Remnants of an ancient severely weathered basement terrain exist at several localities in the Corona area. Weathering affected Triassic metasedimentary rocks, pre-batholithic (Jurassic) hypabyssal rocks, and Cretaceous plutonic rocks of the southern California batholith. The weathered surface is buried in places under early Pliocene nonmarine claystones and sandstones of the Silverado Formation. The kaolinite-dominated paleosols record the weathering effects of a severe leaching environment similar to that found in the modern humid tropics. Thus, the latest Cretaceous and early Paleocene conditions are in sharp contrast to the present semiarid climate of the region.