Some are characterized by volcanic rocks, others by limestone. Over most of the area average annual rainfall is less than 12 inches, but varies greatly from year to year and from lower to higher elevations. July and August are usually the higher rainfall months. Mountain outwash materials have formed the soils of the Trans Pecos. Surface textures and profile characteristics are varied.
Soil reaction is generally alkaline.
Due to the diversity of soils and elevations, many vegetation types exist in the region. The principal plant communities are creosote-tarbush desert scrub, desert grassland, yucca and juniper savannahs, and montane forests of pinon pine and oak. The various subregions reflect the diversity of the Trans Pecos. The Sand Hills area consists of shin oak and mesquite on wind-blown dunes. Soils with high salt content and gypsum dunes are typical of the Salt Basin area.