Geologic units in Maryland (state in United States)

Additional scientific data in this geographic area

Quaternary Deposits Undivided (Quaternary) at surface, covers 18 % of this area

Undifferentiated gray to buff sand and gravel, gray to brown lignitic silt and clay, occasional boulders, and rare shell beds. Surficial deposits occur as intercalated fluvial sands and marsh muds (e.g. in upstream floodplain of the Wicomico and Nanticoke Rivers), well-sorted, stablized dune sands (e.g. eastern Wicomico County), shell-bearing estuarine clays and silts (e.g. lower Dorchester County) and Pocomoke River basin of Worcester County), and beach zone sands (e.g. Fenwick and Assateague Islands). Wisconsin to Holocene in age. Subsurface deposits of pre-Wisconsin age consist of buff to reddish-brown sand and gravel locally incised into Miocene sediments (e.g. Salisbury area), estuarine to marine white to gray sands, and gray to blue, shell-bearing clays (e.g. Worcester County).

Lowland Deposits (Quaternary) at surface, covers 9 % of this area

Gravel, sand, silt, and clay. Medium- to coarse-grained sand and gravel; cobbles and boulders near base; commonly contains reworked Eocene glauconite; varicolored silts and clays; brown to dark gray lignitic silty clay; contains estuarine to marine fauna in some areas (includes in part Pamlico, Talbot, Wicomico and Sunderland Formations of earlier reports); thickness 0 to 150 feet.

Upland Deposits (Eastern Shore) (Quaternary) at surface, covers 8 % of this area

Gravel, sand, silt, and clay. Mostly cross-bedded, poorly sorted, medium- to coarse-grained white to red sand and gravel; boulders near base; minor pink and yellow silts and clays; (Wicomico Formation of earlier reports); thickness 0 to 90 feet, locally thicker in paleochannels.

Wissahickon Formation (Undivided) (Late Precambrian (?)) at surface, covers 6 % of this area

Muscovite-chlorite-albite schist, muscovite-chlorite schist, chloritoid schist, and quartzite; intensely folded and cleaved.

Upland Deposits (Western Shore) (Quaternary) at surface, covers 6 % of this area

Gravel and sand, commonly orange-brown, locally limonite-cemented; minor silt and red, white, or gray clay; (includes Brandywine, Bryn Mawr, and Sunderland Formations of earlier reports); lower gravel member and upper loam member in Southern Maryland; thickness 0 to 50 feet.

Potomac Group, including Raritan and Patapsco Formations, Arundel Clay, and Patuxent Formation (Cretaceous) at surface, covers 4 % of this area

Potomac Group - Interbedded quartzose gravels; protoquartzitic to orthoquartzitic argillaceous sands; and white, dark gray, and multicolored silts and clays; thickness 0 to 800 feet. Includes Raritan and Patapsco Formations - Gray, brown, and red variegated silts and clays; lenticular, cross-bedded, argillaceous, subrounded sands; minor gravels; thickness 0 to 400 feet; Arundel Clay - Dark gray and maroon lignitic clays; abundant siderite concretions; present only in Baltimore-Washington area; thickness 0 to 100 feet; and Patuxent Formation - White or light gray to orange-brown, moderately sorted, cross-bedded, argillaceous, angular sands and subrounded quartz gravels; silts and clays subordinate, predominantly pale gray; thickness 0 to 250 feet.

Chesapeake Group; Calvert Formation (Miocene) at surface, covers 4 % of this area

Plum Point Marls Member: Interbedded dark green to dark bluish-gray, fine-grained argillaceous sand and sandy clay; contains prominent shell beds and locally silica-cemented sandstones; and Fairhaven Member: Greenish-blue diatomaceous clay, weathers to pale gray; pale brown to white, fine-grained argillaceous sand; and greenish-blue sandy clay; total thickness 0 to 150 feet.

Wissahickon Formation; Lower Pelitic Schist (Late Precambrian (?)) at surface, covers 4 % of this area

(Formerly mapped as oligoclase facies of Wissahickon Formation.) Medium- to coarse-grained biotite-oligoclase-muscovite-quartz schist with garnet, staurolite, and kyanite; fine- to medium-grained semipelitic schist; and fine-grained granular to weakly schistose psammitic granulite; psammitic beds increase upward; apparent thickness 5,500 feet or more.

Conemaugh Formation (Pennsylvanian) at surface, covers 3 % of this area

Includes the rocks between the base of the Pittsburgh coal and the top of the Upper Freeport coal; consists of two unnamed members which are separated by the Barton coal; both members are gray and brown claystone, shale, siltstone and sandstone, with several coal beds; lower member also contains redbeds and fossiliferous marine shales; thickness 825 to 925 feet.

"Chemung" Formation, Parkhead Sandstone, Brallier Formation, and Harrell Shale (Devonian) at surface, covers 2 % of this area

"Chemung" Formation - Predominantly marine beds characterized by gray to olive-green graywacke, siltstone, and shale; thickened ranges from 2,000 to 3,000 feet; Parkhead Sandstone - Gray to olive-green sandy shale, conglomeratic sandstone and graywacke; present in Washington County, identification uncertain in west; thickness averages 400 feet; Brallier Fomation - (Woodmont Shale of earlier reports). Medium to dark gray, laminated shale and siltstone; weathers to light olive-gray; grain size coarsens upward; thickness about 2,000 feet in west, about 1,7000 feet in east; and Harrell Shale - Dark gray laminated shale; absent in east where Brallier lies directly on Mahantango, Tully Limestone lies near base in west, in subsurface of Garrett County; total thickness in west 140 to 300 feet.

New Oxford Formation (Triassic) at surface, covers 2 % of this area

Red, maroon, and gray sandstone, siltstone, and shale; basal conglomerate member: From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix; northward, quartz conglomerate with red sandy matrix; estimated total thickness 4,5000 feet.

Baltimore Gabbro Complex (Late Precambrian (?) - Early Paleozoic) at surface, covers 2 % of this area

Hypersthene gabbro with subordinate amounts of olivine gabbro, norite, anorthositic gabbro, and pyroxenite; igneous minerals and textures well preserved in some rocks, other rocks exhibit varying degrees of alteration and recrystallization with a new metamorphic mineral assemblage.

Hampshire Formation (Devonian) at surface, covers 1 % of this area

Interbedded red shale, red mudstone, and red to brown cross-bedded siltstone and sandstone; some thin green shale; greenish-gray sandstone and shale toward top; fragmentary plant fossils; thickness 1,400 to 2,000 feet in west, increases to 3,800 feet in east.

Allegheny Formation and Pottsville Formation (Pennsylvanian) at surface, covers 1 % of this area

Allegheny Formation - Interbedded sandstone, siltstone, claystone, shale, and coal beds; Upper Freeport coal at top; where present, Brookville coal defines base; thickness 275 feet in northeast, increases to 325 feet in south and west. And Pottsville Formation - Interbedded sandstone, siltstone, claystone, shale, and coal beds; conglomeratic orthoquartzite and protoquartzite at base; thickness 60 feet in northeast, increases to 440 feet in southwest

Pamunkey Group; Aquia Formation (Paleocene) at surface, covers 1 % of this area

Dark green to gray-green, argillaceous, highly glauconitic, well-sorted fine- to medium-grained sand; locally indurated shell beds; thickness 0 to 100 feet.

