The emerged part of the Atlantic Coastal Plain is underlain chiefly by Cretaceous and Tertiary sediments above the basement rocks. Some deep 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 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 sands and clays 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.

The basement underlying the eastward- and southeastward-dipping homoclinal beds consists chiefly of crystalline rocks and to a lesser extent Paleozoic and Triassic sedimentary rocks. The basement is a shallow platform beneath the updip portion of the Coastal Plain, but in southern New Jersey and eastern North Carolina the slope steepens where the platform adjoins the western border of a north-trending trough. The Peninsular arch of Florida and the Cape Fear arch of North Carolina are two northwest-trending positive elements. An embayment in southeastern Georgia lies 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) lack of consolidation of sand and clay except at great depth, and (5) decreasing porosity and permeability with depth in coastal areas.

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