The embayment sediments of Kentucky, Illinois, and Missouri consist of the Tuscaloosa gravel, Eutaw sand, and Ripley sand and clay of Cretaceous age, the Porters Creek clay and Lagrange sand and clay of Eocene age, and the "Lafayette" gravel and sand and the colluvium of Pliocene age. The outcrops of these sediments form an inverted U-shaped belt whose apex crosses southern Illinois and whose east and west sides lie respectively in western Kentucky and southeastern Missouri. The outer side of the U is bounded by Paleozoic strata; the center is composed largely of Pleistocene sediments and Recent deposits of Mississippi and Ohio rivers.

The outstanding features of the embayment deposits are: (1) they are almost totally unconsolidated, and (2) calcareous material is very uncommon, suggesting either deposition principally of non-calcareous sediments during Cretaceous-Tertiary times or widespread and pronounced leaching subsequent to deposition.

Lignitic strata contain a few fragmentary plant remains; animal fossils are extremely rare.

Structural data are poor, but the best available suggest a general gentle dip of the sediments toward the deeper part of the embayment, possibly with minor anticlinal folds plunging basinward in the marginal parts. Settling and creep have produced striking local deformation.

The source of the sands and clays of the embayment deposits is thought to have been principally the Pennsylvanian and upper Mississippian sandstones and shales of the adjoining parts of Illinois, Missouri, and Kentucky. The limestones in the same areas have probably also contributed to the clays. The chert gravel making up the Tuscaloosa and Lafayette formations seems to have been derived for the most part locally from cherty limestones and chert formations.