Marburg Schist (Late Precambrian (?)) at surface, covers 1 % of this area

Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

Chesapeake Group; Choptank Formation (Miocene) at surface, covers 1 % of this area

Interbedded brown to yellow very fine-grained to fine-grained sand and gray to dark bluish-green argillaceous silt; locally indurated to calcareous sandstone; prominent shell beds; thickness 0 to 50 feet.

Wissahickon Formation; Boulder Gneiss (Late Precambrian (?)) at surface, covers 1 % of this area

(Formerly mapped as Sykesville and Laurel Formations.) Thick-bedded to massive, pebble- and boulder-bearing, arenaceous to pelitic metamorphic rock; typically a medium-grained, garnet-oligoclase-mica-quartz gneiss; locally an intensely foliated gneiss or schist; apparent thickness 15,000 feet.

Ijamsville Formation and Marburg Schist (Late Precambrian (?)) at surface, covers 1 % of this area

Ijamsville Formation - Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally; and Marburg Schist - Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

Catoctin Metabasalt (Late Precambrian) at surface, covers 1 % of this area

Thick-bedded metabasalt with amygdaloidal layers and secondary veins of quartz, calcite, and epidote; interbedded green tuffaceous phyllite and blue amygdaloidal metaandesite.

Baltimore Gneiss (Precambrian) at surface, covers 1 % of this area

Biotite-quartz-feldspar gneiss and biotite-hornblende gneiss; amphibolite widespread but subordinate; texturally varied; granitic gneiss, veined gneiss, augen gneiss, banded gneiss, and migmatite, in places complexly intermingled; age 1,1000 m.y. * by radiogenic dating. Layered paragneiss in Baltimore City southeast of Relay Quartz Diorite.

Ijamsville Formation (Late Precambrian (?)) at surface, covers 1 % of this area

Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally.

Conococheague Limestone (Upper Cambrian to Lower Ordovician) at surface, covers 1 % of this area

Dark blue, laminated, oolitic, argillaceaous, and siliceous limestone, algal limestone, and flat-pebble conglomerate; siliceous shale partings; some sandstone and dolomite; thickness 1,600 to 1,900 feet.

Port Deposit Gneiss (Paleozoic) at surface, covers 1.0 % of this area

Moderately to strongly deformed intrusive complex composed of gneissic biotite quartz diorite, hornblende-biotite quartz diorite, and biotite granodiorite; all rocks foliated and some strongly sheared; age 550 +/- 50 m.y. * by radiogenic dating.

Pocono Group, including the Purslane Sandstone and Rockwell Formation (Mississippian) at surface, covers 0.9 % of this area

Pocono Group - Gray, white, tan, and brown, thin- to thick-bedded, cross-bedded sandstone, locally conglomeratic; interbedded gray and reddish-brown shale, mudstone, and siltstone; fragmentary plant fossils. Undifferentiated in Garrett and western Allegeny Counties. Includes Purslane Sandstone - White, thick-bedded, coarse-grained sandstone and conglomerate with thin coal beds and red shales. Eastern Allegany and Washington Counties. And also inlcudes Rockwell Formation - Coarse-grained arkosic sandstone, fine-grained conglomerate, and buff shale; dark shale with thin coal beds near base. Eastern Allegany and Washington Counties.

Hamilton Group (including Mahantango Formation and Marcellus Shale), and including Tioga Metabentontite Bed, and Needmore Shale (Devonian) at surface, covers 0.8 % of this area

Hamilton Group including Mahantango Formation - Dark gray, laminated shale, siltstone, and very fine-grained sandstone; thickness 600 feet in west, increases to 1,200 feet in east, and Marcellus Shale - Gray-black, thinly laminated, pyritic, carbonaceous shale; thickness 250 feet in east, increases to 500 feet in west. Also includes Tioga Metabentonite Bed - Brownish-gray, thinly laminated shale containing sand-size mica flakes; thickness less than one foot; and Needmore Shale - Olive-gray to black shale and dark, thin-bedded, fossiliferous, argillaceous limestone; thickness ranges from 70 to 145 feet.

Pamunkey Group; Nanjemoy Formation (Eocene) at surface, covers 0.7 % of this area

Dark green to gray, argillaceous, glauconitic, fine- to medium-grained sand; minor gray to pale brown clay; total thickness 0 to 125 feet; Marlboro Clay member at base: Pink to gray, homogeneous plastic clay with local lenses of very fine-grained white sand; thickness 0 to 30 feet; present west of Chesapeake Bay only; total thickness 0 to 125 feet.

Frederick Limestone (Cambrian) at surface, covers 0.7 % of this area

Blue, slabby, thin-bedded limestone and minor shale; contains Upper Cambrian (Trempealeauian) faunule; thickness approximately 480 feet.

Metagabbro and Amphibolite (Early Paleozoic (?)) at surface, covers 0.7 % of this area

Weakly to strongly lineated metagabbro and epidote amphibolite.

Chilhowee Group; Weverton Formation (Late Precambrian - Cambrian) at surface, covers 0.6 % of this area

Interbedded white to dark gray, thin-bedded, micaceous, ferruginous, and sericitic quartzites, phyllites, and white, thick-bedded, ledge-making quartzites; some gray to brown ferruginous quartz conglomerate and purple-banded phyllite; thickness approximately 100 feet in south, increases to 425 feet in north.

Granodiorite and Biotite Granite Gneiss (Precambrian) at surface, covers 0.6 % of this area

Light gray to pale green, fine-grained, granodiorite gneiss, and dark gray biotite granite gneiss with some augen gneiss; in places a sheared muscovite-biotite gneiss; local biotite schist bands; intruded by metadiabase feeder dikes of Catoctin Metabasalt.

Sams Creek Metabasalt (Late Precambrian (?)) at surface, covers 0.6 % of this area

Grayish-green, massive to schistose, amygdaloidal metabasalt.

Gettysburg Shale (Triassic) at surface, covers 0.6 % of this area

Red shale and soft red sandstone and siltstone; estimated thickness less than 5,000 feet.

Monmouth Formation (Cretaceous) at surface, covers 0.6 % of this area

Dark gray to reddish-brown, micaceous, glauconitic, argillaceous, fine- to coarse-grained sand; basal gravel in Prince Georges County; thickness 0 to 100 feet.

Cockeysville Marble (Late Precambrian (?)) at surface, covers 0.6 % of this area

Metadolomite, calc-schist, and calcite marble are predominant; calc-gneiss and calc-silicate marble widespread but minor; thickness about 750 feet.

Chesapeake Group; St. Marys Formation (Miocene) at surface, covers 0.5 % of this area

Greenish-blue to yellowish-gray sandy clay and fine-grained argillaceous sand; thickness 0 to 80 feet.

