Geologic Map of the Frederick 30’ × 60’ Quadrangle, 

Maryland, Virginia, and West Virginia

Prepared in cooperation with the Maryland Geological Survey

By Scott Southworth, David K. Brezinski, Avery Ala Drake, Jr., William C. Burton, Randall C. Orndorff, 

Albert J. Froelich, James E. Reddy, Danielle Denenny, and David L. Daniels

Pamphlet to accompany

Scientific Investigations Map 2889

U.S. Department of the Interior

U.S. Geological Survey

2007
U.S. Department of the Interior

Dirk Kempthorne, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

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natural hazards, and the environment:

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Cover: Color-shaded-relief image of the Frederick 30 ́ × 60 ́ quadrangle showing high topographic elevations in 

shades of red and low topographic elevations in shades of blue. The hillside is illuminated from the east. The highest 

elevation is 1,920 ft above sea level on Blue Ridge at the southwestern part of the map area in Loudoun County, Va., 

and the lowest elevation is 140 ft above sea level along the Potomac River at Great Falls, Montgomery County, Md. 

The higher elevation regions, such as the ridges of the Blue Ridge province, portions of the Great Valley of the Valley 

and Ridge province, and Sugarloaf Mountain and parts of the Westminster terrane in the Piedmont province, are in 

contrast to the lower elevation region of the Culpeper and Gettysburg basins, Frederick Valley, and Potomac River valley within the Piedmont province in the eastern half of the map area. The elevation differences are a function of rock 

hardness and different erosion histories. Image produced by David L. Daniels, USGS, using a 5-m digital elevation 

model of USGS.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Southworth, Scott, Brezinski, D.K., Drake, A.A., Jr., Burton, W.C., Orndorff, R.C., Froelich, A.J., Reddy, J.E., Denenny, Danielle, and Daniels, D.L., 2007, Geologic map of the Frederick 30 ́ × 60 ́ quadrangle, Maryland, Virginia, and West Virginia: 

U.S. Geological Survey Scientific Investigations Map 2889, scale 1:100,000, 42-p. text.

ISBN 978-1-4113-1165-7
iii

Contents

Introduction.....................................................................................................................................................1

Great Valley Section of the Valley and Ridge Province...........................................................................2

Geologic Setting....................................................................................................................................2

Lower Paleozoic Sedimentary Rocks.......................................................................................4

Tomstown Formation ..........................................................................................................4

Waynesboro Formation......................................................................................................4

Elbrook Limestone ..............................................................................................................4

Conococheague Limestone and Big Spring Station Member ....................................4

Beekmantown Group.............................................................................................................4

Stonehenge Limestone and Stoufferstown Member....................................................4

Rockdale Run Formation....................................................................................................4

Pinesburg Station Dolomite ..............................................................................................5

St. Paul Group......................................................................................................................5

Chambersburg Limestone .................................................................................................5

Martinsburg Formation .....................................................................................................5

Structure.................................................................................................................................................5

Metamorphism ......................................................................................................................................5

Blue Ridge Province......................................................................................................................................5

Geologic Setting....................................................................................................................................5

Mesoproterozoic Rocks..............................................................................................................6

Neoproterozoic Rocks ................................................................................................................6

Cobbler Mountain Alkali Feldspar Quartz Syenite of the Robertson River 

Igneous Suite .........................................................................................................6

Metasedimentary Rocks of the Fauquier Group and Swift Run Formation...............6

Metadiabase and Metarhyolite Dikes.............................................................................7

Metavolcanic and Metasedimentary Rocks of the Catoctin Formation....................7

Paleozoic Clastic Metasedimentary Rocks of the Chilhowee Group..................................7

Loudoun Formation.............................................................................................................7

Weverton Formation...........................................................................................................7

Harpers Formation..............................................................................................................8

Antietam Formation ............................................................................................................8

Carbonaceous Phyllite.......................................................................................................8

Tomstown Formation East of Catoctin Mountain...........................................................8

Structure.................................................................................................................................................8

Mesoproterozoic..........................................................................................................................8

Neoproterozoic ............................................................................................................................8

Paleozoic.......................................................................................................................................9

Folds......................................................................................................................................9

Cleavage...............................................................................................................................9

Faults.....................................................................................................................................9

Mesozoic.......................................................................................................................................9

Metamorphism ......................................................................................................................................9
iv   

Mesoproterozoic..........................................................................................................................9

Paleozoic.....................................................................................................................................10

Piedmont Province.......................................................................................................................................10

Geologic Setting..................................................................................................................................10

Culpeper and Gettysburg Basins ............................................................................................10

Chatham Group.....................................................................................................................10

Manassas Sandstone ......................................................................................................10

Bull Run Formation ...........................................................................................................11

Catharpin Creek Formation .............................................................................................11

Mount Zion Church Basalt ..............................................................................................11

Meriden Group......................................................................................................................11

Midland Formation............................................................................................................11

Hickory Grove Basalt .......................................................................................................11

Turkey Run Formation.......................................................................................................12

Sander Basalt....................................................................................................................12

Thermally Metamorphosed Rocks ................................................................................12

Diabase Dikes and Sills ...................................................................................................12

Frederick Valley Synclinorium.................................................................................................12

Araby Formation................................................................................................................12

Cash Smith Formation ......................................................................................................12

Frederick Formation .........................................................................................................12

Grove Formation................................................................................................................12

Sugarloaf Mountain Anticlinorium and Bush Creek Belt....................................................13

Sugarloaf Mountain Quartzite ........................................................................................13

Urbana Formation .............................................................................................................13

Westminster Terrane.................................................................................................................13

Prettyboy Schist................................................................................................................13

Marburg Formation...........................................................................................................13

Ijamsville Phyllite ..............................................................................................................14

Sams Creek Formation.....................................................................................................14

Wakefield Marble .............................................................................................................15

Potomac Terrane........................................................................................................................15

Metagabbro, Ultramafic Rocks, the Soldiers Delight Ultramafite, and Metatuff...15

Metasedimentary Rocks..................................................................................................15

Cambrian, Ordovician, and Devonian Intrusive Rocks ...............................................16

Structure...............................................................................................................................................17

Western Piedmont.....................................................................................................................17

Central Piedmont .......................................................................................................................17

Thrust Sheets and Faults.................................................................................................17

Foliations ............................................................................................................................17

Folds....................................................................................................................................18

Sugarloaf Mountain Anticlinorium and Bush Creek Belt....................................................18

Eastern Piedmont.......................................................................................................................18

Thrust Sheets and Faults.................................................................................................18

Foliations ............................................................................................................................19
   

Lineations...........................................................................................................................19

Folds....................................................................................................................................19

Metamorphism ....................................................................................................................................19

Sugarloaf Mountain Anticlinorium and Frederick Valley Synclinorium............................19

Westminster Terrane.................................................................................................................20

Potomac Terrane........................................................................................................................20

Coastal Plain Province................................................................................................................................20

Fossils ............................................................................................................................................................20

Surficial Materials .......................................................................................................................................21

Alluvium................................................................................................................................................21

Alluvial Terrace Deposits...................................................................................................................21

Colluvium..............................................................................................................................................21

Residuum and Lag Gravel..................................................................................................................21

Summary of Geologic History and Tectonics ..........................................................................................21

Mineral Resources ......................................................................................................................................22

Limestone and Aggregate .................................................................................................................22

Building and Ornamental Stone .......................................................................................................22

Metals ...................................................................................................................................................23

Aeromagnetic Map......................................................................................................................................23

Introduction..........................................................................................................................................23

The Shaded-Relief Map.....................................................................................................................23

Correlation of Magnetic Data With Geology..................................................................................23

References Cited..........................................................................................................................................25

Description of Map Units............................................................................................................................30

Unconsolidated Surficial Materials.................................................................................................30

Great Valley Section of the Valley and Ridge Province................................................................30

Blue Ridge Province...........................................................................................................................31

Piedmont Province..............................................................................................................................35

Culpeper and Gettysburg Basins ...........................................................................................35

Frederick Valley Synclinorium.................................................................................................37

Sugarloaf Mountain Anticlinorium and Bush Creek Belt....................................................38

Westminster Terrane.................................................................................................................38

Potomac Terrane........................................................................................................................40

Coastal Plain Province.......................................................................................................................42

Figures

1. Index map of the Frederick 30’ × 60’ quadrangle showing 7.5-minute 1:24,000-scale 

quadrangle names and a list of published geologic maps of the areas shown in 

gray .................................................................................................................................................2

2. Map of the Frederick 30’ × 60’ quadrangle showing major physiographic provinces and 

structural features........................................................................................................................3

3. Gray-scale shaded relief image of the Frederick 30’ × 60’ quadrangle showing geologic 

features that correlate with the magnetic data.....................................................................24
vi   

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

meter (m) 1.094 yard (yd)

kilometer (km) 0.6215 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi)

Area

square meter (m2) 0.000247 acre

square kilometer (km2

) 0.3861 square mile (mi2)
Geologic Map of the Frederick 30’ × 60’ Quadrangle,

Maryland, Virginia, and West Virginia

By Scott Southworth,1 David K. Brezinski,2 Avery Ala Drake, Jr.,3 William C. Burton,1 Randall C. Orndorff,1

Albert J. Froelich,4

 James E. Reddy,1 Danielle Denenny,1 and David L. Daniels3

Introduction

The Frederick 30’ × 60’ quadrangle lies within the 

Potomac River watershed of the Chesapeake Bay drainage 

basin. The map area covers parts of Montgomery, Howard, 

Carroll, Frederick, and Washington Counties in Maryland; 

Loudoun, Clarke, and Fairfax Counties in Virginia; and Jefferson and Berkeley Counties in West Virginia. Many geologic 

features (such as faults and folds) are named for geographic 

features that may or may not be shown on the 1:100,000-scale 

base map; see figure 2 for additional features. See page 29 for 

Description of Map Units.

The geology of the Frederick 30’ × 60’ quadrangle, Maryland, Virginia, and West Virginia, was first mapped on the 

32 1:24,000-scale 7.5-minute quadrangle base maps between 

1989 and 1994. The geologic data were compiled manually at 

1:100,000 scale in 1997 and were digitized between 1998 and 

1999. The geologic map and database may be used to support 

activities such as land-use planning, soil mapping, groundwater availability and quality studies, identifying aggregate 

resources, and conducting engineering and environmental 

studies. See the index map of 1:24,000-scale geologic maps 

(fig. 1) for site-specific studies.

The map area covers distinct geologic provinces and 

sections of the central Appalachian region that are defined by 

unique bedrock and resulting landforms (fig. 2). From west 

to east, the provinces include the Great Valley section of the 

Valley and Ridge province, the Blue Ridge province, and the 

Piedmont province; in the extreme southeastern corner, a small 

part of the Coastal Plain province is present. The Piedmont 

province is divided into several sections; from west to east, 

they are the Frederick Valley synclinorium, the Culpeper and 

Gettysburg basins, the Sugarloaf Mountain anticlinorium, the 

Westminster terrane, and the Potomac terrane. The geology 

of the Frederick quadrangle is discussed by geologic province and sections; the geologic units within each province 

are discussed from oldest to youngest. Where applicable, the 

discussion includes information on tectonic origins.

The Blue Ridge province contains Mesoproterozoic 

(1 billion years old, or 1 Ga) paragneiss and granitic gneisses 

that are intruded by a swarm of Neoproterozoic (570 million 

years old, or 570 Ma) metadiabase and metarhyolite dikes. 

Unconformably overlying the gneisses are Neoproterozoic 

metasedimentary rocks and metavolcanic rocks associated 

with the dikes. The Mesoproterozoic gneisses were deformed 

and metamorphosed during the Grenville orogeny. Subsequently, the Neoproterozoic metasedimentary and metavolcanic rocks accumulated during a continental rifting event 

(Rankin, 1976). Clastic metasedimentary rocks of the newly 

formed continental margin were deposited paraconformably 

upon the Neoproterozoic rocks. 

To the east, Neoproterozoic and early Paleozoic metasedimentary and metavolcanic rocks were deposited on the margin of the rifted continent. These rocks underlie the Sugarloaf 

Mountain anticlinorium and Westminster and Potomac terranes. As the rifted continental margin stabilized and became 

a passive margin during the early Paleozoic, carbonate rocks 

were deposited on the broad continental shelf. Those carbonate rocks are now exposed in the Great Valley section and the 

Frederick Valley synclinorium.

The early Paleozoic carbonate platform became unstable 

in response to the Ordovician Taconian orogeny. Deformation 

associated with this tectonic event is recorded in rocks of the 

Piedmont province to the east. These rocks, which are now 

part of the Potomac terrane, were thrust westward onto rocks 

of the Westminster terrane; next, rocks of the Westminster 

terrane were thrust onto rocks now exposed in the Sugarloaf 

Mountain anticlinorium and Frederick Valley synclinorium 

(Drake and others, 1989; Southworth, 1996).

The early Paleozoic sea eventually closed up and disappeared during the continental collision of tectonic plates during the late Paleozoic Alleghanian orogeny. The Alleghanian 

orogeny transported all of the rocks within the map area westward along the North Mountain thrust fault, which is exposed 

immediately northwest of the quadrangle. The Alleghanian 

orogeny produced numerous thrust faults and folds in the rock 

and regional-scale folds that help define the geologic provinces (fig. 2). The Massanutten synclinorium underlies the 

Great Valley section, the Blue Ridge-South Mountain anti1

U.S. Geological Survey, Reston, Va. 20192

2

Maryland Geological Survey, Baltimore, Md. 21218

3

U.S. Geological Survey, retired.

4

Deceased.
    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

SOURCES OF GEOLOGIC DATA

Ashby Gap Gathright and Nystrom (1974) Middleway Dean and others (1990)

Berryville Edmundson and Nunan (1973); Point of Rocks Burton and others (1995); 

Dean and others (1990) Brezinski (2004b)

Bluemont Southworth (1994) Poolesville Southworth (1998)

Buckeystown Southworth and Brezinski (2003) Purcellville Southworth (1995)

Charles Town Dean and others (1990) Round Hill McDowell and Milton (1992)

Frederick Brezinski (2004a) Seneca Drake and others (1999)

Harpers Ferry Southworth and Brezinski (1996b) Shepherdstown Dean and others (1987)

Keedysville Dean and others (1987) Urbana Southworth (1999)

Kensington Drake (1998) Walkersville Brezinski and others (2004)

Martinsburg Dean and others (1987) Waterford Burton and others (1995)

Middletown Brezinski and Fauth (2005)

Figure 1. Index map of the Frederick 30’ × 60’ quadrangle showing 7.5-minute 1:24,000-scale quadrangle names and a list of published 

geologic maps of the areas shown in gray.

clinorium underlies the Blue Ridge province, and the Frederick Valley synclinorium and Sugarloaf Mountain anticlinorium 

underlie the western Piedmont province.

Tens of millions of years after the Alleghanian orogeny, 

early Mesozoic continental rifting formed the Culpeper and 

Gettysburg basins, which once were connected to form a large 

down-faulted basin filled with sediments that eroded from 

the adjacent Blue Ridge and Piedmont highlands. Continued 

rifting resulted in igneous intrusions and extrusive volcanic 

rock at about 200 Ma, and eventually led to the opening of 

the Atlantic Ocean. Sediments eroded from the Appalachian 

highlands were deposited by river systems and transgressing 

seas and now form the Coastal Plain province.

Great Valley Section of the Valley and

Ridge Province

Geologic Setting

The Great Valley section of the Valley and Ridge province is a broad valley underlain mostly by carbonate rocks 

west of the Blue Ridge. The rocks form the Massanutten 

synclinorium, a broad fold with an overturned east limb. 

The western boundary of the Great Valley is not within the 

map area, but is defined by the clastic rocks that underlie 

Great North Mountain to the southwest in the Winchester, 
aphic provinces and structural features. Map of the Frederick 30’ × 60’ quadrangle showing major physiogr . Figure

Great Valley Section of the Valley and Ridge Province    3
    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Va.-W. Va. 30’ × 60’ quadrangle. There, the Alleghanian North 

Mountain thrust fault places the Cambrian and Ordovician 

rocks of the Great Valley onto Silurian and Devonian rocks.

Lower Paleozoic Sedimentary Rocks

Tomstown Formation

The carbonate rocks of the Lower Cambrian Tomstown 

Formation (_t) (Tomstown Dolomite of Stose (1906)) are 

subdivided into the Bolivar Heights (_tbh), Fort Duncan 

(_tf), Benevola (_tb), and Dargan (_td) Members (Brezinski, 

1992). The Tomstown rocks offer the first evidence that the 

eastern part of the rifted continent had evolved into a passive 

continental margin. The vertical sequence of rocks suggests 

a change from a shallow, carbonate shelf to a deep shelf and 

then to a carbonate bank. 

The Bolivar Heights Member (_tbh) is a thin-bedded 

limestone containing wispy dolomitic burrows that increase in 

abundance upsection, where bioturbation was more prevalent. 

The base of the member is a 15-m-thick interval of mylonitic 

marble known as the Keedysville marble bed (Brezinski and 

others, 1996). The marble bed’s lower contact defines a regional 

thrust fault that detached the carbonate rocks from the underlying 

sandstone of the Antietam Formation of the Blue Ridge province 

(Brezinski and others, 1996). The Fort Duncan Member (_tf) is 

a dark, thick-bedded, burrow-mottled dolostone whose base is 

probably an erosional surface at the top of the Bolivar Heights 

Member. The Benevola Member (_tb) is a very thick bedded to 

massive, sugary dolostone with faint crossbedding. The base is 

gradational with the underlying bioturbated dolostone of the Fort 

Duncan Member. This rock is quarried for aggregate because it is 

pure and massive. The Dargan Member (_td) consists of a lower 

bioturbated dolo­stone alternating with intervals of laminated 

dolostone. In the upper part, bioturbated and oolitic dolostone is 

interbedded with laminated limestone and silty dolostone.

Waynesboro Formation

The Lower Cambrian Waynesboro Formation (_wa) 

(Stose, 1906) has been subdivided into the Red Run (_war), 

Cavetown (_wact), and Chewsville (_wac) Members (Brezinski, 1992). These members have distinctive physiographic 

expressions that facilitate field mapping; sandstone of the 

lower Red Run and upper Chewsville Members underlies low 

hills with the intervening, less resistant carbonate rock of the 

Cavetown Member underlying a swale between the hills.

The Red Run Member (_war) consists of interbedded sandstone and laminated, ribbony, sandy dolostone. The 

Cavetown Member (_wact) is typically not well exposed. 

The lower part consists of a thick-bedded, massive limestone 

and bioturbated dolostone; the middle part consists of bioturbated dolomitic limestone and dolostone and thin calcareous 

sandstone and shale; and the upper part consists of thick-bedded, bioturbated dolomite with laminated ribbony dolomite 

at the very top. The Chewsville Member (_wac) is distinctive 

with its dark siltstone, sandstone, and shale. The siltstone 

commonly exhibits ripple marks and mudcracks. The sandstone is crossbedded and contains Skolithos linearis burrows. 

These rocks were deposited in a shallow, subtidal to supratidal 

environment.

Elbrook Limestone

The Middle and Upper Cambrian Elbrook Limestone 

(_e) (Stose, 1906) consists of lower, middle, and upper informal members (Brezinski, 1996) that were not differentiated on 

this map due to poor exposure. The lower part of the formation consists of cyclic intervals of limestone, shale, and shaly 

dolostone; the middle part of the formation is largely composed of thin-bedded to bioturbated limestone; and the upper 

part is a thick sequence of medium-bedded algal limestone, 

dolostone, and dolomitic shale. These rocks were deposited in 

a shallowing-upward, peritidal environment. 

Conococheague Limestone and Big Spring Station 

Member 

The Upper Cambrian and Lower Ordovician Conococheague Limestone (O_c) (Stose, 1910) consists of interbedded limestone, dolostone, and sandstone arranged in cycles. 

The lower 100 m consists of coarse-grained, calcareous to 

dolomitic sandstone, fine-grained limestone with intraformational conglomerate, and fine-grained dolostone of the Upper 

Cambrian Big Spring Station Member (_cb) that forms low 

ridges. Above the Big Spring Station Member, the Conococheague Limestone includes intraformational conglomerates, 

algal bioherms, ribbon rock, and oolites, arranged in cycles. 

Like the underlying rocks of the Elbrook Limestone, these 

rocks represent shallowing-upward marine deposits of a peritidal environment.

Beekmantown Group.—Consisting of Stonehenge 

Limestone and Stoufferstown Member, Rockdale Run 

Formation, and Pinesburg Station Dolomite.

Stonehenge Limestone and Stoufferstown Member

Only the Lower Ordovician lower part of the Stonehenge 

Limestone is mapped in this quadrangle. The basal part of 

the Stonehenge Limestone, the Stoufferstown Member (Obss) 

(Sando, 1958), consists of silty, laminated limestone. The 

remainder of the Stonehenge Limestone (Obs) (Stose, 1910) 

consists of thick-bedded, fossiliferous limestone. Algal bioherms, intraformational conglomerates, and bioclastic beds 

alternate with thin dolostone beds. Unlike the peritidal environments of the underlying Conococheague Limestone, the Stonehenge Limestone was deposited in a subtidal environment.

Rockdale Run Formation

The Lower and Middle Ordovician Rockdale Run Formation (Obrr) (Sando, 1957) consists of cyclically bedded lime-
stone and dolostone. The limestone is thin to medium bedded, 

fossiliferous, and contains intraformational conglomerates, 

algal bioherms, bioclastic zones, burrow mottling, and chert 

nodules. The dolostone is laminated and displays abundant 

mudcracks. These rocks reflect an alternation between subtidal 

to peritidal environments similar to that in the Conococheague 

Limestone (O_c) discussed previously.

Pinesburg Station Dolomite

The Middle Ordovician Pinesburg Station Dolomite 

(Obps) (Sando, 1957) consists of medium- to thick-bedded 

dolostone and laminated dolostone that contains chert nodules. 

