This arid and sparsely populated region contains some of the most exciting geology in Texas. While most of the state is quite flat and less than 2500 feet above sea level, the Trans-Pecos area experienced the Laramide Orogeny--resulting in Basin and Range topography--as well as extensive Tertiary volcanism and faulting. These events elevated the basins to about 4000 feet above sea level with mountain ranges that jut up 2000 to 3000 additional feet. The hydrology of the region is also of interest since most of it is characterized by internal drainage, that is, when most of the water evaporates and the remainder seeps into the basin fill to recharge aquifers. The Trans-Pecos boundaries are roughly defined on the south and west by the Rio Grande River, on the north by the thirty-second parallel (also the boundary with New Mexico), on the northeast by the western margins of the Pecos River valley, and on the southeast by the Edwards Plateau. See also our Big Bend page.