Rockdale Run Formation (Ordovician) at surface, covers 0.5 % of this area

Upper one-third gray, mottled, cherty dolomite and dolomitic limestone; lower two-thirds gray, cherty argillaceous calcarenite and algal limestone with interbedded dolomite and oolitic limestone; thickness at least 1,700 feet east of Conococheague Creek, increases to about 2,500 feet in west.

Tomstown Dolomite (Cambrian) at surface, covers 0.5 % of this area

Interbedded light gray to yellowish-gray, thin- to thick-bedded dolomite and limestone; some shale layers; gradational contact with Antietam; thickness 200 to 1,000 feet.

Mauch Chunk Formation (Mississippian) at surface, covers 0.5 % of this area

Red and green shale, reddish-purple mudstone, and red, green, brown, and gray thin-bedded and cross-bedded sandstones; thickness 500 feet in west, increases to about 800 feet in east.

Chilhowee Group; Harpers Formation (Cambrian) at surface, covers 0.4 % of this area

Brown to dark bluish-gray banded shale, to light bluish-gray, finely laminated phyllite; distinctively pale purple in basal part; bedding obscured by cleavage; increasingly metamorphosed toward east from shale to slate and phyllite; estimated thickness 2,000 feet.

Urbana Formation (Late Precambrian (?)) at surface, covers 0.4 % of this area

Dark gray to green sericite-chlorite phyllite, metasiltstone, and quartzite; thin lenses of impure marble and calcareous phyllite occur locally.

Ultramafic Rocks (Early Paleozoic - Late Precambrian (?)) at surface, covers 0.4 % of this area

Chiefly serpentinite with partly to completely altered dunite, peridotite, pyroxenite, and massive to schistose soapstone; talc-carbonate rock and altered gabbro are common in some bodies.

Oriskany Group including Ridgeley Sandstone and Shriver Chert (Devonian) at surface, covers 0.4 % of this area

Oriskany Group, including Ridgeley Sandstone - White, medium- to coarse-grained, fossiliferous, calcareous orthoquartzite; thickness 160 feet in west. Medium to dark gray cherty, arenaceous limestone in east; thickness 50 feet; and Shriver Chert - Dark gray, brown, and black silty shales, cherty shales, and nodular and bedded black chert; fossiliferous; thickness 170 feet in west, upper boundary gradational with Ridgeley. Thickness 14 feet in east where the lower Shriver intertongues with the Licking Creek Limestone Member of the Helderberg Formation

Wissahickon Formation; Metagraywacke (Late Precambrian (?)) at surface, covers 0.3 % of this area

(Formerly mapped as Peters Creek Formation.) Rhythmically interbedded chlorite-muscovite meta-graywacke and fine-grained chlorite-muscovite schist; graded bedding locally preserved; thickness probably 2,000 to 3,000 feet.

Elbrook Limestone (Cambrian) at surface, covers 0.3 % of this area

Light blue, laminated, argillaceous limestone and calcareous shale; some dolomite; thickness 1,400 to possibly 3,000 feet.

Stonehenge Limestone (Ordovician) at surface, covers 0.3 % of this area

Upper part gray, thin-bedded, coarse-grained to conglomeratic, oolitic calcarenite; some dolomite; lower part gray, thick-bedded, fine-grained algal limestone; thickness 500 to 800 feet.

Martinsburg Formation (Ordovician) at surface, covers 0.3 % of this area

Upper part rhythmically interbedded graywackes, siltstones, and dark shales; lower part dark brown, dark gray, and black, thin-bedded fissile shale; thickness 2,000 to 2,500 feet.

Matawan Formation (Cretaceous) at surface, covers 0.3 % of this area

Dark gray micaceous, glauconitic, argillaceous, fine-grained sand and silt; absent in outcrop southwest of Patuxent River; thickness 0 to 70 feet.

Monongahela Formation (Pennsylvanian) at surface, covers 0.3 % of this area

Interbedded claystone, argillaceous limestone, shale, sandstone, and coal beds; Waynesburg coal at top; Pittsburg coal at base; thickness 240 feet in west, increases to 375 feet in east.

Greenbrier Formation (Mississippian) at surface, covers 0.3 % of this area

Upper part red calcareous shale and sandstone interbedded with greenish-gray and reddish-gray argillaceous limestone; Loyalhanna Limestone Member: Gray to red, cross-bedded, arenaceous calcarenite; total thickness 200 to 300 feet.

Wills Creek Shale and Bloomsburg Formation (Silurian) at surface, covers 0.3 % of this area

Wills Creek Shale - Olive to yellowish-gray, thin-bedded mudstone, calcareous shale, argillaceous limestone, and sandstone; thickness 450 feet in west, increases to 600 feet in east; and Bloomsburg Formation - Bright red, hematitic, thin- to thick-bedded sandstone and shale; some dark sandstone and green shale; Cedar Creek Limestone Member - Dark gray, fine- to medium-grained argillaceous limestone, occurs in middle part of formation; total thickness 20 feet in west, increases to 200 feet in east.

Chilhowee Group; Antietam Formation (Cambrian) at surface, covers 0.3 % of this area

White to dark gray and brown, thick-bedded, fine- to coarse-grained quartzite with thin argillaceous partings; first occurrence of Lower Cambrian fossils; cleavage generally obscures bedding; increasingly metamorphosed and phyllitic toward east; estimated thickness 300 to 800 feet.

Setters Formation (Late Precambrian (?)) at surface, covers 0.3 % of this area

Upper member: Feldspathic mica schist and mica gneiss; total thickness 200 to 500 feet. Middle member: Impure quartzite interstratified with thin beds of mica schist; total thickness 200 to 500 feet. And Lower member: Medium-grained, feldspathic mica schist; locally granitized; total thickness 200 to 500 feet.

Magothy Formation (Cretaceous) at surface, covers 0.3 % of this area

Loose, white, cross-bedded, "sugary", lignitic sands and dark gray, laminated silty clays; white to orange-brown, iron-stained, subrounded quartzose gravels in western Anne Arundel County; absent in outcrop southwest of Patuxent River; thickness 0 to 60 feet.

Metarhyolite and Aasociated Pyroclastic Sediments (Late Precambrian) at surface, covers 0.2 % of this area

Metarhyolite - Dense, blue, cryptocrystalline, with white feldspar phenocrysts and glassy quartz; red porphyritic metarhyolite at contact with Catoctin Metabasalt; and Pyroclastic sediments - tuff breccia, blue slaty tuff, white tuffaceous sericitic schist, and banded green slate.

