Geologic units in Centre county, Pennsylvania

Burgoon Sandstone (Mississippian) at surface, covers 11 % of this area

Buff, medium-grained, crossbedded sandstone; includes shale and coal; in places, contains conglomerate at base; contains plant fossils; equivalent to Pocono Formation of Ridge and Valley province.

Mauch Chunk Formation (Mississippian) at surface, covers 10 % 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.

Reedsville Formation (Ordovician) at surface, covers 8 % 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 7 % 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 6 % 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.

Catskill Formation (Devonian) at surface, covers 6 % of this area

Grayish-red sandstone, siltstone, shale, and mudstone; units of gray sandstone occur in upper part; lithologies in upper part arranged in fining-upward cycles. In the Altoona area, the Catskill Formation is mapped as the Duncannon, Sherman Creek, and Irish Valley Members, which are described under "Central and Eastern Pennsylvania" provinces.

Coburn Formation through Nealmont Formation, undivided (Ordovician) at surface, covers 6 % 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).

Allegheny Formation (Pennsylvanian) at surface, covers 5 % 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.

Bald Eagle Formation (Ordovician) at surface, covers 5 % 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).

Lock Haven Formation (Devonian) at surface, covers 4 % of this area

Interbedded olive-gray mudstone, siltstone, sandstone, and thin conglomerate; marine fossils throughout; "Chemung" of earlier workers. Laterally equivalent to Scherr and Foreknobs Formations.

Lower members of Gatesburg Formation, undivided (Cambrian) at surface, covers 4 % 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).

Bellefonte Formation (Ordovician) at surface, covers 4 % 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.

Rockwell Formation (Mississippian and Devonian) at surface, covers 3 % 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.

Benner Formation through Loysburg Formation, undivided (Ordovician) at surface, covers 3 % 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).

Nittany Formation (Ordovician) at surface, covers 3 % of this area

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

Brallier and Harrell Formations, undivided (Devonian) at surface, covers 3 % 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.

Tuscarora Formation (Silurian) at surface, covers 2 % 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.

Stonehenge/Larke Formation (Ordovician) at surface, covers 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).

Huntley Mountain Formation (Mississippian and Devonian) at surface, covers 1 % of this area

Greenish-gray and light-olive-gray, flaggy, fine-grained sandstone, siltstone, and a few red shale interbeds; includes lower "Pocono" plus "Oswayo" of earlier workers. Forms transition between Catskill Formation and Burgoon Sandstone.

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

Medium-gray fossiliferous limestone.

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

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

Hamilton Group (Devonian) at surface, covers 1 % of this area

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

Clinton Group (Silurian) at surface, covers 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.

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

Gatesburg Formation (Cambrian) at surface, covers 0.8 % of this area

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

Keyser Formation through Clinton Group, undivided (Devonian and Silurian) at surface, covers 0.8 % 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.

Coburn Formation through Loysburg Formation, undivided (Ordovician) at surface, covers 0.6 % of this area

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

Onondaga and Old Port Formations, undivided (Devonian) at surface, covers 0.5 % 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).

Valentine Member of Benner Formation (Ordovician) at surface, covers 0.2 % of this area

Chemically pure limestone; occurs at top of Benner Formation.

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

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

Bellefonte and Axemann Formations, undivided (Ordovician) at surface, covers < 0.1 % of this area

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

Nittany and Stonehenge/Larke Formations, undivided (Ordovician) at surface, covers < 0.1 % of this area

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

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

Grayish-red sandstone, siltstone, and mudstone in fining-upward cycles; conglomerate occurs at base of some cycles.

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

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.