In situ outcrop measurement of magnetic susceptibility with a portable hand-held meter is a quick and useful tool in provenance investigations of siliciclastic strata. The volume percent of magnetite in sandstone and granitoid conglomerate clasts can be approximated and related to potential source areas. Two provenance investigations in southern California illustrate this technique. Magnetic-susceptibility measurements of sandstone and granitoid conglomerate clasts in Upper Cretaceous forearc strata of the Cabrillo Formation in San Diego indicate that the source region(s) were dominated by ilmenite-series granitoid rocks similar to those that underlie the eastern part of the mid-Cretaceous Peninsular Ranges batholith (PRB). These data are consistent with recent thermochronologic studies of the batholith and forearc sandstone that document rapid uplift, cooling, and erosion in the eastern part of the batholith at approximately the same time the Cabrillo Formation was deposited. Magnetic susceptibility of sandstone through part of a nearly equal 6 km-thick section of Neogene rift-related strata in the western Salton Trough clearly record the abrupt arrival in earliest Pliocene time of ancestral Colorado River sediments into the basin. Pre-Colorado River sediments are dominated by local ilmenite-only granitoid source regions of the eastern Peninsular Ranges that contrast with regional magnetite-bearing Colorado River sands. The transition previously has been recognized via painstaking petrographic and micropaleontologic studies. The change in provenance can be recognized in a few minutes in the field using in situ magnetic-susceptibility measurements.