Helderberg Formation and Keyser Limestone (Devonian) at surface, covers 0.2 % of this area

Helderberg Formation, including Licking Creek Limestone Member - (Becraft Limestone of earlier reports.) Present only in east. Medium gray, medium-grained limestone near top; bedded black chert and thin-bedded limestone in middle; silty argillaceous limestone and shale near base; contains tongues of Shriver and Mandata; thickness 110 feet; Mandata Shale Member - Dark brown to black, thin-bedded shale; fossiliferous; thickness 20 to 30 feet in west, intertongues with Licking Creek Limestone Member in east; Corriganville Limestone Member (Head) - (New Scotland Limestone of earlier reports.) Medium gray, medium-grained, medium-bedded limestone, interbedded with chert; fossiliferous; thickness 15 to 30 feet; New Creek Limestone Member - (Coeymans Limestone of earlier reports.) Medium gray, thick-bedded, coarse-grained limestone; fossiliferous; thickness 9 to 10 feet. Limestone changes facies eastward into sandstone, the Elbow Ridge Sandstone Member - Medium-bedded, medium- to coarse-grained, calcarous sandstone; thickness 10 to 18 feet.; and the Keyser Limestone - Dark gray, thin- to thick-bedded, fine- to coarse-grained calcarenite; contains nodular limestone, dolomitic limestone, and calcarous shale; cherty near top; fossiliferous; thickness 200 to 300 feet.

Grove Limestone (Upper Cambrian to Lower Ordovician) at surface, covers 0.2 % of this area

Dark gray to light dove, thick-bedded limestone; dolomite beds in lower part; highly quartzose limestone at base; Upper Cambrian to Lower Ordovician in age; thickness approximately 590 feet.

Tonoloway Limestone (Silurian) at surface, covers 0.2 % of this area

Gray, thin-bedded limestone, dolomitic limestone, and calcareous shale; thin sandstone member in east 20 feet above base; fossiliferous; thickness 400 feet in east, increases to 600 feet in west.

Clinton Group, including Rochester Shale (Silurian) at surface, covers 0.2 % of this area

Gray, thin-bedded calcareous shale and dark gray, thin- to medium-bed7 ded lenticular limestone; thickness 25 to 40 feet; Keefer Sandstone - White to yellowish-gray, thick-bedded protoquartzite and orthoquartzite; calcareous to west; thickness 10 feet in west, increases to 35 feet in east; and Rose Hill Formation - Olive-gray to drab, thin-bedded shale; some purple shale and gray, thin-bedded sandstone; including Cresaptown Iron Sandstone Member - Purple, hematite-cemented, quartzose sandstone; thickness 5 to 30 feet; occurs in lower half of formation; total thickness 300 feet in east, increases to 570 feet in west.

Libertytown Metarhyolite (Late Precambrian (?)) at surface, covers 0.2 % of this area

Purple, bluish-black, and red, dense, fine-grained metarhyolite with feldspar phenocrysts; interbedded with blue and purple amygdaloidal metaandesite; both rhyolite and andesite interbedded with blue, purple, and green phyllitic slates.

Wakefield Marble (Late Precambrian (?)) at surface, covers 0.1 % of this area

White, fine-grained marble; subordinate white, green, and pink variegated marble; and blue marble.

Tuscarora Sandstone (Silurian) at surface, covers 0.1 % of this area

White to light gray, thin- to thick-bedded, cross-stratified subgraywacke and orthoquartzite; thickness 60 feet in east, increases to 400 feet in west.

Norbeck Quartz Diorite (Cambrian) at surface, covers 0.1 % of this area

Ranges from weakly foliated quartz diorite to strongly gneissic and schistose rock with recrystallized textures; more massive rocks have igneous textures; age 510 +/- 10 m.y. * by radiogenic dating.

James Run Gneiss (Late Precambrian (?)) at surface, covers 0.1 % of this area

Thick-bedded, light gray biotite-quartz-plagioclase gneiss with thin interbeds of quartz amphibolite; grades downward into sharply layered, thin- to thick-bedded paragneiss composed of subequal amounts of biotite-quartz-plagioclase gneiss and quartz amphibolite; thickness unknown.

Chilhowee Group; Loudoun Formation (Cambrian) at surface, covers 0.1 % of this area

Basal phyllite member: Pale purple, discontinuous, lenticular; members are in gradational contact; total thickness 0 to 200 feet

McKenzie Formation (Silurian) at surface, covers 0.1 % of this area

Gray, thin-bedded shale and argillaceous limestone; interbedded red sandstone and shale in east; thickness 160 feet in western Washington County, increases to 300 feet in east and 380 feet in west.

Gunpowder Granite (Paleozoic) at surface, covers 0.1 % of this area

Remobilized Baltimore Gneiss. Fine- to medium-grained quartz monzonite with biotitic schlieren; grades into Baltimore Gneiss in core of Towson dome; age 550 +/- 50 m.y. * by radiogenic dating.

Waynesboro Formation (Cambrian) at surface, covers < 0.1 % of this area

Upper part red, gray, and yellowish-brown, thin-bedded siltstone, shale, and ripple-marked, cross-bedded sandstone; lower part interbedded dark gray to red shale and thin-bedded dolomite; thickness approximately 600 feet.

Volcanic Complex of Cecil County (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Metamorphosed andesitic and dacitic volcanic rocks (greenstone, greenschist, quartz amphibolite, and schistose felsite); amygdules and volcano-clastic textures locally preserved; thickness unknown.

Kensington Quartz Diorite (Ordovician) at surface, covers < 0.1 % of this area

Moderately to strongly deformed; igneous textures generally destroyed; composition ranges from quartz diorite to granodiorite; comprises thin concordant sheets or wedges localized along plunging crest of Baltimore anticlinorium.

New Oxford Formation, basal quartz conglomerate (Triassic) at surface, covers < 0.1 % of this area

From vicinity of Maryland Rte. 73 northward, quartz conglomerate with red sandy matrix

New Oxford Formation, basal liimestone conglomerate (Triassic) at surface, covers < 0.1 % of this area

From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix

Chambersburg Limestone and St. Paul Group (including New Market Limestone and Row Park Limestone) (Ordovician) at surface, covers < 0.1 % of this area

Chambersburg Limestone - Dark gray, fine- to medium-grained, thin-bedded argillaceous limestone; nodular and fossiliferous; thickness 225 to 250 feet; and St. Paul Group, including New Market Limestone - Upper part gray, thick-bedded calcilutite; fossiliferous; lower part light gray, thin-bedded, laminated argillaceous calcilutite; thickness 285 feet in south, increases to 700 feet in north; and Row Park Limestone - Light gray, fine-grained, medium- to thick-bedded calcarenite; calcilutite, and dolomitic limestone; interbedded dark gray, cherty, granular limestone; thickness 100 feet in south, increases to 680 feet in north.

Georgetown Mafic Complex (Late Precambrian (?) - Early Paleozoic) at surface, covers < 0.1 % of this area

Poorly exposed complex of tonalite, dark quartz diorite, gabbro, amphibolite, and undifferentiated basic rocks.

Quartz Gabbro and Quartz Diorite Gneiss (Paleozoic) at surface, covers < 0.1 % of this area

Mixed rock zone of greenish-black, uralitized, quartz-bearing gabbro to dark gray, weakly gneissic, pyroxene-hornblende-biotite quartz diorite.

