Geologic units in Dauphin county, Pennsylvania

Mauch Chunk Formation (Mississippian) at surface, covers 24 % 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.

Hamburg sequence rocks (Ordovician) at surface, covers 15 % of this area

Predominantly greenish gray, gray, purple, and maroon shale, siltstone, and graywacke; includes some wildflysch having Martinsburg matrix.

Sherman Creek Member of Catskill Formation (Devonian) at surface, covers 10 % of this area

Alternating grayish-red mudstone and siltstone in poorly defined fining-upward cycles, and minor intervals of gray sandstone; laterally equivalent to Berry Run, Sawmill Run, Packerton, and Long Run Members of eastern Pennsylvania.

Gettysburg Formation (Triassic) at surface, covers 9 % of this area

Reddish-brown to maroon, silty mudstone and shale containing thin red sandstone interbeds; several thin beds of impure limestone.

Pocono Formation (Mississippian) at surface, covers 9 % 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.

Duncannon Member of Catskill Formation (Devonian) at surface, covers 5 % of this area

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

Irish Valley Member of Catskill Formation (Devonian) at surface, covers 4 % 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.

Graywacke of Hamburg sequence (Ordovician) at surface, covers 3 % of this area

Predominantly graywacke.

Pottsville Formation (Pennsylvanian) at surface, covers 3 % of this area

Gray conglomerate, fine- to coarse-grained sandstone, and siltstone and shale containing minable anthracite coals. Includes three members, in descending order: Sharp Mountain--conglomerate and conglomeratic sandstone; Schuylkill--sandstone and conglomeratic sandstone; Tumbling Run--conglomeratic sandstone and sandstone.

Spechty Kopf Formation (Mississippian and Devonian) at surface, covers 3 % of this area

Light- to olive-gray, fine- to medium- grained, crossbedded sandstone, siltstone, and local polymictic diamictite, pebbly mudstone, and laminite; arranged in crude fining-upward cycles in some places; locally has grayish-red shale near top and conglomerate at base and in middle.

Epler Formation (Ordovician) at surface, covers 2 % of this area

Very finely crystalline, light-gray limestone interbedded with gray dolomite; coarsely crystalline limestone lenses present.

Diabase (Jurassic) at surface, covers 2 % of this area

Medium- to coarse-grained, quartz-normative tholeiite; composed of labradorite and various pyroxenes; occurs as dikes, sheets, and a few small flows. Includes the dark-gray York Haven Diabase (high titanium oxide) and the slightly younger Rossville Diabase (low titanium oxide). In chilled margins, the Rossville is distinguished from the York Haven by its lighter gray color and distinctive, sparse, centimeter-sized calcic-plagioclase phenocrysts.

Trimmers Rock Formation (Devonian) at surface, covers 1 % of this area

Olive-gray siltstone and shale, characterized by graded bedding; marine fossils; some very fine grained sandstone in northeast; black shale of Harrell Formation at base in Susquehanna Valley.

Clarks Ferry Member of Catskill Formation (Devonian) at surface, covers 1 % of this area

Gray to yellowish-gray sandstone, siltstone, and conglomerate.

Limestone of Hamburg sequence (Ordovician) at surface, covers 1 % of this area

Conspicuous limestone.

St. Paul Group (Ordovician) at surface, covers 1 % of this area

Very finely crystalline, "birdseye" limestone at top and base, granular fossiliferous limestone, black chert, and dolomite in middle.

Hamilton Group (Devonian) at surface, covers 0.8 % of this area

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

Llewellyn Formation (Pennsylvanian) at surface, covers 0.6 % of this area

Gray, fine- to coarse-grained sandstone, siltstone, shale, conglomerate, and numerous anthracite coals in repetitive sequences.

Martinsburg Formation (Ordovician) at surface, covers 0.6 % of this area

Gray to dark-gray, buff-weathering shale.

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

Dark-gray, thin-bedded, platy argillaceous limestone containing some beds of shale.

Bloomsburg Formation (Silurian) at surface, covers 0.5 % of this area

Grayish-red and greenish-gray shale, siltstone, and very fine to coarse-grained sandstone; some calcareous mudstone in central Pennsylvania; thins to west and is replaced by Mifflintown beds; thickens eastward, replacing overlying Wills Creek and Tonoloway Formations and underlying Mifflintown Formation.

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

Gray, finely crystalline limestone containing dark-gray silty laminations; numerous edgewise conglomerate beds.

Hershey and Myerstown Formations, undivided (Ordovician) at surface, covers 0.5 % of this area

In descending order: Hershey--dark-gray to black, thin-bedded, argillaceous limestone; Myerstown--medium- to dark-gray, platy, medium-crystalline limestone; carbonaceous at base.

Ontelaunee Formation (Ordovician) at surface, covers 0.4 % of this area

Light- to dark-gray, very finely to medium-crystalline dolomite containing interbeds of light-gray limestone; interbedded nodular, dark-gray chert at base.

Buffalo Springs Formation (Cambrian) at surface, covers 0.4 % of this area

Light-gray to pinkish-gray, finely to coarsely crystalline limestone and interbedded dolomite; numerous siliceous and clayey laminae; stromatolitic limestone beds near top; some thin sandy beds.

Tuscarora Formation (Silurian) at surface, covers 0.4 % 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.

Clinton Group (Silurian) at surface, covers 0.3 % 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.

Gettysburg conglomerate (Triassic) at surface, covers 0.3 % of this area

Gray quartz conglomerate, sandstone, and red siltstone and mudstone.

Annville Formation (Ordovician) at surface, covers 0.2 % of this area

Light-gray, massive, high-calcium limestone; mottled at base.

Pinesburg Station Formation (Ordovician) at surface, covers 0.1 % of this area

Light-colored, thick-bedded, finely laminated dolomite; some limestone.

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

Dark-gray, cobbly, argillaceous limestone; abundant irregular shale partings; some metabentonite beds present.

Hammer Creek Formation (Triassic) at surface, covers < 0.1 % of this area

Gray and pale red, fine- to coarse-grained quartzose sandstone, siltstone, and mudstone.

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

Mostly limestone; some dolomite interbeds; some chert near middle and top; stromatolitic limestone in middle; pinkish marbleoid limestone and chert at base.

Limestone fanglomerate (Triassic) at surface, covers < 0.1 % of this area

Yellowish-gray to medium-gray, angular limestone and dolomite pebbles, cobbles, and fragments set in a red, very fine grained quartz matrix; a few shale-clast interbeds.

New Oxford Formation (Triassic) at surface, covers < 0.1 % of this area

Light-gray to buff, commonly arkosic sandstone interbedded with red shale, mudstone, and fine-grained sandstone

Hammer Creek conglomerate (Triassic) at surface, covers < 0.1 % of this area

Cobble and pebble quartz conglomerate interbedded with red sandstone.