Between the Chattanooga shale and the underlying limestone in parts of Tennessee and Kentucky is a clayey gray to brown zone as much as several feet thick. This represents an interval of limestone that has been leached by sulfuric acid formed by oxidation of the abundant pyrite in the black shale. Alteration of the limestone decreases with distance from the base of the black shale; several stages of alteration are recognized and described. The acid also attacks the shale, as indicated by locally conspicuous efflorescences of copiapite, coquimbite, halotrichite, gypsum, and possibly other sulfates. Basaluminite, a hydrous aluminum sulfate previously reported only from England and France, was found in geodal cavities and thin seams in the residuum. The clayey zone has been previously interpreted as an ancient soil formed on the limestone surface during Devonian time.
Petrography and chemical composition of the leached zone are described and illustrated by photomicrographs. The nature of the shale-limestone contact where the leached clayey zone is absent is illustrated by a diamond-drill core from Tennessee. Many fresh outcrops and several dozen cores that penetrate the contact show no clayey material at this position.
The clayey zone does not represent an old soil.