Sugarloaf Mountain Quartzite (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Massive white quartzite interbedded with softer sericitic quartzite, slate, and phyllite.

Ellicott City Granodiorite (Silurian) at surface, covers < 0.1 % of this area

Ranges from biotite granodiorite along margin of body to quartz monzonite in core; age of quartz monzonite core 420 +/- 50 m.y. * by radiogenic dating.

Silver Run Limestone (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Blue, thin-bedded, finely crystalline schistose limestone and calcareous slate.

Muscovite Quartz Monzonite Gneiss (Paleozoic) at surface, covers < 0.1 % of this area

Well foliated to nearly massive quartz monzonite gneiss, generally medium-grained and even textured but locally porphyritic and pegmatitic.

Diabase Sills and Dikes (Triassic) at surface, covers < 0.1 % of this area

Sills: Greenish-gray to black, medium-grained; dikes: Greenish-gray to black, medium- to dine-grained; local contact metamorphic aureoles.

Guilford Quartz Monznoite (Silurian) at surface, covers < 0.1 % of this area

Biotite-muscovite-quartz monzonite; occurs as discontinuous lenticular bodies; age 420 +/- 50 m.y. * by radiogenic dating.

Ultramafic and Gabbroic Rocks (Early Paleozoic (?)) at surface, covers < 0.1 % of this area

Mixed metagabbro, serpentinite, metapyroxenite, and actinolite-, chlorite-, and epidote-bearing schists.

Relay Quartz Diorite (Early Paleozoic - Late Precambrian (?)) at surface, covers < 0.1 % of this area

Intensely foliated, fine-grained, light colored; ranges from quartz diorite to albite granite; age 550 +/- 50 m.y. * by radiogenic dating.

Pinesburg Station Dolomite (Ordovician) at surface, covers < 0.1 % of this area

Light gray, laminated and mottled cherty dolomite; nonfossiliferous; thickness 375 to 500 feet.

Chesapeake Group (Tertiary) at surface, covers < 0.1 % of this area

Bluish gray silt with quartz sand and some shell beds.

Swift Run Formation (Late Precambrian) at surface, covers < 0.1 % of this area

Sericitic quartzite and phyllite; blue and green tuffaceous slate with sericitic blebs; some white marble with interbedded phyllite.

Juniata Formation (Ordovician) at surface, covers < 0.1 % of this area

Red to greenish-gray, thin- to thick-bedded siltstone, shale, subgraywacke, and protoquartzite; interbedded conglomerate; thickness 180 feet in east, increases to 500 feet in west.

Peach Bottom Slate (Ordovician (?)) at surface, covers < 0.1 % of this area

Hard, bluish-black graphitic slate; thin beds of fine-grained black quartzite near base; apparent maximum thickness 1,000 feet.

Dunkard Group (Permian) at surface, covers < 0.1 % of this area

Red and green shale, siltstone, and sandstone, with thin lenticular beds of argillaceous limestone and thin beds of impure coal; thick-bedded, white conglomeratic sandstone at base; thickness greater than 200 feet; occurs only on hilltop

Cardiff Metaconglomerate (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Silvery-gray, well foliated, micaceous quartz-pebble metaconglomerate and quartzite; apparent maximum thickness 700 feet.

Woodstock Quartz Monznoite (Paleozoic) at surface, covers < 0.1 % of this area

Massive biotite-quartz monzonite; well-developed granular textures; age 420 +/- 50 m.y. * by radiogenic dating.

Conemaugh Group (Pennsylvanian) at surface, covers < 0.1 % of this area

Cyclic sequences of red and gray shale, siltstone, and sandstone, with thin limestones and coals. Mostly non-marine. May be divided into Casselman and Glenshaw Formations. Extends from the base of the Pittsburgh coal to the top of the Upper Freeport coal. Includes the Elk Lick, Bakerstown, and Mahoning coals, and the Ames and Brush Creek Limestones.

Quaternary Alluvium (Quaternary) at surface, covers < 0.1 % of this area

Alluvial deposits of sand, gravel, silt, and clay.

Allegheny Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Cyclic sequences of sandstone, siltstone, shale, limestone, and coal. Includes the Freeport, Kittanning and Clarion coals, also, the Princess coals of Kentucky . Extends from the top of the Upper Freeport coal to the top of the Homewood Sandstone.

Chemung Group (Devonian) at surface, covers < 0.1 % of this area

Gray to brown siltstone and sandstone with shale and conglomeratic interbeds; mainly marine and sparingly fossiliferous; boundaries gradational. Can be divided into the Voreknobs and Scherr Formations along the Allegheny Front. Parkhead Sandstone Member near base.

Glenarm Wissahickon"" formation (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Lithologically similar to oligoclase-mica schist of the Wissahickon Formation (PZw), but also includes lenticular amphibolite bodies having ocean-floor basalt chemistry.

Ultramafic rocks (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Includes serpentine, steatite, and other products of alteration of peridotites and pyroxenites.

Banded mafic gneiss (Precambrian) at surface, covers < 0.1 % of this area

Dark, fine to medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgh."

Martinsburg Formation (Ordovician) at surface, covers < 0.1 % of this area

Gray to dark-gray, buff-weathering shale.

Wissahickon Formation; Upper Pelitic Schist (Late Precambrian (?)) at surface, covers < 0.1 % of this area

(Formerly mapped as albite facies of Wissahickon Formation.) Albite-chlorite-muscovite-quartz schist with sporadic thin beds of laminated micaceous quartzite; coarsens from west to east; primary sedimentary structures include normal bedding, graded bedding, and soft-sediment deformational structures; apparent thickness 14,000 feet or more.

Pottsville Group (Pennsylvanian) at surface, covers < 0.1 % of this area

Includes the Kanawha, New River, and Pocahontas Formations. Predominantly sandstones, some of which are conglomeratic, with thin shales and coals. Undivided in northern West Virginia.

Octoraro Formation (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Includes albite-chlorite schist, phyllite, some hornblende gneiss, and granitized members.

Zullinger Formation (Cambrian) at surface, covers < 0.1 % of this area

Interbanded and interlaminated limestone and dolomite, thin- to thick-bedded; stromatolitic limestone; several thin, local quartz sandstone beds.

Hamilton Group (Devonian) at surface, covers < 0.1 % of this area

Includes, in descending order, the Mahantango (Dmh) and Marcellus (Dmr) Formations.

Rockdale Run Formation (Ordovician) at surface, covers < 0.1 % of this area

Mostly limestone; some dolomite interbeds; some chert near middle and top; stromatolitic limestone in middle; pinkish marbleoid limestone and chert at base.

Pegmatite Dikes (Silurian) at surface, covers < 0.1 % of this area

Quartz-albite-microcline-perthite-muscovite granite pegmatites associated with gneiss domes; age 425 +/- 20 m.y. * by radiogenic dating.

Conococheague Formation (Cambrian) at surface, covers < 0.1 % of this area

Predominantly algal and mechanically deposited limestone, with interbeds of aphanitic limestone and dolomite. Contains siliceous and dolomitic laminations. Resistant sandy Big Spring Station Member near the base.

