The Signal Hill Quadrangle is located astride the Balcones Escarpment southwest of Austin. Cretaceous rocks (Albian and Cenomanian) cropping out in the area include the upper two members of the Glen Rose Formation, the Bull Creek and Bee Cave Members of the Walnut Formation, the Kainer and Person Formations of the Edwards Group, and the Georgetown, Del Rio, and Buda Formations. Deposition represented by these formations ranges from supertidal to tidal-flat to open-shelf marine environments. At one location there is a basalt plug, probably of Senonian age. The youngest deposits in the quadrangle are those associated with Quaternary terraces and alluvial sands and gravels. The faults mapped are part of the Balcones Fault Zone, a system of en echelon, northeast-trending, predominantly normal, dip-slip faults. This system was probably active in the middle Tertiary. The Mt. Bonnell Fault is the most important fault in the quadrangle. It is one of the major faults of the Balcones Fault System, having been downthrown 170-350 feet to the southeast. The total displacement of all faulting in the map area is about 800 feet. / text

Seven maps depicting general geology, physical properties, environmental geology, soils, soil thickness, slope, and fracture intensity describe the land in the Lake Travis vicinity. The maps, descriptive text, and interpretive tables provide the basis for evaluations of land-use capability. The general geologic map shows variations in bedrock, in surface deposits, and in the structural-geometrical relations of the units. It is a basic data source for constructing and interpreting the other maps. The physical properties map presents qualitative engineering characteristics of substrate and surface materials. Most of the Lake Travis vicinity is underlain by carbonate rocks that are generally stable foundations for construction. The environmental geologic map is based on variations in processes, landforms, and surface and bedrock materials. Process units represent areas subject to flooding and mass wasting as well as loci of aquifer recharge. These areas will sustain only limited use without detrimental environmental effects. Material-landform units represent areas that have constraints to land use; however, with preventive or corrective engineering the land could probably sustain a variety of uses. Soils, soil thickness, slope, and fracture intensity maps present data at a smaller scale than that of the general geologic, physical properties, and environmental geologic maps. Soil cover is generally thin or absent. Most of the land is moderately steeply to very steeply sloping. High-density fracture zones that allow rapid water infiltration occur in some of the area. The land-use capability map constructed to depict suitability for septic tank operation shows that most of the Lake Travis vicinity is generally unsuited for this use. The current land use map delimits areas of population pressure and attendant competitive uses of the land. These pressures intensify the need for enlightened planning so that land use will be compatible with limitations shown or implied on the basic maps. / text