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Allegheny Plateau and Valley and Ridge

The information contained on this page was adapted from Maryland Geological Survey's Geologic Map of Maryland (1968). This information reflects geologic interpretations from over 20 years ago and do not necessarily represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.

Dunkard Group (Permian)   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 hilltops in center of Georges Creek Basin.
Monongahela Formation (Pennsylvanian)

  Monongahela Formation
Interbedded claystone, argillaceous limestone, shale, sandstone, and coal beds; Waynesburg coal at top; Pittsburgh coal at base; thickness 240 feet in west, increases to 375 feet in east.
Conemaugh Formation (Pennsylvanian)   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.
Allegheny and  Pottsville Formations (Pennsylvanian)

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.

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.

Mauch Chunk Formation (Mississippian)   Mauch Chunk Formation
Red and green shale, reddish-purple mudstone, and red, green, brown, and gray thin-bedded and cross-bedded sandstone; thickness 500 feet in west, increases to about 800 feet in east.
Greenbrier Formation (Mississippian)   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.
Pocono Group (Mississippian)  

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 Allegany Counties.

Purslane Sandstone
White, thick-bedded, coarse-grained sandstone and conglomerate with thin coal beds and red shales. Eastern Allegany and Washington Counties.

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.

Total thickness of Group 250 feet in west, increase to 1,700 feet in east.

Hampshire Formation (Devonian)   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 in east.
"Chemung" Formation,  Parkhead Sandstone, Brallier Formation, Harrell Shale (Devonian)

"Chemung" Formation
Predominantly marine beds characterized by gray to olive-green graywacke, siltstone, and shale; thickness 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 Formation
(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 1,700 feet in east.
Harrell Shale
Dark gray laminated shale; absent in east and 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 (Devonian)

Hamilton Group

Mahantango Formation
dark gray, laminated shale, siltstone, and very fine-grained sandstone; thickness 600 feet in west, increases to 1,200 feet in east.
Marcellus Shale
Gray-black, thinly laminated, pyritic, carbonaceous shale; thickness 250 feet in east, increasing to 500 feet in west.

Tioga Metabentonite Bed
Brownish-gray, thinly laminated shale containing sand-sized mica flakes; thickness less than one foot.
Needmore Shale
Olive-gray to black shale and dark, thin-bedded, fossiliferous, argillaceous limestone; thickness ranges from 70 to 145 feet.
Note: Hamilton Group, Tioga Metabentonite Bed, and Needmore Shale formerly were designated Romney Formation.

Oriskany Group (Devonian)

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.
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 Helderburg Formation.

Helderburg Formation and Keyser Limestone (Devonian-Silurian)

Helderburg Formation
Licking Creek 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, calcareous sandstone; thickness 10 to 18 feet.

Keyser Limestone
Dark gray, thin- to thick-bedded, fine- to coarse-grained calcarenite; contains nodular limestone, dolomitic limestone, and calcareous shale; cherty near top; fossiliferous; thickness 200 to 300 feet.

  Tonoloway Formation (Silurian)   Tonoloway Formation
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.
  Wills Creek Shale and Bloomsburg Formation (Silurian) Wills Creek Shale
Olive to yellowish-gray, thin-bedded sandstone, calcareous shale, argillaceous limestone, and sandstone; thickness 450 feet in west, increases to 600 feet in east.
Bloomsburg Formation
Bright red, hematitic, thin- to thick-bedded sandstone and shale; some dark sandstone and green shale; Cedar Creek 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.
  McKenzie Formation (Silurian)   McKenzie Formation
Gray, thin-bedded shale and argillaceous limestone; interbedded red sandstone and shale in east; thickness 160 in western Washington County, increases to 300 feet in east and 380 feet in west.
  Clinton Group (Silurian) Rochester Shale
Gray, thin-bedded calcareous shale and dark gray, thin- to medium-bedded lenticular limestone; thickness 25 to 40 feet.
Keefer Sandstone
White to yellowish-gray, thick-bedded protoquartzite and orthoquartzite, calcareous to west; thickness 1o feet in west, increase to 35 feet in east.
Rose Hill Formation
Olive-gray to drab, thin-bedded sandstone; Cresaptown Iron Sandstone
  Tuscarora Sandstone (Silurian)   Tuscarora Sandstone
White to light gray, thin- to thick-bedded cross-stratified subgraywacke and orthoquartzite; thickness 60 feet in east, increasing to 400 feet in west.
  Juniata Formation (Ordovician)   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.
  Martinsburg Formation (Ordovician)   Martinsburg Formation
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.
  Chambersburg Limestone (Ordovician)

Chambersburg Limestone
Dark gray, fine- to medium grained, thin-bedded argillaceous limestone; nodular and fossiliferous; thickness 225 to 250 feet.
St. Paul Group

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.
Row Park Limestone
Light gray, fine-grained, medium- to thick-bedded calcerenite, calcilutite, and dolomitic limestone; interbedded dark gray, cherty, granular limestone; thickness 100 feet in south, increases to 680 feet in north.

Pinesburg Station Dolomite (Ordovician)   Pinesburg Station Dolomite
Light gray, laminated and mottled cherty dolomite; nonfossiliferous; thickness 375 to 500 feet.
Rockdale Run Formation (Ordovician)   Rockdale Run Formation
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.
Stonehenge Limestone (Ordovician)   Stonehenge Limestone
Upper part gray, thin-bedded, coarse-grained to conglomeratic, oolitic calcarenite; some dolomite; lower part gray, thick-bedded, fine-grained algal limestone; thickness s500 to 800 feet.
    Conococheague Limestone (Cambro-Ordovician)   Conococheague Limestone
Dark blue, laminated, oolitic, argillaceous and siliceous limestone, algal limestone, and flat-pebble conglomerate; siliceous shale partings; some sandstone and dolomite; thickness 1,6000 to 1,900 feet.
  Elbrook Limestone (Cambrain)   Elbrook Limestone
Light blue, laminated, argillaceous limestone and calcareous shale; some dolomite; thickness 1,400 to possibly 3,000 feet
  Waynesboro Formation (Cambrain)   Waynesboro Formation
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.

Updated 5/18/00

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