Tuscarora Formation (Silurian) at surface, covers < 0.1 % of this area

Light- to medium-gray quartzite and quartzitic sandstone and minor interbedded shale and siltstone, locally conglomeratic in lower part; includes (to the northwest) interbedded red and non-red sandstone (Castanea Member) at top; east of Harrisburg, equivalent to Minsi and Weiders Members of Shawangunk Formation.

Glenshaw Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Cyclic sequences of shale, sandstone, red beds, and thin limestone and coal; includes four marine limestone or shale horizons; red beds are involved in landslides; base is at top of Upper Freeport coal.

Casselman Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Cyclic sequences of shale, siltstone, sandstone, red beds, thin, impure limestone, and thin, nonpersistent coal; red beds are associated with landslides; base is at top of Ames limestone.

Potomac Formation (Cretaceous) at surface, covers < 0.1 % of this area

Variegated silts and clays with beds of quartz sand.

Brallier Formation and Harrell Shale, undivided (Devonian) at surface, covers < 0.1 % of this area

Brallier Formation : predominantly olive-gray to dark, thickly laminated marine shale, with considerable siltstone and thin sandstone lenses; mainly nonfossiliferous. Harrell Shale: dark gray to black thinly laminated to fissile shale. Calcareous shale and limestone lenses near the base (Tully).

Clinton Group (Silurian) at surface, covers < 0.1 % of this area

Predominantly Rose Hill Formation--light-olive-gray to brownish-gray, fossiliferous shale; locally, limestone occurs near top; includes dark-reddish-gray, very fine to coarse-grained, ferruginous sandstone; east of Harrisburg, equivalent to Lizard Creek Member of Shawangunk Formation. Above Rose Hill is Keefer Formation--light- to dark-gray, fossiliferous sandstone, hematitic, oolitic sandstone, and shale; not recognized east of Harrisburg.

Rancocas Formation (Tertiary) at surface, covers < 0.1 % of this area

Grayish-green and green, silty, glauconitic sand.

Tonoloway, Wills Creek, and Williamsport Formations (Silurian) at surface, covers < 0.1 % of this area

Includes the thin-bedded platy argillaceous limestones of the Tonoloway, the thin-bedded shale with fossiliferous limestones of the Wills Creek, the Bloomsburg red clastic facies, and the greenish-brown to white Williamsport Sandstone. The Wills Creek contains anhydrite and rock salt, the latter supplying brine from deep wells along the Ohio River.

Stonehenge Formation (Ordovician) at surface, covers < 0.1 % of this area

Stromatolitic, fine-grained limestone; includes coarser grained and conglomeratic, siliceous, laminated Stoufferstown Formation at base in most of Cumberland Valley.

Hampshire Formation (Devonian) at surface, covers < 0.1 % of this area

Non-marine shales and fine micaceous sandstones, mostly red to brownish-gray, including siltstone, sandstone and conglomerate. Generally distinguishable from the underlying Chemung by non-marine character and red color.

Peters Creek Schist (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Chlorite-sericite schist containing interbedded quartzite.

Stonehenge Limestone (Ordovician) at surface, covers < 0.1 % of this area

Gray, thin-bedded to massive, fossiliferous limestone, largely mechanically deposited, with small black chert nodules and beds of "edgewise" conglomerate. The highly resistant Stoufferstown Limestone member is found at the base.

Catskill Formation (Devonian) at surface, covers < 0.1 % of this area

Grayish-red sandstone, siltstone, shale, and mudstone; locally conglomeratic; contains gray sandstone in upper part; lithologies arranged in fining-upward cycles; equivalent to the Hampshire Formation south of Pennsylvania.

Rockdale Run Formation (Ordovician) at surface, covers < 0.1 % of this area

Bluish to light gray and brown, thick-bedded dolomite and limestone, containing gray chert and zones of Cryptozoa and Lecanospira.

Keyser and Tonoloway Formations, undivided (Devonian and Silurian) at surface, covers < 0.1 % of this area

In descending order: Keyser Formation--medium-gray, crystalline to nodular, fossiliferous limestone; upper part laminated and mud-cracked; not present east of Harrisburg; passes into lower Coeymans, Rondout, and Decker Formations in the east. Tonoloway Formation--medium-gray, laminated, mud-cracked limestone containing some medium-dark- or olive-gray shale interbeds; lower part passes into Wills Creek Formation east and south; passes into Bossardville and Poxono Island beds in the east.

Onondaga and Old Port Formations, undivided (Devonian) at surface, covers < 0.1 % of this area

Includes, in descending order, the Onondaga Formation (Don), Ridgeley Member of Old Port Formation (Dor), and Shriver, Mandata, Corriganville, and New Creek Members of Old Port Formation, undivided (Dosn).

Allegheny Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Cyclic sequences of sandstone, shale, limestone, clay, and coal; includes valuable clay deposits and Vanport Limestone; commercially valuable Freeport, Kittanning, and Brookville-Clarion coals present; base is at bottom of Brookville-Clarion coal.

Elbrook Formation (Cambrian) at surface, covers < 0.1 % of this area

Light-colored calcareous shale and silty limestone at top; medium-gray limestone and dolomite in middle; pure, dark limestone at base.

Unit C (Tertiary) at surface, covers < 0.1 % of this area

Grayish-green, clayey glauconitic silt and sand.

Mahantango Formation (Devonian) at surface, covers < 0.1 % of this area

(part of Millboro Shale) - thickly laminated marine shale, siltstone, very fine sandstone, and some limestone, with an occasional coral reef or biostrome. Contains the Clearville and Chaneysville Siltstone Members of Pennsylvania.

Metabasalt (Precambrian) at surface, covers < 0.1 % of this area

Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.

McKenzie Formation and Clinton Group (Silurian) at surface, covers < 0.1 % of this area

Includes the McKenzie Formation, consisting of shale with thin limestone lenses; the dark Rochester Shale; the white Keefer Sandstone; and the Rose Hill predominantly red shale, with thin sandstone interbeds, some of which are called "iron sandstones" from their reddish-brown color and hematite content.

Mauch Chunk Group (Mississippian) at surface, covers < 0.1 % of this area

Red, green, and medium-gray shale and sandstone, with a few thin limestones.

Tomstown Formation (Cambrian) at surface, covers < 0.1 % of this area

Massive dolomite containing thin shaly interbeds.

Wills Creek Formation (Silurian) at surface, covers < 0.1 % of this area

Variegated gray, grayish-red, yellowish-gray and greenish-gray, interbedded calcareous shale, siltstone, shaly limestone, and dolomite; passes into Bloomsburg Formation in the southeast; not present east of Harrisburg.

Burgoon Sandstone (Mississippian) at surface, covers < 0.1 % of this area

Buff, medium-grained, crossbedded sandstone; includes shale and coal; in places, contains conglomerate at base; contains plant fossils; equivalent to Pocono Formation of Ridge and Valley province.

