The Fort Hancock study area is located 40 mi (65 km) southeast of El Paso, Texas, in the Hueco Bolson on an alluvial slope between the Diablo Plateau and the Rio Grande. The study area spans the drainage divide between Alamo and Camp Rice Arroyos. Since deposition of bolson fill ceased, the arroyos have incised, cutting down to expose the Fort Hancock and Camp Rice Formations in their floors and valley walls.
Quaternary strata younger than the Camp Rice Formation underlying the study area can be divided into four units: a basal gravel, a middle sand, a petrocalcic horizon (Stage IV), and an upper sand. The petrocalcic horizon is interpreted to be the upper surface of the Madden Gravel, and, on the basis of its dense, laminated character, took 25,000 to 75,000 years to form. These Quaternary sediments range in thickness from 20 to 60 ft (6 to 18 m), thinning to the southwest across the study area and toward the edges of arroyos.Bureau of Economic Geolog
