OFR 02-437 Photogallery.This image links to the USGS home page.

Sheet 1 PDF file

Sheet 2 PDF file

 

The geologic map of the Frederick quadrangle uses colors to portray the distribution of different rocks and surficial deposits. Because the map units are classified by rock type, age, and formation, the resulting map (above) is very complex. Because some rock types, such as quartzite, have many different ages and different formational names, there are as many as 12 different map units of this similar type of rock. To help illustrate what the colors on a geologic map represent, we have reclassified the general types of bedrock and surficial materials, provided an index map of their distribution, and show photographs that are representative of the general rock type. 

Bedrock units are classified as sedimentary, metamorphic (metasedimentary, met-igneous and metavolcanic), and igneous rocks (plutonic and volcanic), and miscellaneous rock types. Surficial units of colluvium, alluvium, terrace deposits, lag gravel, and boulders are shown as well as sinkholes. And lastly, photographs that represent the unique landscape of the various physiographic provinces and sections are provided.

BEDROCK

I. Metasedimentary Rocks

Phyllite, Phyllonite, and Slate

Phyllite, Phyllonite, and Slate thumbmail image

 

Phyllite

thumbmail image Variegated phyllite clasts form a conglomerate within the tuffaceous phyllite unit of the Sams Creek Formation (CZsctp), near Libertytown, central Piedmont, Frederick Co., Md.

thumbmail image Folded, laminated phyllite of the Loudoun Formation (Clp), near Furnace Mountain, eastern Blue Ridge, Loudoun Co., Va.

Phyllonite

thumbmail image Phyllonitic rocks of the Mather Gorge Formation (CZmp) along the Pleasant Grove fault, Seneca Creek, central Piedmont, Montgomery Co., Md.
thumbmail image Phyllonitic rocks of the Marburg Formation (CZmbg), near Mount Airy, central Piedmont, Carroll Co., Md.
thumbmail imagePhyllonitic rocks of the Mather Gorge Formation (CZmp) along the Pleasant Grove fault, Seneca Creek, central Piedmont, Montgomery Co., Md.

Slate

thumbmail image Slate of the Ijamsville Phyllite (CZi) was quarried for roofing material, near Hyattstown and Hope Hill, central Piedmont, Frederick Co., Md.

thumbmail image Slate of the Ijamsville Phyllite (CZi) was quarried for roofing material, near Hyattstown and Hope Hill, central Piedmont, Frederick Co., Md.

Metagraywacke, Quartzite, and Metasandstone

Metagraywacke, Quartzite,  and Metasandstone thumbmail image

 

Metagraywacke

thumbmail image Thin-bedded and cross-bedded metagraywacke of the Swain Mountain Formation (Zsm) along Goose Creek, eastern Blue Ridge, Loudoun Co., Va.

thumbmail image Thin- to medium-bedded and graded metagraywacke of the Mather Gorge Formation (CZmg) along the Potomac River, eastern Piedmont, Fairfax Co., Va.

Quartzite

thumbmail image Upper Member of the Sugarloaf Mountain Quartzite (Csqu) on the summit of Sugarloaf Mountain, central Piedmont, Frederick Co., Md.

thumbmail image Cross-bedded quartzite of the Upper Member of the Sugarloaf Mountain Quartzite (Csqu), north of the summit of Sugarloaf Mountain, central Piedmont, Frederick Co., Md.
thumbmail imageCross-bedded quartzite of the Sams Creek Formation (CZscq) south of Monrovia, central Piedmont, Frederick Co., Md.
thumbmail imageQuartzite containing clasts of hematite phyllite of the Sams Creek Formation (CZscq) near Lake Linganore, central Piedmont, Frederick Co., Md.
thumbmail image Clean quartzite of the Buzzard Knob Member of the Weverton Formation (Cwb), Rt. 340 cut through South Mountain, Blue Ridge, Frederick and Washington Counties, Md.
thumbmail image Conglomeratic quartzite of the Owens Creek Member of the Weverton Formation (Cwo), Blue Ridge, Short Hill Mountain, Loudoun Co., Va.

Metasandstone

thumbmail imageFine-grained sandstone with skolithus linearis (vertical worm burrows), Antietam Formation (Ca), along the Shenandoah River, western Blue Ridge, Clarke Co., Va.
thumbmail imageFine-grained, thin-bedded sandstone with skolithus linearis (vertical worm burrows) of the Harpers Formation (Chs), is transitional into the overlying Antietam Formation, near Harpers Ferry, western Blue Ridge, Washington Co., Md.
thumbmail imageMassive, fine-laminated meta-arenite of the Northwest Branch Formation (CZn), near Silver Spring, eastern Piedmont, Montgomery Co., Md.

