LYCOS RETRIEVER 
Edwards Plateau: Texas Agri
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In the Trans-Pecos area (the area west of the Pecos River) and Edwards Plateau area of western and west-central Texas, the Edwards-Trinity aquifer consists of rocks of the Washita, the Fredericksburg, and the Trinity Stages, and the Coahuilan Series (fig. 80). In the Balcones Fault Zone area of south-central Texas, the rocks of the Washita and the Fred-ericksburg Stages are far more permeable than those of the overlying confining unit or the underlying Trinity aquifer and constitute the nearly separate flow system of the Edwards aquifer. Rocks of the Trinity Stage and the Coahuilan Series constitute the Trinity aquifer, which crops out on its updip edge from the Hill Country of south-central Texas into southeastern Oklahoma. In east-central Texas and into Oklahoma, rocks of the Washita and the Fredericksburg Stages that overlie the Trinity aquifer constitute a confining unit.
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The limestone bedrock of the Edwards Plateau helps to contribute to the distinctiveness of the biota. An array of species are specialists on limestone habitats, including caves. Some of the largest assemblages of cave-dwelling bats anywhere in the world, indeed the largest aggregations of mammals anywhere in the world, roost in several of the large caves of this ecoregion. Millions of Mexican free-tailed bats (Tadarida brasiliensis) use these caves as maternity roosts, creating a globally outstanding phenomena. The high number of endemic invertebrate cave species ... qualify this ecoregion as a global hotspot for cave-dwelling species. The aquatic vertebrates endemic to the Plateau include the widemouth blindcat (Satan eurystomus), San Marcos salamander (Eurycea nana), Comal blind salamander (Eurycea tridentifera), Texas blind salamander (Typhlomolge rathbuni), Blanco blind salamander (Typhlomolge robusta), and the Texas salamander (Eurycea neotenes).
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The Edwards Plateau, lying east of the continental divide, is a plateau sloping gently eastward, dropping on average about 180 cm per km, but steeper at the western margin. The southern and eastern margins, and to a lesser extent the other parts, are much dissected by the following rivers: Colorado of Texas, Guadalupe, Nueces, Rio Grande/Pecos, and tributaries thereof (Map 8). This is an ancient evolutionary arena. Most of the land surface has been exposed continuously for occupation by terrestrial biota for at least 65,000,000 years. The plateau consists of three sub-regions:
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The Edwards Plateau consists of flat-lying Cretaceous carbonate and shale units at elevations of 600 to 900 m, which blanket much of south-central Texas. Amistad Reservoir on the Rio Grande River marks the southern limit of the plateau, where it terminates at the Balcones Escarpment. This escarpment is developed along a zone of Late Cretaceous and Cenozoic monoclinal folds and normal en echelon faults. The zone separates the relatively elevated and resistant Cretaceous limestone units of the plateau from the lower and more erodible Upper Cretaceous and Cenozoic sediments of the Coastal Plain to the south and east.
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The Llano Estacado and the Edwards Plateau together constitute in Texas the Plateau of the Plains. This lies within the area inclosed by the Canadian on the north, the Pecos River on the west, the Balcones escarpment on the south and southeast, and an irregular line of scarps along the headwaters of the eastward flowing drainage of the Colorado, Brazos, and Red rivers of Texas. The general outline of this area is shown in the accompanying photograph of a model, P1. XXIII. It is over 500 miles in length and in places 280 miles broad. It is a vast quadrangular mesa, surrounded on all sides by descending escarpments.
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The primary sources of income from rangelands in the Edwards Plateau have historically been livestock production and recreational pursuits, in particular, deer hunting leases (Gerbolini 1995, Rowan and Conner 1994, Rollins and Armstrong 1994). The sale of harvested wood products from the trees and shrubs that naturally grow on the Plateau is another potential income source, but little information is available regarding the situation and outlook for markets for wood products from the Edwards Plateau. These markets dictate the prospects and the incentives to central Texas ranchers to consider wood-harvesting enterprises as part of their ranch management and planning.
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