Geologic units in Massac county, Illinois

Cretaceous undivided (Upper Cretaceous (Campanian to Maastrichtian)) at surface, covers 74 % of this area

Cretaceous undivided, includes Post Creek (Tuscaloosa), McNairy, and Owl Creek Formations (southern Illinois).

Lower Pope Group (Aux Vases Sandstone through Glen Dean Limestone) (Mississippian (Visean)) at surface, covers 14 % of this area

Lower Pope Group (Aux Vases Sandstone through Glen Dean Limestone).

Ste. Genevieve Limestone (Mississippian (Visean)) at surface, covers 10 % of this area

Ste. Genevieve Limestone.

Upper Pope Group (Tar Springs Sandstone through Kinkaid Limestone) (Mississippian (Serpukhovian)) at surface, covers 2 % of this area

Upper Pope Group (Tar Springs Sandstone through Kinkaid Limestone).

St. Louis Limestone (Mississippian (Visean)) at surface, covers < 0.1 % of this area

St. Louis Limestone.