Crosshatched joints weather to form a diagnostic “butcherblock” pattern on the surface. Thin intervals of fine-grained 

limestone occur in the lower part. Near the top of the formation, irregularly bedded, brecciated dolostone that indicates 

paleokarst is common. These rocks reflect a restricted shallow-marine environment that was at times exposed to air.

St. Paul Group

The Middle Ordovician St. Paul Group (Osnr) consists of 

thick-bedded, micritic, fenestral limestone having thin-bedded 

dolomitic limestone and interbedded with laminated dolostone 

in the lower part. These rocks were unconformably deposited 

in a tidal flat to lagoonal environment above the Pinesburg 

Station Dolomite (Obps). The New Market Limestone and Row 

Park Limestone, which are the only parts of the St. Paul Group 

that occur in this quadrangle, are not differentiated on the map.

Chambersburg Limestone

The Middle Ordovician Chambersburg Limestone (Oc) 

(Stose, 1910) consists of argillaceous and nodular limestone 

that is perhaps the most fossiliferous unit in the Great Valley. 

The unit represents a change in depositional environment from 

a passive-continental-margin carbonate platform to an active 

continental-margin shelf; the transition is possibly associated 

with the Ordovician Taconian orogeny.

Martinsburg Formation 

The Middle and Upper Ordovician Martinsburg Formation (Om) consists of shale, siltstone, and sandstone with 

argillaceous limestone at the base. Turbidites exhibiting 

typical Bouma cycles with load clasts are characteristic. The 

turbidites indicate the foundering of the margin and its evolution into a foreland basin (that is, the margin sank at the same 

time that sea level was rising), where shallow-water carbonate 

sedimentation gave way to deepwater clastic sediments. 

Structure

The rocks that underlie the Great Valley form the Massanutten synclinorium, a regional fold that resulted from the late 

Paleozoic Alleghanian orogeny. The core of the synclinorium 

mostly consists of clastic rocks of the Middle and Upper Ordovician Martinsburg Formation. The Massanutten synclinorium, 

Blue Ridge-South Mountain anticlinorium, Frederick Valley 

synclinorium, and Sugarloaf Mountain anticlinorium constitute a fold train that was transported westward above the North 

Mountain thrust fault during the late Paleozoic. The rocks 

of the Great Valley are highly folded with tight and upright 

second-, third-, and fourth-order disharmonic folds that verge 

up the limbs of the higher order folds (see fig. 2). Axial-planar cleavage is most evident in the shale of the Martinsburg 

Formation. There are several late-forming thrust faults on the 

inner limbs of the Massanutten synclinorium that truncate 

units. High-angle tear faults and normal faults cut the folds. 

Metamorphism

The rocks of the eastern part of the Great Valley were 

subjected to the same deformation and lower-greenschistfacies metamorphic conditions as the rocks of the westernmost 

Blue Ridge province. Minor amounts of chlorite and sericite 

are evident in some of the rocks, especially those that exhibit 

slaty cleavage and (or) mylonitic foliation. Most of the carbonate rocks of the Great Valley are not metamorphosed. 

Blue Ridge Province

Geologic Setting

The Blue Ridge province in the Frederick 30’ × 60’

quadrangle consists of rocks of the Blue Ridge-South Mountain anticlinorium, which is a large west-verging complex fold. 

The anticlinorium exposes allochthonous rocks above and 

transported by the North Mountain thrust fault during the late 

Paleozoic Alleghanian orogeny (Evans, 1988). The core of 

the anticlinorium consists of Mesoproterozoic paragneiss and 

granitic gneisses that are intruded by Neoproterozoic granite, a swarm of metadiabase dikes, and several metarhyolite 

dikes. Deposited unconformably on the basement gneisses are 

Neoproterozoic metasedimentary rocks (Fauquier Group and 

Swift Run Formation), which are, in turn, overlain by metavolcanic rocks (Catoctin Formation). A thin unit of phyllite and 

conglomerate (Loudoun Formation) locally is transitional 

between the rocks of the Catoctin Formation and the clastic 

metasedimentary rocks of the overlying Lower Cambrian 

Chilhowee Group (Weverton, Harpers, and Antietam Formations). This cover-rock sequence of metasedimentary and 

metavolcanic rocks mostly underlies the three limbs of the 

Blue Ridge-South Mountain anticlinorium: from east to west, 

these limbs are represented by Catoctin Mountain, Black Oak 

Ridge-Short Hill-South Mountain, and Blue Ridge-Elk Ridge. 

Several outliers of cover rocks in the core of the anticlinorium 

also were preserved from the erosion of folds and faults. 

The boundary between the Blue Ridge province and the 

Great Valley section to the west is the stratigraphic contact 

Blue Ridge Province    
between the ridge-forming metasandstone of the Antietam 

Formation and the overlying limestone and dolostone of the 

Lower Cambrian Tomstown Formation. At the northern part 

of Pleasant Valley in Washington County, Md., the Rohrersville fault (Southworth and Brezinski, 1996a), which separates 

Cambrian limestone of the Great Valley from older metasedimentary and metavolcanic rocks of the Blue Ridge, acts as the 

provincial boundary. To the east, the boundary between the 

Blue Ridge and Piedmont provinces is marked by the lower 

slope of Catoctin and Furnace Mountains along a Mesozoic 

normal fault. Exceptions to this boundary placement are found 

east and south of Furnace Mountain near the Potomac River 

in Loudoun County, Va., where the unnamed carbonaceous 

phyllite unit and dolostone of the Tomstown Formation are 

stratigraphically above Antietam Formation; these rocks are 

discussed under the Blue Ridge province.

Mesoproterozoic Rocks

The Mesoproterozoic rocks in the core of the Blue 

Ridge-South Mountain anticlinorium are paragneisses and 

granitic orthogneisses. The Mesoproterozoic paragneisses are 

poorly exposed and occur as lenses and layers within granitic 

gneiss. They are interpreted to be the country rock that existed 

before intrusion of the granite. The most distinctive unit is 

garnet-graphite paragneiss (Yp). A well-layered texture, flakes 

of specular graphite, and large crystals of garnet (almandine) 

suggest an impure sandstone or graywacke as the protolith 

(Burton and Southworth, 1996). Amphibolite (Ya), a spotted, 

locally well-foliated hornblende-orthopyroxene-plagioclase 

gneiss that contains subordinate sills or dike-like bodies of 

metanorite, metadiorite, and hornblende-biotite gneiss, is 

interpreted to be a mafic metavolcanic rock. Quartzite and 

quartz-sericite tectonite (Yq) have rounded grains of quartz and 

zircon; seams of black, carbonaceous phyllonite; and pods of 

garnet-graphite paragneiss. The quartzite is of sedimentary 

origin and has been locally sheared.

At least four groups of orthogneisses are present in this 

quadrangle; the groupings are based on isotopic age data (Aleinikoff and others, 2000) as follows: (1) 1,153±6 Ma layered 

granitic gneiss (Ylg), 1,149±19 Ma hornblende monzonite 

gneiss (Yhm), 1,144±2 Ma porphyro­blastic metagranite (Ypg), 

and about 1,140 Ma coarse-grained metagranite (Ymc); (2) 

1,111±2 Ma biotitic and 1,112±3 Ma leucocratic Marshall 

Metagranite (Ym); (3) 1,077±4 Ma quartz-plagioclase gneiss 

(Yqp) and 1,077±4 garnetiferous metagranite (Ygt); and (4) 

1,060±2 Ma white leucocratic metagranite (Yg), 1,059±2 Ma 

pink leucocratic metagranite (Yml), 1,055±4 Ma biotite granite 

gneiss (Ybg), and 1,055±2 Ma megacrystic metagranite (Ypb). 

Different groups predominate west (western Blue Ridge) and 

east (eastern Blue Ridge) of the Short Hill fault. Five types 

of granitic rock are mapped in the eastern Blue Ridge. These 

include (from oldest to youngest) coarse-grained metagranite 

(Ymc), the Marshall Metagranite (Ym) (Jonas, 1928; Espenshade, 1986), garnetiferous metagranite (Ygt), white leucocratic metagranite (Yg), and pink leucocratic metagranite (Yml). 

In general, most of these rocks are all light colored, medium 

grained, and distinguished by the presence or absence of 

garnet and (or) biotite. Only one of these units (Ymc) is coarse 

grained.

Six types of granitic rock are present in the western Blue 

Ridge. Massive quartz-hornblende-orthopyroxene-microclineplagioclase charnockitic granite (Yc) is mapped primarily 

on the basis of float of fresh black rock having a distinctive 

orange crust. Charnockitic granite occurs as dike- and pluglike bodies whose age is uncertain (Aleinikoff and others, 

2000). Layered granitic gneiss (Ylg), the oldest dated rock 

in the region, has the variegated texture of a migmatite. The 

protolith of this layered gneiss is interpreted to be a felsic 

volcanic rock. Hornblende monzonite gneiss (Yhm) has a characteristic spotted texture with up to 30 percent hornblende. 

Porphyroblastic metagranite (Ypg) is characterized by ovoid 

porphyroblasts of microcline as much as 3 centimeters (cm) 

long and garnet 1 cm in diameter. Garnetiferous metagranite 

(Ygt) and Marshall Metagranite (Ym) also occur in the western 

Blue Ridge as dikes and sills, and the garnetiferous metagranite also occurs as a large body near the Potomac River.

Neoproterozoic Rocks

Cobbler Mountain Alkali Feldspar Quartz Syenite of the 

Robertson River Igneous Suite

The Robertson River Igneous Suite consists of nine types 

of peralkaline to metaluminous granite and syenite (Tollo 

and Lowe, 1994). Only one member of this suite, the Cobbler 

Mountain Alkali Feldspar Quartz Syenite (Zrc), occurs in the 

Frederick quadrangle where it intrudes Mesoproterozoic rocks 

of the Blue Ridge province within the southwestern part of the 

map area. The syenite is a medium-grained massive rock containing distinctive stubby mesoperthite crystals. The rock lacks 

the Grenvillian foliation seen in the Mesoproterozoic rocks of 

the region because it was intruded about 300 m.y. later (Tollo 

and Lowe, 1994).

Metasedimentary Rocks of the Fauquier Group and Swift 

Run Formation

A sequence of clastic metasedimentary rocks unconformably overlies the Mesoproterozoic gneisses and Neoproterozoic Cobbler Mountain Alkali Feldspar Quartz Syenite. These 

rocks, the Neoproterozoic Fauquier Group and Swift Run 

Formation, occupy the same stratigraphic position but are not 

in contact with each other. Rocks of the Fauquier Group are 

restricted to the east limb of the Blue Ridge-South Mountain 

anticlinorium. The boundary between the Fauquier Group 

rocks and those of the Swift Run Formation are two transverse-normal faults that were active during deposition of both 

groups of strata (Kline and others, 1991).

The stratigraphic nomenclature of Kasselas (1993) is used 

here for rocks of the Fauquier Group, which was formerly 

called the Fauquier Formation by Espenshade (1986). The 
Fauquier Group consists of the Swains Mountain Formation 

and Carter Run Formation. The base of the Swains Mountain 

Formation contains a local, clast-supported boulder conglomerate (Zfsc) consisting of boulders and cobbles of Mesoproterozoic granite gneiss; the conglomerate is interpreted to be 

a debris-flow deposit. The remainder of the Swains Mountain 

consists of gray, coarse- to fine-grained meta-arkose and finer 

grained metamudstone (Zfsm) overlying the boulder conglomerate. The Swains Mountain is overlain by gray metasiltstone 

and meta-arkose of the Carter Run Formation (Zfcr). The rocks 

of the Fauquier Group dramatically increase in thickness to 

the south beyond the quadrangle boundary, and they do not 

occur north of the cross fault that dropped them down against 

the Mesoproterozoic rocks. A small outlier of these rocks near 

Mountville in Loudoun County, Va., probably also was deposited in a fault-bounded basin.

Rocks of the Swift Run Formation (Stose and Stose, 

1946) consist of metasandstone and schist (Zss), phyllite (Zsp), 

and marble (Zsm). The clastic rocks become finer grained 

upward in section and crossbedding suggests a fluvial origin, 

but the marble and some phyllite may be lake or shallowmarine deposits. The three types of rocks are not present in the 

same stratigraphic positions everywhere due to facies variations.

The massive basal metasandstone and schist unit (Zss) 

locally contains pebbles and cobbles of quartz, phyllite, and 

ferruginous sandstone, and grades upward and laterally into 

quartz-sericite schist. The quartz-sericite schist becomes finer 

grained as it grades upward into tan, sandy, sericitic phyllite 

(Zsp). White, pink, and tan, massive to schistose, calcitic and 

dolomitic marble (Zsm) occurs as local pods. Although the 

marble pods have been elongated by deformation, they probably originated as local lake or shallow-marine deposits that 

were not laterally extensive.

Metadiabase and Metarhyolite Dikes

A northeast-trending swarm of tabular dikes intrudes the 

Mesoproterozoic and some Neoproterozoic rocks. The dikes 

consist mostly of green metadiabase (Zmd) with some gray 

metarhyolite (Zrd) (Southworth and others, 2006) and range 

in thickness from 0.5 to 5 m. Texturally, the metadiabase 

ranges from fine grained (most common) to coarse grained 

to porphyritic. The metarhyolite is fine grained to porphyritic 

with plagioclase phenocrysts in a matrix of microcrystalline 

quartz and potassium feldspar. Near Middletown, Md., one of 

these dikes appears to cut metabasalt of the lower part of the 

Catoctin Formation (Zcm). Zircons from a metarhyolite dike 

along the Potomac River yielded a U-Pb date of 571.5±5 Ma 

(Aleinikoff and others, 1995). The dikes were probably feeders to the volcanic flows of the Catoctin Formation.

Metavolcanic and Metasedimentary Rocks of the 

Catoctin Formation

The Catoctin Formation (Keith, 1894) is characterized 

mainly by green, aphanitic, massive to schistose, amygdaloidal metabasalt and chlorite schist (Zcm). In several places at 

or near the base are bodies of white calcitic marble (Zcma) 

similar to, but more extensive than, the marble (Zsm) within 

the underlying Swift Run Formation. Local metabasalt breccia 

contains blocky masses of light-green epidosite. The metabasalt contains interbeds of thin, discontinuous, finely laminated, 

metasedimentary phyllite (Zcs); dark, variegated, vesicular, 

tuffaceous phyllite (Zcp); and phyllite and quartz-muscovite 

schist of metarhyolite composition (Zcr). The rocks of the 

Catoctin Formation show considerable variation in thickness and proportions of rock types along strike. In Maryland, 

metabasalt gives way northward to phyllite, and in Pennsylvania metarhyolite is the dominant unit. Metabasalt of the 

Catoctin Formation in Virginia has a Rb/Sr age of 570±36 Ma 

(Badger and Sinha, 1988), and metarhyolite of the Catoctin 

Formation in Pennsylvania has a U-Pb age of 597±18 Ma 

(Aleinikoff and others, 1995).

Paleozoic Clastic Metasedimentary Rocks of the 

Chilhowee Group

The Lower Cambrian Chilhowee Group (Safford, 1856) 

consists of the Loudoun, Weverton, Harpers, and Antietam 

Formations. The contact between the rocks of the Chilhowee 

Group and the underlying rocks of the Catoctin Formation is 

conformable to locally unconformable. Rip-up clasts of phyllite, metabasalt, and red jasper from the Catoctin Formation 

record a period of erosion before deposition of the Loudoun 

Formation. Rocks of the Chilhowee Group represent a fluvial 

to shallow-marine transgressive sequence exhibiting a depositional transition from rift to passive-continental margin, the 

latter marked by the conformably overlying carbonate rocks.

Loudoun Formation

The Loudoun Formation (Keith, 1894; Stose and Stose, 

1946) locally occurs between the Catoctin and Weverton 

Formations and consists of a thin, discontinuous sequence 

of predominantly phyllite (_clp) and a minor quartz-pebble 

conglomerate and pebbly metasandstone (_clc). Rocks of the 

Catoctin Formation grade into the overlying lower phyllite 

unit. The phyllite is interpreted to include metamorphosed 

volcanic tuff, mudstone, and (or) fossil soil (Reed, 1955; Nickelsen, 1956). The discontinuous and lensoid quartz-pebble 

conglomerate probably represents a local alluvial fan deposit. 

Cross-stratified beds of quartzite occur locally between the 

conglomerate and underlying phyllite. The conglomerate also 

is transitional into the overlying quartzite of the Weverton 

Formation. 

Weverton Formation

The Weverton Formation (Keith, 1894) is divided into 

three members (in ascending order): the Buzzard Knob Member (_cwb), the Maryland Heights Member (_cwm), and the 

Owens Creek Member (_cwo) (Brezinski, 1992). The Buzzard 

Blue Ridge Province    
    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Knob Member is a light, massive, vitreous quartzite interbedded with metagraywacke and metasiltstone. The Maryland 

Heights Member is a dark, granular quartzite interbedded 

with dark metasiltstone. The Owens Creek Member is a dark, 

poorly sorted, crossbedded quartzite and quartz-pebble conglomerate interbedded with dark metasiltstone. The Maryland 

Heights Member on Catoctin Mountain contains a thick bed 

of vitreous quartzite that is mapped separately (_cwmq). The 

Weverton is mapped undivided (_cw) only in the southern part 

of the quadrangle, west of Bull Run Mountain fault.

Harpers Formation

The Harpers Formation (_ch) (Keith, 1894) is mainly a 

dark, foliated, quartz-laminated metasiltstone and biotite-chlorite-muscovite-quartz phyllite. Primary sedimentary structures 

are mostly obscured by recrystallization and cleavage. Rocks 

of the Harpers Formation on Catoctin Mountain are more 

strongly foliated and phyllitic than the Harpers’ metasiltstone 

and metashale on Short Hill-South Mountain or on Blue 

Ridge-Elk Ridge. On Blue Ridge-Elk Ridge, the lower part 

of the Harpers Formation contains thin beds of pebble conglomerate, which indicate that the lower contact is transitional 

above the underlying Weverton Formation. The upper part of 

the Harpers contains thin beds of clean metasandstone (_chs), 

which contains burrows of the trace fossil Skolithos linearis. 

The metasandstone is transitional into the overlying Antietam 

Formation (_ca).

Antietam Formation

The Antietam Formation (_ca) (Keith, 1894) consists 

of a thin-bedded metasandstone and meta-arkose. The unit is 

exposed only along the Potomac River north of Harpers Ferry, 

W. Va., and south of Point of Rocks, Md., on Furnace Mountain. The Antietam underlies prominent ridges that are littered 

with friable float. Locally, the clean metasandstone has worm 

burrows of the trace fossil Skolithos linearis.

Carbonaceous Phyllite

In Loudoun County, Va., south of Point of Rocks, Md., 

a Lower Cambrian lustrous, gray, laminated, carbonaceous 

phyllite (_cp) that produces a distinctive dark soil is locally 

present between rocks of the Antietam Formation and the 

overlying Tomstown Formation. This unit occupies a stratigraphic position similar to that of the dark shale of the Cash 

Smith Formation in the Frederick Valley to the north in Maryland. The organic shale and mudstone mark the end of clastic 

sedimentation prior to the development of an Early Cambrian 

carbonate shelf. 

Tomstown Formation East of Catoctin Mountain

Light-gray dolostone of the Lower Cambrian Tomstown 

Formation (Stose, 1906) (_t) extends from near the town 

of Furnace Mountain south of the Potomac River in Loudoun County, Va., north along Catoctin Mountain to west of 

Frederick, Md. The dolostone is exposed along U.S. Route 15, 

in a pasture south of the Potomac River, as small outcrops in 

streams, and is recorded in drill core (Hoy and Schumacher, 

1956) to the north in Maryland. This dolostone resembles the 

Bolivar Heights Member of the Tomstown that is exposed 

north of Harpers Ferry, W. Va.; however, the Tomstown 

mapped east of Catoctin Mountain differs from the Tomstown 

mapped to the west in the Great Valley section of the Valley 

and Ridge province.

Structure

Mesoproterozoic

Mesoproterozoic foliation in the Blue Ridge province 

is defined by mineral assemblages typical of granulite-facies 

metamorphism. At least three episodes of deformation affected 

the oldest rocks (Burton and others, 1995; Aleinikoff and 

others, 2000). The northwest-trending foliation (D1), which 

locally is parallel to unit contacts, is cut by a weaker foliation 

with mineral lineations and southeast-plunging sheath folds 

(D2). The D2 features were later deformed into broad, northwest-trending folds (D3). The D1 deformation event is younger 

than the age of crystallization (1,153 Ma) of the oldest granitic 

gneisses, but is older than the weakly foliated 1,077±4 Ma 

granite that cuts it. The D2 deformation occurred after the 

latest intrusion at 1,055 Ma, and D3 deformation was still 

later based on the observation that D3 features cut D2 features. 

Outcrops of ductile folds in gneiss are rare. 

The quartz tectonite unit (Yq) that parallels Butchers 

Branch in the southwestern part of the western Blue Ridge 

may be a Mesoproterozoic silicified fault rock because it parallels unit contacts and is transected by Paleozoic schistosity 

and cleavage. Monazite ages from Mesoproterozoic rocks on 

either side of the Short Hill fault suggest that the gneisses on 

either side were not in proximity to each other during intrusion, so the fault may have existed at that time.

Neoproterozoic

Two cross faults and one strike-parallel fault southwest 

of Leesburg, Va., juxtapose the rocks of the Neoproterozoic 

Fauquier Group and Swift Run Formation against Mesoproterozoic gneiss. These are interpreted to be syndepositional 

normal faults (Kline and others, 1991). The faults do not 

appear to cut the rocks of the Neoproterozoic Catoctin Formation, which indicates that they were active prior to extrusion of 

the Catoctin’s metabasalt. 

The presence of a metadiabase dike swarm in the basement rocks of the Blue Ridge is strong evidence that a 50 

percent extension of the crust occurred during crustal rifting, 

even though few normal faults have been recognized in the 

basement gneiss complex. This long period of Neoproterozoic continental extension began with the emplacement of 

the anorogenic granite of the Robertson River Intrusive Suite 
at about 722 Ma and continued through the volcanism of the 

Catoctin Formation at about 565 Ma. The extension ultimately 

resulted in the opening of the Iapetus Ocean. The dikes and 

extrusive lava flows of the Catoctin Formation mark the peak 

of continental rifting activity.

