The Chopawamsic Formation 

A New Stratigraphic Unit 

in the Piedmont of 

Northeastern Virginia

By D. L. SOUTHWICK, JOHN C. REED, JR., and R. B. MIXON

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1324-D

Description of a sequence of felsic, 

intermediate, and mafic metavolcanic 

and metasedimentary rocks
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog card No. 74-610531

UNITED STATES GOVERNMENT PRINTING OFFICE 

WASHINGTON: 1971

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 10 cents (paper cover)
CONTENTS

	Page 

Abstract ___________________________ . Dl

Introduction _____________________________ r________. 1

Type section and reference sections ________-_-___ . 2

Contact relations _________________ _ . 3

Primary features ________________________________ - 7

Mineralogy and petrography _____________________ . 7

Chemical composition ________________________________ - 8

Age and correlation ________________________________ . 9

References ________________________________________. 10

ILLUSTRATION

Page

FIGURE 1. Sketch map showing outcrop belt of the Chopawamsic Formation- D4

TABLES

Page

TABLE 1. Mineral assemblages observed in the Chopawamsic Formation D8 

2. Chemical analyses of rocks from the Chopawamsic Formation _ 9

III

CONTRIBUTIONS TO STRATIGRAPHY

THE CHOPAWAMSIC FORMATION 

A NEW STRATIGRAPHIC UNIT

IN THE PIEDMONT OF 

NORTHEASTERN VIRGINIA

BY D. L. SOUTHWICK, JOHN C. REED, JR., and R. B. MIXON

ABSTRACT

The name Chopawamsic Formation is introduced for a sequence of felsic, intermediate, and mafic metavolcanic and metasedimentary rocks in northeastern Virginia. 

No fossils have been found in the Chopawamsic, but it conformably underlies the 

Quantico Slate of Ordovician age and conformably overlies and interfingers with 

mica gneiss of the Wissahickon Formation of late Precambrian( ?) age. The unit 

is estimated to be 6,000-10,000 feet thick. It is very likely correlative with the 

volcanic rocks beneath the Arvonia Slate in central Virginia and with the James 

Run Gneiss and volcanic complex of Cecil County, Md. It may also be correlative 

with parts of the sedimentary and volcanic sequence of the Carolina slate belt.

INTRODUCTION

The name Chopawamsic Formation is here introduced for a sequence 

of interbedded metavolcanic and metasedimentary rocks that conformably 

underlies the Quantico Slate of Ordovician age (Watson and Powell, 1911) 

on the northwestern limb of the Quantico syncline in the Occoquan, 

Quantico, Joplin, and Stafford 7 /z l -minute quadrangles, Prince William 

and Stafford Counties, Va. The formation rests on and interfingers with 

pebbly mica gneiss of the Wissahickon Formation of late Precambrian (?) 

age. The formation can be divided into three principal lithologic units: 

(1) metamorphosed medium- to thick-bedded mafic to intermediate volcanic rocks derived from andesitic to basaltic flows, coarse breccias, and 

finer tuffaceous clastic rocks; (2) metamorphosed medium- to thick-bedded 

felsic volcanic rocks derived from flows and associated volcaniclastic acDl
D2 CONTRIBUTIONS TO STRATIGRAPHY

cumulations; and (3) metamorphosed thin- to medium-bedded volcaniclastic rocks of felsic to mafic composition, locally containing beds of nonvolcanic quartzose metagraywacke, green to gray phyllite, and felsic to 

mafic flows. Units 1 and 2 grade vertically and laterally into unit 3 and 

appear to be tongues or lenses within a complex volcanic-sedimentary pile.

The geologic structure of the northwestern limb of the Quantico syncline is relatively simple. Bedding strikes N. 25° -35° E., and dips are 

generally vertical but range from 70° NW. to 60° SE. Slaty cleavage in 

the Quantico Slate and schistosity in the Chopawamsic Formation are 

parallel or subparallel to bedding. Graded bedding is recognizable in only 

a few places but indicates tops of beds are consistently to the southeast. 

There is no evidence in either individual outcrops or in map patterns 

of any large-scale repetition of stratigraphic units. If one assumes that 

the average dip is about vertical, the thickness of the Chopawamsic Formation is estimated to range from 6,000 to 10,000 feet.

The rocks here referred to the Chopawamsic Formation were mentioned 

briefly by Watson and Powell (1911, p. 39) as "phyllites and schists." They 

recognized the tuffaceeus character of some beds near the base of the 

Quantico Slate but did not mention the volcanic rocks beneath the slate. 

