Geologic units in Washington county, Maryland

Conococheague Limestone (Upper Cambrian to Lower Ordovician) at surface, covers 21 % 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.

Rockdale Run Formation (Ordovician) at surface, covers 10 % 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 9 % 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.

Elbrook Limestone (Cambrian) at surface, covers 6 % 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 6 % 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 6 % 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.

Chilhowee Group; Harpers Formation (Cambrian) at surface, covers 5 % 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.

"Chemung" Formation, Parkhead Sandstone, Brallier Formation, and Harrell Shale (Devonian) at surface, covers 4 % 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.

Hamilton Group (including Mahantango Formation and Marcellus Shale), and including Tioga Metabentontite Bed, and Needmore Shale (Devonian) at surface, covers 4 % 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.

Wills Creek Shale and Bloomsburg Formation (Silurian) at surface, covers 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.

Hampshire Formation (Devonian) at surface, covers 3 % 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.

Chilhowee Group; Weverton Formation (Late Precambrian - Cambrian) at surface, covers 3 % 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.

Catoctin Metabasalt (Late Precambrian) at surface, covers 3 % 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.

Chilhowee Group; Antietam Formation (Cambrian) at surface, covers 2 % 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.

Waynesboro Formation (Cambrian) at surface, covers 2 % 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.

Granodiorite and Biotite Granite Gneiss (Precambrian) at surface, covers 2 % 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.

Oriskany Group including Ridgeley Sandstone and Shriver Chert (Devonian) at surface, covers 1 % 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

Clinton Group, including Rochester Shale (Silurian) at surface, covers 1 % 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.

Chambersburg Limestone and St. Paul Group (including New Market Limestone and Row Park Limestone) (Ordovician) at surface, covers 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.

Helderberg Formation and Keyser Limestone (Devonian) at surface, covers 1 % 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.

Tonoloway Limestone (Silurian) at surface, covers 0.9 % 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.

McKenzie Formation (Silurian) at surface, covers 0.9 % 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.

Tuscarora Sandstone (Silurian) at surface, covers 0.8 % 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.

Pocono Group, including the Purslane Sandstone and Rockwell Formation (Mississippian) at surface, covers 0.6 % 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.

Pinesburg Station Dolomite (Ordovician) at surface, covers 0.5 % of this area

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

Juniata Formation (Ordovician) at surface, covers 0.3 % 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.

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.

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

Gray to dark-gray, buff-weathering shale.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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

Massive dolomite containing thin shaly interbeds.

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.

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.

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.

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.

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

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.

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.

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.

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).

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.

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.

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.

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

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

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.

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.

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.

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.

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

Ordovician - middle calcareous units.

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.

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.

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).

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.

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.

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.

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.

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.

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.