The emerged part of the Atlantic Coastal Plain is underlain chiefly by Cretaceous and Tertiary sediments. Some deep-lying beds may be of Jurassic Age, and thin deposits of Quaternary age blanket coastal areas. In aggregate, the sediments thicken as a wedge toward the coast; at the extreme tips of southern New Jersey and eastern North Carolina they are about 10,000 feet thick, and in southern Florida they are thicker than 15,000 feet.

Predominantly marine sand and clay characterize the entire sedimentary sequence north of North Carolina, as well as the Cretaceous sequence north of Florida. Near-surface calcareous rocks of Eocene age extend from North Carolina through Florida. Pre-Pleistocene rocks of Florida are largely carbonates.

Eastward- and southeastward-dipping homoclinal beds lie on a basement consisting chiefly of crystalline rocks and to a minor extent of Paleozoic and Triassic sedimentary rocks. The basement is a shallow, nearly flat platform beneath the updip part, or inner half, of the Coastal Plain, but in southern New Jersey and eastern North Carolina, where the platform adjoins the western border of a northward-trending trough, the slope of the basement surface steepens. The Peninsular, or Ocala arch of Florida and the Cape Fear arch of North Carolina are two northwestward-trending positive elements. An embayment in southeastern Georgia is situated between them.

Common tendencies include: (1) downdip change in many formations from coarse clastic to fine clastic to carbonate facies, (2) downdip thickening of beds, (3) downdip increase in number of beds, (4) unconsolidation of sands and clays except at great depth, and (5) decreasing porosity and permeability with depth in coastal areas.