Lonsdale (1927) recognized and described the following rock types among 

those we include in the Chopawamsic Formation: greenstone schist, quartz 

schist with biotite and feldspar, hornblende quartz schists often with 

chlorite, sericite quartz schists often with epidote and chlorite, and arkosic 

quartzite. He suggested that the greenstone schist was of volcanic origin 

and noted the occurrence (p. 61) of "* * * areas which resemble 

amygdaloidal cavities of basalt." On the 1928 geologic map of Virginia 

(Virginia Geol. Survey, 1928), the bulk of the rocks of the Chopawamsic 

Formation are mapped as "Peters Creek quartzite," but some bodies of 

greenstone are mapped separately. The contacts shown are from Lonsdale (1927). On the later State geologic map (Virginia Div. Mineral Resources, 1963), the contacts are virtually the same as on the 1928 map, 

but the Chopawamsic Formation is correctly mapped as "metamorphosed 

volcanic and sedimentary rocks."

TYPE SECTION AND REFERENCE SECTIONS

The Chopawamsic Formation is named for its exposures along Chopawamsic Creek on the Quantico Marine Base in the Joplin 7 1/2 -minute 

quadrangle (@ on fig. 1). The unit crops out along the creek from a 

point 1.13 miles S. 45° W. of the crossroads of Joplin to a point 2.46 miles 

S. 6° E. of Joplin. These exposures are designated as the type section. 

Outcrops of the lower half of the formation are very few because the 

valley has been flooded by Breckenridge Reservoir, but the upper' half of
CHOPAWAMSIC FORMATION, NORTHEASTERN VIRGINIA D3

the formation is well exposed in semicontinuous slabs along the stream 

below the dam. Neither the upper (southeastern) or lower (northwestern) 

contact are exposed in the type section, but both can be located within a 

few feet, on the basis of float and scattered outcrops. The lower contact 

is placed at the abrupt change from medium-grained feldspathic mica 

gneiss of the Wissahickon Formation to fine-grained thinly interlayered 

amphibole schist, amphibolite, and feldspathic schists of the Chopawamsic. 

The upper contact is placed at the first occurrence of dark-gray to black 

graphitic slate typical of the Quantico Slate. The thickness of the formation in the type section is estimated to be about 10,000 feet.

Reference sections of the Chopawamsic Formation are designated as 

follows:

1. Along Quantico Creek in the Quantico 7 l/z -minute quadrangle from 

a point 0.19 mile upstream (west) from Interstate Highway 95 to 

a point 400 feet downstream from the dam near Camp No. 4 in 

Prince William Forest Park. The thickness of the formation in this 

section is about 8,000 feet.

2. Along the north shore of the new reservoir on Aquia Creek in the 

Stafford 7 J^-minute quadrangle and along Aquia Creek above the 

reservoir for about 0.76 mile upstream. The lower contact of the 

formation crosses the creek about 1.3 miles N. 5° E. of Garrisonville; the upper contact is well exposed along the north shore of 

the reservoir about 0.75 mile N. 52° E. of the bridge on which Interstate Highway 95 crosses Aquia Creek. The thickness of the formation in this section is about 6,000 feet.

CONTACT RELATIONS

The contact of the Chopawamsic Formation with the overlying Quantico 

Slate is gradational. Contact relations are well exposed along the north 

shore of the reservoir on Aquia Creek (reference section 2). There the 

black graphitic Quantico Slate contains scattered 1- to 6-inch-thick graded 

sandy and silty (tuffaceous?) layers that indicate top of beds is to the 

southeast. About 100 feet above the base of the slate is a 100-foot-thick 

unit of medium- to fine-grained schistose felsic tuff. This rock is strongly 

cleaved and recrystallized, but contains flattened angular fragments, probably lapilli. The schistose tuff occurs in beds as much as 20 feet thick 

interlayered with very finely interlaminated gray metasiltstone and black 

slate. The tuffaceous unit probably was deposited from submarine pyreclastic ash flows.

