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Geologic units in Arlington county, Virginia

Sykesville Formation (Cambrian) at surface, covers 30 % of this area

Sykesville Formation - Metasedimentary melange

Lithology: melange

Potomac Formation (Cretaceous) at surface, covers 29 % of this area

Potomac Formation - Pebbly, poorly sorted quartzo-feldspathic sand interbedded with sandy clay and silt; minor organic-rich clay and silt

Lithology: sand; clay or mud; silt

Pliocene Sand and Gravel (Tertiary) at surface, covers 14 % of this area

Pliocene Sand and Gravel - Sandy gravel, gravelly sand, poorly to well-sorted sands, and thin- to medium-beds of clay and silt, at altitudes from 170-320 ft. (top of unit).

Lithology: gravel; sand; clay or mud; silt

Shirley Formation (Quaternary) at surface, covers 7 % of this area

Shirley Formation - Interbedded gravel, sand, silt, clay, and peat; at altitudes to 35-45 ft. (top of unit).

Lithology: gravel; sand; silt; clay or mud; peat

Miocene Sand and Gravel (Tertiary) at surface, covers 5 % of this area

Miocene Sand and Gravel - Sandy gravel, sand, silt, and clay.

Lithology: gravel; sand; silt; clay or mud

Indian Run Formation (Cambrian) at surface, covers 5 % of this area

Indian Run Formation - Metasedimentary melange

Lithology: melange

Artificial Fill (Quaternary) at surface, covers 4 % of this area

Artificial Fill - Areas filled for construction and waste disposal

Bacons Castle Formation (Tertiary) at surface, covers 1 % of this area

Bacons Castle Formation - Gravel grading upward into sand and sandy clayey silt

Lithology: gravel; sand; silt; clay or mud

Lowland Deposits (Quaternary) at surface, covers 0.1 % of this area

Lowland Deposits - 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.

Lithology: gravel; sand; silt; clay or mud