The Van Horn-Sierra Blanca region lies athwart part of the boundary between two contrasting geologic provinces. The contrast between these two provinces, splendidly displayed by Holocene landforms in the present map area, has a history that dates back to at least Late Precambrian time.

The Bofecillos Mountains area of Trans-Pecos Texas contains a Tertiary volcanic vent and a varied sequence of lava flows, tuff, ash-flow tuff, and associated conglomerate, sandstone, and mudrock; after most of the volcanic activity had ceased, the area was block faulled and later dissected into a

At the northern end of the Coahuila platform, the thinned Comanchean sequence consists of the newly named Sixshooter group of carbonate formations underlain by the newly named Yearwood formation.

The rim rock of the Vieja Rim, the quartz pantellerite of Lord, is named the Bracks Rhyolite.