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
Variegated phyllite clasts form a conglomerate within the tuffaceous
phyllite unit of the Sams Creek Formation (CZsctp), near Libertytown,
central Piedmont, Frederick Co., Md.
Folded, laminated phyllite of the Loudoun Formation (Clp), near Furnace
Mountain, eastern Blue Ridge, Loudoun Co., Va.
Phyllonite
Phyllonitic rocks of the Mather Gorge Formation (CZmp) along the Pleasant
Grove fault, Seneca Creek, central Piedmont, Montgomery Co., Md.
Phyllonitic rocks of the Marburg Formation (CZmbg), near Mount Airy,
central Piedmont, Carroll Co., Md.
Phyllonitic rocks of the Mather Gorge Formation (CZmp) along the Pleasant
Grove fault, Seneca Creek, central Piedmont, Montgomery Co., Md.
Slate
Slate of the Ijamsville Phyllite (CZi) was quarried for roofing material,
near Hyattstown and Hope Hill, central Piedmont, Frederick Co., Md.
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
Thin-bedded
and cross-bedded metagraywacke of the Swain Mountain Formation (Zsm) along
Goose Creek, eastern Blue Ridge, Loudoun Co., Va.
Thin- to medium-bedded
and graded metagraywacke of the Mather Gorge Formation (CZmg) along the
Potomac River, eastern Piedmont, Fairfax Co., Va.Quartzite
Upper Member of the
Sugarloaf Mountain Quartzite (Csqu) on the summit of Sugarloaf Mountain,
central Piedmont, Frederick Co., Md.
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.
Cross-bedded quartzite of the Sams
Creek Formation (CZscq) south of Monrovia, central Piedmont, Frederick
Co., Md.
Quartzite containing clasts of hematite phyllite of the Sams Creek
Formation (CZscq) near Lake Linganore, central Piedmont, Frederick Co.,
Md.
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.
Conglomeratic quartzite of the Owens Creek Member of the Weverton
Formation (Cwo), Blue Ridge, Short Hill Mountain, Loudoun Co., Va.
Metasandstone
Fine-grained sandstone
with skolithus linearis (vertical worm burrows), Antietam Formation (Ca),
along the Shenandoah River, western Blue Ridge, Clarke Co., Va.
Fine-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.
Massive, fine-laminated meta-arenite
of the Northwest Branch Formation (CZn),
near Silver Spring, eastern Piedmont, Montgomery Co., Md.
Metasiltstone, Schist
Metasiltstone
Finely laminated beds
of metasiltstone perpendicular to cleavage in rocks of the Urbana
Formation (Cu), east of New Market, central Piedmont, Frederick Co., Md.
Highly cleaved metasiltstone of
the Harpers Formation (Ch) is folded with
inclined spaced cleavage, western Blue Ridge near Harpers Ferry, Jefferson
Co., W.Va.
Folded and cleaved finely-laminated metasiltstone of the Carter Run
Formation (Zcrs) along Goose Creek, eastern Blue Ridge, Loudoun Co., Va.
Schist
Garnetiferous quartz-muscovite-biotite-schist of the
Loch Raven Schist (CZl) east of Triadelphia Reservoir, eastern Piedmont,
Montgomery Co., Md.
Quartz-sericite schist of the
Swift Run Formation (Zss), Blue Ridge, Loudoun Co., Va.
II. Sedimentary Rocks
Conglomerates
Conglomerates
Conglomerate 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.
Conglomerate of
gravel-sized vein quartz in a chlorite-rich matrix in the Ijamsville
Phyllite (CZic) near Chestnut Grove, central Piedmont, Frederick Co., Md.
Conglomerate of cobbles of vein quartz and red jasper of the Loudoun
Formation (Clc), western Blue Ridge near Washington Monument State Park,
Frederick Co., Md.
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.
Conglomerate of limestone clasts of the Lime Kiln Member of the Frederick
Formation (Cfl) near Greenfield Mills, western Piedmont, Frederick Co.,
Md.
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.
Conglomerate
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
Thin-bedded siltstone of the Balls
Bluff Member of the Bull Run Formation (Trbb),
Bluff National Cemetary, Culpeper basin, Loudoun Co., Va.Shale
Massive, 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
Limestone
Folded thin-bedded
limestone of the Frederick Formation (Cf), Furnace Mountain, Blue
Ridge-western Piedmont, Loudoun County, Va.
Folded, thin-bedded limestone and
marble of the Tomstown Formation (Ct), Furnace
Mountain, Blue Ridge-western Piedmont, Loudoun County, Va.
Folded, thin-bedded limestone of the Sams Creek Formation (CZscl) west of
Woodville, central Piedmont, Frederick Co., Md.
Thin-bedded limestone of the Tomstown Formation near Rohersville, eastern
Great Valley, Washington Co., Md. Marble
Foliated crystalline marble of the Tomstown Formation near
Keedysville, eastern Great Valley, Washington Co., Md.
Thin- to medium- bedded marble interbedded with phyllite of the Sams Creek
Formation (CZscm), near Libertytown, northcentral Piedmont, Frederick Co.,
Md.
Massive, non-bedded, marble and blocks of marble in a sandy matrix, Sams
Creek Formation (CZscm), excavated near New Market, central Piedmont,
Frederick Co., Md.
