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Geologic units in Huntingdon county, Pennsylvania

Clinton Group (Silurian) at surface, covers 11 % 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

Hamilton Group (Devonian) at surface, covers 10 % of this area

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

Brallier and Harrell Formations, undivided (Devonian) at surface, covers 9 % 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.

Wills Creek Formation (Silurian) at surface, covers 8 % 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.

Mauch Chunk Formation (Mississippian) at surface, covers 7 % 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.

Bloomsburg and Mifflintown Formations, undivided (Silurian) at surface, covers 7 % 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.

Pocono Formation (Mississippian) at surface, covers 6 % of this area

Pocono Formation - 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.

Reedsville Formation (Ordovician) at surface, covers 5 % of this area

Reedsville Formation - Olive-gray to dark-gray shale, siltstone, and fine-grained, thin-bedded sandstone having graded bedding; upper sandstone is very fossiliferous; includes Antes Formation (black calcareous shale) at base along Nittany Arch.

Keyser and Tonoloway Formations, undivided (Devonian and Silurian) at surface, covers 4 % 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 Formation (Silurian) at surface, covers 3 % 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.

Pottsville Formation (Pennsylvanian) at surface, covers 3 % 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.

Shriver, Mandata, Corriganville, and New Creek Members of Old Port Formation, undivided (Devonian) at surface, covers 3 % of this area

Shriver, Mandata, Corriganville, and New Creek Members of Old Port Formation, undivided - Limestone, chert, shale, and siliceous siltstone. In Fulton County, limestone and chert of the Licking Creek Member replaces the Shriver and Mandata.

Ridgeley Member of Old Port Formation (Devonian) at surface, covers 3 % of this area

Ridgeley Member of Old Port Formation - Fine- to very coarse grained, light-gray sandstone ("Oriskany" of earlier workers).

Lithology: sandstone

Keyser Formation through Mifflintown Formation, undivided (Devonian and Silurian) at surface, covers 2 % of this area

Keyser Formation through Mifflintown Formation, undivided - In descending order: Keyser Formation--limestone; Tonoloway Formation--limestone and interbedded shale; Wills Creek Formation--interbedded shale, siltstone, limestone, and dolomite; Bloomsburg Formation--grayish-red and greenish-gray shale, siltstone, sandstone, and mudstone; Mifflintown Formation--interbedded shale and limestone.

Allegheny Formation (Pennsylvanian) at surface, covers 2 % 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.

Gatesburg Formation (Cambrian) at surface, covers 2 % of this area

Gatesburg Formation - Gray dolomite, limestone, and sandstone. Includes the Mines Member (CAgm) and four lower members (CAgl).

Lower members of Gatesburg Formation, undivided (Cambrian) at surface, covers 2 % of this area

Lower members of Gatesburg Formation, undivided - Includes, in descending order: cyclic repetitions of sandstone and dolomite ("upper sandstone'' member); fossiliferous, laminated to massive limestone and dolomite (Ore Hill Member); cyclic repetitions of sandstone and dolomite (''lower sandstone" member); and thick-bedded crystalline dolomite (Stacy Member).

Irish Valley Member of Catskill Formation (Devonian) at surface, covers 2 % of this area

Irish Valley Member of Catskill Formation - 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.

Onondaga and Old Port Formations, undivided (Devonian) at surface, covers 2 % 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).

Coburn Formation through Loysburg Formation, undivided (Ordovician) at surface, covers 1 % of this area

Coburn Formation through Loysburg Formation, undivided - Includes, in descending order, the Coburn Formation through Nealmont Formation, undivided (Ocn) and Benner Formation through Loysburg Formation, undivided (Obl).

Juniata Formation (Ordovician) at surface, covers 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

Bellefonte and Axemann Formations, undivided (Ordovician) at surface, covers 1 % of this area

Bellefonte and Axemann Formations, undivided - Includes, in descending order, the Bellefonte (Obf) and Axemann (Oa) Formations.

Bald Eagle Formation (Ordovician) at surface, covers 1 % of this area

Bald Eagle Formation - Gray to olive-gray and grayish-red, fine- to coarse-grained, crossbedded sandstone, siltstone, and shale; some conglomerate (Lost Run Member); not present east of Susquehanna River, except at Spitzenberg Hill area (Berks County).

Keyser Formation through Clinton Group, undivided (Devonian and Silurian) at surface, covers 0.9 % 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.

Rockwell Formation (Mississippian and Devonian) at surface, covers 0.7 % of this area

Rockwell Formation - 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.

Warrior Formation (Cambrian) at surface, covers 0.7 % of this area

Warrior Formation - Gray, thin- to medium-bedded, fossiliferous, cyclic limestone bearing stromatolites; interbedded with thick-bedded crystalline dolomite and some sandstone.

Bellefonte Formation (Ordovician) at surface, covers 0.7 % of this area

Bellefonte Formation - Medium-gray, brownish-weathering, medium-bedded dolomite and minor sandstone; very fine grained Tea Creek Member above, and crystalline Coffee Run Member below.

Coburn Formation through Nealmont Formation, undivided (Ordovician) at surface, covers 0.7 % of this area

Coburn Formation through Nealmont Formation, undivided - In descending order: Coburn Formation--medium-gray to very dark gray, very fossiliferous limestone and shaly limestone; Salona Formation--very dark gray to black, nonfossiliferous shaly limestone and calcareous shale containing metabentonite beds; Nealmont Formation--medium-gray fossiliferous limestone (calcarenite--Rodman Member) overlying thin-bedded shaly limestone (calcilutite--Center Hall Member).

Nittany Formation (Ordovician) at surface, covers 0.7 % of this area

Nittany Formation - Medium- to dark-gray, thick-bedded dolomite containing chert and siliceous oolites.

Scherr Formation (Devonian) at surface, covers 0.6 % of this area

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

Nittany and Stonehenge/Larke Formations, undivided (Ordovician) at surface, covers 0.5 % of this area

Nittany and Stonehenge/Larke Formations, undivided - Includes, in descending order, the Nittany (On) and Stonehenge/Larke (Osl) Formations.

Foreknobs Formation (Devonian) at surface, covers 0.4 % 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.

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers 0.4 % of this area

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

Mines Member of Gatesburg Formation (Cambrian) at surface, covers 0.4 % of this area

Mines Member of Gatesburg Formation - Gray dolomite containing siliceous "oolites" and chert having cryptozoon stromatolites.

Onondaga Formation (Devonian) at surface, covers 0.3 % of this area

Onondaga Formation - Medium-gray calcareous shale; marine fossils; medium-gray argillaceous limestone of Selinsgrove Member at top; called "Needmore Formation" west of 78° longitude; Tioga bentonite at top.

Lithology: shale; limestone; bentonite

Stonehenge/Larke Formation (Ordovician) at surface, covers 0.3 % of this area

Stonehenge/Larke Formation - Medium-gray, medium-bedded to laminated, fossiliferous, oolitic limestone containing edgewise conglomerate; to the west, Stonehenge is laterally equivalent to medium- to dark-gray, coarsely crystalline dolomite (Larke).

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

Axemann Formation - Medium-gray fossiliferous limestone.

Lithology: limestone

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

Pleasant Hill Formation - Gray, thin-bedded, argillaceous limestone interbedded with shale, siltstone, and sandstone.