The South Texas Plains, which occupy the southern tip of Texas, may be separated from the rest of the state roughly by a line drawn from Del Rio eastward to Austin and from Austin southeastward to Corpus Christi. This is a region characterized by considerable variety. Climatically, the South Texas Plains region is subhumid to dry. Topographically, it consists of the three major divisions characterizing the Coastal Plain at large: the Interior Inner Lowlands Belt, the Coastal Belt, and the Central Dissected Belt. The Interior Inner Lowlands Belt, although sculptured into Upper Cretaceous outcrops, is largely covered with younger alluvial materials carried out from the erosion of the Edwards Plateau. Lying adjacent to the Balcones Escarpment, the Interior Lowlands Belt is characterized by black soils that extend from Austin to San Antonio and thence westward, although discontinuously, to Uvalde. Those areas having typical black-earth soils are highly important for farming purposes. The Coastal Belt comprises the low, flat country just interior from the coast. South of Falfurrias, this belt is blanketed with a thick cover of sand, some of which occurs as "live" or active dunes. Most of the sand area, however, is stabilized by vegetation, which, where the sand is deepest, includes live oak and, to the southward where the sands are shallower, mesquite shrubs and grasses. Still farther south in the Coastal Belt is the lower Rio Grande country, consisting of a smooth undulating to very slightly rolling upland of black-earth soils and the lowland of the Rio Grande, including its delta area.
Occupying the wide extent of variegated intervening country between the Inner Lowland Belt and the Coastal Belt is the Central Dissected Belt, which is further divisible into a number of sub-belts. Interior from the low Coastal Belt is a fairly wide strip underlain in considerable part by calcareous clays. South of the Guadalupe River this strip comprises the caliche plateau-a rolling upland with shallow soils underlain by a thick accumulation of caliche. The interior edge of the caliche plateau is an inward-facing scarp known as the Bordas Escarpment. The vegetation of the caliche plateau comprises one phase of the chaparral growths of South Texas. North of the Guadalupe River this strip includes a lowland in which the town of Yorktown is situated and which is continuous with the Brenham-Schulenburg Prairies; this lowland is characterized by black earth. Interior from the caliche plateau and its plains extension northeastward is a series of plains developed on the soft, unconsolidated formations of the Lower Tertiary. One of these plains includes the Winter Garden area. Near the interior margin of the Central Dissected Belt is a ridge of deep sands formed from the outcrops of the Carizzo sands and characterized as far south as the Lytle area by the presence of hardwoods. Toward the Rio Grande, the Central Dissected Belt merges into the dry country known as the Brasada, characterized by a vegetative growth of the chaparral type. The South Texas Plains region is important for its production and reserves of oil and natural gas. The fault-line fields, including Luling, Darst Creek, and Salt Creek, are well known in the history of the oil industry. Production in this district is from the top of the Edwards limestone of the Lower Cretaceous. The other two important oil and gas districts of the South Texas Plains include the Laredo district and the Corpus Christi, the latter also being known as the Lower Gulf District. See also GEOLOGY.