Below (northwest of) the schistose felsic tuff unit is another 50-100 

feet of black graphitic slate, interbedded near the base with 1- to 6-inchthick beds of metamorphosed coarse tuffaceous sandstone that contain
D4 CONTRIBUTIONS TO STRATIGRAPHY

77°30' ~r~i * "\ c' WASHING wAoniiw TON i c 

// 1O /3 MIL£S

INDEX TO 71/2-MINUTE 

QUADRANGLES
CHOPAWAMSIC FORMATION, NORTHEASTERN VIRGINIA D5

EXPLANATION

Sedimentary deposits of the

Atlantic Coastal Plain, river

Sedimentary rocks

Granitic rocks

Ull 1

Mafic intrusive rocks J

II

DCO

o

Pelitic schist

o

Contact

Dashed where approximately located; 

dotted where concealed

FIGURE 1. Sketch map showing outcrop belt of the Chopawamsic Formation in 

the Occoquan, Quantico, and Joplin quadrangles and part of the Stafford quadrangle. Circled symbols refer to sections mentioned in text: (T), type section 

along Chopawamsic Creek; (T), reference section 1 along Quantico Creek; ©, 

reference section 2 along Aquia Creek. Geology in the Quantico and adjacent 

parts of Joplin and Stafford quadrangles based on detailed mapping by the 

authors; geology in part of the Occoquan quadrangle based on mapping by V. M. 

Seiders. Other geologic map details sketched from State geologic map (Virginia Div. Mineral Resources, 1963).

angular fragments of fine-grained light-colored sericite phyllite (probably 

pumice lapilli), granules of light-blue quartz 0.1-0.25 inch in diameter, and 

angular chips of black slate as much as 1 inch in diameter. The sequence 

of interbedded slate and tuffaceous sandstone is about 100 feet thick and 

is underlain by mafic volcanic breccia and amygdaloidal greenstone typical 

of the mafic parts of the Chopawamsic Formation. The contact between 

the Quantico Slate and the Chopawamsic Formation is placed at the 

lowest continuous bed of graphitic slate in the interbedded slate-sandstone 

sequence.
D6 CONTRIBUTIONS TO STRATIGRAPHY

Nowhere else is the upper contact of the Chopawamsic Formation well 

exposed, but similar relations can be inferred from float and scattered 

outcrops along Quantico Creek (reference section 1) and Neabsco Creek, 

both in the Quantico 7 J^-minute quadrangle.

No exposures of the lower contact of the Chopawamsic Formation 

have been located, but its position can be inferred within a few feet in 

the type section and in both reference sections. In these sections the 

Wissahickon Formation northwest of the contact is a massive nonlayered 

and weakly foliated medium- to coarse-grained feldspathic biotite-muscovite 

gneiss containing scattered pebbles of quartz. It closely resembles the 

boulder gneiss lithofacies of the Wissahickon Formation of Southwick and 

Fisher (1967, p. 12-13). Locally the gneiss contains pebbles, cobbles, and 

boulders of metagraywacke, biotite schist, laminated metasiltstone, and 

chlorite-actinolite schist, some of them identical with rock types found in 

the Chopawamsic Formation. This body of pebble gneiss has an outcrop 

width of at least 15,000 feet in the Quantico and Joplin 7 l/z-minute 

quadrangles. It has been traced along strike from Neabsco Creek in the 

Occoquan 7 ^ -minute quadrangle as far southwest as Aquia Creek in the 

Stafford quadrangle, a distance of about 10 miles. Northwestward it apparently grades into a fine-grained pelitic schist or phyllite facies of the 

Wissahickon Formation. In Quantico Creek (reference section 1) and in 

all sections to the southwest, the contact between the Chopawamsic Formation and the Wissahickon is apparently sharp. There is no evidence of 

interbedding or gradation but neither is there evidence of structural discordance.

Northeast of Quantico Creek, in the northwestern part of the Quantico 

and southwestern part of the Occoquan quadrangles, map patterns strongly 

suggest that parts' of the Wissahickon Formation interfinger northeastward 

with and pass laterally into the basal part of the Chopawamsic Formation. 

Along Neabsco Creek southeast of Virginia Highway 640, pebble gneiss of 

the Wissahickon Formation containing numerous pebbles of fine-grained 

greenstone is locally interbedded with bedded mafic tuff, mafic volcanic 

breccia, and a medium-grained strongly foliated greenish-gray biotite 

gneiss containing distinctive round granules of sky-blue quartz 0.25-0.5 

inch in diameter. The last-named rock was described by Lonsdale (1927, p. 

68-69) as arkosic quartzite. The rock probably was derived from a volcanic sediment, but it is not clear whether the quartz granules are clastic 

grains, resorbed phenocrysts, or possibly amygdule fillings. It is here included in the Chopawamsic Formation.

