Geologic units in Adams county, Pennsylvania

Gettysburg Formation (Triassic) at surface, covers 30 % of this area

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

Metarhyolite (Precambrian) at surface, covers 13 % of this area

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

New Oxford Formation (Triassic) at surface, covers 12 % of this area

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

Diabase (Jurassic) at surface, covers 11 % 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.

Heidlersburg Member of Gettysburg Formation (Triassic) at surface, covers 8 % of this area

Red, green, and gray shale and argillite, and minor thin beds of gray arkosic sandstone; some quartz conglomerate and limestone conglomerate.

Metabasalt (Precambrian) at surface, covers 5 % of this area

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

Quartz fanglomerate (Triassic) at surface, covers 4 % of this area

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

Conestoga Formation (Ordovician and Cambrian) at surface, covers 3 % of this area

Light-gray, thin-bedded, impure, contorted limestone having shale partings; conglomeratic at base; in Chester Valley, includes micaceous limestone in upper part, phyllite in middle, and alternating dolomite and limestone in lower part.

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

Gray quartzite.

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers 2 % of this area

Includes, in descending order, the Antietam (CAa) and Harpers (CAh) Formations. Antietam Formation - gray, buff-weathering quartzite. Harpers Formation - Dark-greenish-gray phyllite and schist containing thin quartzite layers; includes Montalto Member (CAhm) - gray quartztite

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

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

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

Base--dark-brown shale; middle--gray and white spotted limestone and, locally, marble having irregular partings; top--sandy limestone which weathers to a fine-grained, friable, porous, sandy mass.

Beekmantown Group (Ordovician) at surface, covers 0.8 % of this area

Includes, in descending order, the Pinesburg Station (Ops), Rockdale Run (Orr), and Stonehenge (Os) Formations.

Marburg Schist (Probably lower Paleozoic) at surface, covers 0.8 % of this area

Gray-green phyllite, mica-chlorite schist, and conglomerate.

Limestone fanglomerate (Triassic) at surface, covers 0.7 % 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.

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

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

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

Light-gray, locally mottled, massive, pure, coarsely crystalline dolomite; siliceous in middle part.

Chickies Formation (Cambrian) at surface, covers 0.3 % of this area

Light-gray, hard, massive, Scolithus-bearing quartzite and quartz schist; thin, interbedded dark slate at top; conglomerate (Hellam Member) at base.

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

Quartz or quartzite pebbles, cobbles, and rare boulders set in a red, sandy, ferruginous matrix; some silica cement; some feldspar clasts.

Antietam Formation (Cambrian) at surface, covers 0.2 % of this area

Gray, buff-weathering quartzite.

Vintage Formation (Cambrian) at surface, covers 0.2 % of this area

Dark-gray, knotty, argillaceous dolomite; impure light-gray marble at base locally.

Sedimentary strata at Jacksonwald and Aspers (Jurassic) at surface, covers 0.1 % of this area

Arkosic sandstone; fossiliferous, gray to black shale and limestone; ripple-cross-laminated siltstone; and boulder conglomerate.

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

Gray, buff-weathering quartzite and quartz schist.

Chadakoin Formation (Devonian) at surface, covers < 0.1 % of this area

Light-gray or brownish siltstone and some sandstone, interbedded with medium-gray shale; included in Conneaut Group and "Chemung" of earlier workers; marine fossils common; includes "pink rock" of drillers.

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

Massive dolomite containing thin shaly interbeds.

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

Red, maroon, and gray sandstone, siltstone, and shale; basal conglomerate member: From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix; northward, quartz conglomerate with red sandy matrix; estimated total thickness 4,5000 feet.

Marburg Schist (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

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

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

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

Red shale and soft red sandstone and siltstone; estimated thickness less than 5,000 feet.

Chilhowee Group; Harpers Formation (Cambrian) at surface, covers < 0.1 % of this area

Brown to dark bluish-gray banded shale, to light bluish-gray, finely laminated phyllite; distinctively pale purple in basal part; bedding obscured by cleavage; increasingly metamorphosed toward east from shale to slate and phyllite; estimated thickness 2,000 feet.

Chilhowee Group; Antietam Formation (Cambrian) at surface, covers < 0.1 % of this area

White to dark gray and brown, thick-bedded, fine- to coarse-grained quartzite with thin argillaceous partings; first occurrence of Lower Cambrian fossils; cleavage generally obscures bedding; increasingly metamorphosed and phyllitic toward east; estimated thickness 300 to 800 feet.

Diabase Sills and Dikes (Triassic) at surface, covers < 0.1 % of this area

Sills: Greenish-gray to black, medium-grained; dikes: Greenish-gray to black, medium- to dine-grained; local contact metamorphic aureoles.

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

From vicinity of Maryland Rte. 73 northward, quartz conglomerate with red sandy matrix

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

Thick-bedded metabasalt with amygdaloidal layers and secondary veins of quartz, calcite, and epidote; interbedded green tuffaceous phyllite and blue amygdaloidal metaandesite.