West-central Texas is strategically located along the Texas Comanche outcrop belt. On the east and north, the Comanche section is influenced by terrigenous

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clastics allowing the classic formational breakdown of this region. On the south and west, the section is almost entirely limestone. The area of study is a convenient avenue for correlation between these two areas.

The Cretaceous of west-central Texas consists of a basal quartz sand, the Antlers Sand, and an overlying carbonate sequence. The Antlers is laterally equivalent to both Fredericksburg and Trinity sequences at the northeast. The southern limit of the usefulness of the term is at the latitude of the Brady Mountains, Menard County. The carbonate sequence above the Antlers can be divided into two geologically distinct areas; a northern area, generally coincident with the Callahan Divide, and a southern area underlain by the northern Edwards Plateau.

The carbonate sequence on the Callahan Divide may be broken into two units: (1) a basal, nodular, marly unit, and (2) an overlying massive, rudistid-bearing limestone. The basal unit has characteristics of, and is laterally equivalent to, the Walnut and Comanche Peak Formations of central Texas. These units cannot be recognized and the sequence is termed the Walnut-Comanche Peak undifferentiated. The overlying massive rudistid-bearing limestone is continuous with the Edwards of north-central Texas and the term Edwards is used across the Callahan Divide.

South of the northern Edwards Plateau, the Edwards Limestone replaces the Walnut-Comanche Peak by facies change to occupy the entire Fredericksburg interval. The term "Edwards" should include the beds which extend southward into the Edwards Plateau proper until they are lost at the position of the Devils River Limestone in Edwards and Val Verde Counties. The top of the Edwards is an unconformity from Edwards and Val Verde Counties northward to an east-west line running through central Irion, Tom Green, and Concho Counties. North of this line the contact appears to be conformable. The base of the unit lying above the Edwards (unnamed upper formation of Lozo and Smith) is termed the "Dr. Burt ammonite beds." This stratigraphic datum can be extended northward from the Edwards Plateau to he Callahan Divide south of Abilene in Nolan County.

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