A series of kaolinitic claybeds marks the transition from marine to non-marine sediments in the Silverado Formation exposed in San Joaquin Hills. These claybeds are identified as kaolinitic and lateritic paleosol developed in arkosic sandstones, and indicate a change in climate from arid to tropical occurred during the middle to late Paleocene. High precipitation rates found in humid tropical environments provide a nearly constant influx of water, and all minerals except quartz are leached out. When ferrallitic soils develop in quartz-rich rocks, kaolinite forms in subsurface horizons. Poor drainage within this kaolinitic horizon produces red and white mottling.
Watertable fluctuations promotes the development of a ferricrete crust.
The lowermost paleosol comprises four distinct beds and is best exposed in the northernn part of the field area. All units contain quartz in a kaolinitic matrix. Hematite staining of the kaolinite matrix produces red to lavender mottles. Paleosol 1 grades laterally to the south into a saprolite. Mono- and polycrystalline quartz grains are partially dissolved, etched, and angular.