Post-batholithic geology of the volcanic hills and vicinity, San Diego County, California

Scale of accompanying map 1:24,000Title of accompanying map on Plate I: Geologic map of the volcanic hills and vicinity.Center coordinates of accompanying map in decimal degrees [longitude/latitude]: -116.158334?? and 32.801667??Carrizo Mountain 7.5-minute Quadrangle, scale 1:24,000, contour interval 40 feet; mapped, edited, and published by the Geological Survey Control by USGS, USC&GS and USCE, Topography from aerial photographs by ER-55 plotter, Aerial photographs taken 1956. Advance field check 1957, Polyconic projection. 1927 North American datum 10,000-foot grid based on California coordinate system, zone 6. 1000-meter Universal Transverse Mercator grids ticks, zone 11. National geodetic vertical datum of 1929Sweeney Pass 7.5-minute Quadrangle, scale 1:24,000, contour interval 40 feet; mapped, edited, and published by the Geological Survey Control by USGS, USC&GS, USCE and State of California, Topography from aerial photographs by photogrammetric methods, Aerial photographs taken 1957. Field check 1959, Polyconic projection. 1927 North American datum, 10,000-foot grid based on California coordinate system, zone 6. 1000-meter Universal Transverse Mercator grids ticks, zone 11. Vertical datum is mean sea level.Bibliography: leaves [55]-57Also available in print and on microfiche.The post-batholithic rocks of the Volcanic Hills are composed of a thin veneer of basal conglomerates of the Split Mountain Formation that are overlain by olivine basalts of the Alverson Canyon Formation. Interflow sediments and mudflows are capped by an upper basalt flow. Basaltic and andesitic dikes cut all previous units.\ud The volcanic rocks are unconformably overlain by the fine sands and muds of the Palm Springs Formation and the conglomerates of the Canebrake Conglomerate. A terrace deposit unconformably caps the section. Recent uplift has allowed the older sediment to be incised by the modern drainage.\ud Several generations of faulting have occurred in the Volcanic Hills. Middle Alverson faulting has fractured the lower flows and localized the later intrusions along these zones of weakness. Post-Palm Springs-Canebrake faulting has formed most of the present landforms. Recent faulting is present with fault traces uncovered by modern alluvium. \ud Volcanic rocks of similar composition and stratigraphic position are found seven miles to the south at Jacumba. A conflict in the stratigraphic nomenclature between corresponding units found in the Peninsular Ranges and in the Salton Trough should be resolved by downgrading the Jacumba-Peninsular Ranges units to member rank within the Salton Trough terminology