Geologic units in Franklin county, Pennsylvania

Martinsburg Formation (Ordovician) at surface, covers 34 % of this area

Gray to dark-gray, buff-weathering shale.

Rockdale Run Formation (Ordovician) at surface, covers 7 % 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.

St. Paul Group (Ordovician) at surface, covers 7 % of this area

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

Elbrook Formation (Cambrian) at surface, covers 6 % 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 5 % of this area

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

Tuscarora Formation (Silurian) at surface, covers 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.

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers 4 % of this area

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

Clinton Group (Silurian) at surface, covers 4 % 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.

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

Massive dolomite containing thin shaly interbeds.

Graywacke and shale of Martinsburg Formation (Ordovician) at surface, covers 3 % of this area

Abundant impure sandstone (graywacke) interbeds.

Montalto Member of Harpers Formation (Cambrian) at surface, covers 3 % of this area

Gray quartzite.

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

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

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.

Bloomsburg and Mifflintown Formations, undivided (Silurian) at surface, covers 2 % 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.

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

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

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

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

Weverton and Loudoun Formations, undivided (Cambrian) at surface, covers 2 % 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.

Bellefonte Formation (Ordovician) at surface, covers 2 % 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.

Metarhyolite (Precambrian) at surface, covers 1 % of this area

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

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

Gray, buff-weathering quartzite.

Harpers Formation (Cambrian) at surface, covers 0.9 % of this area

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

Shadygrove Formation (Cambrian) at surface, covers 0.9 % of this area

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

Wills Creek Formation (Silurian) at surface, covers 0.6 % 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.

Hamilton Group (Devonian) at surface, covers 0.6 % of this area

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

Keyser and Tonoloway Formations, undivided (Devonian and Silurian) at surface, covers 0.5 % 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.

Metabasalt (Precambrian) at surface, covers 0.3 % of this area

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

Onondaga and Old Port Formations, undivided (Devonian) at surface, covers 0.3 % 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).

Keyser Formation through Mifflintown Formation, undivided (Devonian and Silurian) at surface, covers < 0.1 % 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.

Reedsville Formation (Ordovician) at surface, covers < 0.1 % 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.

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

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

Brallier and Harrell Formations, undivided (Devonian) at surface, covers < 0.1 % 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.