Geologic units in Huntingdon county, Pennsylvania

Hamilton Group (Devonian) at surface, covers 9 % of this area

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

Clinton Group (Silurian) at surface, covers 9 % 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.

Tuscarora Formation (Silurian) at surface, covers 8 % 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.

Brallier and Harrell Formations, undivided (Devonian) at surface, covers 7 % 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.

Wills Creek Formation (Silurian) at surface, covers 6 % 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.

Mauch Chunk Formation (Mississippian) at surface, covers 5 % 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.

Bloomsburg and Mifflintown Formations, undivided (Silurian) at surface, covers 5 % 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.

Catskill Formation (Devonian) at surface, covers 5 % 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.

Scherr Formation (Devonian) at surface, covers 5 % of this area

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

Foreknobs Formation (Devonian) at surface, covers 4 % 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.

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

Pocono Formation (Mississippian) at surface, covers 4 % 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.

Reedsville Formation (Ordovician) at surface, covers 4 % of this area

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.

Juniata Formation (Ordovician) at surface, covers 3 % 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).

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

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

Shriver, Mandata, Corriganville, and New Creek Members of Old Port Formation, undivided (Devonian) at surface, covers 2 % of this area

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 2 % of this area

Fine- to very coarse grained, light-gray sandstone ("Oriskany" of earlier workers).

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

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 Mifflintown Formation, undivided (Devonian and Silurian) at surface, covers 1 % of this area

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 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.

Gatesburg Formation (Cambrian) at surface, covers 1 % of this area

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 1 % of this area

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 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.

Nittany and Stonehenge/Larke Formations, undivided (Ordovician) at surface, covers 1 % of this area

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

Coburn Formation through Loysburg Formation, undivided (Ordovician) at surface, covers 0.9 % of this area

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

Bellefonte and Axemann Formations, undivided (Ordovician) at surface, covers 0.9 % of this area

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

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

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).

Keyser Formation through Clinton Group, undivided (Devonian and Silurian) at surface, covers 0.7 % 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.

Rockwell Formation (Mississippian and Devonian) at surface, covers 0.5 % 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.

Warrior Formation (Cambrian) at surface, covers 0.5 % of this area

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 0.5 % of this area

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

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

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

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers 0.3 % of this area

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

Mines Member of Gatesburg Formation (Cambrian) at surface, covers 0.3 % of this area

Gray dolomite containing siliceous ""oolites"" and chert having cryptozoon stromatolites.

Onondaga Formation (Devonian) at surface, covers 0.2 % of this area

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

Conemaugh Group (Pennsylvanian) at surface, covers 0.2 % of this area

Includes in descending order, the Casselman Formation (PAcc) and the Glenshaw Formation (PAcg), which are described separately below. Casselman: 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. Glenshaw: 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.

Stonehenge/Larke Formation (Ordovician) at surface, covers 0.2 % of this area

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).

Benner Formation through Loysburg Formation, undivided (Ordovician) at surface, covers 0.1 % of this area

In descending order: Benner Formation--light- to dark-gray, thick-bedded limestone (calcilutite); includes chemically pure Valentine Member (Obv) at top, and, below, the less pure Valley View Member, which contains metabentonite beds--all laterally equivalent to impure limestones of Oak Hall Member; Stover Member at base is dark-gray limestone (calcilutite) having dolomite streaks; Benner is called "Linden Hall" by some workers. Snyder Formation--light- to medium-gray limestone, laminated to medium-bedded; has mud cracks, oolites, and dolomitic layers. Hatter Formation--medium-gray, fossiliferous, argillaceous limestone, laminated and dolomitic. Loysburg Formation--light- to medium-gray, medium-bedded limestone (Clover Member) overlying laminated, alternating limestone, dolomitic limestone, and dolomite (Milroy ["tiger-striped"] Member).

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

Medium-gray fossiliferous limestone.

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

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