The Embayment Mega Group is found from the Gulf of Mexico north to southern Illinois.

[1] The Grover Gravel (Rubey, 1952) is found in western Illinois.

It preserves marine fossils dating back to the Cretaceous period.

The upper and lower parts consists of cross-bedded white to light grey micaceous sand.

The middle section is made up of grey to blackish silt, with beds of lignite.

It is composed of chert gravel and has moderate cementations by silica or kaolinite.

[5] The Baylis Formation is Cretaceous in age and found in the western sections of the Embayment Megagroup.