Metasiltstone, Schist

Metasiltstone,  Schist thumbmail image

 

Metasiltstone

thumbmail imageFinely laminated beds of metasiltstone perpendicular to cleavage in rocks of the Urbana Formation (Cu), east of New Market, central Piedmont, Frederick Co., Md.
thumbmail image Highly cleaved metasiltstone of the Harpers Formation (Ch) is folded with inclined spaced cleavage, western Blue Ridge near Harpers Ferry, Jefferson Co., W.Va.
thumbmail image Folded and cleaved finely-laminated metasiltstone of the Carter Run Formation (Zcrs) along Goose Creek, eastern Blue Ridge, Loudoun Co., Va.

Schist

thumbmail image Garnetiferous quartz-muscovite-biotite-schist of the Loch Raven Schist (CZl) east of Triadelphia Reservoir, eastern Piedmont, Montgomery Co., Md.
thumbmail image Quartz-sericite schist of the Swift Run Formation (Zss), Blue Ridge, Loudoun Co., Va.

II. Sedimentary Rocks

Conglomerates

Conglomerates thumbmail image

 

Conglomerates

thumbmail imageConglomerate of carbonate clasts in a matrix of red siltstone, Leesburg Member of the Bull Run Formation (Trbl), Culpeper and Gettysburg basins, western Piedmont, Loudoun Co., Va., and Frederick Co., Md.
thumbmail imageConglomerate of gravel-sized vein quartz in a chlorite-rich matrix in the Ijamsville Phyllite (CZic) near Chestnut Grove, central Piedmont, Frederick Co., Md.
thumbmail imageConglomerate of cobbles of vein quartz and red jasper of the Loudoun Formation (Clc), western Blue Ridge near Washington Monument State Park, Frederick Co., Md.
thumbmail image Conglomerate of cobbles of limestone, dolomite, sandstone, and greenstone in a silt matrix of the Goose Creek Member of the Catharpin Creek Formation (Jtrcg) near the Loudoun County Landfill, western Culpeper basin, western Piedmont, Va.
thumbmail imageConglomerate of limestone clasts of the Lime Kiln Member of the Frederick Formation (Cfl) near Greenfield Mills, western Piedmont, Frederick Co., Md.
thumbmail image Boulder conglomerate member of the Swains Mountain Formation (Zsmc) containing rounded boulders of Mesoproterozoic granitic gneiss along Goose Creek, eastern Blue Ridge, Loudoun Co., Va.

thumbmail imageConglomerate composed of phyllite clasts (light) supported in a matrix (dark) of heavy minerals in the Ijamsville Phyllite (CZid), near Monrovia, central Piedmont, Frederick Co., Md.

Siltstone & Shale

Siltstone, Shale thumbmail image

 

Siltstone

thumbmail imageThin-bedded siltstone of the Balls Bluff Member of the Bull Run Formation (Trbb), Bluff National Cemetary, Culpeper basin, Loudoun Co., Va.

Shale

thumbmail imageMassive, well-bedded shale of the Martinsburg Formation (Om), folded with prominent inclined axial plane cleavage, Great Valley of the Valley and Ridge, Jefferson Co., W.Va. Photo by Jack Epstein.

Carbonate Rocks

Carbonate Rocks thumbmail image

 

Limestone

thumbmail image Folded thin-bedded limestone of the Frederick Formation (Cf), Furnace Mountain, Blue Ridge-western Piedmont, Loudoun County, Va.
thumbmail imageFolded, thin-bedded limestone and marble of the Tomstown Formation (Ct), Furnace Mountain, Blue Ridge-western Piedmont, Loudoun County, Va.
thumbmail image Folded, thin-bedded limestone of the Sams Creek Formation (CZscl) west of Woodville, central Piedmont, Frederick Co., Md.
thumbmail imageThin-bedded limestone of the Tomstown Formation near Rohersville, eastern Great Valley, Washington Co., Md. 

Marble

thumbmail imageFoliated crystalline marble of the Tomstown Formation near Keedysville, eastern Great Valley, Washington Co., Md.
thumbmail image Thin- to medium- bedded marble interbedded with phyllite of the Sams Creek Formation (CZscm), near Libertytown, northcentral Piedmont, Frederick Co., Md.
thumbmail imageMassive, non-bedded, marble and blocks of marble in a sandy matrix, Sams Creek Formation (CZscm), excavated near New Market, central Piedmont, Frederick Co., Md.
thumbmail image Fine-grained crystalline marble of the Catoctin Formation (Zcm) along Goose Creek, eastern Blue Ridge, Loudoun Co., Va.
thumbmail image Thin-bedded crystalline marble within phyllite of the Sams Creek Formation (CZscm), near Libertytown, central Piedmont, Frederick Co., Md.

