Geologic units in Union county, Mississippi

Ripley formation (Upper Cretaceous) at surface, covers 29 % of this area

(Selma group), Gray to greenish-gray fine glauconitic sand, clay, and sandy limestone; south of Oktibbeha County is very sandy micaceous chalk.

Porters Creek formation (Paleocene) at surface, covers 27 % of this area

(Midway group), Dark-gray clay, north of Clay County contains slightly glauconitic, micaceous sand lenses.

Prairie Bluff chalk and Owl Creek formation (Upper Cretaceous) at surface, covers 11 % of this area

(Selma group), Prairie Bluff chalk, compact brittle chalk, sandy chalk, and calcareous clay; at base contains many phosphatic molds of fossils; in Ponotoc and Union Counties merges northward into Owl Creek formation, tough blue glauconitic sandy clay.

Wilcox formation (Eocene) at surface, covers 11 % of this area

Irregularly bedded fine to coarse sand, more or less lignitic clay and lignite; includes bauxite bearing Fearn Springs sand member at base.

Clayton formation (Paleocene) at surface, covers 10 % of this area

(Midway group), Upper part, greenish-gray coarsely glauconitic sandy clay and marl; lower part, crystalline sandy limestone and loose sand, represented south of Houston by a discontinuous bed of indurated calcareous sandstone.

Demopolis chalk (Upper Cretaceous) at surface, covers 8 % of this area

(Selma group), Chalk and marly chalk containing fewer impurities than underlying and overlying formations.

Ripley formation (McNairy sand member) (Upper Cretaceous) at surface, covers 4 % of this area

(Selma group), Red and white cross-bedded micaceous sand and white sandy clay.