Paleozoic

The rocks of the Blue Ridge province were deformed 

and metamorphosed during the Paleozoic to produce the Blue 

Ridge-South Mountain anticlinorium. This event most likely 

was the result of the late Paleozoic (Permian?) Alleghanian 

orogeny, although some argon data suggest an earlier Devonian to Mississippian thermal event may have been responsible (Kunk and Burton, 1999). The anticlinorium is a broad, 

asymmetrical, west-verging, and gently north-plunging fold 

complex. Gently plunging folds occupy the homoclinal east 

limb, and more complexly deformed and tightly folded rocks 

are located on the overturned west limbs. The stratigraphy on 

the west limb of the anticlinorium is repeated by the Short Hill 

fault, which is an early Paleozoic (or older?) normal fault that 

was contractionally reactivated and folded during the formation of the anticlinorium. The formation of the anticlinorium 

was accompanied by a main fold phase (F1) and associated 

axial-planar cleavage (S1), and a later, minor fold phase (F2) 

and associated crenulation cleavage (S2).

Folds

The Blue Ridge-South Mountain anticlinorium is a large 

west-verging F1 fold. The east limb underlies Catoctin Mountain and Furnace Mountain, and the fault-repeated west limbs 

underlie Blue Ridge-Elk Ridge and the sequence of Black Oak 

Ridge, Short Hill, and South Mountain. Map-scale F1 parasitic 

folds include those on Purcell Knob (which is east of Keys 

Gap on Blue Ridge), the Hillsboro syncline that underlies 

Short Hill Mountain and Black Oak Ridge, several synclines 

of the Swift Run Formation east of Short Hill Mountain, and 

the Furnace Mountain syncline. These F1 parasitic folds contain second- and third-order folds.

Later deformation produced open to tight F2 folds accompanied by axial-planar crenulation cleavage and spaced cleavage, all of which may have been a continuation of the earlier 

phase of deformation (Nickelsen, 1956; Southworth and others, 2006). F2 folds superimposed on F1 folds and S1 cleavage 

are exposed on Blue Ridge from Wilson Gap to its terminus 

north of Rohrersville, Md., and on Catoctin Mountain south of 

Point of Rocks, Md.

Cleavage

A regional, penetrative, northeast-striking, southeast-dipping S1 cleavage (South Mountain cleavage of Cloos (1951) 

and Mitra and Elliott (1980)) overprints Mesoproterozoic 

foliation and bedding in Neoproterozoic and Cambrian rocks. 

The S1 cleavage is axial planar to the Blue Ridge-South Mountain anticlinorium and coeval with its formation. The foliation 

is slaty to schistose, depending on the host rock.

The S2 cleavage is discontinuous, is commonly axial 

planar to F2 folds, did not involve extensive recrystallization, 

and formed mostly by pressure solution. S2 has about the same 

strike as S1 but at a slightly steeper dip.

Faults

Paleozoic faults were mapped on the basis of stratigraphic truncation and omission, mylonitic foliation, and 

localized deformational fabrics. The most regionally extensive 

faults are also the most enigmatic. The Short Hill fault can be 

traced about 60 kilometers (km) from a shear zone in Mesoproterozoic rocks in Fauquier Co., Va., south of the Frederick 

quadrangle, to an area near Rohrersville, Md., where limestone of the Middle and Upper Cambrian Elbrook Formation 

structurally overlies metabasalt of the Neoproterozoic Catoctin 

Formation along a flat-lying fault that plunges northward with 

the Blue Ridge-South Mountain anticlinorium. This fault is 

interpreted to be an early Paleozoic normal fault that was contractionally reactivated as a thrust fault and then was folded 

with the anticlinorium (Southworth and Brezinski, 1996a). 

The Keedysville marble bed is a mylonite derived from 

the Bolivar Heights Member of the Tomstown Formation. The 

mylonite indicates that a regional detachment was present 

between the carbonate rock sequence and the underlying clastic rocks of the Chilhowee Group before the anticlinorium was 

folded (Brezinski and others, 1996). 

Other intraformational thrust faults, strike-slip faults, and 

normal faults are either synchronous with or postdate F1 folding of the anticlinorium. The Mesoproterozoic gneiss contains 

zones of mylonite and phyllonite that superficially resemble 

fine-grained, metasedimentary rocks but actually were formed 

by ductile faulting. The mylonite contains minerals typical 

of lower-greenschist-facies metamorphism and is probably 

contemporaneous with the development of coplanar S1 cleavage. Several shear zones are as much as about 1 km wide and 

juxtapose different basement gneiss units.

Mesozoic

Several small, brittle normal faults in the Mesoproterozoic rocks may be early Mesozoic (Burton and others, 1995), 

because they are similar to the family of brittle normal faults 

that cuts the east limb of the anticlinorium near Point of 

Rocks, Md. Elsewhere in the Blue Ridge province, the only 

evidence of Mesozoic structures is the north-trending Early 

Jurassic diabase dikes. 

Metamorphism

Mesoproterozoic

Plutonism, metamorphism, and deformation in the Blue 

Ridge province spanned about 150 m.y., from about 1,153 to 

Blue Ridge Province    
10    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

1,000 Ma (Aleinikoff and others, 2000). Mesoproterozoic deformation and metamorphism ended no earlier than about 1,055 

Ma, which is the age of the youngest deformed unit (megacrystic metagranite, Ypb). Granulite-facies mineral assemblages of 

(1) orthopyroxene and microcline in charnockitic granite and 

(2) orthopyroxene and brown hornblende in amphibolite show 

granoblastic texture and triple-junction grain boundaries. Monazite ages (Aleinikoff and others, 2000) indicate that there was no 

regional thermal event hotter than 720°C that affected the rocks 

after the Grenville orogeny. The 40Ar/39Ar age of hornblende 

from a Grenville rock provides a cooling date of 1,000 to 920 

Ma (Kunk and others, 1993), which could be designated as the 

minimum age for regional metamorphism in this area. 

Paleozoic

Paleozoic deformation was accompanied by recrystallization under lower-greenschist-facies metamorphic conditions. 

The formation of S1 cleavage was accompanied by the growth 

of fine-grained muscovite, chlorite, biotite, epidote, actinolite, 

and accessory minerals in the rocks of the Neoproterozoic and 

Lower Cambrian cover sequence. Biotite is only recognized in 

cover rocks on the east limb of the Blue Ridge-South Mountain anticlinorium. Some of the cover rocks show multiple 

generations of white muscovite-sericite, suggesting a complex 

metamorphic history. Retrograde mineral assemblages of 

muscovite, biotite, and chlorite replaced feldspar, brown biotite, and garnet in Mesoproterozoic gneisses; Neoproterozoic 

metadiabase dikes and metabasalt contain actinolite, chlorite, 

and epidote. The age of metamorphism and deformation is 

poorly constrained but is usually attributed to the late Paleozoic Alleghanian orogeny (Mitra and Elliott, 1980). The Blue 

Ridge province rocks were transported westward during the 

Alleghanian orogeny (Evans, 1988), but the metamorphism 

and deformation that they exhibit may be older. 

Piedmont Province

Geologic Setting

The Piedmont, which means “foot of the mountain,” is 

the province east of the lower slope of Catoctin Mountain in 

the Blue Ridge province. In the eastern part of the province, 

crystalline rocks of the Potomac terrane were thrust onto metamorphic rocks of the Westminster terrane along the Pleasant 

Grove fault. The metasedimentary and metavolcanic rocks of 

the Westminster terrane were thrust westward along the Martic 

fault onto metasedimentary rocks now exposed in the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium. Except for some valleys underlain by carbonate rocks, 

the Piedmont is a dissected plateau. Rocks of the Sugarloaf 

Mountain anticlinorium and Frederick Valley synclinorium 

are correlative with rocks of the Blue Ridge anticlinorium and 

the Massanutten synclinorium (Great Valley), respectively, 

and they also have similar landforms. Mesozoic sedimentary 

rocks of the Culpeper and Gettysburg basins unconformably 

overlie the older rocks. The sedimentary rocks were intruded 

by diabase dikes and sills. 

Culpeper and Gettysburg Basins

In the Frederick 30’ × 60’ quadrangle, Triassic and Jurassic sedimentary and volcanic rocks were formerly assigned 

to the Culpeper Group (Lee, 1979, 1980), which was in turn 

divided informally into a lower part and an upper part. The 

former lower part was revised as the Chatham Group, and 

the former upper part was revised as the Meriden Group 

by Weems and Olsen (1997). In this quadrangle, the Chatham Group includes the mainly Upper Triassic sequence of 

continental sedimentary rocks and some basalt; the Meriden 

Group includes the Lower Jurassic series of basalt flows and 

intercalated sedimentary rocks (Midland Formation, Hickory 

Grove Basalt, Turkey Run Formation, and Sander Basalt) (Lee 

and Froelich, 1989). Both the Chatham and Meriden Groups 

belong to the Newark Supergroup (Froelich and Olsen, 1984).

Chatham Group.—Consisting of Manassas Sandstone, 

Bull Run Formation, Catharpin Creek Formation, and Mount 

Zion Church Basalt.

Manassas Sandstone

The Upper Triassic Manassas Sandstone (Lee, 1977, 

1979) is divided into the Reston Member (Lee, 1977) (^cmr), 

the Tuscarora Creek Member (Lee, 1977) (^cmt), and the 

Poolesville Member (Lee and Froelich, 1989) ( The ^cmp). 

rocks assigned here to the Tuscarora Creek Member and the 

Poolesville Member in the Culpeper basin have been called the 

New Oxford Formation by workers in Maryland and Pennsylvania (Cleaves and others, 1968; Berg and others, 1980). 

These rocks were deposited by braided streams that flowed 

across a lowland.

The Reston Member (^cmr) is the basal conglomerate that 

is faulted against and unconformably overlies older metasedimentary rocks of the Potomac and Westminster terranes. The 

conglomerate is composed of cobbles, pebbles, and boulders 

of micaceous quartzite, phyllite, metagraywacke, and schist in 

an arkosic matrix of sand, clay, muscovite, and feldspar. The 

deposits are lensoid and discontinuous, which indicates that they 

are probably channel deposits of a river system.

The Tuscarora Creek Member (^cmt) is a basal conglomerate that unconformably overlies carbonate rocks of 

the Upper Cambrian Frederick Formation (and rocks of the 

Urbana Formation at one locality). Although the Tuscarora 

Creek is laterally equivalent to the Reston Member, the clasts 

are from different source rocks. The Tuscarora Creek contains 

limestone and dolostone clasts derived from the Frederick 

and possibly the Grove Formations. The conglomerate locally 

is lensoidal and discontinuous as it represents debris-flow 

channels incised into an alluvial fan. In the subsurface along 
the western margin of the basin, the conglomerate is probably 

the base of the stratigraphically higher conglomerate in the 

Leesburg Member of the Bull Run Formation.

The Poolesville Member (^cmp) is an arkosic, muscovite-rich sandstone interbedded with siltstone containing 

sparse quartz pebbles that fines upward. The base of the unit 

is transitional with the underlying Reston and Tuscarora Creek 

Members. The Poolesville Member also grades up into the 

overlying Balls Bluff Member of the Bull Run Formation; 

therefore, the contact between the two units is gradational and 

approximately located.

Bull Run Formation

Rocks previously assigned to the Upper Triassic Balls 

Bluff Siltstone by Lee (1977, 1979) are now assigned to the 

Bull Run Formation, which consists of the Leesburg Member, 

the Balls Bluff Member, and the Groveton Member by Weems 

and Olsen (1997). The three members are laterally equivalent, 

but are characterized by different lithologies that represent 

variable depositional environments. As the climate changed 

from wet to dry, alluvial fans formed and extended from the 

highlands west of the border fault onto playas with lakes and 

intermittent streams. The resulting deposits are conglomerates 

interbedded with sandstone and siltstone at every scale.

The Leesburg Member (^cbl) is a conspicuous carbonate 

conglomerate composed of subangular to subrounded boulders, cobbles, and pebbles of limestone and dolostone in a 

sandy siltstone matrix. The source of the carbonate rock clasts 

includes the limestone and dolostone of the Tomstown, Frederick, and Grove Formations. The conglomerate intertongues 

with the sandstone and siltstone of the Balls Bluff Member 

and forms a complex map pattern. The sandstone and siltstone 

then pinch out to the west. This unit is similar to the conglomerate of the Tuscarora Creek Member (^cmt) of the Manassas 

Sandstone and is probably stratigraphically continuous with it 

in the subsurface.

The Balls Bluff Member (^cbb) is mainly feldspathic, 

silty sandstone interbedded with clayey and sandy siltstone in 

cyclic sequences as much as 3 m thick. The unit is gradational 

with the underlying Poolesville Member of the Manassas 

Sandstone, intertongues laterally with rocks of the Groveton 

Member and Leesburg Member of the Bull Run Formation, 

and is overlain unconformably by rocks of the Upper Triassic 

and Lower Jurassic Catharpin Creek Formation (J^cc).

The Groveton Member (^cbg) is mostly a thin-bedded 

to laminated, silty and sandy lacustrine shale interbedded 

with clayey and sandy siltstone in cyclic sequences 3 to 9 m 

thick. The unit is gradational with the underlying Balls Bluff 

Member and is considered to be a lateral equivalent. The rocks 

are also conformably overlain by rocks of the Catharpin Creek 

Formation. The shale is poorly exposed but can be traced by 

shale chips in soil.

Catharpin Creek Formation

The Upper Triassic and Lower Jurassic Catharpin Creek 

Formation (J^cc) consists of cyclic sequences of interbedded 

sandstone, siltstone, and conglomerate as much as 30 m thick. 

The contact with the underlying Upper Triassic Bull Run 

Formation is gradational and intertonguing, but the contact 

with the overlying Lower Jurassic Mount Zion Church Basalt 

(Jcm) is a sharp disconformity. When this unit was deposited, 

the climate had once again become wetter; therefore, mainly 

fluvial to lacustrine deposits accumulated.

The Goose Creek Member (J^ccg) of the Catharpin Creek Formation is a lenticular body of conglomerate 

composed of subrounded pebbles and cobbles of quartzite, 

metabasalt, metasiltstone, gneiss, limestone, and vein quartz, 

all derived from rocks of the Blue Ridge province. The conglomerate is interbedded with pebbly arkosic sandstone and 

its matrix is locally rich in calcite. Like the conglomeratic 

rocks of the Tuscarora Creek Member (^cmt) of the Manassas 

Sandstone and the Leesburg Member (^cbl) of the Bull Run 

Formation, the Goose Creek Member probably represents a 

debris-flow channel incised into an alluvial fan; the distinction between the Goose Creek Member and the other two 

units is the source and composition of its clasts. The resistant 

clasts exposed at the surface of this unit remain as a lag gravel 

deposit. 

Mount Zion Church Basalt

The Lower Jurassic Mount Zion Church Basalt (Jcm) is a 

high-titanium, quartz-normative tholeiitic basalt. Vesicles and 

amygdules at the tops indicate one or two flows. The basalt 

underlies a low ridge but is poorly exposed. 

Meriden Group.—Consisting of Midland Formation, 

Hickory Grove Basalt, Turkey Run Formation, and Sander 

Basalt.

Midland Formation

The Lower Jurassic Midland Formation (Jmm) consists 

of cyclic sequences of interbedded siltstone, sandstone, and 

shale. Lenticular deposits of variegated cobble and pebble 

conglomerate and conglomeratic arkosic sandstone (Jmmc) are 

mapped separately. The conglomeratic strata represent alluvial 

fans that prograded eastward into lakes situated on playas. 

Shale and siltstone contain the remains of the abundant and 

diverse fish that lived in the lakes.

Hickory Grove Basalt

The Lower Jurassic Hickory Grove Basalt (Jmh) consists 

of two or three flows of high-titanium, high-iron, quartz-normative tholeiitic basalt with vesicles and amygdules locally at 

the tops of flows. Locally, the flows are separated by sandstone and siltstone (Jmhs) that, in places, are disconformable 

with both the underlying and overlying strata.

Piedmont Province    11
1    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Turkey Run Formation

The Lower Jurassic Turkey Run Formation (Jmt) consists 

of cyclic sequences of interbedded sandstone, siltstone, conglomerate, and shale. The conglomerate (Jmtc) is composed 

of subrounded boulders, cobbles, and pebbles of metabasalt, 

quartzite, marble, quartz, and basalt. Like the Midland Formation, these deposits represent alluvial fans that prograded into 

lakes on a playa that were later covered with basaltic lava 

flows.

Sander Basalt

The Lower Jurassic Sander Basalt (Jms) is the uppermost 

unit of basalt flows and sedimentary rocks and is the youngest bedrock in the map area. The stratigraphically lower flows 

of the Sander are high-titanium, high-iron, quartz-normative 

basalts. Poorly exposed sandstone and siltstone (Jmss) separates the lower flows from the stratigraphically higher flows of 

low-titanium, quartz-normative basalt. The Sander Basalt has 

distinctive curved columnar joints.

Thermally Metamorphosed Rocks 

The Upper Triassic and Lower Jurassic strata (J^tm) of 

the Newark Supergroup were thermally metamorphosed by 

diabase intrusions (Jd, Jdc, Jdh, Jdg). Zoned contact aureoles 

in the country rock may be seen adjacent to the diabase. The 

largest contact aureoles are found adjacent to the diabase 

sills along the Dulles Greenway (Virginia State Route 267) at 

Belmont Ridge in Loudoun County, Va., and near Boyds, Md. 

Thin aureoles may be seen adjacent to narrow diabase dikes. 

Siltstone and shale are altered to cordierite-spotted hornfels 

in the inner aureole; epidote-chlorite hornfels characterizes 

the outer aureole. Sandstone is metamorphosed to tourmaline 

granofels and (or) quartzite; carbonate conglomerate is metamorphosed to marble.

Diabase Dikes and Sills

Early Jurassic diabase from three varieties of magma is 

mapped in the Culpeper and Gettysburg basins (Froelich and 

Gottfried, 1988). Both the olivine-normative diabase and the 

low-titanium, quartz-normative tholeiitic diabase occurs as 

dikes (Jd). High-titanium, quartz-normative tholeiitic diabase 

(Jdh) occurs as both narrow dikes and thick, differentiated sills 

that contain cumulates (mapped separately as unit Jdc) in the 

lower parts and late-stage differentiates in the higher parts. 

The differentiates include granophyre (mapped separately as 

unit Jdg), ferrogabbro, diorite, syenite, and aplite. The thin 

sills and thicker sheets of diabase are irregular in shape. The 

dikes are linear, discontinuous, and were emplaced en echelon 

along dilated fractures. The diabase dike exposed along the 

Maryland side of the Potomac River, between Sandy Hook and 

Weverton, has an 40Ar/39Ar age of 200 Ma (Kunk and others, 

1992).

Frederick Valley Synclinorium

Araby Formation

The Lower and Middle Cambrian Araby Formation (_ar) 

(Reinhardt, 1974, 1977) consists of burrow-mottled sandy 

metasiltstone whose bedding is obscured by structural foliations. The metasiltstone is interpreted as a deepwater-slope 

facies of a starved clastic basin (Reinhardt, 1974). The Araby 

Formation is conformably overlain by limestone and limestone 

breccia of the Rocky Springs Station Member of the Frederick 

Formation. In the northeastern part of the Frederick Valley, 

within the map area, metashale beds thicken near the top of 

the unit.

Cash Smith Formation

The Middle and Upper Cambrian Cash Smith Formation 

(Edwards, 1988) (_cs) is a dark shale and slate that grades 

up into thin-bedded, calcareous shale with limestone nodules. 

The Cash Smith is transitional between the metasiltstone of 

the underlying Araby Formation and the shale of the overlying 

Rocky Springs Station Member (_frs) of the Frederick Formation. Although the Cash Smith is similar to the dark shale beds 

within the mostly carbonate Rocky Springs Station Member, 

neither unit can be continuously mapped across the synclinorium.

Frederick Formation

The Upper Cambrian Frederick Formation (Frederick 

Limestone of Keyes, 1890; Stose and Stose, 1946; Reinhardt, 

1974, 1977) is a thick interval of limestone and dolostone that 

contains thin intervals of shale and sandstone. The Frederick 

Limestone was renamed Frederick Formation by Southworth 

and Brezinski (2003), and the three members of Reinhardt 

(1974, 1977) are shown on the map; in ascending order, 

they are the Rocky Springs Station, Adamstown, and Lime 

Kiln Members. The Rocky Springs Station Member (_fr) is 

characterized by intervals of polymictic breccia that may be 

traced locally. This lower member was deposited by offshelf 

submarine slides that accumulated at the toe of a paleoslope 

(Reinhardt, 1974). An interval of grayish-black shale (_frs) 

that is similar to the shale of the Cash Smith Formation (which 

locally underlies the unit to the north) is interbedded with the 

limestone.

The Adamstown Member (_fa) consists of thinly bedded 

limestone and thin intervals of shale that were probably deposited in an oceanic basin (Reinhardt, 1974). The Lime Kiln 

Member (_fl) consists of thinly bedded limestone interbedded 

with algal limestone at the top of the formation. This upper 

member appears to record aggradation from ocean-basin deposition to shallow-shelf deposition (Reinhardt, 1974).

Grove Formation

The Upper Cambrian and Lower Ordovician Grove 

Formation (Stose and Jonas, 1935; Jonas and Stose; 1938b; 
Reinhardt, 1974) is an interval of carbonate rock in the center 

of the Frederick Valley. The Grove consists of two informal 

members. The lower member (O_gl) consists of crossbedded, sandy limestone; sandy dolomitic limestone; and sandy 

dolostone that, as a package, is distinct from the underlying 

and overlying strata. These rocks were probably deposited on 

the shelf edge as a sand wave complex.

