Geologic units in Saint Mary's county, Maryland

Upland Deposits (Western Shore) (Quaternary) at surface, covers 44 % of this area

Gravel and sand, commonly orange-brown, locally limonite-cemented; minor silt and red, white, or gray clay; (includes Brandywine, Bryn Mawr, and Sunderland Formations of earlier reports); lower gravel member and upper loam member in Southern Maryland; thickness 0 to 50 feet.

Lowland Deposits (Quaternary) at surface, covers 28 % of this area

Gravel, sand, silt, and clay. Medium- to coarse-grained sand and gravel; cobbles and boulders near base; commonly contains reworked Eocene glauconite; varicolored silts and clays; brown to dark gray lignitic silty clay; contains estuarine to marine fauna in some areas (includes in part Pamlico, Talbot, Wicomico and Sunderland Formations of earlier reports); thickness 0 to 150 feet.

Chesapeake Group; Choptank Formation (Miocene) at surface, covers 16 % of this area

Interbedded brown to yellow very fine-grained to fine-grained sand and gray to dark bluish-green argillaceous silt; locally indurated to calcareous sandstone; prominent shell beds; thickness 0 to 50 feet.

Chesapeake Group; St. Marys Formation (Miocene) at surface, covers 10 % of this area

Greenish-blue to yellowish-gray sandy clay and fine-grained argillaceous sand; thickness 0 to 80 feet.

Chesapeake Group; Calvert Formation (Miocene) at surface, covers 3 % of this area

Plum Point Marls Member: Interbedded dark green to dark bluish-gray, fine-grained argillaceous sand and sandy clay; contains prominent shell beds and locally silica-cemented sandstones; and Fairhaven Member: Greenish-blue diatomaceous clay, weathers to pale gray; pale brown to white, fine-grained argillaceous sand; and greenish-blue sandy clay; total thickness 0 to 150 feet.