III. Meta-igneous and Metavolcanic Rocks

Greenstone

thumbmail image

 

Greenstone

Greenstone thumbmail imageMetabasalt with epidosite bodies of the Sams Creek Formation (CZscg), west of Fountain Mills, central Piedmont, Frederick Co., Md.
thumbmail image Metabasalt with flattened pillows, Sams Creek Formation (CZscg), near Monrovia, central Piedmont, Frederick Co., Md.
thumbmail image Schistose and well-cleaved greenstone overlies quartzite, Sams Creek Formation (CZscg) along Linganore Creek, east of McKaig, central Piedmont, Frederick Co., Md.
thumbmail image Hyaloclasite, or breccia of metabasalt with nodular epidosite, Sams Creek Formation (CZscg), central Piedmont, Montgomery Co., Md.
thumbmail image Folded, schistose metabasalt of the Catoctin Formation (Zc), near Loudoun Heights, western Blue Ridge, Loudoun Co., Va.

Mafic Igneous Rock

thumbmail image Ultramafic rocks (CZu), like pyroxenite, occur as blocks and large bodies within metagraywacke and schist, eastern Piedmont, Montgomery Co., Md.
thumbmail image Very dense ultramafic rock (CZu) occurs as small blocks within phyllite, eastern Piedmont near Germantown, eastern Piedmont, Montgomery Co., Md.

Metavolcanic

thumbmail image Tuffaceous phyllite with clasts of metarhyolite of the Sams Creek Formation (CZscf) along Linganore Creek, east of McKaig, central Piedmont, Frederick Co., Md.
thumbmail imageWell-foliated metarhyolite and tuffaceous phyllite (Zcr) north of Middletown, Blue Ridge, Frederick Co., Md.
thumbmail imageFelsic tuffaceous schist with clasts of metarhyolite of the Sams Creek Formation (CZscf) near Fountain Mills, central Piedmont, Frederick Co., Md.

Amphibolite

thumbmail imageDark bodies of amphibolite (CZa) occur as blocks within rocks of the Sykesville Formation, Potomac River, eastern Piedmont, Montgomery Co., Md.
thumbmail image Dark bodies of amphibolite (CZa) occur as blocks within rocks of the Sykesville Formation, Potomac River, eastern Piedmont, Montgomery Co., Md.

Metadiabase Dike

thumbmail image Well-cleaved, dark, fine-grained chlorite-actinolite metadiabase (Ymd) intrudes light granitic gneiss along Catoctin Creek, eastern Blue Ridge, Loudoun Co., Va.

Plutonic Rock

Plutonic  Rock thumbmail image

 

Granite Gneiss

thumbmail imageLayered granitic gneiss (Ylg) near Trapp, western Blue Ridge, Loudoun Co., Va.
thumbmail image Porphyroblastic granite gneiss (Ypg) intruded by garnetiferous metagranite  (Ygt) near Howardsville, western Blue Ridge, Loudoun Co., Va. 
thumbmail image Large feldspars of porphyroblastic granite gneiss (Ypg) near Howardsville, western Blue Ridge, Loudoun Co., Va.
thumbmail image Massive, pink, leucocratic metagranite (Yml) near Philomont, Blue Ridge, Loudoun Co., Va.
thumbmail imageFolded, migmatitic(?), layered granitic gneiss (Ylg) south of Howardsville, western Blue Ridge, Loudoun Co., Va.
thumbmail image Dark, massive, well-foliated hornblende monzonite gneiss (Yhm) underlies Loudoun Valley near Neersville, western Blue Ridge, Loudoun Co., Va.
thumbmail imageWell-foliated biotite granite gneiss (Ybg) along Goose Creek near Leithtown, Blue Ridge, Loudoun Co., Va.
thumbmail image Cobbler Mountain Alkali Feldspar Quartz Syenite (Zrc) of the Robertson River Igneous Suite near Paris, western Blue Ridge, Loudoun Co., Va.
thumbmail image Sawed hand specimens of dark hypersthene-bearing charnockite (Yc)(upper) and speckled amphibolite (Ya), western Blue Ridge, Loudoun Co., Va.
thumbmail imageMassive, unfoliated granodiorite of the Dalecarlia Intrusive Suite (Odm), Potomac River, eastern Piedmont, Montgomery Co., Md.
thumbmail image Massive, unfoliated, biotite-hornblende tonalite of the Goergetown Intrusive Suite (Ogh), eastern Piedmont, Potomac River, Montgomery Co., Md.
thumbmail image Migmatitic garnetiferous granite gneiss (Ygt) near the Potomac River, western Blue Ridge, Frederick Co., Md.

