Running in an arc from west of Uvalde to north of Austin, this fault zone comprises the primary recharge zone where water enters the Edwards Aquifer, the sole source of drinking water for San Antonio and many other communities in this region.

EdwardsAquifer in Bexar County

Time and erosion have created an intricate fringe of steep, rugged hills and narrow, protected valleys and canyons on the southern and eastern edge of the plateau. This is known as the Texas Hill Country and is filled with a delightful diversity of microclimates, plants and animals.

Within Friedrich Wilderness Park, much of the younger Edwards limestone has eroded away; only small remnant caps may be found on top of the hills. The majority of the park is the older Upper Glenn Rose limestone, which is composed of alternating harder and softer layers. The softer layers erode more rapidly, exposing large bands of the harder limestone. This gives the hills their terraced or balconied appearance. Hence the Spanish name "Balcones".

Another stone found in the park is "chert" or "flint". This is also a sedimentary rock formed from the remains of plankton and diatoms. It began formation on the sea floor as a colloidal mat. Particles began to coalesce and underwent chemical changes to become a gel, then finally became the hard, glassy solid called "chert".

The soils formed from the limestone deposits are generally dark brown loam with much sand, gravel, and rock. These soils are very high in pH (alkaline). Many plants cannot survive in alkaline soils where critical nutrients may be unavailable for plant nutrition. In addition, the steep hillsides have very shallow and unstable soils. It is easily eroded, washing down to collect in ravines and bottomlands to form pockets of deeper soil. Studying the vegetation on between the balconies, in the ravines, and along the bottom of the slopes reveals how this effects vegetation.