Trilobites collected during the past 20 years from the Morgan Creek, Point Peak, and San Saba Members of the Wilberns Formation comprise 89 species assigned to 45 genera belonging to zones of the upper Franconian and Trempealeauan Stages of the Upper Cambrian Croixan Series.

This work was begun in 1964 and substantially completed in 1965; at that time conodont zones had been established in North America only for the Upper Devonian, by Clark and Becker (1960) for the Great Basin and Collinson, Scott, and Rexroad (1962) for the mid-continent.

Based on need for convenient, small-scale cartographic units, the basal Cretaceous rocks in Texas from Red River to Burnet County and on the Callahan Divide are herein divided into three distinctive lithologic sequences.

Sediments of the Fredericksburg Division in south-central Texas were deposited on the slowly subsiding west flank of the Tyler basin. In this region there are three stratigraphically distinct areas. The southern area has a thick Edwards Limestone unit overlying a thin Walnut Formation.

Forty-three species belonging to 28 genera, 12 species of brachipods belonging to 8 genera, and 1 species of gastropods are described from the Morgan Creek, Point Peak, and San Saba Members of the Wilberns Formation in the Llano Uplift.

The Colorado River Industrial Development Association area consists of ten counties [Bastrop, Blanco, Burnet, Colorado, Fayette, Llano, Matagorda, San Saba, Travis, and Wharton counties] in the lower watershed of Colorado River in Texas.

The first entry in the Guidebook Series details a field excursion to the Eastern Llano Region of Central Texas in 1958. Retrace the footsteps of Virgil Barnes and the other state geologists in attendance: 

Stop 1. Honeycut Bend

The Devonian-Mississippian transition outcrops of central Texas are here described summarily and assigned to a new stratigraphic unit, the Houy Formation. The beds included are mainly Upper Devonian, but partly Lower Mississippian. Locally a basal fraction may be Middle Devonian.