Above the basal limestone and dolostone member are 

limestone and dolostone of the upper member (O_gu). The 

strata are arranged in cycles consisting of thrombolitic and 

stromatolitic limestone and laminated dolostone that were 

deposited in a peritidal environment. 

Sugarloaf Mountain Anticlinorium and Bush 

Creek Belt

Sugarloaf Mountain Quartzite

The Lower Cambrian Sugarloaf Mountain Quartzite 

(Jonas and Stose, 1938b) is a medium-bedded to massive 

quartzite cemented by silica and sericite. Crossbeds, graded 

beds, and sparse ripple marks show that these are continentalmargin deposits laid down in shallow water. Poorly exposed 

laminated metasiltstone is interbedded with the quartzite. The 

Sugarloaf Mountain Quartzite is subdivided into informal 

lower (_sql), middle (_sqm), and upper (_squ) members. The 

upper member is well exposed at the summit of Sugarloaf 

Mountain, the middle member forms the prominent ridge 

north of the summit (which may be traversed by following the 

Northern Peaks trail), but the lower member is poorly exposed 

(Southworth and Brezinski, 2003).

Urbana Formation

The Lower Cambrian(?) Urbana Formation (Urbana 

Phyllite of Jonas and Stose, 1938b; Edwards, 1986) conformably overlies the Sugarloaf Mountain Quartzite and is structurally overlain by rocks of the Ijamsville Formation along the 

Martic fault. The Urbana Formation (_u) consists of quartzite, 

calcareous metasandstone, metagraywacke, and metasiltstone, 

all of which cannot be differentiated because of poor exposure 

and the interbedding of all the rock types; however, there are 

mappable bodies of marble (_um) and quartzite (_uq). Most 

of the rock units in the Urbana Formation are discontinuous 

lenses that are sandy, calcareous, friable, or deeply weathered 

and thus are poorly exposed. The quartzite, metasandstone, 

and metasiltstone are crossbedded and ripple-marked, which 

indicates that they were deposited in shallow water. Schistose 

to massive, sandy, chlorite-rich marble (_um) is interbedded 

with metasiltstone and metasandstone. Massive quartzite (_uq) 

interbedded with vuggy calcareous metasandstone mapped 

within broad belts of metasiltstone are probably local channel 

deposits and not a continuous stratigraphic horizon. 

Metasiltstone, calcareous metasandstone, marble, and 

quartzite crop out along Bush Creek. These rocks are lithologically and structurally identical to rocks of the Urbana Formation mapped to the west, and they are surrounded by Ijamsville 

Phyllite. The rocks along Bush Creek are interpreted to be the 

Urbana Formation exposed beneath the Martic thrust sheet in a 

tectonic window (Southworth, 1999).

Westminster Terrane

Prettyboy Schist

The Neoproterozoic(?) and Lower Cambrian(?) Prettyboy 

Schist (Crowley, 1976) (_Zp) is quartz-muscovite-chloritealbite schist and muscovite-quartz-albite schist containing 

characteristic white, euhedral albite porphyroblasts and oxidized cubes of pyrite. The Prettyboy Schist may be a coarser, 

distal-facies equivalent of the rocks of the Marburg Formation 

(Drake, 1994; Howard, 1994). However, the Prettyboy rocks 

exhibit a higher metamorphic grade, thus making the gradational contact between the two units a metamorphic isograd 

(Fisher, 1978). The Prettyboy Schist crops out in two areas 

interpreted to be antiforms within the northeastern part of the 

Westminster terrane in the northeastern corner of the quadrangle. Massive to schistose metabasalt (_Zpb) forms a lensoidal 

body within the schist of the northernmost antiform. Granular 

quartzite (_Zpq), as much as 1 m thick, is interbedded with the 

schist (_Zp) in sequences as thick as 30 m.

Marburg Formation

The Neoproterozoic(?) and Lower Cambrian(?) Marburg Formation (Drake, 1994) (Marburg Schist of Jonas and 

Stose, 1938b) consists of phyllitic metasiltstone (_Zmbs) and 

small bodies of metagraywacke (_Zmbg), metabasalt (_Zmbb), 

and quartzite (_Zmbq). Quartz-sericite-chlorite metasiltstone contains local chloritoid and albite porphyroblasts, and 

chlorite-albite metasiltstone contains ribbons and laminae of 

quartz 0.25 cm thick. Both of these varieties of the Marburg 

are impregnated with vein quartz and are highly folded. Thin 

layers and lenses of muscovite-chlorite-paragonite-hematite 

phyllite that are lithologically similar to rocks of the Ijamsville 

Phyllite are locally seen within the fine-grained rocks but are 

too small to map. The quartz laminae in the metasiltstone may 

be the result of both sedimentary (turbidite) and metamorphic 

processes. 

Metagraywacke (_Zmbg) consists of a dark, quartz-rich 

granular graywacke locally interbedded with the phyllite and 

metasiltstone (_Zmbs). The small metabasalt body south of 

Barnesville, Md. (_Zmbb), is interpreted to be an epiclastic 

deposit. The quartzite (_Zmbq) is medium to coarse grained 

with a high chlorite content. 

The contact between the rocks of the Marburg Formation 

and the Sams Creek Formation is the Hyattstown thrust fault, 

but rocks of the Marburg Formation may stratigraphically 

underlie rocks of the Sams Creek Formation in the northern 

part of the map near Sams Creek, Md. North of Winfield, Md., 

the Marburg Formation is interpreted to structurally overlie 

the Prettyboy Schist. The Marburg Formation contains a minor 

Piedmont Province    13
1    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

amount of hematitic phyllite that resembles rocks of the Ijamsville Phyllite, although the rock units never come into contact. 

The Marburg Formation contains some fine-grained albite 

schist that is similar to the coarse-grained albite schist of the 

Prettyboy Schist. Because of these two lithologic similarities, 

the Marburg Formation may be a transitional facies between 

the Ijamsville Phyllite and Prettyboy Schist; otherwise, the 

Marburg rocks are unique to this region. The rocks mapped 

here as Marburg Formation were previously assigned to the 

Wissahickon Formation by Fisher (1978) and to the Gillis 

Group by Edwards (1986, 1994).

Ijamsville Phyllite

The Neoproterozoic(?) and Lower Cambrian(?) Ijamsville 

Phyllite (Jonas and Stose, 1938b) consists of eight mappable 

varieties: phyllite (_Zip), metabasalt (_Zib), quartzite (_Ziq), 

marble (_Zim), metalimestone (_Zil), phyllitic conglomerate 

(_Zic), conglomeratic metagraywacke (_Zicm), and chloritic 

phyllite (_Zicp). The Ijamsville is mapped in four distinct belts 

as follows: (1) from the type locality at Ijamsville, Md., south 

to Beallsville, Md., (2) in a klippe of the Martic fault west of 

Sugarloaf Mountain, (3) between the klippe and rocks of the 

Sams Creek Formation near Chestnut Grove, and (4) above the 

Martic fault east of Sugarloaf Mountain.

The muscovite-chlorite-paragonite-chloritoid phyllite 

variety (_Zip) often has composite foliations and abundant 

quartz veins that are folded and transposed to form phyllonites 

with dull-green clots and patches of retrograde chlorite. The 

least deformed fine-grained rocks are slates that were quarried for roofing material as early as 1880 (Dale and others, 

1914). Bedding in the slate is locally expressed as thin bands 

of opaque minerals on cleavage surfaces. The dark color of 

the phyllite and slate results from abundant minute hematite 

crystals. 

Metabasalt (_Zib) contains nodules of epidosite and 

sparsely crops out as flattened pillows north of Beallsville, 

Md. Locally, centimeter-thick metabasalt is interlayered with 

phyllite. Yellowish-gray, massive, fine- to medium-grained 

quartzite (_Ziq) is interbedded with phyllite, but the bodies are 

discontinuous and probably represent channel deposits. 

Thin-bedded metalimestone (_Zil) containing black carbonaceous phyllite is surrounded by phyllite near the mouth 

of the Monocacy River. The thin, argillaceous metalimestone 

may be olistoliths (large clasts) deposited in deepwater mud. 

The metalimestone resembles the Silver Run Limestone of 

Jonas and Stose (1938b) and Fisher (1978) (Southworth, 

1998). Three pods of sandy limestone (_Zim), 10 cm to 3 m 

long, are found within conglomeratic phyllite (_Zip) west of 

the type locality at Ijamsville. The marble is also interpreted to 

be olistoliths deposited by debris flows in deepwater mud. 

An unusual outcrop of phyllitic conglomerate (_Zic) 

north of Bush Creek and west of Monrovia, Md., is composed 

of fist-sized, flat chips of muscovite-chlorite phyllite supported by a dark matrix rich in detrital heavy minerals that 

include zircon, ilmenite, and magnetite. The protolith of this 

rock is interpreted to be a submarine debris flow. 

Chloritic phyllite and metasiltstone (_Zicp) are interbedded with conglomeratic metagraywacke (_Zicm) near Chestnut 

Grove, Md. The conglomeratic metagraywacke has distinctive 

subrounded pebbles of white quartz and tan lithic clasts in a 

green, chloritic siltstone matrix. Large boulders of the distinctive rock litter the uplands, but exposure is poor. The detrital 

material in the conglomeratic metagraywacke was derived 

from greenschist-facies metasedimentary rocks with abundant 

vein quartz, but the source is unknown.

The chloritic phyllite unit (_Zicp) is interlayered with 

the hematitic phyllite at the type locality of the formation. 

Northward from the type locality, the phyllite is conglomeratic, contains phyllite and marble clasts and marble lenses, and 

is interbedded with quartzite of the Sams Creek Formation. 

Near Linganore Creek, phyllite interbedded with 0.5-m-thick 

quartzite transitions northward into thick-bedded quartzite that 

is interbedded with thin layers of phyllite and rip-up clasts of 

phyllite. 

Sams Creek Formation

The Neoproterozoic(?) and Lower Cambrian(?) Sams 

Creek Formation (Jonas and Stose, 1938b) contains 11 mappable varieties: metabasalt (_Zsb), felsic schist (_Zsf), marble 

(_Zsm), tuffaceous phyllite (_Zstp), muscovitic phyllite 

(_Zsmp), hematitic phyllite (_Zshp), metalimestone (_Zsl), 

quartzite interbedded with phyllite (_Zsqp), quartzite (_Zsq), 

metasiltstone (_Zss), and calcareous metasandstone (_Zsc). 

Metabasalt (_Zsb) is mostly massive, aphanitic to porphyritic, 

and contains calcite-filled amygdules and epidosite nodules. In 

several places, the metabasalt is a hyaloclastite breccia composed of fragmented metabasalt, a conglomerate of elongated 

metabasalt clasts, and epiclastic deposits of weathered metabasalt. The metabasalt is interpreted to be metamorphosed basalt 

flows that were intercalated with beds of quartzite, metasiltstone, phyllite, and marble. The flows are chemically identical to tholeiitic basalt of the Catoctin Formation of the Blue 

Ridge-South Mountain anticlinorium to the west (Southworth, 

1999). Felsic metavolcanic and metavolcaniclastic schist 

(_Zsf) is interlayered with metabasalt in one place, just west of 

Fountain Mills.

Massive to thin-bedded, calcitic and dolomitic marble 

(_Zsm) occurs as linear pods in three different types of Sams 

Creek phyllite as well as in metabasalt and quartzite in different stratigraphic positions. These marble pods are interpreted 

to be olistoliths. Several bodies that are as long as 1 km and as 

wide as 0.5 km are within a “calico” phyllite-clast conglomerate. 

Tuffaceous phyllite (_Zstp) is a metavolcaniclastic phyllite that was formerly called the Libertytown Metarhyolite 

(Jonas and Stose, 1938b). Muscovitic phyllite (_Zsmp) is 

extensive in the eastern belt of the Sams Creek Formation, 

adjacent to the thrust sheet of rocks of the Marburg Formation. Hematitic phyllite (_Zshp) closely resembles rocks of the 
Ijamsville Phyllite but is interlayered with tuffaceous phyllite 

that is not recognized in Ijamsville Phyllite to the south. Metalimestone (_Zsl) is identical to metalimestone of the Ijamsville Phyllite and occurs mostly within phyllite (muscovitic, 

tuffaceous, or hematitic varieties), but one body is adjacent to 

metabasalt (_Zsb) and another body is above marble (_Zsm). 

The quartzite interbedded with phyllite (_Zsqp) is mostly 

medium-grained quartzite interbedded with phyllite, phyllite-clast conglomerate, and tuffaceous phyllite. The unit is 

mapped near Lake Linganore, southeast of McKaig, Md. The 

quartzite, which is similar to that of the Weverton Formation 

and the Sugarloaf Mountain Quartzite, is interbedded with 

rocks that can be traced to the type locality of the Ijamsville 

Phyllite. The variegated phyllite-clast conglomerate and tuffaceous phyllite are associated with marble and metavolcanic 

rocks of the Sams Creek elsewhere.

Medium-grained quartzite of the Sams Creek Formation (_Zsq) is locally crossbedded and calcareous and probably represents channel deposits within all the other units of 

the formation. Metasiltstone (_Zss) locally interbedded with 

quartzite and calcareous metasandstone resembles rocks of 

the Urbana Formation (Jonas and Stose, 1938a), but the Sams 

Creek rocks contain metabasalt and the Urbana rocks do not. 

Friable, coarse-grained, calcareous metasandstone (_Zsc) with 

large, high-angle crossbeds appears to be overlain by metabasalt (_Zsb) south of Monrovia, Md.

Wakefield Marble

The Neoproterozoic(?) and Lower Cambrian(?) Wakefield Marble (Jonas and Stose, 1938b) (_Zw) is a dolomitic 

to calcitic marble that can be traced continuously from the 

northern edge of the map area northward to the type locality 

in Wakefield Valley in the adjacent New Windsor 7.5-minute 

quadrangle, where it stratigraphically overlies metabasalt of 

the Sams Creek Formation (Fisher, 1978). The Wakefield 

Marble is interpreted to represent shallow-water deposits on 

submerged islands of basalt (Fisher, 1978). Because the Wakefield is identical to large clasts of marble within both the Sams 

Creek Formation and Ijamsville Formation, the significance of 

its stratigraphic position is uncertain.

Potomac Terrane

Metagabbro, Ultramafic Rocks, the Soldiers Delight 

Ultramafite, and Metatuff

Dark-green and black, fine- to medium-grained metagabbro containing layers of metapyroxenite (_Zg), as well as 

serpentinite, soapstone, chlorite-tremolite-epidote schist, 

talc-chlorite schist, and talc-calcite-actinolite schist (mapped 

together as _Zu), constitute blocks of mafic and ultramafic 

rock that were emplaced during deposition of turbidites of the 

Mather Gorge Formation. The Soldiers Delight Ultramafite 

(_Zsd) (Drake, 1994) is serpentinite that has been sheared and 

altered to talc schist and soapstone between the Brinklow and 

Henryton thrust faults. The ultramafite constitutes a fault block 

that separates the Sykesville Formation (_s) to its west from 

the Loch Raven Schist (_Zl). Chemical data suggest that the 

rock originally was pyroxenite that formed beneath the ocean 

floor (Drake, 1994). 

Metasedimentary Rocks

The Mather Gorge Formation (Drake and Froelich, 1997) 

mostly consists of quartz-rich schist (_Zms), metagraywacke 

(_Zmg), migmatite (_Zmm), phyllonite (_Zmp), and metatuff 

(_Zmt). The metagraywacke and schist are interpreted to be 

deepwater turbidites deposited in a large submarine fan under 

fairly high-energy conditions (Drake, 1989). In the eastern 

part of the unit, some of these metasedimentary rocks were 

later partially melted to form migmatites and sheared to form 

phyllonites.

The schist unit (_Zms) includes a quartz-muscovitechlorite-plagioclase-epidote-magnetite-hematite mineral 

assemblage. Locally, the unit is a muscovitic gneiss containing 

interbedded metagraywacke and some small blocks of calcsilicate rock. 

The well-bedded metagraywacke unit (_Zmg) also 

contains semipelitic schist, quartzose schist, and some calcsilicate rock interbeds. Metagraywacke beds range from 3 

cm to as much as 3 m thick but average about 15 cm. Metagraywacke beds are graded and have sole marks and slump 

features that are preserved even though the original rocks 

were subjected to sillimanite-grade metamorphism (Hopson, 

1964). In the eastern part of the map unit’s areal extent, some 

of the schist and metagraywacke was partially melted to form 

stromatic and, less commonly, phlebitic migmatite (_Zmm) 

(Mehnert, 1971) consisting of a quartz-plagioclase leucosome 

and a quartz-rich schist paleosome. In the extreme eastern part 

of the map unit’s areal extent, the schist and metagraywacke 

were locally sheared in broad fault zones. The resulting rock 

is a lustrous, fine-grained chlorite-sericite phyllonite (_Zmp) 

containing knots and pods of white vein quartz.

Metatuff (_Zmt) consists of hornblende-plagioclasequartz-muscovite, felsic, schistose metamorphosed tuff. It 

is interpreted to be part of a sparse sequence of interlayered 

mafic and felsic metavolcanic rocks near Great Falls, Va.

The Northwest Branch Formation (_Zn) (Drake, 1998) 

consists of well-bedded meta-arenite and lesser quartzite and 

calc-silicate rock. Beds range from 1.3 cm to 1.2 m in thickness and are interbedded with schist similar to that in the 

overlying Loch Raven Schist.

The Loch Raven Schist (_Zl) (Crowley, 1976) is a thinbedded, lustrous, quartz-muscovite-biotite-plagioclase schist 

that, in places, contains garnet, staurolite, and (or) chlorite. 

The schist is interbedded with semipelitic schist and meta-arenite (_Zlm) and is similar to the underlying Northwest Branch 

Formation into which it grades.

The Oella Formation (_Zo) (Crowley, 1976) is quartzplagioclase-biotite-muscovite meta-arenite interbedded with 

quartz-muscovite-plagioclase schist similar to the underlying 

Piedmont Province    1
16    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Loch Raven Schist into which it grades. The meta-arenite beds 

range from 1 cm to about 1 m in thickness.

The Laurel Formation (_l) (Hopson, 1964) is a sedimentary mélange consisting of a quartzofeldspathic matrix that 

contains quartz grains and fragments of meta-arenite and muscovite-biotite schist, and large porphyroblasts of muscovite. 

The upper part of the formation (_lu) contains more than 50 

percent olistoliths of meta-arenite and muscovite-biotite schist 

that are locally as long as 5 to 10 m.

The Sykesville Formation (_s) (Hopson, 1964) is also a 

sedimentary mélange consisting of a quartzofeldspathic matrix 

that contains distinctive round, eye-shaped quartz lumps and 

blocks of phyllonite (_Zmp), migmatite (_Zmm), and metagraywacke (_Zmg) of the Mather Gorge Formation; as well as 

mafic and ultramafic rocks (_Zu), metafelsite, and plagiogranite. The upper part of the formation locally contains 50 percent 

or more of phyllonite and schist lenses interpreted to be olistoliths derived from the Mather Gorge Formation (Drake, 1989; 

Muller, 1994).

Cambrian, Ordovician, and Devonian Intrusive Rocks

Intrusive rocks include Ordovician quartz bodies; the 

Ordovician(?) Bear Island Granodiorite; the Early Ordovician 

Georgetown Intrusive Suite and Dalecarlia Intrusive Suite; 

the Middle Ordovician Kensington Tonalite; the Late Ordovician Norbeck Intrusive Suite; and the Late Devonian Guilford 

Granite and pegmatites.

The Ordovician(?) Bear Island Granodiorite (Cloos and 

Cooke, 1953) (Ob) is a leucocratic muscovite-biotite granodiorite and related pegmatite composed of quartz, albite, and 

microcline. The granodiorite and pegmatite form small- to 

medium-size sheets, sills, and dikes in migmatite and phyllonite of the Mather Gorge Formation. These bodies extend a 

distance of 18 km from Bear Island northeast to the Plummers 

Island fault.

The Early Ordovician Dalecarlia Intrusive Suite (Drake 

and Fleming, 1994) consists of biotite monzogranite and 

granodiorite (Odm), leucocratic muscovite-biotite monzogranite (Odl), and muscovitic trondhjemite (Odt) that intrudes 

rocks of the Sykesville Formation, Georgetown Intrusive 

Suite, and Norbeck Intrusive Suite. Biotite monzogranite and 

lesser granodiorite contain lenses, zones, and irregular pods of 

leucocratic muscovite-biotite monzogranite. The biotite monzogranite has a U-Pb single crystal sensitive high-resolution 

ion microprobe (SHRIMP) age of 478±6 Ma (Aleinikoff and 

others, 2002). South of Olney, Md., muscovitic trondhjemite 

encloses a very small body of leucocratic monzongranite; 

these two Dalecarlia units are in turn enclosed by biotitehornblende tonalite of the Norbeck Intrusive Suite (described 

below). 

The Early Ordovician Georgetown Intrusive Suite (Fleming and others, 1994) consists of biotite-hornblende tonalite 

(Ogh), quartz gabbro (Ogg), and metapyroxenite (Ogp) that 

intrude rocks of the Sykesville Formation in Washington, 

D.C., to the south. In this quadrangle, they are found within 

the Sykesville Formation in southern Rockville, Md. Biotitehornblende tonalite has a strong relict-igneous-flow foliation 

in places. The tonalite contains many ultramafic and mafic 

xenoliths and (or) autoliths, as well as xenoliths of metasedimentary rocks; mafic minerals, therefore, typically make up 

50 percent of the tonalite. Radiometric dating methods have 

yielded a concordant conventional U-Pb age of 466±3 Ma and 

a single-crystal U-Pb age of 472±4 Ma (Aleini­koff and others, 

2002). Quartz gabbro (Ogg) also contains lesser diorite and 

even lesser quartz norite. At many places, the unit contains 

thin cumulate layers of metapyroxenite and augite-hornblende 

rock. Metapyroxenite (Ogp) is mostly altered to dark-grayishgreen serpentinite. Small pods and a swarm of metapyroxenite 

xenoliths occur within or along the borders of the larger tonalite and quartz-gabbro plutons.

