BOFECILLOS CANYON. Bofecillos Canyon, in southern Presidio County, begins on the north side of Elephant Mountain (at 29°28' N, 104°02' W) and runs west for ten miles to its mouth on the Rio Grande, eight miles southeast of Fort Leaton State Historic Site (at 29°29' N, 104°12' W). It crosses the eroded volcanic terrain of the Bofecillos Mountains area and terminates in the gravel terrace of the Rio Grande floodplain. The soils in the area are generally sands and clay loams, often rough and stony. The vegetation consists primarily of sparse desert shrubs.