Petrology and structure of the Crystal Lake area, Los Angeles County, California

Abstract

Crystal Lake is located near the head of the north of the San Gabriel River in the San Gabriel Mountains. Igneous and syntectic rocks are the dominant types in this locality. However, remnants and inclusions of the older metamorphic complex of meta-sediments and dike rocks, which is dominant both north and south of the area studied form a large part of the exposures. The complex is intruded by a quartz diorite similar to the Wilson diorite described by Miller several miles to the southwest. The dioritic rook is in turn intruded and replaced by granitic material, the only intrusive evidence of which consists of a network of pegmatite and aplite dikes. Migmatization is indicated by gradations in structure and composition from the quartz diorite to the granite porphyroblastic development in the intermediate rock, and replacement textures in the granite and intermediate rock. Two parallel high-angle normal faults with displacements of 200 to 400 feet cross the area in a northeast southwest direction. Associated with the major faults are innumerable minor faults and fractures with displacements from a fraction of an inch to a foot or more in magnitude. The formation of Pine Flat Basin, the broad valley containing Crystal Lake, has been ascribed to glaciation, but the author believes that structural movements and stream erosion with accompanying landslides were the responsible factors. This explanation accounts for all of the physiographic features of the basin and is more compatible with the geographic location and climatic conditions.