The Edwards Plateau is a geographic region at the crossroads of Central, South, and West Texas. It is bounded by the Balcones Fault to the south and east, the Llano Uplift and the Llano Estacado to the north, and the Pecos River and Chihuahuan Desert to the west. San Angelo, Austin, San Antonio, and Del Rio roughly outline the area. The southeast portion of the plateau is known as the Texas Hill Country. Natural history The bedrock consists primarily of limestone, with elevations ranging between 100 and 3000 ft. Caves are numerous. The landscape of the plateau is mostly savanna scattered with trees. It mostly lacks deep soil suitable for farming, though the soil type is fertile mollisol, so some cotton, grain sorghum, and oats are grown. For the most part, though, the thin soil and rough terrain areas are primarily grazing regions, with cattle, sheep, and goats (Angora and meat types) predominant. Several rivers cross the region, which generally flow to the south and east through the Texas Hill Country toward the Gulf of Mexico; springs occur (in wet years), but permanent surface water supplies are sparse throughout the area, except for man-made reservoirs. The area is well drained; rainwater flows into the Edwards Aquifer recharge zone at the south of the plateau to feed rivers to the south. Rainfall varies from 15 to 33 inches per year, typically, from northwest to southeast, and the area has a moderate temperature and a reasonably long growing season. Flora Trees of the savanna include junipers (locally called mountain cedar), oak species scattered over grasses, a vegetation type historically shaped by droughts and regular fires. Some pecan trees are found near the springs and rivers, which are typically lined with stands of bald cypress. The Balcones Fault is associated with the Edwards Plateau formation. This fault line is an ecological demarcation for the range definition of a number of species. Fauna Caves of the Edwards Plateau are important habitats for a great deal of wildlife. The area is home to some of the largest colonies of bats in the world, including millions of Mexican free-tailed bats. The largest colony of these inhabits Bracken Cave near San Antonio, while the Congress Avenue Bridge in Austin is the summer home for over half a million (they winter in Mexico) and is the largest bat colony anywhere in an urban area. The Edwards Plateau is