletin, pt. 1, v. 90, 

p. 525-539. 

Hazlett, J.M., 1978, Petrology and provenance of the Triassic 

limestone conglomerate in the vicinity of Leesburg, Virginia: 

Washington, D.C., The George Washington University, M.S. 

thesis, 100 p. 

Herz, Norman, and Force, E.R., 1984, Rock suites in Grenvillian 

terrane of the Roseland district, Virginia, in Bartholomew, 

M.J., ed., The Grenville event in the Appalachians and related 

topics: Geological Society of America Special Paper 194, 

p. 187-214. 

Hillhouse, D.N., 1960, Geology of the Piney River-Roseland titanium area, Nelson and Amherst Counties, Virginia: Blacksburg, Virginia Polytechnical Institute and State University, 

Ph.D. dissertation, 129 p. 

Holden, R.J., 1907, Iron, in Watson, T.L., ed., Mineral resources of 

Virginia: Lynchburg, Va., J.P. Bell Co., p. 402-491. 

Howard, J.L., 1991, Lithofacies of the Precambrian basement 

complex in the northernmost Blue Ridge province of Virginia: Southeastern Geology, v. 31, no. 4, p. 191-202. 

Jonas, A.l., and Stose, G.W., 1938, Geologic map of Frederick 

County and adjacent parts of Washington and Carroll Counties: Maryland Geological Survey, scale 1:62,500. 

Keith, Arthur, 1894, Geology of the Catoctin Belt: U.S. Geological 

Survey Annual Report, no. 14, p. 285-395. 

Klein, S.W., Lyttle, P.T., and Froelich, A.J., 1990, Geologic map of 

the Loudoun County portion of the Middleburg quadrangle, 

Virginia: U.S. Geological Survey Open-File Report 90-641, 

scale 1 :24,000. 

Klein, S.W., Lyttle, P.T., and Schindler, J.S., 1991, Late Proterozoic sedimentation and tectonics in northern Virginia, in 

Schultz, Arthur, and Compton-Gooding, Ellen, eds., Geologic 

evolution of the eastern United States, Northeast-Southeast 

Section of the Geological Society of America Field Trip 

Guidebook, no. 2: Virginia Museum of Natural History, 

p. 263-294. 

Kunk, M.J., Froelich, A.J., and Gottfried, David, 1992, Timing of 

emplacement of diabase dikes and sheets in the Culpeper 

basin and vicinity, Virginia and Maryland-40Arf39Ar age 

spectrum results from hornblende and K-feldspar in granophyres [abs.]: Geological Society of America Abstracts with 

Programs, v. 24, no. 2, p. 25. 

Kunk, M.J., Lyttle, P.T., Schindler, J.S., and Burton, W.C., 1993, 

Constraints on the thermal history of the Blue Ridge in northernmost Virginia-40Arf39Ar age dating results [abs.]: Geological Society of America Abstracts with Programs, v. 25, 

no. 2, p. 31. 

Laczniak, R.J., and Zenone, Chester, 1985, Ground-water 

resources of the Culpeper basin, Virginia and Maryland: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-1313-F, scale 1:125,000. 

Leavy, B.D., 1984, Map showing planar and linear features in the 

Culpeper basin and vicinity, Virginia and Maryland: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-1313-G, scale 1:125,000. 

Leavy, B.D., Froelich, A.J., and Abram, E.C., 1983, Bedrock map 

and geotechnical properties of rocks of the Culpeper basin 

and vicinity, Virginia and Maryland: U.S. Geological Survey 

Miscellaneous Investigation Series Map 1-1313-C, scale 

1:125,000. 

Lee, K. Y., 1977, Triassic stratigraphy in the northern part of the 

Culpeper basin, Virginia and Maryland: U.S. Geological Survey Bulletin 1422-C, 17 p. 

---1979, Geology of the Culpeper basin in the Waterford 

quadrangle, Loudoun County, Va.-Montgomery County, 

Md., in Triassic-Jurassic geology of the northern part of the 

Culpeper basin, Virginia and Maryland: U.S. Geological Survey Open-File Report 79-1557 (parts of 16 quadrangles, 

scale 1:24,000), 19 p. 

---1982, Thermal metamorphism of Triassic and Jurassic sedimentary rocks in the Culpeper basin, Virginia [abs.]: Geo-
30 GEOLOGY OF WATERFORD QUAD, VA AND MD, AND VIRGINIA PART OF POINT OF ROCKS QUAD 

logical Society of America Abstracts with Programs, v. 14, 

no. 1-2, p. 34. 

Lee, K.Y., and Froelich, A.J., 1989, Triassic-Jurassic stratigraphy 

of the Culpeper and Barboursville basins, Virginia and Maryland: U.S. Geological Survey Professional Paper 1472, 52 p. 

Lindholm, R.C., 1979, Geologic history and stratigraphy of the 

Triassic-Jurassic Culpeper basin, Virginia: Geological Society of America Bulletin, pt. 2, v. 90, p. 1702-1736. 

Lindholm, R.C., Hazlett, J.M., and Fagin, S.W., 1979, Petrology of 

Triassic-Jurassic conglomerates in the Culpeper basin, 

Virginia: Journal of Sedimentary Petrology, v. 49, no. 4, 

p. 1245-1262. 

Lukert, M.T., and Nuckols, E.B., IlL 1976, Geology of the Linden 

and Flint Hill quadrangles, Virginia: Virginia Division of 

Mineral Resources Report of Investigations 44, 83 p. 

