Geology of the Signal Hill quadrangle, Hays and Travis Counties, Texas

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.