The Middle Ordovician Kensington Tonalite (Cloos and 

Cooke, 1953; Fleming and others, 1994) (Ok) intruded rocks 

in the eastern belt of the Sykesville Formation near the Rock 

Creek shear zone and the Brinklow thrust fault. In and near 

these fault zones, the unit is intensely foliated to mylonitic 

muscovite-biotite granodiorite that contains augen and coarse 

porphyroblasts of microcline. The rock has a single-crystal 

U-Pb age of 463±8 Ma (Aleinikoff and others, 2002).

Veins, lenses, and irregular bodies of massive, foliated, 

and jointed vein quartz of several different ages have intruded 

rocks in the eastern Piedmont. Map-scale quartz bodies (Oq) 

intruded schist and phyllonite of the Mather Gorge Formation, mélange of the Sykesville Formation, schist of the Oella 

Formation, tonalite of the Norbeck Intrusive Suite, and the 

Kensington Tonalite. Loose boulders of quartz commonly litter the upland as the silica is very resistant to physicochemical 

weathering. One of the largest bodies of quartz is Annapolis 

Rock, a prominent hill located immediately northeast of the 

Patuxent River, east of Damascus, Md.

The Late Ordovician Norbeck Intrusive Suite (Drake, 

1998) consists of biotite-hornblende tonalite (Ont), quartz 

gabbro (Ong), ultramafic rocks (Onu), and trondhjemite (Ontr). 

Most of these bodies intrude rocks of the Sykesville Formation. Biotite-hornblende tonalite contains many xenoliths and 

(or) autoliths of more mafic rock and locally has a strong 

relict-igneous-flow foliation. This rock has a concordant conventional U-Pb age of 467±4 Ma and a single-crystal SHRIMP 

age of 449±7 Ma (Aleinikoff and others, 2002). Quartz gabbro 

forms small bodies within biotite-hornblende tonalite. Ultramafic rocks include small bodies of serpentinite and soapstone. Trondhjemite is very siliceous and consists largely of 

plagioclase and quartz.

The Late Devonian Guilford Granite (Dg) (Cloos and 

Broedal, 1940) is a nonfoliated, homogeneous monzogranite 

that forms thin bodies that crosscut bedding and schistosity in 

the Northwest Branch Formation just north of the Burnt Mills 

fault. The Guilford Granite has a U-Pb age of 362±3 Ma

(Aleinikoff and others, 2002).

Late Devonian pegmatite (Dp) is composed of coarse 

grains of muscovite, microperthitic microcline, albite, and 

quartz. The rock is nonfoliated and crosscuts schistosity and 
foliation in the Sykesville and Laurel Formations east of the 

Rock Creek shear zone. 

Structure

The Piedmont province is subdivided into the western, central, and eastern parts, each of which is bounded by 

regional faults. The central and western parts of the Piedmont 

share a similar structural history, but the rocks of the eastern 

Piedmont are more complex and polydeformed. Therefore, 

the geologic structure of each region is presented with little 

attempt to correlate the events. As discussed in the section on 

metamorphism (below), ongoing argon geochronological studies (Kunk and others, 2005) and metamorphic fabric studies 

(Robert P. Wintsch, Indiana University, 2001, oral and written 

commun.) will attempt to unravel this complex history.

Western Piedmont

The Culpeper and Gettysburg basins are half grabens 

bounded on the west by the east-dipping, normal Bull Run Mountain fault. The strata on the east side of the basins unconformably 

overlie pre-Mesozoic rocks of the Piedmont province but some 

contacts are normal faults. The strata within the basins mostly 

dip gently west with the greatest thicknesses found in the western 

part of the basins. The basins deepened and the strata thickened 

during slip movement along the normal faults (Smoot, 1991). The 

minimal displacement on the Bull Run Mountain fault is equal 

to the greatest thickness of the basins’ strata, or about 10 km, as 

determined by our cross sections. The west-dipping strata, the 

conglomerates on the western margin, and the fact that all units 

are truncated by the Bull Run Mountain fault all show that the 

faulting was active throughout Triassic and Jurassic time. Both 

the broad folds near the western margin of the basins and the 

transverse folds formed because displacement along the Bull Run 

Mountain fault varied along strike. Strike-parallel folds formed as 

a result of drag along the Bull Run Mountain fault. The northern 

margin of the Culpeper basin terminates along many normal 

faults that are both parallel to and transverse to the regional strike.

The rocks that underlie the Frederick Valley have been 

folded into a doubly plunging synclinorium that is overturned to 

the west. The east limb of the synclinorium contains a series of 

northeast- and southwest-plunging anticlines and synclines of 

the Araby Formation, Cash Smith Formation, and Rocky Springs 

Station Member of the Frederick Formation. These folds display axial-plane cleavage. The upper member of the Frederick 

Formation and the lower member of the Grove Formation define 

second-order folds in the core of the synclinorium, but cleavage is 

not as pervasive as that on the overturned east limb.

Central Piedmont

The central Piedmont includes the Westminster terrane, 

the Sugarloaf Mountain anticlinorium, and the Bush Creek 

belt.

Thrust Sheets and Faults

The rocks of the Westminster terrane are lithotectonic 

units in a stack of imbricate thrust sheets, the base of which 

is floored by the Martic fault (Southworth, 1996). The thrust 

sheets are defined by deformation fabrics in distinct rock 

types, domains of different fold orientations, and the truncation of rock units. The major faults are the Martic, the Barnesville-Monrovia, and the Hyattstown thrust faults (from west 

to east and structurally lowest to highest). The faults were 

probably active during all the orogenies of the Paleozoic Era. 

Shear-band cleavage and steep folds locally suggest some 

dextral strike-slip motion along some of the faults (Southworth, 1996). The Martic thrust fault cuts upsection westward, 

placing the Ijamsville Phyllite on rocks of the Urbana, Araby, 

and Frederick Formations; the fault is folded with the footwall 

rocks. The Barnesville-Monrovia thrust fault places rocks of 

the Sams Creek Formation on rocks of the Ijamsville Phyllite. 

The Hyattstown thrust fault places rocks of the Marburg Formation on rocks of the Sams Creek Formation. In addition to 

the named thrust faults discussed above, there are early thrust 

faults that are now folded, late thrust faults that cut existing 

faults, reactivated thrust faults, and numerous small intraformational faults of limited displacement in the Westminster 

terrane.

Foliations

The Martic thrust sheet is composed completely of the 

Ijamsville Phyllite. The Ijamsville displays a composite foliation that consists of a transposition foliation that is overprinted 

with phyllonitic foliation and several generations of cleavage. Vein quartz that impregnates the rock has been sheared, 

transposed, and folded into steep isoclines. These steep F1

folds were deformed by westward-verging, inclined F2 folds 

that plunge steeply to gently in all directions. These F2 folds 

exhibit an attendant northeast-dipping, axial-planar, pressuresolution crenulation cleavage.

In the Barnesville-Monrovia thrust sheet, foliated metabasalt of the Sams Creek Formation is structurally aligned parallel to the fault. Sheath folds of metabasalt plunge moderately 

south to southeast in a zone of high strain near Monrovia, Md. 

The map pattern just north of Monrovia yields a cross section 

of the tubular folds. 

The dominant foliation in rocks of the Marburg Formation above the Hyattstown fault is transposition foliation. 

Transposed beds are folded by west-verging, inclined to 

recumbent folds. Crenulation cleavage that is axial planar to F2

folds plunges steeply and gently northeast and southwest. All 

rocks of the Marburg Formation are sheared.

The primary foliation in the Prettyboy Schist is a schistosity that displays transposition. In the northern antiform this 

foliation is shallow dipping and folded upright with crenulation cleavage. Along the Pleasant Grove fault, the schistosity 

of the Prettyboy Schist is steep, and pervasive to spaced shearband cleavage transects the early foliation to form a tectonite. 

Piedmont Province    1
1    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Folds

Folds are recognized in both outcrop and map patterns 

Jonas, 1937). Fisher (1978) described three fold phases that 

re nearly coaxial: early Wakefield Valley folds (isoclinal folds 

f bedding with axial-plane schistosity), a Marston fold phase 

steep north-northeast-trending folds with axial-planar cleavge), and late kinks of the Jasontown fold phase. The Wakeield Valley and Marston folds enter the northern part of the 

ap and are defined by the contacts of the Prettyboy Schist, 

arburg Formation, Sams Creek Formation, and Wakefield 

arble. To the south, deformation has obscured evidence of 

hese folds. Some outcrops in the north-central part of the 

ams Creek Formation between the Hyattstown and Martic 

aults show refolded folds. Here, the early schistosity is vertial, trends east to west, and is folded with northeast-striking 

leavage that is axial planar to steep F2 antiforms that plunge 

n all directions. Similar high strain between the Barnesvilleonrovia fault and the Hyattstown fault has produced tubular 

heath folds of metabasalt and phyllite of the Sams Creek 

ormation.

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Sugarloaf Mountain Anticlinorium and Bush 

Creek Belt

The Sugarloaf Mountain Quartzite and the stratigraphically overlying Urbana Formation were folded to create the 

doubly plunging Sugarloaf Mountain anticlinorium, which is 

overturned to the northwest (Scotford, 1951; Thomas, 1952). 

The north-plunging nose of the anticlinorium is an anticlinesyncline-anticline triplet of Sugarloaf Mountain Quartzite. To 

the south, the structure ends as a plunging anticline. An intraformational thrust fault places part of the main body of the 

anticlinorium onto its west limb. The numerous small, normal 

faults that cut the folded quartzite beds are related to Mesozoic 

extension associated with the formation of the Culpeper basin. 

The Sugarloaf Mountain Quartzite and Urbana Formation display a cleavage that is axial planar to the single fold 

that constitutes the anticlinorium. Metasiltstone of the Urbana 

Formation is characterized by bedding, one cleavage, and a 

conspicuous bedding and cleavage intersection lineation. This 

lineation plunges northward away from Sugarloaf Mountain. 

Bedding-cleavage intersection lineations in the Urbana Formation, which is mapped in a window of the Martic thrust fault in 

the Bush Creek belt, plunge gently southward.

Eastern Piedmont

Thrust Sheets and Faults

Metasedimentary and metavolcanic rocks of the Potomac 

terrane constitute a stack of thrust sheets (Drake, 1989). The 

thrust sheet of rocks represented by the Mather Gorge Formation was placed over previously undeformed sediments of 

the Sykesville Formation by the Plummers Island fault. The 

rocks of the Mather Gorge Formation were polydeformed 

and polymetamorphosed prior to emplacement (Drake, 1989; 

Muller, 1994). Olistoliths (clasts) of the Mather Gorge Formation (migmatite, schist, folded metagraywacke, and phyllonite) 

within the Sykesville Formation demonstrate that deformation 

and metamorphism of the Mather Gorge Formation predated 

the sedimentation and deposition of the Sykesville Formation 

(Drake, 1989; Muller, 1994). From the Plummers Island fault 

west to Bear Island, ductile fault fabrics in the Mather Gorge 

Formation are restricted to phyllonite, which is interpreted to 

be derived from the shearing of schist, metagraywacke, and 

migmatite of the Mather Gorge Formation (Drake, 1989) as 

the rocks were transported westward above the rocks of the 

Sykesville Formation. Along the Potomac River just south 

of the map area, exposures of strongly sheared rocks in both 

the Mather Gorge and Sykesville Formations across the steep 

west-dipping Plummers Island fault suggest that the fault was 

reactivated, possibly as a back thrust (Fisher, 1970). Argon 

geochronology suggests that the Plummers Island fault was 

reactivated in the Carboniferous (Kunk and others, 1998, 

2005). 

The Loch Raven thrust sheet consists of the Northwest 

Branch Formation, Loch Raven Schist, and Oella Formation (Drake, 1989). The Laurel Formation is a sedimentary 

mélange that contains locally abundant olistoliths of previously deformed rocks. The Loch Raven thrust sheet was 

emplaced along the Burnt Mills fault during deposition of the 

Laurel Formation. The Loch Raven thrust sheet and Laurel 

Formation were next overthrust by rocks of the Sykesville Formation along the Brinklow thrust fault (Fleming and Drake, 

1998). Both hanging wall and footwall rocks are marked by 

mylonitic foliation (Drake, 1994). Rocks of the Loch Raven 

thrust sheet and Sykesville Formation are locally separated by 

the Rock Creek shear zone (Fleming and others, 1994). The 

Rock Creek shear zone is characterized by phyllonitized and 

mylonitized rocks that cover an area as much as 3 km wide. 

The rocks contain kinematic indicators of thrust faulting, as 

well as dextral and sinistral strike-slip motion (Fleming and 

others, 1994; Fleming and Drake, 1998). To the south of the 

map area, near the National Zoo in Washington, D.C., are 

some discrete younger thrust faults of the Rock Creek shear 

zone that have placed crystalline rocks against Quaternary 

and Tertiary sediments (Fleming and Drake, 1998). The Rock 

Creek shear zone locally reactivated segments of the Brinklow 

thrust fault and localized emplacement of the Kensington 

Tonalite (Fleming and Drake, 1998; Drake, 1998). 

Rocks of the Potomac terrane were transported westward onto rocks of the Westminster terrane along the Pleasant 

Grove fault. The Pleasant Grove fault is a ductile shear zone as 

much as 1 to 2 km wide that initially formed as a thrust fault 

during deformation associated with the Ordovician Taconian 

orogeny (Drake, 1989). K-Ar geochronology suggests that 

deformation-induced recrystallization was associated with 

shear-band foliation having dextral strike-slip movement in 

the late Paleozoic (Pennsylvanian to Permian, 311.4±3.2 Ma) 

Alleghanian orogeny (Krol and others, 1999). An 40Ar/39Ar 

date on white mica in the shear zone also indicates a 364 
to 348 Ma episode of deformation, possibly related to the 

Acadian orogeny (Krol and others, 1999). Rocks within and 

on either side of this fault zone have phyllonitic foliation and 

a spaced shear-band cleavage having dextral shear sense. 

40Ar/39Ar data near the Potomac River define a Late Devonian 

deformation zone about 12 km wide that transects the Pleasant 

Grove fault (Kunk and others, 2005). There is no isotopic evidence of pre-Devonian or significant post-Devonian movement 

along the Pleasant Grove fault (Kunk and others, 2005). 

Foliations

Rocks of the Mather Gorge Formation contain several 

foliations (Drake, 1989). The first schistosity forms a fan along 

the Potomac River; it dips gently eastward near Seneca, Md., is 

near vertical at Great Falls, and dips westward near the Maryland 

border with Washington, D.C. This schistosity has been folded, 

faulted, and overprinted by several younger foliations. Rocks of 

the Northwest Branch Formation, Loch Raven Schist, and Oella 

Formation have a primary schistosity that has been overprinted 

by a spaced cleavage at many places. Rocks of the Sykesville and 

Laurel Formations have a crude foliation, defined commonly by 

aligned clasts, that is overprinted by schistosity in some spots. 

Rocks of the Georgetown, Norbeck, and Dalecarlia Intrusive 

Suites have a primary flow foliation that is overprinted by a 

secondary metamorphic foliation. Rocks of the Guilford Granite 

are not foliated, but rocks of the Bear Island Granodiorite are 

locally sheared. Sheared leucocratic granitoid (Bear Island Granodiorite?) near and along the Plummers Island thrust fault at the 

Potomac River suggests post-Early Ordovician deformation along 

this fault. Shear strain in all these rocks has resulted in transposition, phyllonitic, and mylonitic foliation (Fleming and Drake, 

1998). Rocks of the Mather Gorge Formation along the Pleasant 

Grove fault have a shear-band cleavage with dextral kinematics 

of undetermined displacement. Similar movement indicators are 

only locally found along the Plummers Island fault.

Lineations

The first schistosity in rocks of the Mather Gorge Formation, Northwest Branch Formation, Loch Raven Schist, and 

Oella Formation is marked by intersecting beds and forms a 

prominent intersection lineation. Later schistosities in these 

rocks have intersected the first schistosity, which formed additional intersection lineations. Higher grade rocks have mineral 

lineations that plunge down the dip of the schistosity.

Rocks of the Sykesville and Laurel Formations are also 

lineated. Small olistoliths in these rocks are elongated down 

the dip of the foliation. Profile views of the olistoliths show 

that they are shaped like pancakes that were flattened, not 

stretched. Minerals on foliation surfaces are also aligned, 

which indicates that the rocks have been stretched. Many of 

the rocks of the Georgetown, Dalecarlia, and Norbeck Intrusive Suites have a downdip mineral lineation that formed in a 

stretching regime.

Folds

Rocks of the Mather Gorge Formation were folded at 

least three times (Drake, 1989). The map patterns of the 

metagraywacke and schist units of the Mather Gorge Formation near the Potomac River illustrate the early west-trending Captain Hickory fold phase. These isoclinal folds were 

refolded by the northerly trending, upright Potomac folds and 

northwest-trending Reston folds. The local outcrop pattern 

of the Plummers Island and Burnt Mills faults, as well as the 

interpreted tectonic windows, indicate that these faults have 

been folded. The reclined folded thrust may have occurred 

during the Potomac fold phase, whereas open folds may be 

related to later strike-slip transpression. 

Metamorphism

The rocks of the Piedmont province were metamorphosed 

at different times and grades, and they have been subsequently juxtaposed and transported westward during Paleozoic orogenies. In general, the rocks are of the highest grade 

of metamorphism (sillimanite) in the east and of the lowest 

grade (chlorite-sericite) in the west. However, the grades of 

metamorphism do not follow a simple Barrovian sequence 

as has been described along discrete sections of the Potomac 

River near Great Falls (Fisher, 1970; Drake, 1989). Recent 

analysis of 40Ar/39Ar dates on hornblende and muscovite from 

locations across the Maryland Piedmont demonstrate that the 

rocks experienced a complex polymetamorphic history (Krol 

and others, 1999; Kunk and others, 2004, 2005). In short, the 

core of the Potomac terrane at Great Falls records Ordovician 

cooling ages. To the east of Great Falls, the rocks were structurally deformed and then retrograded in the Early Devonian 

and Late Pennsylvanian. West of Great Falls, the rocks of the 

Potomac terrane contain Late Devonian foliations. West of the 

Potomac terrane and beneath the Pleasant Grove fault, rocks of 

the Westminster terrane contain Early Silurian foliations, and 

there is no evidence of Devonian or later deformation.

Sugarloaf Mountain Anticlinorium and Frederick 

Valley Synclinorium

The Sugarloaf Mountain Quartzite and rocks of the 

Urbana Formation are at chlorite grade resulting from a 

single, greenschist-facies metamorphic event. The matrix of 

the quartzites is rich in sericite and contains sparse chlorite. 

Cleavage in metasiltstone and marble of the Urbana Formation 

are marked by aligned sericite crystals and chlorite porphyroblasts that give the rocks a green hue.

Chlorite-grade rocks of the Araby, Cash Smith, Frederick, 

and Grove Formations locally contain sparse, tiny crystals of 

sericite and chlorite porphyroblasts that are crudely aligned 

in the cleavage plane. Cleavage and chlorite crystals are most 

evident in clay-rich rocks of the overturned east limb of the 

synclinorium. 

Piedmont Province    1
Westminster Terrane

Rocks of the Westminster terrane had a complex metamorphic history under greenschist-facies conditions (Kunk 

and others, 2004). Phyllites of the Ijamsville Phyllite, Marburg 

Formation, and Sams Creek Formation contain varying proportions of muscovite, chlorite, paragonite, quartz, magnetite, 

and calcite, all resulting from the first metamorphic event. 

Metabasalt of the Sams Creek Formation contains varying 

proportions of actinolite, epidote, and stilpnomelane. Quartzose rocks in all the formations contain sericite and chlorite. 

Phyllitic rock in fault zones was retrogressively metamorphosed and sheared into phyllonitic diaphthorites (Knopf, 

1931). Rocks described by Knopf (1931) as “sick-looking” 

are characterized by chlorite in multiple shades of green, white 

quartz, and abundant leucoxene. Phyllites of the Ijamsville 

Phyllite, Sams Creek Formation, and Marburg Formation 

locally experienced a later prograde metamorphic event that 

caused chloritoid and albite to overgrow the earlier foliation. 

The contacts between the phyllite of the Sams Creek Formation, rocks of the Marburg Formation, and the Prettyboy Schist 

in the northeastern part of the map area may be isograds as 

suggested by Fisher (1978). Static chloritoid in phyllite of the 

Marburg Formation increases in size and abundance to the 

southeast and is rotated by pressure-solution cleavage. Albite 

porphyroblasts also increase in size and abundance to the 

east, from rocks of the Marburg Formation into the Prettyboy 

Schist. Locally, albite encloses crinkles of the Wakefield and 

Marston fold phases; thus, the peak of metamorphism postdates the major folding events. 

Potomac Terrane

Rocks of the Mather Gorge Formation are polymetamorphic. They experienced a prograde metamorphic event 

characterized by mineral assemblages ranging from a chlorite 

zone in the west to a sillimanite zone in the east (Fisher, 1970; 

Drake, 1989). In the sillimanite zone the rocks are migmatites. 

This Barrovian metamorphism occurred before the intrusion 

of the Ordovician(?) Bear Island Granodiorite and maybe 

before the Middle Ordovician Taconian orogeny (Becker and 

others, 1993). The high-grade rocks to the east were locally 

sheared and retrogressively metamorphosed to chlorite-sericite 

phyllonites. This retrograde metamorphic event was followed 

by a secondary prograde event that crystallized the chloritoid, muscovite, and biotite in the shimmer aggregate that 

had formed during the retrograde event. The 40Ar/39Ar white 

mica dates suggest that rocks of the Mather Gorge Formation 

immediately above the Pleasant Grove fault record thermal 

evidence of the Taconian (Ordovician), Acadian (Devonian), 

and Alleghanian (Pennsylvanian to Permian) orogenies (Krol 

and others, 1999).

