The Cap Rock Escarpment forms the eastern boundary of the vast Llano Estacado, or "Staked Plains," an elongated oval and one of the world's most perfect plains regions, with about three quarters of its twenty million acres in Texas and the rest in eastern New Mexico. Its naturally treeless surface, broken mainly by several canyons, slopes gradually from about 2,700 feet at its eastern edge to more than 4,000 feet along the New Mexico border. The Cap Rock Escarpment, created by surface erosion beginning in the early pleistocene period some 750,000 years ago, is composed of tough caliche that protected the softer materials beneath it and allowed it to resist erosion with varying success. Beginning in Borden County twenty-five miles south of this point, the escarpment extends northward in a sweeping arc one hundred seventy miles into the Texas Panhandle and rises from three hundred to one thousand feet above the lower plains at its base. In 1541 the Spanish conquistador Francisco de Coronado crossed the Great Plains while searching for the fabled Seven Cities of Cibola and was struck by the sea of grass that left no permanent trace of his expedition; according to tradition, Spaniards staked their route for the return trip, giving rise to the name "Staked Plains." Because surface water was scarce, buffalo and Indians generally avoided the Llano until settlers in lower areas pushed them onto it. After the buffalo disappeared, the native grasses supported an immense ranching empire, and more recently the Llano became one of the nation's leading areas for producing cotton, wheat, and grain sorghum.