Helderberg Group (Devonian) at surface, covers < 0.1 % of this area

Mostly cherty limestone, with some sandstone and shale. Contains several named stratigraphic units, including the Keyser Formation, which is partly Silurian and includes the Clifton Forge Sandstone and Big Mountain Shale Members.

Bloomsburg and Mifflintown Formations, undivided (Silurian) at surface, covers < 0.1 % of this area

Includes, in descending order, the Bloomsburg Formation (Sb) and the Mifflintown Formation--interbedded dark-gray shale and medium-gray fossiliferous limestone; equivalent to "McKenzie" and "Rochester" of earlier workers; not present east of Harrisburg.

Graywacke and shale of Martinsburg Formation (Ordovician) at surface, covers < 0.1 % of this area

Abundant impure sandstone (graywacke) interbeds.

Martinsburg Formation (Ordovician) at surface, covers < 0.1 % of this area

Predominantly gray to dark shale, yellowish in the upper portion. Contains scattered thin limestone and sandstone interbeds, particularly in the lower portion. The upper portion constitutes the Reedsville Shale.

Wissahickon Schist (Paleozoic) at surface, covers < 0.1 % of this area

Dense micaceous schist, gneiss and migmatite.

Oriskany Sandstone and Huntersville Chert (Devonian) at surface, covers < 0.1 % of this area

Oriskany Sandstone: sometimes designated Ridgeley in eastern West Virginia. White to brown coarse- to fine-grained, partly calcareous sandstone, locally pebbly or conglomeratic, and ridge-forming. May be white, nearly pure silica, and a source of glass sand, as at Berkeley Springs, Morgan County. Huntersville Chert (part of Onesquethaw ("Onondaga") Group): ranges from a nearly pure slightly calcitic or dolomitic chert to an inter-tonguing of such chert and the Needmore Shale. Grades westward in the subsurface to a limestone, commonly considered as "Onondaga". Contains the "glauconitic" Bobs Ridge Sandstone Member. Not mappable at scale of this map. Included with Do.

Port Deposit Granodiorite (Paleozoic) at surface, covers < 0.1 % of this area

Granodiorite

Tomstown Dolomite (Cambrian) at surface, covers < 0.1 % of this area

Thick-bedded, blue-gray, aphanitic, saccharoidal, and splintery dolomite, weathering coffee-brown; some siliceous limestone and black chert. Contains commercial beds of white, pure, low-silica dolomite.

Weverton and Loudoun Formations, undivided (Cambrian) at surface, covers < 0.1 % of this area

In descending order: Weverton--gray to purplish-gray quartzite and quartzose conglomerate containing rounded pebbles. Loudoun--sericitic slate and purplish-gray, crumbly, poorly sorted, arkosic sandstone and conglomerate.

Rockwell Formation (Mississippian and Devonian) at surface, covers < 0.1 % of this area

Buff, fine- to medium-grained, crossbedded, argillaceous sandstone and dark-gray shale; includes some carbonaceous shale, sporadic conglomerate beds, and diamictite; included in lower "Pocono" of earlier workers.

Brallier and Harrell Formations, undivided (Devonian) at surface, covers < 0.1 % of this area

In descending order: Brallier--medium-gray, planar-bedded siltstone interbedded with light-olive shale; sparse marine fauna; Harrell--black shale (Burket Member) and dark-gray shale.

Mauch Chunk Formation (Mississippian) at surface, covers < 0.1 % of this area

Grayish-red shale, siltstone, sandstone, and some conglomerate; some local nonred zones. Includes Loyalhanna Member (crossbedded, sandy limestone) at base in south-central and southwestern Pennsylvania; also includes Greenbrier Limestone Member, and Wymps Gap and Deer Valley Limestones, which are tongues of the Greenbrier. Along Allegheny Front from Blair County to Sullivan County, Loyalhanna Member is greenish-gray, calcareous, crossbedded sandstone.

Foreknobs Formation (Devonian) at surface, covers < 0.1 % of this area

Interbedded sandstone, siltstone, and shale; gray to olive gray, red near top; substantial brownish-gray sandstone; some marine fossils; a few conglomerate beds at base and top.

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers < 0.1 % of this area

Includes, in descending order, the Juniata (Oj) and Bald Eagle (Obe) Formations.

Pocono Group (Mississippian) at surface, covers < 0.1 % of this area

Predominantly hard gray massive sandstones, with some shale. In the Eastern Panhandle, has been divided into the Hedges, Purslane, and Rockwell Formations.

Shadygrove Formation (Cambrian) at surface, covers < 0.1 % of this area

Pure, light-colored limestone, stromatolitic in part; abundant pinkish limestone and cream-colored chert.

Pottsville Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Predominantly gray sandstone and conglomerate; also contains thin beds of shale, claystone, limestone, and coal; includes Olean and Sharon conglomerates of northwestern Pennsylvania; thin marine limestones present in Beaver, Lawrence, and Mercer Counties; minable coals and commercially valuable high-alumina clays present locally.

Mt. Laurel-Navesink Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark greenish brown and dark gray, highly glauconitic sandy silt and silty sand.

St. Paul Group (Ordovician) at surface, covers < 0.1 % of this area

Very finely crystalline, "birdseye" limestone at top and base, granular fossiliferous limestone, black chert, and dolomite in middle.

Chambersburg Formation (Ordovician) at surface, covers < 0.1 % of this area

Dark-gray, cobbly, argillaceous limestone; abundant irregular shale partings; some metabentonite beds present.

Harpers Formation (Cambrian) at surface, covers < 0.1 % of this area

Dark-greenish-gray phyllite and schist containing thin quartzite layers; includes Montalto Member (CAhm).

Waynesboro Formation (Cambrian) at surface, covers < 0.1 % of this area

Interbedded red to purple shale and sandstone and some beds of dolomite and impure limestone.

Merchantville Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark gray to bluish-gray slightly glauconitic, micaceous silty, very fine sand.

Antietam Formation (Cambrian) at surface, covers < 0.1 % of this area

(Chilhowee Group) - gray to white quartzite, sandstone and quartz-schist; thick-bedded, and resistant; contains Scolithus.

Elbrook Formation (Cambrian) at surface, covers < 0.1 % of this area

Thin-bedded blue-gray argillaceous limestone and platy shale, with some siliceous limestone and minor dolomite.

Pinesburg Station Dolomite (Ordovician) at surface, covers < 0.1 % of this area

Fine- to medium-crystalline, brown to light gray dolomite, containing nodular chert.

Pinesburg Station Formation (Ordovician) at surface, covers < 0.1 % of this area

Light-colored, thick-bedded, finely laminated dolomite; some limestone.

Greenbrier Group (Mississippian) at surface, covers < 0.1 % of this area

Marine limestone and marine and non-marine red and gray shale, and minor sandstone beds in numerous formational units.

Magothy Formation (Cretaceous) at surface, covers < 0.1 % of this area

White and buff quartz sand with beds of gray and black clayey silt.

