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Rocks of the South Mountain Anticlinorium and Frederick Valley

The information contained on this page was adapted from Maryland Geological Survey's Geologic Map of Maryland (1968). This information reflects geologic interpretations from over 20 years ago and do not necessarily represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.

Diabase Sills and Dikes (Triassic) Diabase Sills and Dikes
Sills: greenish-gray to black, medium-grained;
dikes: greenish-gray to black, medium- to fine-grained; local contact metamorphic aureoles.
Gettysburg Shale (Triassic) Gettysburg Shale
Red shale and soft red sandstone, siltstone; estimated thickness less than 5,000 feet.
New Oxford Formation (Triassic) New Oxford Formation
Red, maroon, and gray sandstone, silts tone 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 4,500 feet.
  Grove Limestone (Cambro-Ordovician) Grove Limestone
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.
  Frederick Limestone (Cambrian) Frederick Limestone
Blue, slabby, thin-bedded limestone and minor shale; contains Upper Cambrian (Trempealeauian) fuanule; thickness approximately 480 feet.
  Tomstown Dolomite (Cambrian) Tomstown Dolomite
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.
Antietam Formation (Cambrian) Antietam Formation
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 ft.
Harpers Formation  (Cambrian) Harpers Formation
Brown to dark bluish-gray banded shale, 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.
Weverton Formation (Late Precambrian ?)

Weverton Formation
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.

Loudoun Formation (Late Precambrian ?) Loudoun Formation
Upper conglomerate member: Quartz and granitic pebbles in pale purple phyllitic matrix; basal phyllite member: Pale purple, discontinuous, lenticular; members are in gradational contact; total thickness 0 to 200 feet.
Metarhyolite and Associated Pyroclastic Sediments (Late Precambrian ?) Metarhyolite and Associated Pyroclastic Sediments
Metarhyolite: Dense, blue, cryptocrystalline, with white feldspar phenocrysts and glassy quartz; red porphyritic metarhyolite at contact with Catoctin Metabasalt; Pyroclastic sediments: Tuff breccia, blue slaty tuff, white tuffaceous sericitic schist, and banded green slate.
Catoctin Metabasalt (Late Precambrian ?) Catoctin Metabasalt
Thick-bedded metabaslt with amygdaloidal layers and secondary veins of quartz, calcite, and epidote; interbedded green tuffaceous phyllite and blue amygdaloidal metaandesite.
Swift Run Formation (Late Precambrian ?) Swift Run Formation
Sericitic quartzite and phyllite; blue and green tuffaceous slate with sericitic blebs; some white marble with interbedded phyllite.
Granodiorite and Biotite Granite Gneiss (Precambrian) Granodiorite and Biotite Granite Gneiss
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.

Updated 6/1/00

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