Geologic units in Frederick county, Maryland

Catoctin Metabasalt (Late Precambrian) at surface, covers 16 % 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.

Ijamsville Formation (Late Precambrian (?)) at surface, covers 13 % of this area

Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally.

Frederick Limestone (Cambrian) at surface, covers 10 % of this area

Blue, slabby, thin-bedded limestone and minor shale; contains Upper Cambrian (Trempealeauian) faunule; thickness approximately 480 feet.

New Oxford Formation (Triassic) at surface, covers 9 % 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.

Gettysburg Shale (Triassic) at surface, covers 8 % of this area

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

Granodiorite and Biotite Granite Gneiss (Precambrian) at surface, covers 8 % of this area

Light gray to pale green, fine-grained, granodiorite gneiss, and dark gray biotite granite gneiss with some augen gneiss; in places a sheared muscovite-biotite gneiss; local biotite schist bands; intruded by metadiabase feeder dikes of Catoctin Metabasalt.

Chilhowee Group; Weverton Formation (Late Precambrian - Cambrian) at surface, covers 7 % of this area

Interbedded white to dark gray, thin-bedded, micaceous, ferruginous, and sericitic quartzites, phyllites, and white, thick-bedded, ledge-making quartzites; some gray to brown ferruginous quartz conglomerate and purple-banded phyllite; thickness approximately 100 feet in south, increases to 425 feet in north.

Urbana Formation (Late Precambrian (?)) at surface, covers 5 % of this area

Dark gray to green sericite-chlorite phyllite, metasiltstone, and quartzite; thin lenses of impure marble and calcareous phyllite occur locally.

Marburg Schist (Late Precambrian (?)) at surface, covers 3 % of this area

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

Metarhyolite and Aasociated Pyroclastic Sediments (Late Precambrian) at surface, covers 3 % of this area

Metarhyolite - Dense, blue, cryptocrystalline, with white feldspar phenocrysts and glassy quartz; red porphyritic metarhyolite at contact with Catoctin Metabasalt; and Pyroclastic sediments - tuff breccia, blue slaty tuff, white tuffaceous sericitic schist, and banded green slate.

Sams Creek Metabasalt (Late Precambrian (?)) at surface, covers 3 % of this area

Grayish-green, massive to schistose, amygdaloidal metabasalt.

Grove Limestone (Upper Cambrian to Lower Ordovician) at surface, covers 3 % of this area

Dark gray to light dove, thick-bedded limestone; dolomite beds in lower part; highly quartzose limestone at base; Upper Cambrian to Lower Ordovician in age; thickness approximately 590 feet.

Chilhowee Group; Harpers Formation (Cambrian) at surface, covers 2 % 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 2 % 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.

Libertytown Metarhyolite (Late Precambrian (?)) at surface, covers 2 % of this area

Purple, bluish-black, and red, dense, fine-grained metarhyolite with feldspar phenocrysts; interbedded with blue and purple amygdaloidal metaandesite; both rhyolite and andesite interbedded with blue, purple, and green phyllitic slates.

Chilhowee Group; Loudoun Formation (Cambrian) at surface, covers 2 % of this area

Basal phyllite member: Pale purple, discontinuous, lenticular; members are in gradational contact; total thickness 0 to 200 feet

New Oxford Formation, basal liimestone conglomerate (Triassic) at surface, covers 1 % of this area

From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix

Wakefield Marble (Late Precambrian (?)) at surface, covers 0.7 % of this area

White, fine-grained marble; subordinate white, green, and pink variegated marble; and blue marble.

Sugarloaf Mountain Quartzite (Late Precambrian (?)) at surface, covers 0.6 % of this area

Massive white quartzite interbedded with softer sericitic quartzite, slate, and phyllite.

New Oxford Formation, basal quartz conglomerate (Triassic) at surface, covers 0.4 % of this area

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

Swift Run Formation (Late Precambrian) at surface, covers 0.3 % of this area

Sericitic quartzite and phyllite; blue and green tuffaceous slate with sericitic blebs; some white marble with interbedded phyllite.

Diabase Sills and Dikes (Triassic) at surface, covers 0.2 % of this area

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

Tomstown Dolomite (Cambrian) at surface, covers 0.2 % of this area

Interbedded light gray to yellowish-gray, thin- to thick-bedded dolomite and limestone; some shale layers; gradational contact with Antietam; thickness 200 to 1,000 feet.

Ijamsville Formation and Marburg Schist (Late Precambrian (?)) at surface, covers 0.2 % of this area

Ijamsville Formation - Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally; and Marburg Schist - Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

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

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

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

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

Silver Run Limestone (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Blue, thin-bedded, finely crystalline schistose limestone and calcareous slate.

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.