Hornfels is a hard, brownish-black to black, fine-to-coarse grained stone resulting from thermal metamorphism, or the baking of sediments adjacent to volcanic dikes and rills. In the eastern Trans-Pecos region, outcrops of hornfels are most common in the south—in Big Bend National Park and immediately adjacent areas—where they invariably occur at the margins of volcanic masses such as laccoliths. Major sources of black hornfels are found in the foothills of the Chisos, Christmas, and Rosillos Mountains in and near the park, as well as in desert pavement deposits along the margins of drainages such as Tornillo Creek. High quality hornfels cobbles are also commonly found in the floors of arroyos as well as in gravel bars of the adjoining Rio Grande, where they have been moved downstream at least as far as the Lower Pecos River.
Prehistoric peoples of the southern Big Bend fashioned tools from hornfels for thousands of years, and it may have been a preferred stone for some classes of implements. Because of its high silica content, lack of impurities, and a general availability of large-sized boulders and cobbles, hornfelses were well-suited for the manufacture of large expediency tools—such as those used in the processing of plant foods. Less frequently, more formal tools such as bifacial knives, end scrapers, perforators, and projectile points were fashioned from hornfels as well. The extensive hornfels quarries found sprinkled throughout the northern half of Big Bend National Park invariably contain massive quantities of worked stone, including a wide range of core types and hand-sized or larger flakes and blade-like flakes. The faces of outcropping hornfels boulders often are covered with the negative scars of flake removals, and worked pieces can often be refitted onto the parent cores. Unlike higher quality toolstone such as jasper, chert, and chalcedony, the somewhat coarser hornfels was only rarely carried significant distances from its primary source area by prehistoric peoples.