Large surfaces polished by wind-borne dust on bedrock and boulders are abundant along the margins of the Davis, Quitman, Finlay, and other mountains in the Trans-Pecos region. At present the polished surfaces are being destroyed by weathering, and new polishing is not taking place. The polished surfaces are thus relics of a past time when wind-borne dust was a more effective agent of corrasion than at present.
Indian petroglyphs engraved on and partially destroying wind-polished surfaces in the Finlay Mountains are unmodified by later wind action. Near-by middens contain fragments of pottery which, according to the archeological chronology, were made between 1200 and 1300 A. D. Granting that the associated petroglyphs and pottery are contemporaneous, the picture-writings are 600 or 700 years old, and the wind-polished surfaces are still older.
Alternation of relatively humid and relatively arid intervals during the late Quaternary has already been postulated on evidence provided by alluvial deposits in the Davis Mountain area. It is suggested that the wind-worn surfaces here described were formed during one of the dry periods. The longest and most important such interval intervened between deposition of the Neville and Calamity formations. The polished surfaces presumably date back to this dry period which is to be correlated with the so-called Postglacial Optimum 5000 to 7500 years ago.