The gas at Jackson, Mississippi, is produced from hard, porous limestone between the Selma chalk (Upper Cretaceous) and the Porters Creek clay of the Midway group (Eocene). Minor structural features of the gas-producing horizon are determined by the topography of an old erosion surface at the top of the Selma. Directly underlying the Selma chalk is the Tuscaloosa formation which contains volcanic and intrusive material in its lower part. This igneous activity took place early in Tuscaloosa time, probably at approximately the same time as that in Arkansas, which suggests that the lower part of the Tuscaloosa is equivalent to part of the Woodbine sand.

Underlying the Tuscaloosa formation is a series of hard, steeply dipping rocks that are considered to be of Carboniferous age on the evidence of plant material and close lithologic similarity to rocks of the Pottsville formation which crop out near Tuscaloosa, Alabama.

Discovery of steeply dipping Paleozoic rocks at Jackson suggests that the belt of Appalachian folds passes through Mississippi near this point.