The suggestion that parts of the Wissahickon and Chopawamsic Formations are lateral equivalents is further substantiated by the presence 

of pebbles similar to the volcanic rocks of the Chopawamsic Formation in
CHOPAWAMSIC FORMATION, NORTHEASTERN VIRGINIA D7

Wissahickon gneiss that is northwest of the Chopawamsic outcrop belt. 

This indicates that some of the pebble gneiss is younger than some of the 

volcanic rocks of the Chopawamsic. Excellent exposures of gneiss containing such pebbles occur along Quantico Creek just east of the footbridge south of Camp No. 1 in Prince William Forest Park (Quantico 

7/2-minute quadrangle).

PRIMARY FEATURES

Shearing and recrystallization have erased sedimentary and volcanic 

structures and textures from some parts of the Chopawamsic Formation, 

but in many places primary features are well preserved.

Amygdular flows of andesitic greenstone are exposed in the floor of a 

shallow gravel pit near the south branch of Quantico Creek just below 

Camp No. 3 in Prince William Forest Park (Quantico 7J/2 -minute quadrangle), and at several localities in the gorge farther downstream (reference section 1). The amygdules are as large as 1 cm (centimeter), 

tectonically flattened, and filled with quartz, calcite, albite, epidote, or 

combinations of these minerals. Farther downstream, about 0.2 mile 

northwest of Interstate Highway 95, are exposures of a coarse mafic breccia 

consisting of ovoid clasts as long as 2 feet. The clasts are all composed 

of slightly arnygdular greenstone and are in a fine-grained matrix of nearly 

identical composition. Interbedded with a breccia are units of laminated 

siltstone (now schistose) a few inches to a few feet thick.

Felsic rocks with relict phehocrysts of oligoclase and quartz occur in the 

formation east of Camp No. 4 in Prince William Forest Park (Quantico 

7/2-minute quadrangle). The phenocrysts generally are smaller than 1 

mm (millimeter) and invariably show some evidence of granulation and 

recrystallization. The oligoclase phenocrysts that have withstood these 

effects show only faint zoning and are in general simply twinned. Groundmass textures have not survived metamorphism.

Some parts of the formation have well-preserved thin beds of intimately 

interlayered felsic and mafic schists. Some beds locally show internal laminations. Subangular to subrounded grains of clastic quartz are preserved 

in a few sandy beds, and in a few places graded beds are found. Locally 

some thin beds have complex internal deformation, probably due to slumping. These features are well shown in outcrops in the type section along 

Chopawamsic Creek about 0.15 mile below the dam.

MINERALOGY AND PETROGRAPHY

The rocks of the Chopawamsic Formation are fine-grained gray-green, 

gray, or white schists or granofels composed of minerals typical of the 

greenschist facies of metamorphism. Quartz, sodic plagioclase, epidote,
D8 CONTRIBUTIONS TO STRATIGRAPHY

TABLE 1. Mineral assemblages observed in the Chopawamsic Formation

Minerals

Chlorite _____ __

Biotite __ ________

Actinolite2 __ __ _.

Assemblage

1

v

2

X 

X

X 

X

X

3

X 

X

X 

X 

X 

X

4

X 

X

X 

X

X 

X

5

X

X

X

X 

X 

X

X

6

X 

X

X

X 

X

7

X 

X

X 

X 

X

X

8

X 

X

X 

X 

X

9

X 

X

X 

X

X 

X

10

X 

X

X 

X

X

11

X

X

X 

X 

X

X

12

X

X

X 

X 

X

X

13

X 

X 

X 

X 

X 

X

X

14

X 

X

X 

X 

X 

X

X 

X

15

X 

X 

X

X 

X

X

1 The composition of the sodic plagioclase has not been determined precisely. In some 

rocks it is albite, but in others it contains some anorthite.

2 All pale-bluish-green amphiboles are lumped under the term actinolite, even though 

some are probably aluminous and may properly be called hornblendes.

chlorite, biotite, actinolite, and muscovite are the principal minerals 

(table 1). The felsic rocks differ mineralogically from the mafic ones 

chiefly in having more plagioclase and quartz. Chlorite, biotite, and actinolite all occur in the felsic rocks, but are less abundant there than in the 

mafic rocks. The commonest mineral assemblages in mafic to intermediate 

rocks are Nos. 2 and 3 of table 1, whereas the most common assemblage 

in felsic rocks is No. 8. Sphene, apatite, iron-titanium oxides, pyrite, 

and zircon are the chief accessory minerals.

