The Edwards Plateau, in South Central Texas east of the Pecos River and west of the Colorado, is the southernmost unit of the Great Plains. Physiographically, it is an erosional region with thin soil over beveled Comanchean limestone exposures that extend as limestone beds to constitute the underpinning of the High Plains, lying above the Permian and Triassic beds and beneath the more recent unconsolidated Pliocene and Pleistocene deposits, the latter forming the constructional surface of the High Plains. If any such loose cover ever mantled the Edwards Plateau, it has long since been carried away by erosion, although remnantal summit areas in the northwest portion of the plateau, in the transition zone adjacent to the High Plains, are characterized by a cover of unconsolidated silty materials. On such areas a deep black soil of the Chernozem type occurs. By and large, however, the Edwards Plateau is erosional with the margins of the region frayed rather deeply so that the plateau as a whole is perceptibly higher than adjacent areas.
Its distinctive physical features, especially its lack of deep soils suitable for farming, cause the Edwards Plateau to be an outstanding grazing region of Texas. Its cattle, sheep, and goat industries are of national importance. The distribution of livestock in the region serves to indicate the various types of the natural environment, particularly as reflected in the different kinds of natural vegetation. Cattle, for instance, are grazed on the typical mesquite-shrub, short-grass areas, characterized by deeper soils. Such areas are the best grazing lands of the region. The next best grazing areas support large numbers of sheep, while the poorest support large numbers of goats. Limited farming is carried on in the deeper soil areas along the broader valleys in the northeast quarter of the Edwards Plateau, as well as in the black-earth soil district on a remnantal summit area in the vicinity of Eldorado. Although some cotton is grown, much of the agriculture is devoted to grain sorghum production as an adjunct to the predominant livestock enterprises.
Along the northeastern edge of the plateau occurs a distinctive district unlike any other part of Texas. This is the Llano country, from which the radically dipping Comanchean limestones and older sedimentaries have been removed by erosion, thereby unroofing the pre-Cambrian rocks that form the central and higher portion of the area. The Llano country is a circumscribed basin, rimmed in by interior-facing escarpments, particularly of the Comanchean limestones. The inner portion is underlain by granitic and metamorphosed pre-Cambrian rocks; the outer portion, rimming the granitic outcrops, is underlain by Paleozoic sedimentaries that dip beneath the surrounding and geologically younger Comanchean limestones. Though structurally an uplifted area, the rough and broken Llano district is topographically a basin threaded by the Llano and Colorado rivers. It is timbered, but grasses are practically absent on the rougher portions of the granitic central section.
Historically, the Edwards Plateau as a whole, like the High Plains, is a region in which supplies of permanent surface water are sparse. Before modern transportation and the means of tapping underground supplies of water were developed, travel across the Edwards Plateau country was difficult. Even in 1950 no railway line crossed the entire region.