Pliocene series - Mounds Gravel (Lafayette Formation) - ferruginous, chert gravel, max of 60 ft. Eocene series - Wilcox group - includes Holly Springs Formation - bedded sandstone, clay and gravel, max of 1200 ft.; Ackerman Formation - clay with lens of sand and gravel, max of 100ft. Paleocene series - Midway Group - includes Porters Creek Clay - bentonitic clay, max of 200 ft.; Clayton Formation - marl, fossiliferous, calcareous, limonitic, glauconitic sand and clay, max of 20 ft.