Granoblastic textures are typical of all rocks of the formation. Rocks 

rich in quartz and feldspar commonly are granular and very fine grained, 

except for scattered remnants of phenocrysts. The mafic rocks tend to be 

somewhat more coarsely recrystallized and more strongly foliated. Porphyroblastic textures are limited to some of the well-layered actinolitebearing schists, in which sieved needlelike porphyroblasts of actinolite as 

long as 1 cm are randomly oriented on the foliation surfaces.

CHEMICAL COMPOSITION

The volcanic parentage of the many rocks in the Chopawamsic Formation, amply demonstrated by relict structures and textures, is further 

substantiated by two chemical analyses (table 2). Both analyzed samples 

are chemically similar to albitized and zeolitized flows and tuffs of comparable SiO2 content from several younger volcanic-tectonic belts (for 

example, Oregon: Dickinson, 1962; Japan: R. S. Fiske, oral commun., 

1966; the Aleutians: Hamilton, 1963, p. 71; Puerto Rico: Glover, 1970) 

but are chemically rather unlike typical nonvolcanic shales and graywackes 

(Pettijohn, 1957).
GHOPAWAMSIC FORMATION, NORTHEASTERN VIRGINIA D9

TABLE 2. Chemical analyses of rocks from the Chopawamsic Formation, 

Quantico T l/i-minute quadrangle, Virginia

[Analyses by S. D. Botts, G. W. Chloe, James Kelsey, Lowell Artis, Hezekiah Smith, and

J. L. Glenn, U.S. Geol. Survey, using methods described by Shapiro

and Brannock (1962), supplemented by atomic absorption]

SiO2

A12O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

H20 +

H2O 

TiO2

P205

MnO

GO2

S as SO3 _

Volatiles other than CO2 and H2O _

Total

1

74.4

11.5

2.9

.69

2.4

3.0

1.1

1.1

.11

.35

.11

.09

1.0

<.01

1.0

100.1

2

56.3

13.0

2.5

4.9

6.2

8.3

1.9

.35

2.5

.17

.67

.14

.17

2.3

<.01

.00

99.9

1. Schistose felsite composed of quartz, albite, biotite, chlorite, and muscovite with traces of 

carbonate. Located on Powells Creek 1.5 miles N. 70° W. from Interstate 95 bridge 

(Q-30-85).

2. Well-foliated greenstone composed of quartz, sodic plagioclase, biotite, chlorite, and epidote. 

Located on Powells Creek 1.6 miles N. 70° W. from Interstate 95 bridge (Q-31-87).

AGE AND CORRELATION

The stratigraphic position of the Chopawamsic Formation conformably 

beneath the Quantico Slate of Late Ordovician age (Watson and Powell, 

1911) and conformably above the Wissahickon Formation of late Precambrian(?) age (Southwick and Fisher, 1967) suggests that the Chopawamsic includes rocks of early Paleozoic and possibly latest Precambrian 

age. Metavolcanic rocks of the same general age and character occur in 

the Evington Group beneath the Arvonia Slate of Ordovician age in 

central Virginia (Smith and others, 1964; Brown, 1969), but there they 

are apparently separated from the overlying slate by an unconformity 

(Brown, 1970; Brown and Griswold, 1970). Metamorphosed volcanic 

rocks are associated with the Wissahickon Formation in the northeastern 

part of the Piedmont in Maryland the James Run Gneiss in Baltimore 

and Harford Counties and the "Cecil County volcanic complex" in Cecil 

County (Marshall, 1937; South wick, 1966, 1969; Southwick and Fisher, 

1967). Zircons from the James Run Gneiss have yielded discordant ages 

that would be consistent with an original age of about 550 million years 

(Tilton and others, 1970). All these volcanic units may be at least in 

part correlative with the Chopawamsic Formation. The Chopawamsic 

Formation and other volcanic units in the northern Piedmont may be a
D10 CONTRIBUTIONS TO STRATIGRAPHY

northern extension of the Carolina slate belt where volcanic and sedimentary rocks of Ordovician and Cambrian age have been recognized 

(White and others, 1963; St. Jean, 1965; Stromquist and Sundelius, 1969; 

Sundelius, 1970).

