Geologic units in Cumberland county, Pennsylvania

Martinsburg Formation (Ordovician) at surface, covers 25 % of this area

Gray to dark-gray, buff-weathering shale.

Rockdale Run Formation (Ordovician) at surface, covers 9 % 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.

Montalto Member of Harpers Formation (Cambrian) at surface, covers 8 % of this area

Gray quartzite.

Elbrook Formation (Cambrian) at surface, covers 7 % of this area

Light-colored calcareous shale and silty limestone at top; medium-gray limestone and dolomite in middle; pure, dark limestone at base.

Zullinger Formation (Cambrian) at surface, covers 7 % of this area

Interbanded and interlaminated limestone and dolomite, thin- to thick-bedded; stromatolitic limestone; several thin, local quartz sandstone beds.

St. Paul Group (Ordovician) at surface, covers 6 % of this area

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

Tomstown Formation (Cambrian) at surface, covers 6 % of this area

Massive dolomite containing thin shaly interbeds.

Hamburg sequence rocks (Ordovician) at surface, covers 6 % of this area

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

Weverton and Loudoun Formations, undivided (Cambrian) at surface, covers 4 % of this area

In descending order: Weverton--gray to purplish-gray quartzite and quartzose conglomerate containing rounded pebbles. Loudoun--sericitic slate and purplish-gray, crumbly, poorly sorted, arkosic sandstone and conglomerate.

Metarhyolite (Precambrian) at surface, covers 4 % of this area

Light gray, green, and purple; commonly schistose, containing quartz augen and veins.

Shadygrove Formation (Cambrian) at surface, covers 3 % of this area

Pure, light-colored limestone, stromatolitic in part; abundant pinkish limestone and cream-colored chert.

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

Interbedded red to purple shale and sandstone and some beds of dolomite and impure limestone.

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

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

Graywacke and shale of Martinsburg Formation (Ordovician) at surface, covers 2 % of this area

Abundant impure sandstone (graywacke) interbeds.

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers 2 % of this area

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

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

Stromatolitic, fine-grained limestone; includes coarser grained and conglomeratic, siliceous, laminated Stoufferstown Formation at base in most of Cumberland Valley.

Tuscarora Formation (Silurian) at surface, covers 1 % 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.

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

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

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

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

Antietam Formation (Cambrian) at surface, covers 1.0 % of this area

Gray, buff-weathering quartzite.

Gettysburg Formation (Triassic) at surface, covers 0.7 % of this area

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

Clinton Group (Silurian) at surface, covers 0.6 % 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.

Limestone of Hamburg sequence (Ordovician) at surface, covers 0.5 % of this area

Conspicuous limestone.

Graywacke of Hamburg sequence (Ordovician) at surface, covers 0.4 % of this area

Predominantly graywacke.

Epler Formation (Ordovician) at surface, covers 0.3 % of this area

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

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

Dark-greenish-gray phyllite and schist containing thin quartzite layers; includes Montalto Member (CAhm).

Hershey and Myerstown Formations, undivided (Ordovician) at surface, covers < 0.1 % 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.

Diabase (Jurassic) at surface, covers < 0.1 % 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.

Greenstone schist (Precambrian) at surface, covers < 0.1 % of this area

Fine to medium grained, light to medium green; includes probable metavolcanic rocks.

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

Well-rounded quartzite pebbles, cobbles, and rare boulders set in a reddish-brown, sandy matrix.

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.

Metabasalt (Precambrian) at surface, covers < 0.1 % of this area

Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.

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

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

Bloomsburg Formation (Silurian) at surface, covers < 0.1 % 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.