Geology and Mineral Resources of McLennan County by W. S. Adkins

"The Del Rio formation is at most places on the outcrop in McLennan county the highest formation of the Comanchean, and at such places directly underlies the Eagle Ford...

The Del Rio is about seventy feet thick in McLennan County. It contains a well defined sequence of fossil zones essentially similar to that observed at Fort Worth and Denison.

The lower part of the formation is distinguished by its thin calcareous and sandy seams and contains Turrilites sp., Kingena wacoensis (Roemer), and Exogyra arietina (Roemer). The middle portion contains a zone of abundance of Exogyra arietina, above which this fossil is rare. Above this is a well marked zone of abundance of Gryphea mucronata. At this level are hematite layers and abundant pyrite or limonite fossils. Peltastes sp. is associated with these. The top part contains few fossils, mainly echinoids, Pecten texanus, and a few ammonites. At this level at Georgetown, an Exogyra, probably E. cartledgei Bose was found."