The Llano Estacado is that part of the High Plains south of the Canadian River in northwest Texas and eastern New Mexico. Encompassing more than 30,000 square miles, this vast, semiarid tableland is one of the flattest parts of the United States. Its surface slopes gently toward the southeast at about eight to ten feet per mile, and elevations range from around 2,500 feet on the eastern and southern margins to more than 4,200 feet in the northwest.
The Llano is a coalescent alluvial plain composed of Tertiary and Pleistocene deposits carried east from the Rocky Mountains by east.west trending streams. This alluvium also forms the massive Ogallala Aquifer that underlies the Llano and is the region's major water source. Later the Llano was isolated hydrologically from the rest of the High Plains by the downcutting of the Canadian River and was deprived of runoff from the Rockies by the headward erosion and stream piracy of the Pecos River. Distinct physical boundaries mark three sides: the rugged valley of the Canadian River in the north and the prominent Caprock and Mescalero Escarpments on the east and west respectively. To the south, the Llano merges imperceptibly with the Edwards Plateau. The surface of the Llano is covered with widespread eolian deposits of sands, marls, and loams. Sand sheets and dunes are found in many places, with the largest dune field extending across the New Mexican Llano into Bailey and Lamb Counties, Texas. Among the Llano's topographic features are numerous playa basins and a few shallow draws that drain southeastward and form the headwaters of the Red, Brazos, and Colorado Rivers. These intermittent streams have cut scenic canyons into the eastern escarpment, most notably Palo Duro Canyon. Nearly all runoff on the Llano, however, accumulates in the thousands of ephemeral, freshwater playa lakes, which capture as much as two to three million acre-feet of water annually, although most of it soon evaporates.
Several theories explain the origin of the name Llano Estacado, though none is universally accepted. At present, a favored theory is that "estacado" refers to the palisaded or stockaded appearance of the caprock in many places, especially the west-facing escarpment in New Mexico.
See also WATER: Playa Lakes.
Otis W. Templer Texas Tech University
Hunt, Charles B. Natural Regions of the United States and Canada. San Francisco: W. H. Freeman and Company, 1974.
Leatherwood, Art, and Otis W. Templer. "The Llano Estacado: A Geographic Overview." In Land of the Underground Rain: Water Usage on the High Plains, edited by Donald W. Whisenhunt. Portales, NM: Eastern New Mexico State University, 1974: 12–22.