Geologic features of a section across the Casa Loma fault, exposed in an aqueduct trench near San Jacinto, California

The active Casa Loma fault, which closely parallels the San Jacinto fault, was briefly exposed in an aqueduct trench across San Jacinto Valley in southern California. Physical features, a gravity survey, profile levels, and ground-water data show that San Jacinto Valley is an active graben, which causes concern for the engineer designing large hydraulic structures here. A detail section across the fault reveals dip-slip drag in the bedded sand on the valley side, but contortion folds in the 20-ft thick fault gouge are not compatible. Minor shear planes with offsets to 3 1/2 ft extend 210 ft W. of the fault.