Various parts of the Ellenburger (Cambro-Ordovician) formation have been penetrated within the West Texas area bounded by Latitude 32° and 30° and Longitude 101° and 103°. The formation consists of fine to coarsely crystalline dolomites and dense limestones. These lithologic units are of little value even in local correlations when accurate work is desired.

A careful study has been made of the insoluble residues from most of the well cuttings available. The chief criterion for correlations is chert. Four main classes of chert are recognized: smooth, granular, chalky, and drusy. Each class may have several secondary characteristics, and gradational types are present. From this work the complete Ellenburger section, which is approximately 1,335 feet thick in western Crane County, has been divided into five zones.

Chert zones have made it possible to locate accurately the stratigraphic position entered in the dolomite section. The upper part of the Ellenburger has been truncated in four wells in the Big Lake field (Reagan County). Restoration of the truncated part shows the axis of the folding to be parallel with the Permian folding (San Andres) which extends northwest and southeast rather than north and south.

Approximately 275 feet of Ellenburger is absent from the base against the Apco Ridge (Pecos County) probably due to overlap, and about 900 feet has been eroded from the top of the section in this area.