Description
The mapped area in the southern part of the Alpine topograprtc quadrangle, San Diego County, California contains a few small roof pendants, batholithic rocks, and alluvium. The roof pendants are the oldest rocks in tne area. One is composed of quartz-muscovite-silli manite schist, containing dumortierite. Two others are composed of quartz-mica schist and quartzite. Exposures are mainly batholithic rock, probably of Cretaceous age. These, in order of dect'easing age, are Cuyamaca Gabbro, Diorite, Green Valley Tonalite, Bonsall Tonalite, and Woodson Mountain Granodiorite, aplite and pegmatite dikes. QU.aternary alluvium has been deposited in stream channels. The structural features include foliations, inclusions, and joints. The foliations were developed by the flowage of magma and appear near contacts and parallel to them. They are particularly pronounced in Green Valley Tonalite and Woodson Mountain Granodiorite in the northvest pa.rt of the area. Inclusions are abundant in tonalites and are believed to have been formed from gabbroic rock. They bear no prefeITed orientation. Joints are well-developed in Woodson Mountain Granodiorite striking north and northwest and dipping steeply. Dikes are numerous in all units and frequently strike in the same direction as joints. These primary structures together with the contacts of the batholithic rocks were useful in the determination of the intrusive sequence. The field evidence and petrographic studies indicate an origin from differentiated magma.
