From about 280 million years ago and earlier, an ancient ocean covered West Texas and some portions of New Mexico. Today, evidence of the Permian Period still exists in the form of the ever-present limestone and the fossils that characterize a once healthy, thriving marine ecosystem. Over the eons, a truly remarkable transformation has occurred, and the region known as the Trans-Pecos is now a desert.

North America has four desert ecosystems: The Great Basin, Mojave, Sonoran, and the Chihuahuan Desert. The latter is distinguished geographically by the Permian Basin, once an ocean floor, and also by more recent volcanic and block fault activity - giving rise to the regions' numerous mountains and canyon lands. Ecological zones in the Trans-Pecos are distinguished primarily by the Chihuahuan Desert with ecotones meeting the prairies of the South Plains to the north and the woodland savannas of the Edward's Plateau to the east.