The Sykesville Formation was metamorphosed at biotite±garnet±staurolite grade, but its olistoliths were metamorphosed (some at partial melt conditions) and deformed prior to 

deposition. Foliations containing biotite and garnet are seen in 

the Loch Raven Schist and Laurel Formation east of the Rock 

Creek shear zone, which indicates that these rocks experienced 

two prograde events. Static porphyroblastic staurolite and 

coarse muscovite have overgrown this foliation as well as the 

mylonitic and phyllonitic foliations in the shear zone (Fleming 

and Drake, 1998). Staurolite occurs along fractures in olistoliths of ultramafic and mafic rocks. Rocks of the Middle to 

Early Ordovician Georgetown, Dalecarlia, and Norbeck Intrusive Suites, and the Ordovician(?) Bear Island Granodiorite 

have been metamorphosed, but the Late Devonian Guilford 

Granite has not been metamorphosed.

Coastal Plain Province

The Lower Cretaceous Potomac Formation (Kps) consists 

of unconsolidated gravel, sand, silt, and clay that unconformably overlies rocks of the Laurel Formation (_l) in the extreme 

southeastern corner of the map area, near Silver Spring, Md. 

This is the only remnant in this quadrangle of a fluvial and 

deltaic deposit that once may have extended as far west as the 

Blue Ridge province. 

Fossils

Some of the sedimentary rocks in the quadrangle 

contain a variety of fossils that become more abundant 

and diverse as the rocks get younger. The Cambrian Harpers and Antietam Formations contain burrow traces called 

Skolithos linearis. The Cambrian Antietam Formation also 

contains Olenellus (Walcott, 1896), the oldest trilobite fossil 

in the central Appalachian region. Other Cambrian rocks 

(Tomstown, Frederick, and Waynesboro Formations; and 

Elbrook and Conococheague Limestones) contain parts 

of trilobites, algal colonies (stromatolites), and conodont 

microfossils. Ordovician rocks (Grove Formation, Conococheague Limestone, Stonehenge Limestone, Rockdale Run 

Formation, Pinesburg Station Dolomite, St. Paul Group, and 

Chambersburg Limestone) contain fewer trilobites and conodonts, but do have more snails, brachiopods, echinoderms, 

bryozoans, and cephalopods. When the carbonate deposition 

was replaced by clastic sedimentation of the Martinsburg 

Formation, mainly snails, clams, and trilobites survived 

because they could filter out the clay and silt.

Upper Triassic and Lower Jurassic rocks of the Culpeper 

Group yield phytosaur teeth; coelacanth fish; footprints of 

crocodiles, lizards, and carniverous dinosaurs; plant impressions, pollen, and spores; insect parts; conchostracans; 

ostracodes; and fish scales. These fossils occur in the Manassas Sandstone, Bull Run Formation (Balls Bluff Member), 

Midland Formation, Catharpin Creek Formation, and Turkey 

Run Formation (Weems, 1987, 1993). 
Surficial Materials

Alluvium

Well to poorly sorted Holocene alluvium (Qa) composed 

of stratified mixtures of unconsolidated clay, silt, sand, gravel, 

and cobbles underlies flood plains of nearly all rivers and 

tributaries. The channel of the tributary commonly is incised 

into bedrock with alluvium exposed along the banks. The 

thickness of the alluvium is highly variable and is a function 

of bedrock, topography, and land-use practices. Locally, thick 

deposits of alluvium resulted from erosion due to agricultural 

practices of the 19th century; mill dams also caused a buildup 

of eroded sediment in places. Recent suburban development 

has introduced large quantities of sediment into tributaries and 

into the Chesapeake Bay.

Alluvial Terrace Deposits

The map area includes locally preserved remnants of 

high-level Pleistocene and older(?) terraces (QTt) and lowlevel Pleistocene and Holocene terraces (Qt) of the ancestral 

Potomac, Shenandoah, and Monocacy Rivers. These conspicuous terraces consist of rounded sandstone and quartzite 

cobbles and boulders that contain the trace fossil Skolithos 

linearis, and rounded to angular chert pebbles, cobbles, and 

boulders whose sources are Early Cambrian and younger rocks 

of the Valley and Ridge province. The deposits are isolated 

remnants of former, more extensive terraces. The remnants 

of older deposits may only be a veneer of coarse clasts; 

younger, more well-preserved deposits may be several meters 

thick. Some of the highest deposits—such as Mount Sterling 

in Loudoun County, Va.; east of Lucketts, Va.; and south of 

Dickerson, Md.—are the result of the topographic inversion 

of an ancient channel. The oldest deposits at Sterling, Va., are 

several meters thick and are deeply oxidized and leached.

Colluvium

Fine and coarse colluvium cover the hill slopes across the 

quadrangle. The colluvium was derived from the escarpment 

of the east limb of the Blue Ridge-South Mountain anticlinorium. The deposits occur as lensoid aprons that cover the 

bedrock of the western margin of the Culpeper basin. 

Fine colluvium (Qcf) consists of subangular clasts of 

quartzite, phyllite, metabasalt, epidosite, and vein quartz. 

Coarse colluvium (Qcc) consists of cobbles and boulders of 

predominantly quartzite. Gravity, debris-flow, and freeze-thaw 

processes acting on the landscape resulted in coarse colluvium 

concentrations in hillslope depressions on Blue Ridge-Elk 

Ridge, Short Hill-South Mountain, Catoctin Mountain, and 

Sugarloaf Mountain. Only the largest boulder streams, boulder 

fields, and rock slides are portrayed on this map because a 

thin veneer of colluvium covers virtually all of the mountain 

slopes. The surface of the deposits consists completely of 

clasts with no visible matrix supporting them because the finer 

grained material has washed away, but matrix-supported clasts 

may occur at depth.

Residuum and Lag Gravel

The limestone of the eastern Great Valley section (Hack, 

1965) and the Frederick Valley section (Southworth and Brezinski, 2003), gneiss of the Blue Ridge province, and schist of 

the Piedmont province uplands (Froelich, 1975) have a thick 

regolith developed on their surfaces. This regolith consists 

partly of a residuum of in situ weathered bedrock (saprolite). 

Some of the regolith is more than 30 m thick, which accounts 

for the flat topography (note the contours on the base map of 

the quadrangle) and the lack of outcrop. 

Pebbles and cobbles of angular to euhedral white quartz 

from possible in situ weathering of the underlying carbonate 

bedrock superficially resemble terrace deposits (Qt or QTt); 

however, these deposits are lag concentrates from residual 

material (Qr). Presumably, the vein quartz originally filled 

fractures and cavities in the carbonate rock in the western and 

central Piedmont.

Another distinctive residuum, Pleistocene and Holocene 

lag gravel (Ql), formed from the in situ weathering of conglomeratic rocks of the Goose Creek Member (J^ccg) of the 

Catharpin Creek Formation in the western part of the Culpeper 

basin, south of Leesburg, Va. Natural surface accumulations of 

rounded quartz pebbles and cobbles superficially resemble terrace deposits. Excavations near the Loudoun County Landfill 

reveal a thick regolith of saprolitized cobbles and boulders 

around pinnacles of bedrock. 

Summary of Geologic History and

Tectonics

The rocks of the Frederick 30’ × 60’ quadrangle reflect several Wilson cycles (Wilson, 1966) of opening and closing ocean 

basins. Rocks of the Blue Ridge province record evidence of a 

billion-year-old mountain range that resulted from the Grenville 

orogeny. This continent, called Laurentia, rifted about 500 m.y. 

later to create an ocean known as Iapetus. Rocks of the central 

and western Piedmont province (west of the Pleasant Grove 

fault) as well as rocks of the Great Valley section of the Valley 

and Ridge province were deposited in various depths and environments within the Iapetus Ocean. For example, slate, phyllite, 

and quartzite of the Westminster terrane of the central Piedmont 

were deposited in deep water on the continental rise, whereas 

carbonate rocks of the Great Valley were deposited in shallow 

water on a continental shelf platform. Carbonate rocks of the 

Frederick Valley in the western Piedmont reflect the continental 

slope that marks the transition between the two settings. 

Summary of Geologic History and Tectonics    1
Rocks of the Potomac terrane in the eastern Piedmont are 

interpreted to be deepwater deposits containing debris of the 

oceanic crust (Drake, 1989). Parts of this terrane show evidence of deformation and metamorphism not seen elsewhere 

(Drake, 1989). The earliest thrust sheets were emplaced during 

sedimentation in a deep oceanic trench. One thrust sheet contains the older Mather Gorge Formation overlying the younger 

Sykesville Formation; the other thrust sheet contains the older 

Loch Raven Schist overlying the younger Laurel Formation 

(Fleming and others, 1992, 1994).

The deformed rocks of the Potomac terrane were intruded 

by tonalitic to granitic rocks during the Ordovician Taconian 

orogeny (Drake, 1989). The highly deformed rocks of the 

Westminster terrane also may have been subjected to late 

Taconian deformation and may have been transported westward 

onto Cambrian rocks of the Frederick Valley along the Martic 

fault during this tectonic event (Southworth, 1996). Mylonitic 

Keedysville marble at the base of the Cambrian carbonate rock 

sequence (Tomstown Formation) in the Blue Ridge province 

and Great Valley section of the Valley and Ridge province is 

interpreted to be a regional detachment that formed during the 

Taconian orogeny (Brezinski and others, 1996). Other evidence of the Taconian orogeny is the change in deposition from 

carbonate rocks to clastic rocks in the Ordovician Martinsburg 

Formation of the Great Valley section, indicating an uplifted 

source that was eroding. However, argon geochronology in the 

rocks of the Piedmont province suggests that deformation and 

metamorphism occurred in the Silurian, Devonian, and Mississippian (Krol and others, 1999; Kunk and others, 2004).

All of the pre-Mesozoic rocks were folded, faulted, and 

metamorphosed in several events throughout the Paleozoic, 

culminating in the late Paleozoic (Pennsylvanian to Permian?) 

Alleghanian orogeny. This orogeny resulted from the collision 

of the continental plates of Laurentia (North America) and 

Gondwana (Eurasia) that closed the Iapetus Ocean. The rocks 

were transported westward above the North Mountain thrust 

fault that crops out at the surface just northwest of the Frederick quadrangle. From west to east, the Massanutten synclinorium, Blue Ridge-South Mountain anticlinorium, Frederick 

Valley synclinorium, and Sugarloaf Mountain anticlinorium 

are a west-verging fold train above this thrust fault.

The Appalachian highlands that were created by the converging fold-and-thrust belt collapsed as the continent began 

to rift and pull apart, creating the Atlantic Ocean during the 

Triassic and Jurassic Periods. Alluvial-fan debris shed from 

scarps along normal faults filled the lowlands that eventually 

evolved into basins by possibly renewed listric fault movement along older thrust faults. Basalt flows were extruded and 

diabase was intruded in the last stages of the rifting prior to the 

formation of the Atlantic Ocean. 

Drainage systems that flowed to the west into the Appalachian basin gradually reversed as a result of the rifting and 

flowed east toward the Atlantic Ocean. Debris that was shed 

from the eroding Appalachian highlands formed thick deposits 

of unconsolidated sediments that eventually became the Atlantic Coastal Plain. Modern rivers and their tributaries continue 

to erode bedrock and carry sediment down the Potomac River 

to the Chesapeake Bay.

Mineral Resources

Active quarries in the Frederick 30’ × 60’ quadrangle 

include those that mine (1) high-calcium limestone for cement 

and agricultural lime, (2) diabase, limestone, marble, shale, 

and serpentinite for aggregate, and (3) gneiss, schist, and 

quartzite for dimension building stone (Edwards, 1995). Only 

the locations of quarries, mines, and prospect pits for aggregate, precious metals, dimension and ornamental building 

stone, and iron or manganese minerals that existed in historical 

times are shown on the map.

Limestone and Aggregate

In the Great Valley section of the Valley and Ridge 

province, there are at least 26 quarries in the Great Valley 

that mine high-calcium limestone of the Cambrian Tomstown 

Formation, Waynesboro Formation, and Elbrook Limestone; 

and the Ordovician Stonehenge Limestone. In the Blue Ridge 

province, small pods of calcitic to dolomitic marble were quarried historically in four places for agricultural lime and ornamental stone. In the Frederick Valley of the western Piedmont 

province, there are seven quarries that produce crushed stone 

and Portland cement. In the Westminster terrane of the central 

Piedmont, more than 10 small quarries and pits in marble were 

recognized and they often were in close proximity to historical 

kilns that were used to heat the marble to produce agricultural 

lime.

Several quarries are located in sills of Early Jurassic 

diabase and in beds of basalt, which are used for aggregate 

in road metal and for concrete in Loudoun County, Va. Shale 

and limestone are quarried and crushed for aggregate in the 

Frederick Valley. Serpentinite is crushed for aggregate in 

Montgomery Co., Md., in the eastern Piedmont. 

Building and Ornamental Stone

Historic homes and other structures in the area were 

constructed from virtually every type of locally exposed indigenous rock. Those rocks mined for commercial quantities are 

mostly in the eastern Piedmont and were quarried and used for 

the construction of canals, bridges, and buildings in the early 

1800s. The most popular dimension stones used have included 

(1) Kensington Tonalite, Georgetown Intrusive Suite, and 

Sykesville Formation (known as “Potomac blue stone”), (2) 

serpentinite, (3) sandstone of the Poolesville Member of the 

Manassas Sandstone (often called “Seneca red sandstone”), (4) 

quartzite of the Weverton Formation and Sugarloaf Mountain 

Quartzite, and (5) various limestones in the Great Valley. Slate 
of the Ijamsville Phyllite was quarried in at least six locations 

for roofing material in the early 1900s (Dale and others, 1914).

Gneiss of the Sykesville Formation, limestone conglomerate of the Leesburg Member of the Bull Run Formation, and 

marble of the Swift Run and Catoctin Formations have been 

the most popular ornamental stones. Conglomerate of the 

Leesburg Member (called the “Potomac marble” or “Calico 

rock”), quarried in Loudoun County, Va., and Frederick 

County, Md., was used in 1815 by Benjamin Latrobe for the 

columns in Statuary Hall of the U.S. Congress on Capitol Hill 

(U.S. Geological Survey, 1998).

Metals

In the late 1700s and 1800s, there were abundant prospects for limonite, hematite, and manganese for the production of iron located throughout the quadrangle. In the Blue 

Ridge-South Mountain anticlinorium, near the contact of the 

Antietam Formation with the overlying Tomstown Formation, 

are at least 24 manganese prospects. On the eastern side of 

the Frederick Valley, there are at least five limonite prospects 

along the contact between the Araby Formation and the overlying Frederick Formation. 

Both gold mines and placer deposits (Bernstein, 1980; 

Kuff, 1983) may be found within the Piedmont province. Vein 

quartz was mined for gold near Great Falls, Md. (Reed and 

Reed, 1969). Gold was also a byproduct of mining for lead, 

zinc, and silver from marble and metavolcanic rocks in the 

Linganore mining district of the Westminster terrane in the 

central western Piedmont (Heyl and Pearre, 1965). There are 

three reported occurrences of gold in the Blue Ridge province: 

one placer deposit, one associated with metabasalt of the Catoctin Formation, and a third located in rocks of the Chilhowee 

Group near the border fault of the Gettysburg basin. Gold, 

silver, and copper prospects were located in sandstone and 

diabase in the Culpeper basin in Virginia and Maryland (Robinson and Sears, 1988). 

Aeromagnetic Map

Introduction

A digital aeromagnetic grid of the Frederick 30’ × 60’

quadrangle was assembled from data resulting from 10 aeromagnetic surveys that were flown between 1958 and 1989. 

Three of the surveys were recorded and processed digitally; 

the remainder were analog surveys, and the magnetic values 

were digitized from hand-drawn contour maps. Surveys in 

Maryland and Virginia were flown along east-west flightlines, 

with the spacing between the flightlines ranging from 0.125 

to 1 mile (mi) (200–1,600 m). All but one survey were flown 

at 500 feet (ft) above ground. In West Virginia, flightlines 

trended northwest to southeast at 1,000 ft above ground with 2 

mi of separation.

The total magnetic field measurements from each survey 

were interpolated into separate grids. The International Geomagnetic Reference Field was then calculated in grid form for 

the time and altitude of each survey and subtracted from the 

corresponding grids of observed magnetic data. The resulting 

residual grids were then compared with adjacent grids and 

adjusted for level differences. Finally, the adjusted grids were 

joined into one grid.

The Shaded-Relief Map

The residual magnetic field grid is displayed as a shadedrelief map overlain with selected geologic features (fig. 3). 

The synthetically produced illumination is from the east at a 

simulated sun angle of 45o; therefore, shadows appear along the 

western sides of the magnetic anomalies. The relative intensities of the magnetic anomalies are displayed as gray scales. see 

Southworth and others (2002) for a color-shaded-relief map 

with red corresponding to higher magnetic values and blue 

corresponding to lower magnetic values. The shading technique 

greatly enhances small-amplitude magnetic anomalies. 

Correlation of Magnetic Data With Geology

The aeromagnetic anomalies correlate closely with the 

mapped geology because some lithologic units have contrasting magnetic properties. The dominant features of the aeromagnetic map are (1) a series of large-amplitude magnetic 

anomalies in parallel belts that are associated with metabasalt 

of the Catoctin Formation in the Blue Ridge province and 

(2) a broad swath of high-amplitude values corresponding 

to metasedimentary, metavolcanic, and mafic rocks of the 

Mather Gorge Formation in the eastern Piedmont. Lower 

amplitude magnetic anomalies are associated with metabasalt 

of the Sams Creek Formation and metasedimentary rocks of 

the Urbana Formation in the western Piedmont. The complex 

pattern of segmented magnetic highs within the Catoctin 

Formation may represent individual or groups of basalt flows. 

Smaller wavelength high-amplitude magnetic anomalies are 

associated with Mesoproterozoic gneiss units (Southworth and 

others, 2006) in the western part of the Blue Ridge province 

and with Jurassic diabase dikes and sills.

The aeromagnetic anomalies also correlate with mapped 

faults. The most obvious anomaly is associated with the 

Pleasant Grove fault that places rocks of the Mather Gorge 

Formation against rocks of the Marburg Formation in the 

central Piedmont. The fault zone is marked by magnetitebearing phyllonites and schists. The Plummers Island fault 

(rocks of the Mather Gorge Formation juxtaposed against 

rocks of the Sykesville Formation) is roughly coincident to a 

boundary between large- and small-amplitude anomalies. The 

low-amplitude lineament to the east of the highly magnetic 

phyllonites of the Mather Gorge Formation corresponds to 

weakly magnetic rocks of part of the Sykesville Formation. 

The Mather Gorge Formation in the Potomac terrane of the 

Aeromagnetic Map    3
Figure 3. Gray-scale shaded relief image of residual aeromagnetic field grid data. High magnetic anomalies are displayed as dark gray 

(values as much as 430 nanotestlas). Geologic features that correlate with the magnetic data are shown.
Piedmont has irregular, short-wavelength anomalies that indicate an internal complexity. In the eastern Piedmont, the Rock 

Creek shear zone is marked by a prominent large-amplitude 

aeromagnetic anomaly associated with highly magnetic schists 

of the Laurel Formation and Loch Raven Schist. In the western 

Piedmont, the Barnesville-Monrovia and Hyattstown faults, 

and segments of the Martic fault are marked by moderateamplitude linear anomalies that are spatially associated with 

magnetite-bearing phyllonites.

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References Cited    
30    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Description of Map Units

[For thickness of units, see cross sections.]

Unconsolidated Surficial Materials

Qa Alluvium (Holocene)—Well to poorly sorted, stratified mixtures of unconsolidated clay, silt, sand, 

gravel, and cobbles underlying flood plains 

of nearly all rivers and tributaries. Channel of 

tributary commonly is incised into bedrock with 

alluvium exposed along the banks. Thickness 

of alluvium is highly variable and is a function 

of bedrock, topography, and land-use practices. 

Locally, thick deposits of alluvium resulted 

from erosion due to agricultural practices of the 

19th century; mill dams also caused a buildup 

of eroded sediment in places. Recent suburban 

development has introduced large quantities of 

sediment into tributaries

Qt Terrace deposit, low level (Holocene and 

Pleistocene)—Sand, gravel, and boulders on flat 

benches located as much as 50 meters (m) above 

the flood plain

 Colluvium (Holocene and Pleistocene)

Qcf Fine colluvium—Lensoid aprons of poorly sorted, 

subangular clasts of quartzite, phyllite, metabasalt, epidosite, and vein quartz supported in a silt 

matrix. Covers bedrock of the western margin 

of the Culpeper basin along the lower slope of 

Catoctin Mountain

Qcc Coarse colluvium—Cobbles and boulders of predominantly quartzite, concentrated in hillslope 

depressions on Blue Ridge-Elk Ridge, Short Hill 

Mountain-South Mountain, Catoctin Mountain, 

and Sugarloaf Mountain. Emplacement was by 

gravity, debris flows, and freezing and thawing

Qr Residuum (Holocene and Pleistocene)—

Unconsolidated mixture of moderately reddish-brown soil, and pebbles and cobbles of 

grayish-pink to white, angular, locally euhedral 

quartz derived from in situ weathering of underlying carbonate rocks. Located primarily in 

the Frederick Valley north of Potomac River. 

Thickness ranges from a thin veneer to 3 m. 

