Tests of Rock Cores, Bergstrom Study Area, Texas

Laboratory tests were conducted on rock core samples from nine core holes in the Bergstrom area of Mason, Llano, Gillespie, and Burnet Counties, Texas. The results were used to determine the quality and uniformity of the rock to depths of 200 feet below the ground surface. Petrographic examination indicated that the samples represented a very complex geologic area. Five general groups of material were identified: red granite, light gneiss, dark gneiss, schist, and contact zone rock. The predominant material was red granite. Based on physical characteristics (specific gravity, Schmidt hardness, wave velocity, and compressive strength), four groups of material were present: poor, marginal, good, and excellent quality rock. Although the better quality rock is predominant, the poorer quality rock is scattered throughout the upper elevation, and one may expect to remove up to 50 feet of material in some areas before competent rock is reached. The wide area represented by the drill holes and the complex nature of the material preclude assessment of the area on a hole-to-hole basis. In general, however, the Bergstrom area appears to be one of complex geologic nature, but one yielding sufficiently competent physical test results to merit further study as a competent hard rock medium. (Author)