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Mineral Resources > Online Spatial Data > Geology > by state > Maryland Geology

Geologic units in Allegany county, Maryland

[Additional scientific data in this geographic area]

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

Allegheny Formation - 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.

Lithology: sandstone; shale; limestone; clay or mud; coal

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

Keyser and Tonoloway Formations, undivided - 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.

Lithology: limestone; shale

Tuscarora Sandstone (Silurian) at surface, covers 2 % of this area

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

Lithology: orthoquartzite; graywacke

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

Tonoloway, Wills Creek, and Williamsport Formations - 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.

Lithology: limestone; shale; sandstone; clastic; chemical

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

Mahantango Formation (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.

Lithology: shale; sandstone; siltstone; limestone; coral

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

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

Lithology: siltstone; sandstone; shale; mudstone

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

Bloomsburg and Mifflintown Formations, undivided - 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.

Lithology: shale; siltstone; sandstone; limestone; mudstone

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

Hampshire Formation - 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.

Lithology: shale; sandstone; siltstone; conglomerate

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

Mauch Chunk Group - red, green, and medium-gray shale and sandstone, with a few thin limestones.

Lithology: shale; sandstone; limestone

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

Brallier Formation and Harrell Shale, undivided - 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).

Lithology: shale; siltstone; sandstone; limestone

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

Oriskany Sandstone and Huntersville Chert - 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.

Lithology: sandstone; chert; limestone

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

Tuscarora Formation - 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.

Lithology: quartzite; sandstone; shale; siltstone; conglomerate

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

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

Lithology: sandstone; shale

Mauch Chunk Formation (Mississippian) at surface, covers 1.0 % of this area

Mauch Chunk Formation - 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.

Lithology: shale; mudstone; sandstone

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

Helderberg 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.

Lithology: limestone; sandstone; shale

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

Pottsville Group - 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.

Lithology: sandstone; shale; coal

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

Pottsville Formation - 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.

Lithology: sandstone; conglomerate; shale; siltstone; claystone; limestone; coal

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.

Lithology: shale; mudstone; limestone; sandstone

Hampshire Formation (Devonian) at surface, covers 9 % of this area

Hampshire Formation - 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.

Lithology: shale; sandstone; mudstone; siltstone

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

Wills Creek Formation - 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.

Lithology: shale; siltstone; limestone; dolostone (dolomite)

Oriskany Group including Ridgeley Sandstone and Shriver Chert (Devonian) at surface, covers 7 % 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

Lithology: orthoquartzite; shale; chert; limestone; black shale

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

Keyser Formation through Clinton Group, undivided - 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.

Lithology: limestone; shale; sandstone; siltstone; mudstone; dolostone (dolomite)

Tonoloway Limestone (Silurian) at surface, covers 3 % of this area

Tonoloway Limestone - 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.

Lithology: limestone; shale; sandstone

Chemung Group (Devonian) at surface, covers 0.2 % of this area

Chemung Group - 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.

Lithology: siltstone; sandstone; shale; conglomerate

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

Juniata Formation - 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.

Lithology: sandstone; shale; graywacke; siltstone; conglomerate

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

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

Lithology: limestone; shale; sandstone

Monongahela Formation (Pennsylvanian) at surface, covers 6 % of this area

Monongahela Formation - 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.

Lithology: claystone; limestone; shale; sandstone; coal

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

Catskill Formation - 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.

Lithology: sandstone; siltstone; shale; mudstone; conglomerate

Greenbrier Formation (Mississippian) at surface, covers 0.6 % of this area

Greenbrier Formation - 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.

Lithology: limestone; sandstone; shale; calcarenite

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

Juniata Formation - 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).

Lithology: sandstone; shale; siltstone

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

McKenzie Formation and Clinton Group - 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.

Lithology: shale; sandstone; limestone

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

Glenshaw Formation - 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.

Lithology: shale; sandstone; limestone; clastic; coal

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

Onondaga and Old Port Formations, undivided - 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).

Lithology: shale; sandstone; limestone; chert; shale; siltstone; bentonite

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

Clinton Group - 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.

Lithology: shale; sandstone; limestone

Dunkard Group (Permian) at surface, covers 0.4 % of this area

Dunkard Group - 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

Lithology: shale; siltstone; sandstone; limestone; coal

"Chemung" Formation, Parkhead Sandstone, Brallier Formation, and Harrell Shale (Devonian) at surface, covers 30 % 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.

Lithology: shale; siltstone; sandstone; graywacke; limestone

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

Casselman Formation - 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.

Lithology: shale; siltstone; sandstone; clastic; limestone; coal

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

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

Lithology: black shale; shale; siltstone; limestone; sandstone; bentonite

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

Foreknobs Formation - 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.

Lithology: sandstone; siltstone; shale; conglomerate

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

Mauch Chunk Formation - 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.

Lithology: shale; siltstone; sandstone; conglomerate; limestone

Conemaugh Formation (Pennsylvanian) at surface, covers 12 % of this area

Conemaugh Formation - 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.

Lithology: claystone; shale; siltstone; sandstone; coal

Helderberg Formation and Keyser Limestone (Devonian) at surface, covers 3 % 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.

Lithology: limestone; chert; shale; black shale; sandstone

Pocono Group, including the Purslane Sandstone and Rockwell Formation (Mississippian) at surface, covers 2 % 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.

Lithology: sandstone; shale; mudstone; siltstone; conglomerate; coal

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

Allegheny Formation - 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.

Lithology: sandstone; siltstone; shale; limestone; coal

Clinton Group, including Rochester Shale (Silurian) at surface, covers 2 % of this area

Clinton Group, including Rochester Shale - 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.

Lithology: shale; limestone; quartzite; sandstone

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

Pocono and Rockwell Formations, undivided: 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.

Lithology: sandstone; conglomerate; shale

Quaternary Alluvium (Quaternary) at surface, covers 0.2 % of this area

Quaternary Alluvium - alluvial deposits of sand, gravel, silt, and clay.

Lithology: alluvium

Hamilton Group (including Mahantango Formation and Marcellus Shale), and including Tioga Metabentontite Bed, and Needmore Shale (Devonian) at surface, covers 14 % 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.

Lithology: shale; siltstone; sandstone; black shale; limestone

Allegheny Formation and Pottsville Formation (Pennsylvanian) at surface, covers 3 % 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

Lithology: sandstone; shale; claystone; siltstone; coal; limestone

McKenzie Formation (Silurian) at surface, covers 1 % of this area

McKenzie Formation - 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.

Lithology: shale; limestone; sandstone

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

Brallier and Harrell Formations, undivided - 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.

Lithology: siltstone; shale; black shale