Superficially resembles terrace deposits (Qt and 

QTt)

Ql Lag gravel (Holocene and Pleistocene)—Distinctive residuum formed from in situ weathering 

of the Goose Creek Member (J^ccg) of the 

Catharpin Creek Formation, the Poolesville 

Member (^cmp) of the Manassas Sandstone, 

and the upper member (O_gu) of the Grove 

Formation. Natural surface accumulations consisting of rounded quartz pebbles and cobbles 

superficially resemble terrace deposits (Qt and 

QTt). Excavations reveal a thick regolith of saprolitized cobbles and boulders around bedrock 

pinnacles

QTt Terrace deposit, high level (Pleistocene and 

Tertiary)—Conspicuous, high-level terraces 

of the ancestral Potomac, Shenandoah, and 

Monocacy Rivers preserved locally on isolated 

hillocks. Composed of rounded sandstone and 

quartzite whose sources are Lower Cambrian 

and younger rocks of the Valley and Ridge 

province; rock fragments contain remains of 

trace fossil Skolithos linearis. Deposits are 

isolated remnants of former, more extensive 

terraces. Some of the highest deposits, such as 

Mount Sterling in Loudoun County; one east of 

Lucketts, Va.; and one south of Dickerson, Md., 

are the result of topographic inversion of an 

ancient channel

Great Valley Section of the Valley and Ridge 

Province

Om Martinsburg Formation (Upper and Middle 

Ordovician)—Light-brown shale, calcareous 

shale, and siltstone. Contains thin to medium 

beds of sandstone and metagraywacke in the 

upper part; gray argillaceous limestone at base

Oc Chambersburg Limestone (Middle Ordovician)—

Light-gray, argillaceous, nodular limestone

Osnr St. Paul Group (New Market and Row Park 

Limestones, undivided) (Middle Ordovician)—Light- to medium-gray, thick-bedded 

micritic limestone and bioclastic limestone containing bedded black chert nodules

Beekmantown Group

Obps Pinesburg Station Dolomite (Middle Ordovician)—Light-gray, medium- to thick-bedded 

dolostone and dololaminate containing white 

and light-gray chert nodules. Characteristic 

“butcher-block” weathered surface (caused by 

cross-hatched joints). Irregular bedding resulting 

from collapse breccia and paleokarst near top
Obrr

Obss Stoufferstown Member—Light-gray, laminated, silty 

limestone containing thin interbeds of platy 

limestone and coarse-grained bioclastic limestone

O_c Conococheague Limestone (Lower Ordovician 

and Upper Cambrian)—Interbedded dark- to 

light-gray, laminated, algal limestone; dolomitic 

limestone; light-brown dolostone and sandstone

_cb Big Spring Station Member (Upper Cambrian)—

Light-gray, calcareous to dolomitic sandstone; 

medium-gray, fine-grained limestone with intraformational conglomerate; and light-gray, finegrained dolostone

_e Elbrook Limestone (Upper and Middle Cambrian)—Medium-gray, thinly bedded limestone 

interbedded with (1) white, mylonitic marble; 

(2) light-brown, laminated dolostone; and (3) 

thin, calcareous shale to shaly dolostone

 Waynesboro Formation (Lower Cambrian)

_wac Chewsville Member—Interbedded dusky-red shale, 

mudstone, and argillaceous sandstone; light-gray 

sandstone; and light-brown sandy, dolomitic 

limestone and dolostone

_wact Cavetown Member—Interbedded gray, bioturbated 

dolostone, dolomitic limestone, and laminated 

limestone, with a few thin, sandy limestone beds 

near the middle

	_war Red Run Member—Interbedded light-olive-gray 

shale; light-gray, fine-grained sandstone; and 

medium- to dark-gray, sandy, dolomitic limestone

_wa Undivided—Shown in cross section only

_t Tomstown Formation (Lower Cambrian)—Lightto dark-gray, limestone, dolostone, and marble

_td Dargan Member—Lower part is dark-gray, bioturbated dolostone interbedded with intervals of 

dark-gray, laminated dolostone and dark-gray 

limestone. Upper part is dark-gray, bioturbated, 

oolitic dolostone, interbedded with laminated 

limestone and silty dolostone

_tb Benevola Member—Light-gray, very thick bedded 

to massive, sugary dolostone with faint crossbedding. Base is gradational with the underlying bioturbated dolostone of the Fort Duncan 

Member (_tf). Pure and massive dolomite is 

quarried as aggregate

_tf Fort Duncan Member—Dark-gray, thick-bedded, 

burrow-mottled dolostone. Base is probably 

an erosional surface on top of Bolivar Heights 

Member (_tbh)

_tbh Bolivar Heights Member—Dark-gray, fine-grained, 

thin-bedded limestone with wispy dolomitic 

burrows that increase in abundance upsection. Base is a 15-m-thick interval of gray to 

white, mylonitic marble (informally known as 

Keedysville marble bed) interpreted to be a 

regional thrust fault that detached the carbonate rocks from the underlying rocks of the 

Antietam Formation (_ca) (Brezinski and others, 1996)

Blue Ridge Province

_t Tomstown Formation (Lower Cambrian)—Poorly 

exposed medium-light- to medium-gray, saccharoidal dolostone and dolomitic marble containing thin layers of sericite 

_cp Carbonaceous phyllite (Lower Cambrian)—

Medium- to dark-gray, fine-grained muscovitegraphite phyllite. Locally contains alternating 

light- and dark-gray thin beds ranging from 

several mm to 1 cm in thickness. Produces distinctive light-gray soil. Found only as float or as 

small, slumped outcrops in discontinuous lenses 

above Antietam Formation (_ca) near Furnace 

Mountain. Interfingers laterally with dolostone 

of the Tomstown Formation

Chilhowee Group

_ca Antietam Formation (Lower Cambrian)—Lightolive- to olive-gray, medium- to coarse-grained, 

medium-bedded, locally ferruginous, micaceous, 

Description of Map Units    31
3    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

silty metasandstone interbedded with very fine 

grained, silty metasandstone to sandy metasiltstone. Local ferruginous horizons with abundant 

botryoidal hematite and limonite are located near 

contact with overlying Tomstown Formation (_t) 

or carbonaceous phyllite (_cp)

_ch Harpers Formation (Lower Cambrian)—Darkgreenish-gray to brownish-gray, fine- to very 

fine grained, laminated to massively bedded 

chlorite-muscovite-quartz phyllitic metasiltstone. 

Magnetite-rich, sandy metasiltstone, fine-pebble 

conglomerate, and thin-bedded metasandstone 

(_chs) found locally in the western Blue Ridge. 

On Catoctin Mountain, the rock is a phyllite 

with transposed bedding and metamorphic foliation consisting of alternating millimeter-scale 

quartzose and micaceous laminae

_chs Metasandstone—Light-gray to brown, thin-bedded 

metasandstone, with Skolithos linearis burrows. 

Interbedded with metasiltstone of the main body 

of the Harpers (_ch) west of Blue Ridge-Elk 

Ridge near Harpers Ferry, W. Va.

 Weverton Formation (Lower Cambrian)

	_cwo Owens Creek Member—Dusky-blue to dark-gray to 

dark-purplish-gray, very coarse grained quartzite 

and quartz-pebble conglomerate. Poorly sorted, 

thick bedded, with graded beds and crossbeds; 

contains local accumulations of magnetite, 

ilmenite, and pebbles of red jasper, red and purple quartz, and phyllite. Interbedded with poorly 

exposed, dark-greenish-gray, phyllitic metasiltstone. Gradational with overlying Harpers 

Formation 

_cwm Maryland Heights Member—Greenish-gray to lightgray, medium-grained to granular, massive 

quartzite in beds 5 to 10 m thick. Forms topographic ledges but is not well exposed. Includes 

unnamed thick quartzite bed (_cwmq) mapped 

separately on Catoctin Mountain in Maryland 

	_cwb Buzzard Knob Member—Light- to medium-gray, 

fine- to medium-grained, well-sorted, graded, 

variously bedded (crossbedded, thick bedded, 

or massive), vitreous quartzite interbedded 

with light-gray metagraywacke and metasiltstone; locally arkosic where unconformable to 

Mesoproterozoic gneiss on Blue Ridge. White to 

gray, massive to thickly bedded, vitreous quartzite with thin (less than 0.3 m thick) interbeds of 

dark phyllite is restricted to Catoctin Mountain 

in Virginia
contain coarse-grained actinolite, tremolite, and 

chlorite. Two thin bodies mapped within metabasalt unit south of Potomac River

Zmd

Zrd Metarhyolite dike (Neoproterozoic)—White- to 

gray-weathering, aphanitic to fine-grained 

quartz-sericite schist and very fine grained, flinty 

rock with plagioclase-feldspar phenocrysts in a 

felted groundmass consisting of quartz, feldspar, 

biotite, and epidote

 Swift Run Formation (Neoproterozoic)

Zsm Marble—White, pink, and tan, gray- to buff-weathering, medium- to fine-grained, calcitic and 

dolomitic marble with local thin, discontinuous, 

sandy layers

Zsp Phyllite—Grayish-reddish-purple phyllite, grayishgreen finely laminated phyllite, dark-greenishgray to brownish-gray, sandy, sericitic phyllite; 

and medium-dark-gray slate in fining-upward 

sequence. Pinkish-gray to light-brownish-gray, 

fine-grained, dolomitic marble locally found 

near top

Zss Metasandstone and schist—Pinkish- to greenish-gray, 

very coarse to medium-grained, crossbedded 

metasandstone and quartzite containing quartz 

phyllite and sandstone pebbles and cobbles; 

brownish-green chlorite-sericite-feldspar-quartz 

metagraywacke; and lustrous, silvery quartz-sericite schist in fining-upward sequence. Grayishbrown arkosic metasandstone (fossil soil?) locally found at lower contact with basement rocks

Fauquier Group

Zfcr Carter Run Formation (Neoproterozoic)—Lightto medium-gray and olive-gray, light-yellowishgray-weathering, coarse- to fine-grained sericitepotassium feldspar-quartz metasiltstone and 

arkosic metasandstone. Beds range from 0.3 to 

2 m thickness and are defined by fining-upward 

sequences; tangential and trough crossbeds are 

common. Gravel and isolated, cobble-sized 

clasts occur locally. Mapped south of Steptoe 

Hill in Virginia, near southern edge of quadrangle

 Swains Mountain Formation (Neoproterozoic)

Zfsm Metamudstone—Light-gray, light-brown-weathering, 

thinly laminated (less than 2 cm) sericite-quartz 

metamudstone containing very fine quartz sand; 

and lustrous sericitic phyllite. Upper contact 

is gradational with rocks of the Carter Run 

Formation (Zfcr)

Zfsc Boulder conglomerate—Light- to dark-gray, boulder 

and cobble conglomerate and metasandstone 

with biotite-chlorite-sericite-potassium feldspar-quartz matrix. Bedding thickness varies 

from 0.5 to 6 m and is defined by finingupward sequen- ces. Boulders and cobbles 

consist of locally derived Mesoproterozoic 

granite gneiss. Trough crossbeds of coarse- to 

medium-grained sandstone are common. Lower 

contact is an unconformity that has as much as 

100 m relief and is distinct and abrupt; elsewhere, it is obscured within a zone of schist 

that is possibly metamorphosed fossil soil. 

Upper contact is gradational into metasandstone

Zrc Cobbler Mountain Alkali Feldspar Quartz 

Syenite of the Robertson River Igneous Suite 

(Neoproterozoic)—Gray- to buff-weathering, 

medium-grained, massive alkali-feldspar quartz 

syenite. Consists of stubby, euhedral mesoperthite grains (2–4 mm in diameter) intergrown 

with anhedral quartz; amphibole breaks down 

into quartz, plagioclase, and oxides; and minor 

interstitial plagioclase. Mesoperthite crystals 

conspicuous on weathered surface. Locally cut 

by dikes of fine-grained granite of presumably 

coeval magma source. Mapped only in southwestern corner of quadrangle

Ypb Megacrystic metagranite (Mesoproterozoic)—

Light- to medium-gray, medium- to coarsegrained quartz-monzonite gneiss with large, 

pink potassium-feldspar porphyroblasts. Strong 

Paleozoic schistosity is the dominant foliation. 

Mapped only along the southern edge of the 

quadrangle in the southwestern corner

Description of Map Units    33
3    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

Ybg sive to well-foliated biotite-plagioclase-quartzmicrocline metagranite. Characterized by white 

or pink, 1- to 2-cm-long microcline porphyroblasts and aggregates of blue quartz; biotite 

content ranges from 0 to 10 percent. Commonly 

contains pronounced augen texture due to overprinting by Paleozoic schistosity

Ypg
grained, massive to weakly foliated, spotted 

hornblende-orthopyroxene-plagioclase amphibolite gneiss. Contains subordinate sill- or dikelike bodies of metanorite, metadiorite, and hornblende-biotite gneiss

Yp Garnet­graphite paragneiss (Mesoproterozoic)—

Reddish-brown (rusty-weathering), mediumfine- to fine-grained, well-foliated to layered 

graphite-biotite-garnet-plagioclase-quartz gneiss 

and schist. Layering defined by alternating millimeter-scale, garnet-rich zones containing garnets 0.1 to 1 cm in diameter, garnet aggregates, 

and centimeter-scale quartzofeldspathic layers. 

Garnets (almandine) typically deformed and retrograded to green, lensoid clots of fine-grained 

chlorite and muscovite. Graphite occurs as disseminated, small, rounded flakes. Distinctive 

orange-red stain produced by secondary hematite 

after accessory magnetite. Retrograded schistose varieties include quartz-chlorite-magnetite 

schist and carbonaceous phyllonite. Probably 

the remnant of sedimentary rock that predates 

Mesoproterozoic granites

Yq Quartzite and quartz­sericite tectonite 

(Mesoproterozoic)—Light-gray to white, 

fine- to medium-grained, massive quartzite and 

quartz-sericite tectonite. No primary textures 

recognized such as bedding or early metamorphic foliation. Rounded zircons in a quartz 

mylonite are seen in thin section; commonly 

contains strong Paleozoic penetrative cleavage. 

Thin lenses of graphite may be seen in outcrop; 

contains larger mappable pods of paragneiss 

(Yp) as seen along Butchers Branch, east of 

Bluemont, Va.

Piedmont Province

Culpeper and Gettysburg Basins 

Jd Diabase dikes and sills (Early Jurassic)—Mediumto dark-gray, medium crystalline and equigranular, massive diabase with characteristic orangebrown-weathered surface. Derived from several 

magma types as follows: (1) olivine-normative 

(olivine-plagioclase-pyroxene) tholeiitic diabase 

was intruded as dikes (Jd); (2) low-titanium, 

quartz-normative tholeiitic diabase containing 

centimeter-size phenocrystic clusters of calcic 

plagioclase in a fine-grained groundmass of 

pyroxene and plagioclase also was intruded as 

dikes (Jd); and (3) high-titanium, quartz-normative tholeiitic diabase was intruded as differentiated sheets (Jdh). Igneous differentiation of 

unit Jdh produced early-stage, bronzite-bearing 

cumulates (Jdc) lower in the sheets and latestage differentiated bodies higher in the sheets 

such as granophyre (Jdg), ferrogabbro, diorite, 

syenite, and aplite, all of which contain pink 

potassium feldspar, albite, hornblende, biotite, 

and quartz. Dike dimensions vary from a wedge 

edge to more than 150 m wide. Diabase sheets 

exceed 600 m in thickness

J^tm Thermally metamorphosed rocks (Lower Jurassic 

and Upper Triassic)—Variegated hornfels, 

quartzite, marble, meta-arkose, and metaconglomerate occurring as zoned contact aureoles 

adjacent to diabase intrusions. Includes darkgray to olive-black, cordierite-spotted hornfels; bluish-gray epidote and chlorite hornfels; 

white to pinkish-gray tourmaline granofels or 

quartzite; greenish- to purplish-gray epidote 

and chlorite meta-arkose; and white, light-gray, 

and pink, crystalline marble. Contact aureoles 

a few meters thick adjoin narrow dikes, but can 

be more than 213 m thick and as much as 1.6 

km wide adjacent to thick diabase sheets; rocks 

closer to the contacts are hard, brittle, fractured, 

and unweathered; sedimentary structures commonly preserved

Meriden Group

Jms Sander Basalt (Lower Jurassic)—Dark-gray to 

bluish-gray, finely to moderately crystalline, porphyritic to equigranular basalt with plagioclase, 

augite, and pigeonite phenocrysts. Tops of flows 

are locally vesicular and amygdaloidal. Lower 

basalt flows locally separated from stratigraphically higher basalt flows by poorly exposed reddish-brown sandstone and siltstone (Jmss). Both 

the lower and upper basalts are characterized by 

distinctive curved columnar joints locally overprinted by closely spaced fractures. Apparently 

paraconformable with underlying and intercalated sedimentary rocks. Estimated thickness less 

than 300 m. Single occurrences of Jms and Jmss

west of U.S. Route 15, south of Oatlands, Va.

Jmt Turkey Run Formation (Lower Jurassic)—

Reddish-brown and dark-gray sandstone, siltstone, conglomerate, and shale, interbedded in 

cyclic sequences. Sandstone is fine to coarse 

grained, locally pebbly and crossbedded, poorly 

sorted, and micaceous. Siltstone is ripple laminated and contains very fine detrital muscovite. 

Shale is fissile, laminated, fossiliferous, and carbonaceous. Conglomerate is mapped separately 

(Jmtc) and consists of variegated, subrounded 

Description of Map Units    3
36    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

boulders, cobbles, and pebbles of metabasalt, 

quartzite, marble, quartz, and subangular basalt 

clasts derived from the Goose Creek Member 

(J^ccg) of the Catharpin Creek Formation. 

Cycles consist of meter-scale, coarser layers 

alternating with finer grained layers. Unit is 

deeply weathered and poorly exposed. Mapped 

west of U.S. Route 15, south of Oatlands, Va.

Jmh Hickory Grove Basalt (Lower Jurassic)—Mediumto dark-gray, finely to moderately crystalline, 

microporphyritic to equigranular basalt with plagioclase, augite, and pigeonite phenocrysts. Tops 

of flows are locally vesicular and amygdaloidal; 

vugs are filled by zeolites, calcite, and prehnite. 

Consists of two or three separate flows of basalt 

(Jmh) locally separated by poorly exposed, 

reddish-brown sandstone and siltstone (Jmhs). 

Unit is locally disconformable (but regionally 

paraconformable) to underlying and overlying 

sedimentary rocks. Mapped along U.S. Route 15 

around Oatlands, Va.

Jmm Midland Formation (Lower Jurassic)—Reddishbrown and light- to dark-gray siltstone, sandstone, and shale (Jmm) and conglomerate (Jmmc) 

interbedded in cyclic sequences. Siltstone is 

micaceous and commonly ripple laminated; 

locally bioturbated, calcareous, carbonaceous, 

and fossiliferous. Sandstone is fine to coarse 

grained, locally pebbly, crossbedded and ripplelaminated, and feldspathic. Shale is silty, microlaminated, burrowed (with dessication cracks), 

carbonaceous, pyritic, calcareous, and fossiliferous. Lenticular deposits of variegated cobble 

and pebble conglomerate and coarse-grained 

conglomeratic, arkosic sandstone locally are 

near border fault of both basins. Unit is poorly 

exposed but is paraconformable to basalt formations above and below. Mapped east of Bull Run 

Mountain fault, south of Leesburg, Va.

Chatham Group

Jcm Mount Zion Church Basalt (Lower Jurassic)—

Dark-gray to black, aphanitic to very finely 

crystalline, microporphyritic, high-titanium, 

quartz-normative tholeiitic basalt. Contains 

phenocrysts of augite and plagioclase. Tops of 

flows are vesicular and contain amygdules filled 

by calcite, zeolites, and prehnite. Weathers to 

reddish-brown or gray saprolite in uplands. Unit 

is poorly exposed but probably conformable or 

paraconformable to underlying and overlying 

sedimentary rocks. Mapped east of U.S. Route 

15, south of Oatlands, Va.

J

J^ccg Goose Creek Member—Variegated, lenticular 

body of conglomerate and interbedded reddish-brown to grayish-green, pebbly sandstone. 

Conglomerate is thick bedded to massive, with 

subrounded pebbles and cobbles of mainly 

quartzite, metabasalt, metasiltstone, gneiss, vein 

quartz, and limestone in medium- to coarsegrained, arkosic and calcitic sandstone matrix. 

Sandstone is fine to coarse grained, poorly 

sorted, medium to thick bedded, micaceous, 

arkosic, pebbly, and silty. Deeply weathered to 

thick, orange-brown saprolite mantled by lag 

gravel. Intertongues laterally into main body 

of Catharpin Creek Formation. Mapped east of 

U.S. Route 15, south of Leesburg, Va.

 Bull Run Formation (Upper Triassic)

^cbg Groveton Member—Light- to dark-gray, light-greenish-gray, and black, thin-bedded to laminated, 

locally ripple marked, mud-cracked, calcareous and dolomitic, sparsely fossiliferous, silty 

and sandy shale; interbedded with dusky-red, 

thin-bedded, calcareous, micaceous, feldspathic, 

bioturbated, clayey and sandy siltstone in cyclic 

sequences, 3 to 9 m thick. Grades northward into 

fluvial and deltaic sandstones and siltstones, but 

is partly repeated by faulting. Laterally equivalent to Balls Bluff Member (^cbb). Mapped 

along Broad Run and to its west in the southern 

part of the quadrangle

	^cbb Balls Bluff Member—Reddish-brown, fine- to medium-grained, thin- to medium-bedded, locally 

crossbedded, feldspathic, silty sandstone interbedded with dusky-red, thin-bedded, calcareous, 

micaceous, feldspathic, bioturbated, clayey and 

sandy siltstone in repetitive sequences, 1 to 3 

m thick. Intertongues laterally with carbonate 

conglomerate of Leesburg Member (^cbl) to 

the northwest and with the Groveton Member 

(^cbg) to the south
^cbl Leesburg Member—Variegated, light-gray-weathering, crudely bedded carbonate conglomerate 

containing conspicuous subangular to subrounded boulders, cobbles, and pebbles of grayish and 

reddish lower Paleozoic limestone and dolostone 

in a matrix of reddish-brown, pebbly sandstone 

and calcareous, sandy siltstone. Intercalations of 

calcareous sandstone and siltstone thicken to the 

southeast where they intertongue with the main 

body of sandstone and siltstone of the Balls 

Bluff Member (^cbb)

 Manassas Sandstone (Upper Triassic)

^cmp Poolesville Member—Predominantly gray, pinkishgray, and reddish-brown, fine- to coarse-grained, 

thick-bedded, arkosic, micaceous sandstone; 

locally pebbly and crossbedded where it fills 

channels; commonly interbedded with reddishbrown, calcareous siltstone in upward-fining 

sequences in upper part of unit. Thickness is as 

much as 900 m. Gradational and intertonguing 

contacts with overlying and underlying units

^cmt Tuscarora Creek Member—Light- to dark-gray and 

light-red, variegated conglomerate composed 

of very fine to very coarse grained, angular to 

subangular pebbles and cobbles of limestone and 

dolostone within a matrix consisting chiefly of 

limestone and dolostone granules and dusky-red 

to grayish-red clayey sand and silt with calcite 

cement. Limestone and dolostone clasts derived 

from Cambrian and Ordovician strata

^cmr Reston Member—Light-gray to pinkish-gray, variegated pebble, cobble, and boulder conglomerate 

containing clasts of phyllite, schist, quartzite, 

metagraywacke, and quartz in a poorly sorted, 

coarse-grained, arkosic sandstone matrix. 