Scherr Formation (Devonian) at surface, covers < 0.1 % of this area

Chiefly siltstone; some fine-grained sandstone, shale, and mudstone; light olive gray; marine fossils.

Irish Valley Member of Catskill Formation (Devonian) at surface, covers < 0.1 % of this area

Nonmarine, grayish-red siltstone and mudstone, and gray and grayish-red sandstone interbedded with minor, thin, light-olive-gray marine siltstone; arranged in fining-upward cycles. Lower part of member has conglomeratic sandstones.

Oriskany Sandstone and Helderberg Group, undivided (Devonian) at surface, covers < 0.1 % of this area

Oriskany Sandstone: sometimes designated Ridgeley in eastern West Virginia. White to brown coarse- to fine-grained, partly calcareous sandstone, locally pebbly or conglomeratic, and ridge-forming. May be white, nearly pure silica, and a source of glass sand, as at Berkeley Springs, Morgan County. Helderburg Group: mostly cherty limestone, with some sandstone and shale. Contains several named stratigraphic units, including the Keyser Formation, which is partly Silurian and includes the Clifton Forge Sandstone and Big Mountain Shale Members.

Ordovician, middle calcareous units (Ordovician) at surface, covers < 0.1 % of this area

Ordovician - middle calcareous units.

Keyser Formation through Clinton Group, undivided (Devonian and Silurian) at surface, covers < 0.1 % of this area

Same as Keyser-through-Mifflintown (DSkm) interval, plus Clinton Group at base. Clinton includes the following, in descending order: Keefer Formation--fossiliferous sandstone and hematitic, oolitic sandstone and shale; Rose Hill Formation--fossiliferous shale.

Unit B (Cretaceous-Tertiary) at surface, covers < 0.1 % of this area

Greenish-gray sandy and clayey glauconitic silt.

Amphibolite (Paleozoic) at surface, covers < 0.1 % of this area

Amphibolite

Wenonah Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray and rust-brown fine to medium, micaceous, sparingly glauconitic quartz sand.

Redbank Formation (Cretaceous) at surface, covers < 0.1 % of this area

Reddish-brown, slightly micaceous and glauconitic, fine to medium sand.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers < 0.1 % of this area

Fine- to coarse-grained, reddish-brown to gray, primary bedding features such as cross-beds, channel lags, and ripple marks , minor conglomerate, siltstone, and shale beds.

Antietam Formation (Cambrian) at surface, covers < 0.1 % of this area

Gray, buff-weathering quartzite.

Waynesboro Formation (Cambrian) at surface, covers < 0.1 % of this area

Variegated shale and sandstone, commonly red or brown. Middle zone contains much dolomite and limestone. Contains the Olenellus fauna of Early Cambrian age, although its upper portion may be of Middle Cambrian age.

Pocono and Rockwell Formations, undivided (Mississippian and Devonian) at surface, covers < 0.1 % of this area

Buff, medium-grained, crossbedded sandstone and some conglomerate (Pocono), overlying buff to olive-gray, fine- to medium-grained, crossbedded sandstone containing a few beds of shale and conglomerate (Rockwell); shown in southwestern Bedford County only.

Metarhyolite (Precambrian) at surface, covers < 0.1 % of this area

Light gray, green, and purple; commonly schistose, containing quartz augen and veins.

Marcellus Formation and Needmore Shale, undivided (Devonian) at surface, covers < 0.1 % of this area

Marcellus Formation (part of Millboro Shale): predominantly gray-black to black thinly laminated non-calcareous pyritic shale. Contains one or more thin-bedded limestones, including the Purcell Member of Pennsylvania. Needmore Shale (part of Onesquethaw ("Onondaga") Group): predominantly dark grey or green, calcitic, mostly non-fissile shale. Gives strong "kick" on gamma ray logs. Tioga Bentonite near the top. Includes the black Beaver Dam Shale Member. Grades westward into the Huntersville Chert. Not mappable at scale of this map. Included with Dmn.

Greenstone schist (Precambrian) at surface, covers < 0.1 % of this area

Fine to medium grained, light to medium green; includes probable metavolcanic rocks.

Juniata Formation (Ordovician) at surface, covers < 0.1 % of this area

Grayish-red, very fine to medium-grained, crossbedded sandstone, and grayish-red siltstone and shale; merges with underlying Bald Eagle Formation to the south; not present east of Susquehanna River, except at Spitzenberg Hill area (Berks County).

Pocono Formation (Mississippian) at surface, covers < 0.1 % of this area

Light-gray to buff or light-olive-gray, medium-grained, crossbedded sandstone and minor siltstone; commonly conglomeratic at base and in middle; medial conglomerate, where present, is used to divide into Mount Carbon and Beckville Members; equivalent to Burgoon Sandstone of Allegheny Plateau.

Marsh and Intertidal Mud Deposits (Quaternary) at surface, covers < 0.1 % of this area

Medium to dark-gray soft mud, and grayish-brown peat, comprising sediment of marshes in coastal areas and Chesapeake Bay; thickness is 0 to 10 feet. Also, sandy mud and muddy fine sand, light- to dark-gray. Locally, contains abundant shell material characterized by Crassostrea virginica and Mercenaria mercenaria. Comprises sediments of shallow bays and flats in area of Atlantic coastal lagoons of the Eastern Shore.

Sams Creek Metabasalt (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Altered basaltic flows, some amygdaloidal; green, schistose, containing hornblende, epidote, chlorite, and quartz.

Mather Gorge Formation - Metagraywacke (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Metagraywacke, light- to medium-gray, yellowish- to red dish-brown-weathering, fine- to medium-grained, generally well-bedded, and lesser semi-pelitic schist; contains interbedded quartzose schist and some calc-silicate rock; mineral assemblages as in schist (CZms). Beds range from about 3 cm to 3 m, averaging about 20 cm; graded bedding, sole marks, and slump features are abundant. Mather Gorge is unconformable beneath Popes Head Formation, which is intruded by Occoquan Granite; includes rocks previously mapped in northern Virginia as Peters Creek Schist.

Trenton Group and Black River Group, undivided (Ordovician) at surface, covers < 0.1 % of this area

Trenton Group: dark, crystalline, nodular, and argillaceous limestones, with some metabentonite streaks. Includes the Nealmont, Oranda, Edinburg, and the upper part of the Chambersburg Limestones of northeastern West Virginia; also the Moccasin and Eggleston Formation of Mercer and Monroe Counties. Black River Group: predominantly gray aphanitic limestones, with many bioclastic streaks; siliceous in the lower part.

Chadakoin Formation (Devonian) at surface, covers < 0.1 % of this area

Light-gray or brownish siltstone and some sandstone, interbedded with medium-gray shale; included in Conneaut Group and "Chemung" of earlier workers; marine fossils common; includes "pink rock" of drillers.

St. Paul Group (Ordovician) at surface, covers < 0.1 % of this area

Includes the New Market and Row Park Limestones. Predominantly medium-gray aphanitic limestone, containing very low-silica, cream-colored member of considerable economic importance. Chert nodules and dolomite occur in the Row Park.