Hornfels, Basalt, Diabase

Hornfels, Basalt,  Diabase thumbmail image

 

Hornfels

thumbmail imageHornfels (JTrtm) in Belmont Ridge along the Dulles Greenway, western Piedmont, Loudoun Co., Va. These are thin-bedded siltstone and sandstone (Trbb) altered by contact metamorphism of hot diabase (Jd) that intruded as sills above and below the rocks.
thumbmail imageClose-up of fig 65 shows normal fault (dipping to the lower right) that offsets bedding and disrupts the brittle fracture pattern in the hornfels (JTrtm).

Basalt

thumbmail image Dark, fine-grained Hickory Grove basalt (Jhg), western Culpeper basin,  western Piedmont, near Oatlands, Loudoun Co., Va.

Diabase

thumbmail imageDark, fine-grained diabase (Jd) weathers to boulders that litter the surface that have a distinctive orange-brown, iron-rich weathering rind, Culpeper basin, western Piedmont, Loudoun Co., Va.

Miscellaneous Rock Types

Miscellaneous Rock Types thumbmail image

 

Migmatite

thumbmail imagePhyllonitic schist of the Mather Gorge Formation (CZmp) partially melted with bodies of granodiorite and pegmatite of the Bear Island Granodiorite (Ob), eastern Piedmont, Potomac River, Montgomery Co., Va.

Garnet Graphite Schist (Paragneise)

thumbmail imageRusty weathered, schistose garnet graphite paragneiss (Yp) near Airmont, Blue Ridge, Loudoun Co., Va.
thumbmail image Sawed hand specimen of rusty weathered, schistose garnet graphite paragneiss (Yp), along Catoctin Creek, Blue Ridge, Loudoun Co., Va.

SURFICIAL

Surficial rocks thumbmail image

Colluvium

thumbmail image Fine colluvium (Qcf) of subrounded vein quartz supported in a silty matrix forms aprons along the slope of Catoctin Mountain, western Culpeper basin, western Piedmont, Loudoun Co., Va., and Frederick Co., Md.
thumbmail image Coarse colluvium (Qc) of sub-rounded meter-long boulders of quartzite form block streams in hollows along mountainous areas of the western Blue Ridge, Loudoun Co., Va., and Washington Co., Md.
thumbmail image Coarse, angular blocks of quartzite are coarse colluvium (Qc) and scree on the upper slopes near outcrop on the ridges in the Blue Ridge, Loudoun Co., Va., Jefferson Co., W.Va., and Washington and Frederick Counties, Md.

Alluvium

thumbmail image Stratified alluvium (Qa) along the Potomac River, Culpeper basin, western Piedmont, Frederick Co., Md. Gravel approximately 1 m thick at river level grades up into silt and clay.
thumbmail image Stratified gravel, silt, and clay of a lower level terrace (Qt) of the Potomac River, Culpeper basin,  Loudoun Co., Va. Weathered siltstone bedrock is beneath shovel.

Terrace Deposits

thumbmail image Terrace deposit (QTt) of stratified quartzite clasts and fine matrix on siltstone bedrock in the Culpeper basin, central Piedmont, Loudoun Co., Va. Deposit is about 2 m thick and approximately 4 km from the present Potomac River.

Lag Gravel and Boulders

thumbmail image Lag gravel (Ql) of vein quartz, derived from weathering of the conglomerate of the Reston Member of the Manassas Sandstone, eastern Culpeper basin, near Reston, Va., central Piedmont, Fairfax Co., Va.
thumbmail image Lag gravel (Ql) of vein quartz, derived from weathering of the conglomerate of the Reston Member of the Manassas Sandstone, eastern Culpeper basin, near Poolesville, central Piedmont, Montgomery Co., Md.
thumbmail image Large boulder (see foot long hammer on top) of quartzite approximately 3 km from the Potomac River, eastern Culpeper basin, Loudoun Co., Va., that was transported by the ancestral Potomac River.

Sinkholes

thumbmail image Regolith of limestone of the Frederick Formation (Cf), Frederick Valley, western Piedmont, Frederick Co., Md., has been stripped during quarry operations to reveal pinnacles around sinkholes.

thumbmail image Sinkhole developed in regolith of limestone of the Frederick Formation, Frederick Valley, western Piedmont, Frederick Co., Md.

Landscape

thumbmail image Great Falls of the Potomac River as seen from Great Falls Park, Fairfax Co., Va. The 25 m falls are on resistant, massive metagraywacke of the Mather Gorge Formation, eastern Piedmont.
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