Description
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. 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. 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. 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.