REFERENCES

Brown, W. R., 1969, Geology of the Dilwyn quadrangle, Virginia: Virginia Div.

Mineral Resources Rept. Inv. 10, 77 p. 

 1970, Investigations of the sedimentary record in the Piedmont and Blue

Ridge of Virginia, in Fisher, G. W., Pettijohn, F. J., Reed, J. G., Jr., and

Weaver, K. N., eds., Studies of Appalachian geology central and southern:

New York, Interscience Publishers, p. 335-349. 

Brown, W. R., and Griswold, T. B., 1970, Superposed folding in the Arvonia Slate

district, Virginia [abs.]: Geol. Soc. America Abstracts with Programs, v. 2,

no. 3, p. 198. 

Dickinson, W. R., 1962, Petrology and diagenesis of Jurassic andesitic strata in

central Oregon: Am. Jour. Sci., v. 260, no. 7, p. 481-500. 

Glover, Lynn, III, 1970, Geology of the Coamo area, Puerto Rico: U.S. Geol. Survey

Prof. Paper 636. [In press.] 

Hamilton, Warren, 1963, Metamorphism in the Riggins region, western Idaho: U.S.

Geol. Survey Prof. Paper 436, 95 p. 

Lonsdale, J. T., 1927, Geology of the gold-pyrite belt of the northeastern Piedmont,

Virginia: Virginia Geol. Survey Bull. 30, 110 p. 

Marshall, John, 1937, The structures and age of the volcanic complex of Cecil

County, Maryland: Maryland Geol. Survey [Rept.], v. 13, p. 189-213. 

Pettijohn, F. J., 1957, Sedimentary rocks [2d ed.]: New York, Harper & Bros., 718 p. 

St. Jean, Joseph, Jr., 1965, New Cambrian trilobite from the Piedmont of North

Carolina [abs.]: Geol. Soc. America Spec. Paper 82, p. 307-308. 

Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analysis of silicate, carbonate,

and phosphate rocks: U.S. Geol. Survey Bull. 1144-A, 56 p.

Smith, J. W., Milici, R. C., and Greenberg, S. S., 1964, Geology and mineral resources of Fluvanna County: Virginia Div. Mineral Resources Bull. 79, 62 p. 

Southwick, D. L., 1966, Paragneisses of the northeast Piedmont some facts and

speculation [abs.]: Geol. Soc. America Spec. Paper 101, p. 279. 

______1969, Crystalline rocks of Harford County, in Maryland Geological Survey,

Geology of Harford County, Maryland: Baltimore, Md., Maryland Geol. Survey,

p. 1-76.

Southwick, D. L., and Fisher, G. W., 1967, Revision of the stratigraphic nomenclature of the Glenarm Series in Maryland: Maryland Geol. Survey Rept. Inv. 6,

19 p. 

Stromquist, A. A., and Sundelius, H. W., 1969, Stratigraphy of the Albemarle Group

of the Carolina slate belt in central North Carolina: U.S. Geol. Survey Bull.

1274-B, 22 p. 

Sundelius, H. W., 1970, The Carolina slate belt, in Fisher, G. W., Pettijohn, F. J.,

Reed, J. C., Jr., and Weaver, K. N., eds., Studies of Appalachian geology 

central and southern: New York, Interscience Publishers, p. 351-367. 

Tilton, G. W., Doe, B. R., and Hopson, C. A., 1970, Zircon age measurements in

the Maryland Piedmont, with special reference to Baltimore gneiss problems,

in Fisher, G. W., Pettijohn, F. J., Reed, J. C., Jr., and Weaver, K. N., eds.,
CHOPAWSMSIG FORMATION, NORTHEASTERN VIRGINIA Dll

Studies of Appalachian geology central and southern: New York, Interscience

Publishers, p. 429-434. 

Virginia Division of Mineral Resources, 1963, Geologic map of Virginia: Gharlottesville, scale 1:500,000. 

Virginia Geological Survey, 1928, Geologic map of Virginia: Gharlottesville, scale

1:500,000. 

Watson, T. L., and Powell, S. L., 1911, Fossil evidence of the age of the Virginia

Piedmont slates: Am. Jour. Sci., 4th ser., v. 31, p. 33-44. 

White, A. M., Stromquist, A. A., Stern, T. W., and Westley, Harold, 1963, Ordovician age for some rocks of the California slate belt in North Carolina: U.S.

Geol. Survey Prof. Paper 475-C, 475, p. C107-C109.

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