Fine-grained crystalline marble of the Catoctin Formation (Zcm) along
Goose Creek, eastern Blue Ridge, Loudoun Co., Va.
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
Greenstone
Metabasalt with epidosite bodies of the Sams Creek Formation (CZscg), west
of Fountain Mills, central Piedmont, Frederick Co., Md.
Metabasalt with flattened pillows, Sams Creek Formation (CZscg), near
Monrovia, central Piedmont, Frederick Co., Md.
Schistose and well-cleaved greenstone overlies quartzite, Sams Creek
Formation (CZscg) along Linganore Creek, east of McKaig, central Piedmont,
Frederick Co., Md.
Hyaloclasite, or breccia of metabasalt with nodular epidosite, Sams Creek
Formation (CZscg), central Piedmont, Montgomery Co., Md.
Folded, schistose metabasalt of the Catoctin Formation (Zc), near Loudoun
Heights, western Blue Ridge, Loudoun Co., Va.Mafic Igneous Rock
Ultramafic rocks (CZu), like pyroxenite, occur as blocks and large bodies
within metagraywacke and schist, eastern Piedmont, Montgomery Co., Md.
Very dense ultramafic rock (CZu) occurs as small blocks within phyllite,
eastern Piedmont near Germantown, eastern Piedmont, Montgomery Co., Md.Metavolcanic
Tuffaceous phyllite
with clasts of metarhyolite of the Sams Creek Formation (CZscf) along
Linganore Creek, east of McKaig, central Piedmont, Frederick Co., Md.
Well-foliated
metarhyolite and tuffaceous phyllite (Zcr) north of Middletown, Blue
Ridge, Frederick Co., Md.
Felsic tuffaceous schist with clasts of metarhyolite of the Sams Creek
Formation (CZscf) near Fountain Mills, central Piedmont, Frederick Co.,
Md.Amphibolite
Dark bodies of
amphibolite (CZa) occur as blocks within rocks of the Sykesville
Formation, Potomac River, eastern Piedmont, Montgomery Co., Md.
Dark bodies of
amphibolite (CZa) occur as blocks within rocks of the Sykesville
Formation, Potomac River, eastern Piedmont, Montgomery Co., Md.Metadiabase Dike
Well-cleaved, dark, fine-grained chlorite-actinolite metadiabase (Ymd)
intrudes light granitic gneiss along Catoctin Creek, eastern Blue Ridge,
Loudoun Co., Va.Plutonic Rock
Granite Gneiss
Layered granitic
gneiss (Ylg) near Trapp, western Blue Ridge, Loudoun Co., Va.
Porphyroblastic granite gneiss (Ypg) intruded by garnetiferous metagranite
(Ygt) near Howardsville, western Blue Ridge, Loudoun Co., Va. 
Large feldspars of porphyroblastic granite gneiss (Ypg) near Howardsville,
western Blue Ridge, Loudoun Co., Va.
Massive, pink, leucocratic metagranite (Yml) near Philomont, Blue Ridge,
Loudoun Co., Va.
Folded, migmatitic(?), layered granitic gneiss (Ylg) south of Howardsville,
western Blue Ridge, Loudoun Co., Va.
Dark, massive, well-foliated hornblende monzonite gneiss (Yhm) underlies
Loudoun Valley near Neersville, western Blue Ridge, Loudoun Co., Va.
Well-foliated biotite granite gneiss (Ybg) along Goose Creek near
Leithtown, Blue Ridge, Loudoun Co., Va.
Cobbler Mountain Alkali Feldspar Quartz Syenite (Zrc) of the Robertson
River Igneous Suite near Paris, western Blue Ridge, Loudoun Co., Va.
Sawed hand specimens of dark hypersthene-bearing charnockite (Yc)(upper)
and speckled amphibolite (Ya), western Blue Ridge, Loudoun Co., Va.
Massive, unfoliated granodiorite of the Dalecarlia Intrusive Suite (Odm),
Potomac River, eastern Piedmont, Montgomery Co., Md.
Massive, unfoliated, biotite-hornblende tonalite of the Goergetown
Intrusive Suite (Ogh), eastern Piedmont, Potomac River, Montgomery Co., Md.
Migmatitic garnetiferous granite gneiss (Ygt) near the Potomac River,
western Blue Ridge, Frederick Co., Md. Hornfels, Basalt, Diabase
Hornfels
Hornfels
(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.
Close-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
Dark, fine-grained Hickory Grove basalt (Jhg), western Culpeper basin,
western Piedmont, near Oatlands, Loudoun Co., Va.Diabase
Dark, 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
Migmatite
Phyllonitic
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)
Rusty weathered, schistose garnet graphite paragneiss (Yp) near Airmont,
Blue Ridge, Loudoun Co., Va.
Sawed hand specimen of rusty weathered, schistose garnet graphite
paragneiss (Yp), along Catoctin Creek, Blue Ridge, Loudoun Co., Va.SURFICIAL
Colluvium
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.
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.
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
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.
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
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
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.
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. 
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
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.
Sinkhole developed in regolith of limestone of the Frederick Formation,
Frederick Valley, western Piedmont, Frederick Co., Md.
Landscape
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.