Locally interbedded with pale-reddish-brown 

sandstone and siltstone. Basal conglomerate 

unconformably overlies metasedimentary rocks 

of the central and eastern Piedmont. Mapped as 

very small or thin bodies west of and south of 

Sugarloaf Mountain and around Seneca, Md.

Frederick Valley Synclinorium

 Grove Formation (Lower Ordovician and Upper 

Cambrian)

O_gu Upper member—Thickly interbedded medium-lightgray, locally sandy, thrombolitic and stromatolitic, algal limestone; medium-gray, laminated, 

dolomitic limestone; and olive-gray dolostone

O_gl Lower member—Medium-light- to medium-gray, 

thick-bedded and crossbedded, sandy limestone; 

and sandy dolomitic limestone. Limestones contain 0.3-m-thick interbeds of medium-light-gray 

sandy dolostone 

 Frederick Formation (Upper Cambrian)

_fl Lime Kiln Member—Dark-gray, fine-grained limestone and calcareous shale. Interbedded with 

fine-grained, thinly laminated to thin-bedded 

and medium-bedded limestone near the base. 

More thickly interbedded toward the top with 

medium-dark-gray, fine-grained, wavy-bedded 

limestone locally containing stromatolitic 

algal beds. Near the top, interbedded with 

medium-light-gray, crossbedded, sandy limestone

_fa Adamstown Member—Medium-dark- to dark-gray, 

fine-grained, argillaceous limestone thinly interbedded with dusky-yellow to medium-dark-gray, 

silty dolostone. Several dark-greenish-gray to 

greenish-black, light-olive-brown-weathering, 

silty, calcareous shale intervals 2 to 15 m thick 

are present throughout the member 

_fr Rocky Springs Station Member—Dark-gray, nodular 

to lumpy-bedded, argillaceous, dolomitic limestone at the base of the Frederick Formation. 

Contains an interval of grayish-black, platy shale 

(_frs) as much as 20 m thick that is mapped 

along the eastern flank of the Frederick Valley. 

Upsection becomes dark-gray, laminated to 

flaggy-bedded limestone with dusky-yellow to 

light-olive-gray, silty, dolomitic partings and 

laminations, and intervals as much as 10 m thick 

of medium-dark-gray, polymictic breccia that 

grade upsection into medium-gray, planar-bedded, arenaceous limestone. Clast sizes range 

from sand size to 0.5 m in diameter on the western side of the Frederick Valley and diminish to 

less than 1 cm in diameter on the eastern side of 

the Frederick Valley

_f Undivided—Light-gray limestone. Single occurrence 

along Potomac River just east of the U.S. Route 

15 bridge

_cs Cash Smith Formation (Upper and Middle 

Cambrian)—Grayish-black, fissile, platy, 

cleaved shale and slate, grading upward into 

thin-bedded, calcareous shale with limestone 

nodules. Mapped only along northern edge of 

quadrangle in an anticlinal ridge adjacent to 

Israel Creek, near Walkersville, Md.

Description of Map Units    3
3    Geologic Map of the Frederick 30’ × 60’ Quadrangle, Maryland, Virginia, and West Virginia

 

_Zsb Metabasalt—Dark greenish- to medium-bluish-gray, 

aphanitic to porphyritic, massive to schistose, 

vesicular metabasalt composed of chlorite, epidote, quartz, altered plagioclase, actinolite, hornblende, and albite. Igneous texture is locally preserved. Pods of epidosite are common. Includes 

some metaconglomerate composed of metabasalt 

pebbles and cobbles. Contains local pillow structures and hyaloclastite

_Zsf Felsic schist—Medium-bluish-gray, grayish-blue, 

very pale blue, and light-olive-gray, fine-grained, 

tuffaceous, fragmental, metavolcanic and 

metavolcaniclastic quartz-muscovite-feldspar 

schist of intermediate composition. Locally 

interbedded with metabasalt. Only one small 

occurrence mapped with muscovitic phyllite 

unit (_Zsmp) southwest of the intersection of 

Maryland State Routes 75 and 80 on either side 

of Fahrney Branch

_Zsm Marble—Brownish-gray to grayish-reddish-purple, 

massive to thin-bedded, calcitic and dolomitic 

marble containing quartz sand. Includes minor 

calcareous metasiltstone

_Zstp Tuffaceous phyllite—Variegated grayish-reddishpurple and bluish-gray, vesicular phyllite with 

light-gray streaks and blebs

_Zsmp Muscovitic phyllite—Light-bluish-gray, dusky-yellow, and medium-orangish-pink muscovite-chlorite phyllite containing albite porphyroblasts, 

quartz, and hematite dust. Contains minor metasiltstone. Lithologically distinct from rocks of 

the Ijamsville Phyllite and Marburg Formation 

by its lighter color

_Zshp Hematitic phyllite—Bluish-purple, hematite-rich 

phyllite. Resembles Ijamsville Phyllite

_Zsl Metalimestone—Light-gray, thinly layered, argillaceous metalimestone 

_Zsqp Quartzite interbedded with phyllite—Light-gray 

quartzite interbedded with purple phyllite and 

slate; with variegated, conglomeratic phyllite; 

and with bluish-gray, tuffaceous phyllite

_Zsq Quartzite—Light-gray to grayish-green, mediumgrained, thin-bedded to massive, crossbedded quartzite and minor calcareous sandstone 

containing detrital plagioclase, orthoclase, and 
polymictic quartz. Bedding is defined by concentrations of heavy minerals. Interbedded with 

other units of the Sams Creek Formation, including phyllite (_Zsmp), metabasalt (_Zsb), and 

metasiltstone and metagraywacke (_Zss) 

_Zss Metasiltstone—Metasiltstone, phyllite, quartzite, and 

metagraywacke, undifferentiated. Contains other 

units of the Sams Creek Formation, including 

marble (_Zsm) and quartzite (_Zsq), and appears 

to overlie the metabasalt unit (_Zsb). Bedding 

can be recognized except where transposed 

in shear zones adjacent to faults. Muscovitic 

phyllite containing albite porphyroblasts and 

elongated blebs of chlorite is interpreted to be 

a metatuff. Locally, light-brown metasiltstone 

is interbedded with quartzite and calcareous 

metasandstone

_Zsc Calcareous metasandstone—Light-brown and gray, 

cross-stratified, calcareous metasandstone and 

quartzite. Mapped in two small areas just south 

of Bush Creek near Monrovia, Md.

 Ijamsville Phyllite (Lower Cambrian? and 

Neoproterozoic?)

_Zip Phyllite—Dusky-blue, grayish-blue, very dusky 

reddish-purple, and greenish-gray to pale-olive 

phyllite; phyllonite containing abundant pods 

and folded stringers of white vein quartz; and 

minor slate. Intensely folded and sheared. Finely 

laminated beds seen only in slate. Phyllite consists mostly of muscovite and chlorite, but also 

contains paragonite and chloritoid. Paragonite 

(determined by X-ray diffraction) gives unit a 

lustrous sheen. Dark coloration is caused by 

abundant hematite dust

_Zib Metabasalt—Dark-green and schistose metabasalt 

composed of chlorite, epidote, and quartz. Rare 

pillows locally preserved. Mapped as thin unit 

east of Martic thrust fault contact with Urbana 

Formation (_u); one small body mapped in fault 

slice near confluence of Potomac and Monocacy 

Rivers

_Ziq Quartzite—Yellowish-gray, fine- to mediumgrained, massive sericitic quartzite locally 

intervening between phyllite (_Zip) and metabasalt (_Zib). Mapped as very thin units east 

of the Martic thrust fault contact with Urbana 

Formation (_u)

_Zim Marble—Light-olive-gray, sandy limestone and 

dusky-red, calcareous quartzite occurs as three 

very small pods within phyllite (_Zip) on the 

southeast limb of the Sugarloaf Mountain anticlinorium

_Zil Metalimestone—Light-bluish-gray to medium-lightgray, thin-bedded, laminated, carbonaceous and 

argillaceous metalimestone; and minor, medium-dark-gray, finely laminated, carbonaceous 

phyllite. Thin unit mapped from confluence of 

Potomac and Monocacy Rivers for about 5 km 

to the southeast. Small body located above flood 

plain of Linganore Creek, east of Frederick, Md.

_Zic Phyllitic conglomerate—Light-olive-gray muscovitic 

phyllite clasts supported in a dark-gray matrix of 

chlorite, quartz, ilmenite, magnetite, and zircon. 

Occurs just north of Bush Creek to the west of 

Monrovia, Md.

_Zicm Conglomeratic metagraywacke—Light-gray and 

green, medium- to coarse-grained metagraywacke with white quartz pebbles and variegated 

phyllite chips, in a green, chloritic matrix. Found 

within chloritic phyllite unit (_Zicp)

_Zicp Chloritic phyllite—Light-olive-gray and greenishgray chloritic phyllite and metasiltstone

 Marburg Formation (Lower Cambrian? and 

Neoproterozoic?)

_Zmbs Metasiltstone—Greenish-gray to light-olive-gray, 

quartz-sericite-chlorite phyllitic metasiltstone 

containing thin (0.25 cm) light-gray quartz laminae and ribbons; and dusky-blue, grayish-blue, 

dusky-reddish-purple, and greenish-gray to paleolive muscovite-chlorite-paragonite-hematite 

phyllite similar to that of Ijamsville Phyllite 

(_Zip). Porphyroblasts of albite and chloritoid 

occur locally. Much of unit is transposed, phyllonitized, and has abundant pods and folded 

stringers of white vein quartz

_Zmbg Metagraywacke—Grayish-green to black, massive graywacke interbedded with dark phyllite. 

Mapped in southernmost area of Marburg

_Zmbb Metabasalt—Dark-greenish-gray to dusky-yellowish-green, aphanitic to porphyritic metabasalt 

containing epidosite. Only one small body in 

quadrangle mapped along tributary to Bucklodge 

Branch south of Barnesville, Md.

_Zmbq Quartzite—Light-gray to grayish-green, coarsegrained, massive quartzite

Description of Map Units    3
_Zp Prettyboy Schist (Lower Cambrian? and 

Neoproterozoic?)—Dark-greenish-gray quartzmuscovite-chlorite-albite schist with white, 

euhedral albite crystals

_Zpb Metabasalt—Dark-green, massive, porphyritic metabasalt composed of chlorite, epidote, and quartz. 

Located only in northeastern corner of quadrangle

_Zpq Quartzite—Brown, medium-grained quartzite interbedded with quartz-muscovite-chlorite schist. 

Single occurrence on map along Interstate Route 

26, just east of intersection with Maryland State 

Route 97

Potomac Terrane

Dp Pegmatite (Late Devonian)—Light-grayLight-gray and white 

muscovite-microcline-albite-quartz pegmatite. 

Locally contains minor amounts of biotite, 

tourmaline, allanite, monzonite, magnetite, and 

pyrite. Most of the microcline is microperthitic. 

Crosscuts schistosity and foliation of the country 

rocks and is not deformed

Dg Guilford Granite (Late Devonian)—Gray, medium- to fine-grained, nonfoliated, homogeneous 

granite of monzogranite composition. About 90 

percent of the rock consists of equal parts of 

microcline, plagioclase, and quartz; remainder 

consists of equal amounts of biotite and muscovite and minor amounts of apatite, garnet, clinozoisite, zircon, and magnetite. Forms thin sheets 

that crosscut bedding and schistosity in the 

Northwest Branch Formation (_Zn). Two small 

bodies mapped on north side of Burnt Mills fault 

along eastern edge of quadrangle

 Oq Quartz bodies (Ordovician)—White, massive, and 

fractured quartz probably intruded as large veins 

which are now preserved as smaller remnants. 

Found scattered throughout map area east of 

Pleasant Grove fault

 Norbeck Intrusive Suite (Late Ordovician)

Ont Biotite-hornblende tonalite—Gray, medium- to 

coarse-grained, fairly massive to foliated biotitehornblende tonalite consisting largely of quartz, 

andesine, hornblende, and biotite; and lesser 

chlorite, epidote, apatite, and magnetite. In many 

places, has a strong relict-igneous-flow structure. 

Contains many xenoliths and (or) autoliths of 

more mafic rock. Petrographic and chemical data 

are given in Hopson (1964)

Ong Quartz gabbro—Dark-gray, medium-grained quartzaugite-hornblende metagabbro. Forms small 

bodies within biotite-hornblende tonalite (Ont) 

Onu Ultramafic rocks—Dark-green, well-foliated serpentinite and lesser soapstone. Probably altered 

from pyroxenite. Forms three small bodies within biotite-hornblende tonalite (Ont) 

Ontr Trondhjemite—Light-gray, medium-grained muscovitic trondhjemite. One small body mapped 

within biotite-hornblende tonalite southeast of 

Olney, Md.

Ok Kensington Tonalite (Middle Ordovician)—

Medium-gray, coarse-grained, weakly to moderately foliated, locally garnetiferous, muscovitebiotite tonalite. In and west of the Rock Creek 

shear zone and other sheared areas, the rock is 

intensely foliated to mylonitic muscovite-biotite 

granodiorite containing augen and coarse porphyroblasts of microcline that suggest metasomatism during the shearing event. Petrographic 

and chemical data are given in Hopson (1964) 

and Fleming and others (1994)

 Georgetown Intrusive Suite (Early Ordovician)

Ogh Biotite-hornblende tonalite—Gray, medium- to 

coarse-grained, massive to foliated biotite-hornblende tonalite that has a strong relict-igneousflow structure at many places. Unit contains 

many ultramafic and mafic xenoliths and (or) 

autoliths, and xenoliths of metasedimentary 

rocks. Typically contains 40 to 50 percent dark 

minerals. Petrographic and chemical data are 

given in Drake and Froelich (1997) and Fleming 

and others (1994). Mapped along southern edge 

of quadrangle

Ogg Quartz gabbro—Dark-gray, mostly medium- to 

coarse-grained quartz-augite-hornblende 

metagabbro, lesser quartz diorite, and much lesser quartz norite. In many places, the unit contains thin cumulate layers of metapyroxenite and 

augite-hornblende rock. Petrographic and chemical data are given in Fleming and others (1994). 

Mapped along southern edge of quadrangle

Ogp Metapyroxenite—Dark-green to black, medium- to 

coarse-grained, massive to well-foliated metapyroxenite occurring within metagabbro (Ogg). 

Much of the rock has been altered to dark-green 

to grayish-green serpentinite. Unit forms small 

pods and xenolith swarms within or along the 

borders of larger tonalite (Ogh) and quartz gab-
bro (Ogg) plutons. Mapped along southern edge 

of quadrangle

 Dalecarlia Intrusive Suite (Early Ordovician)

Odm Biotite monzogranite and granodiorite—Mediumgray, medium- to coarse-grained, massive to 

well-foliated biotite monzogranite and lesser 

granodiorite. Locally contains plagioclase phenocrysts. Mapped bodies contain widespread 

lenses, zones, and irregular bodies of leucocratic 

muscovite-biotite monzogranite (Odl). Two 

small bodies are mapped, one within biotitehornblende tonalite (Ont) where it is completely 

enclosed by muscovitic trondhjemite (Odt) and 

one at the southern edge of the quadrangle west 

of Maryland State Route 187

Odl Leucocratic muscovite-biotite monzogranite—Lightgray, leucocratic, medium- to coarse-grained, 

foliated muscovite-biotite monzogranite. Locally 

grades into monzogranite (Odm) with which it 

commonly occurs. Mapped only at southern 

edge of quadrangle west of Maryland State 

Route 355

Odt Muscovitic trondhjemite—Light-gray, fine- to medium-grained, sugary textured, massive to weakly 

foliated muscovitic trondhjemite dikes, sheets, 

and irregular bodies. Petrographic and chemical data are given in Drake and Fleming (1994). 

Mapped in one location within biotite-hornblende tonalite (Ont) where it encloses a small 

body of biotite monzogranite and granodiorite 

(Odm) east of Maryland State Route 97, south of 

Olney, Md.

Ob Bear Island Granodiorite (Ordovician?)—Lightgray, fine-grained muscovite-biotite granodiorite and related pegmatite composed largely of 

quartz, albite, and microcline. Forms small- to 

medium-size, crosscutting sheets and bodies in 

rocks of the Mather Gorge Formation

_s Sykesville Formation (Lower Cambrian)—

Variegated, gray metamorphosed sedimentary 

mélange consisting of a quartzofeldspathic 

granofels matrix containing quartz and feldspar 

grains, and fragments and bodies of metamorphosed sedimentary, volcanic, and intrusive 

rocks

_l Laurel Formation (Lower Cambrian)—Light- to 

medium-gray, medium- to coarse-grained, moderately to well-foliated sedimentary mélange 

consisting of a quartzofeldspathic matrix 

that contains elliptical quartz fragments and 

fragments of meta-arenite and biotite schist. 

Typically spangled with very large porphyroblasts of muscovite. Upper part (_lu) contains 

more than 50 percent olistoliths of meta-arenite 

and muscovite-biotite schist that are locally as 

long as 5 to 10 m. Located only in the southwestern corner of the quadrangle

_Zo Oella Formation (Lower Cambrian and (or) 

Neoproterozoic)—Gray, fine- to mediumgrained quartz-plagioclase-biotite-muscovite 

meta-arenite interbedded with lesser schist. 

Mapped adjacent to Loch Raven Schist (_Zl) 

south of Triadelphia Reservoir in Maryland

_Zl Loch Raven Schist (Lower Cambrian and (or) 

Neoproterozoic)—Medium-gray, medium- to 

coarse-grained, thin-bedded, lustrous quartzmuscovite-biotite-plagioclase schist that, in places, contains garnet, staurolite, and (or) chlorite. 

Contains some interbedded semipelitic schist 

and meta-arenite (_Zlm) similar to those of the 

underlying Northwest Branch Formation (_Zn)

_Zn Northwest Branch Formation (Lower Cambrian 

and Neoproterozoic)—Light-gray, mediumgrained, well-bedded quartz-plagioclase-biotite 

meta-arenite and lesser quartzite and calc-silicate rock. Both the meta-arenite and quartzite 

have a good schistosity marked by biotite flakes. 

Beds range from 1.3 cm to 1.2 m in thickness. 

Contains interbedded schist similar to that of the 

overlying Loch Raven Schist (_Zl); upper contact grades into overlying Loch Raven Schist

 Mather Gorge Formation (Lower Cambrian and 

(or) Neoproterozoic)

_Zms Quartz-rich schist—Greenish-gray to gray, reddishbrown-weathering, very fine grained, lustrous 

quartz-muscovite-chlorite-plagioclase-epidotemagnetite-hematite schist containing interlayers 

of phyllitic metasiltstone, muscovitic gneiss, 

metagraywacke, and calc-silicate rock. Thin to 

very thin beds are largely transposed into early 

cleavage and contain folded white quartz veins 

_Zmg Metagraywacke—Light- to medium-gray, yellowishto reddish-brown-weathering, fine- to mediumgrained, generally well-bedded metagraywacke 

and semipelitic schist. Beds range from about 3 

cm to as much as 3 m thick, averaging about 20 

cm. Many beds are graded; sole marks and slump 

features are abundant. Contains interbedded 

quartzose schist and lenses of calc-silicate rock 

Description of Map Units    1
_Zmm Migmatite—Light-gray quartz-plagioclase leucosome 

and dark-gray, quartz-rich schist 

_Zmp Phyllonite—Greenish-gray, chlorite-sericite phyllonite containing white vein quartz. Highly 

folded

_Zmt Metatuff—Dark-gray, hornblende-plagioclase-quartz- Dark-gray, hornblende-plagioclase-quartz- Dark-gray, hornblende-plagioclase-quartzmuscovite, felsic, schistose metatuff. Only one metatuff. Only one

small body is mapped near the southern edge of 

the quadrangle in Great Falls, Va.

_Zsd Soldiers Delight Ultramafite (Lower Cambrian 

and (or) Neoproterozoic)—Green serpentinite, talc schist, and soapstone. Interpreted to 

be mostly altered pyroxenite. Mapped adjacent 

to Loch Raven Schist north of Triadelphia 

Reservoir in Maryland

_Zu Mafic and ultramafic rocks (Lower Cambrian 

and Neoproterozoic)—Greenish-gray serpentinite, soapstone, and talc schist that occur as 

large bodies or smaller blocks within rocks of 

the Mather Gorge Formation

_Zg Metagabbro and metapyroxenite (Lower Cambrian and Neoproterozoic)—Dark-green and 

black metagabbro and metapyroxenite that 

occur as two small blocks within rocks of the 

Mather Gorge Formation. The larger body is 

located at the headwaters of Cattail Creek south 

of Interstate Route 70 in Howard County, Md.; 

the smaller body is located along Great Seneca 

Creek west of Gaithersburg, Md.

Coastal Plain Province

Kps Potomac Formation (Lower Cretaceous)—

Unconsolidated, compact, well-sorted, stratified 

gravel, sand, silt, and clay that unconformably 

overlies rocks of the Laurel Formation (_l) in 

the extreme southeastern corner of the map area 

near Silver Spring, Md.