Cienega brown chert, a name coined here, comes from the panhandle of Big Bend Ranch State park (BBRSP) in Presidio County. It has been found in a single location immediately adjacent to an alkali basalt or hawaiite dike that lies perpendicular to Cienega Creek, southwest of the Cienega Mountains. The dike in this area has cut through a deposit of air fall tuff, which, among other things, includes Cretaceous sedimentary rocks (which, in this case, includes chert). Two documented outcrops of the Cienega brown chert occur in this location, both about a meter in diameter.
This material was sought after by prehistoric peoples because it has been naturally heat treated by volcanic activity. Technically a metamorphosed rill, brown chert was “baked” when heated by molten igneous material flowing through cracks in the surrounding limestone within which the chert had originally formed. This gave the chert a glossy brown sheen and improved its quality, making it easier for prehistoric peoples to knap. A Folsom point made of this material and found within a kilometer of the quarry shows that the source area was known by early Paleoindian times. Undoubtedly later peoples continued to make use of the material as well, as the outcrop area is strewn with flakes and chips of this distinctive stone.