McBirney, A.R., 1984, Igneous petrology: San Francisco, Freeman, Cooper, and Co., 509 p. 

McClelland, J., and Chiarenzelli, J., 1990, Geochronological studies in the Adirondack Mountains and implications of a Middle Proterozoic tonalitic suite, in Gower, C., Rivers, T., and 

Ryan, B., eds., Mid-Proterozoic geology of the southern margins of proto-Laurentia-Baltica: Geological Association of 

Canada Special Paper 38, p. 175-194. 

Mitra, Gautam, 1979, Ductile deformation zones in Blue Ridge 

basement rocks and estimation of finite strains: Geological 

Society of American Bulletin, pt. I, v. 90, p. 935-951. 

Mitra, Gautam, and Elliott, David, 1980, Deformation of basement 

in the Blue Ridge and the development of the South Mountain cleavage, in Wones, D.R., ed., The Caledonides in the 

USA; Proceedings, International Geological Correlation Program Project 27-Caledonide Orogen, Blacksburg, Va., 

1979: Virginia Polytechnic Institute and State University 

Department of Geological Sciences Memoir 2, p. 307-311. 

Mitra, Gautam, and Lukert, M.T., 1982, Geology of the CatoctinBlue Ridge anticlinorium in northern Virginia, in Lyttle, PT., 

ed., Central Appalachian geology, Northeast-Southeast Section of the Geological Society of America Field Trip Guidebooks, field trip 4: Falls Church, Va., American Geological 

Institute, p. 83-108. 

Moore, J.M., and Thompson, PH., 1980, The Flinton Group-A 

late Precambrian metasedimentary succession in the Grenville province of eastern Ontario: Canadian Journal of Earth 

Sciences, v. 17, p. 1685-1707. 

Nickelsen, R.P., 1956, Geology of the Blue Ridge near Harpers 

Ferry, West Virginia: Geological Society of America Bulletin, 

v.67,no. 3,p. 239-269. 

Nunan, W.E., 1979, Stratigraphy of the Weverton Formation, 

northern Blue Ridge anticlinorium: Chapel Hill, University of 

North Carolina, Ph.D. thesis, 215 p. 

Rankin, D.W., 1975, The continental margin of eastern North 

America in the southern Appalachians-The opening and 

closing of the proto-Atlantic Ocean: American Journal of Science, v. 275-A, p. 298-336. 

Ratcliffe, N.M., Aleinikoff, J.N., Burton, W.C., and Karabinos, P, 

1991, Trondhjemitic, 1.35-1.31 Ga gneisses of the Mount 

Holly Complex of Vermont-Evidence for an Elzevirian 

event in the Grenville basement of the U.S. Appalachians: 

Canadian Journal of Earth Sciences, v. 28, p. 77-93. 

Reed, J.C., Jr., 1955, Catoctin Formation near Luray, Virginia: 

Geological Society of America Bulletin, v. 66, no. 7, 

p. 871-896. 

Reed, J.C., Jr., and Morgan, B.A., 1971, Chemical alteration and 

spilitization of Catoctin greenstones, Shenandoah National 

Park, Virginia: Journal of Geology, v. 79, no. 5, p. 526-548. 

Reinhardt, J., 1974, Stratigraphy, sedimentology, and CambroOrdovician paleogeography of the Frederick Valley, Maryland: Maryland Geological Survey Report of Investigations 

23, 12 p. 

Roberts, J.K., 1923, Triassic basins of northern Virginia: The PanAmerican Geologist, v. 39, no. 3, p. 185-200. 

Sinha, A.K., and Bartholomew, M.J ., 1984, Evolution of the Grenville terrane in the central Virginia Appalachians, in Bartholomew, M.J., ed., The Grenville event in the Appalachians 

and related topics: Geological Society of America Special 

Paper 194, p. 175-186. 

Smoot, J.P., 1991, Sedimentary facies and depositional environments of early Mesozoic Newark Supergroup basins, eastern 

North America-Palaeogeography, palaeoclimatology, palaeoecology: Amsterdam, Elsevier Science Publishers, v. 84, 

p. 369-423. 

Southworth, C.S., 1991, Geologic map of the Loudoun County, 

Virginia, part of the Harpers Ferry quadrangle: U.S. Geological Survey Miscellaneous Field Studies Map MF-2173, scale 

1:24,000. 

Stose, A.J., and Stose, G.W., 1946, Geology of Carroll and Frederick Counties, in The physical features of Carroll County and 

Frederick County: State of Maryland Department of Geology, 

Mines, and Water Resources, p. 11-131. 

Sutter, J.F., 1985, Progress on geochronology of Mesozoic diabase 

and basalts, in Proceedings of the Second U.S. Geological 

Survey Workshop on the early Mesozoic basins of the eastern 

United States: U.S. Geological Survey Circular 946, 

p. 10--114. 

---1988, Innovative approaches to the dating of igneous 

events in the early Mesozoic basins of the eastern United 

States, in Froelich, A.J., and Robinson, G.R., Jr., Studies of 

the early Mesozoic basins of the eastern United States: U.S. 

Geological Survey Bulletin 1776, p. 194-200. 

Weigand, P.W., and Ragland, P.C., 1970, Geochemistry of Mesozoic dolerite dikes from eastern North America: Contributions to Mineralogy and Petrology, v. 29, no. 3, p. 195-214. 

Whittaker, J.C., 1955, Geology of Catoctin Mountain, Maryland 

and Virginia: Geological Society of America Bulletin, v. 66, 

p. 435-462.