Preliminary geologic map of the El Cajon 30' x 60' quadrangle, Southern California

This data set maps and describes the geology of the El Cajon 30' x 60' quadrangle, southern California. Compilation of the El Cajon quadrangle is based upon published mapping at various scales, unpublished mapping at 1:24,000 scale, and reconnaissance mapping. Mapping was done by fieldwork and the use of aerial photographs at 1:24,000 scale. The El Cajon quadrangle includes parts of two physiographic provinces: the Peninsular Ranges Province on the west underlies the major part of the quadrangle; the western Colorado Desert (locally called the Anza-Borrego Desert) underlies the northeastern corner. The approximate boundary between these two provinces is the Neogene Elsinore Fault Zone, the westernmost on-land strand of the San Andreas Fault System. Movements within the Elsinore Fault Zone are believed to have resulted in the uplift and westward rotation of the Peninsular Ranges block relative to the western Colorado Desert (Gastil and others, 1975). As a result, elevations in the El Cajon quadrangle increase from less than 100 m in the westernmost part of the quadrangle to ~2000 m along the irregular mountainous spine of the Peninsular Ranges (the Cuyamaca, Laguna, Tierra Blanca, and Jacumba Mountains); elevations then decrease rapidly eastward to