Knowledge of the regional sand distribution and its relationship to formation temperature and pressure is a preliminary step in evaluating the geothermal resources of the Frio Formation. At depths generally greater than 7,000 feet, the sands and shales of the Frio Formation are overpressured and undercompacted. The insulating effect of these overpressured and undercompacted sediments results in the accumulation of subsurface heat and, thus, high temperature water.

Nelson and others (1962) define a bank as " ... a skeletal deposit formed by organisms which do not have the ecologic potential to erect a rigid wave-resistant structure." They explain that a bank may have any geometry. The principal types or end members are biostromes which are thin, flat to lenticular deposits, or bioherms which are mounds.

Cracked pavements, undulating road surfaces, broken curbs, stairstep fractures of brick and stone building walls, and tilted power poles are common occurrences in areas underlain by cracking, expansive clay soils of the Vertisol order. These soils, which underlie 15 to 20 percent of the Coastal Plain of Texas, are composed predominately of montmorillonite and develop distinctive microtopographic features known as gilgaies. Relief between adjacent microknolls or ridges and microdepressions ranges up to 18 inches (45.7 cm).

Historical monitoring along Brazos and south Padre Islands records the nature and magnitude of changes in position of the shoreline and vegetation line and provides insight into the factors affecting those changes. Documentation of changes is accomplished by the compilation of shoreline and vegetation line position from topographic maps, aerial photographs, and coastal charts of various vintages.