The current wildcatting campaign in southwestern Michigan has revealed many irregularities in the interval between the "Red Rock" horizon at the base of the Coldwater formation of Mississippian age, and the Traverse limestone of Devonian age. It is the purpose of this paper to analyze these irregularities, and toward this end various isopachous maps and cross sections have been prepared. The results and conclusions derived from these studies are summarized as follows.

Regionally, the "Red Rock" to Traverse limestone interval thickens toward the west and northwest, due primarily to thickening of the Ellsworth formation, and possibly due to some thickening of the Antrim formation in these directions. Locally, the thickening off structure, which takes place throughout the complete section between the Traverse limestone and the Coldwater "Red Rock," tends to prevent minor structural or topographic irregularities on the surface of the Traverse limestone from being reflected in the overlying beds, and to minimize in the younger beds the degree of dip off the flanks of the larger structural features.

Evidence for the existence of an Ellsworth basin in southwestern Michigan is fairly definite, and the possibility that such a basin may have existed during the deposition of the Antrim formation is suggested.