The thickest known columnar section of Pleistocene in the world (about 5000 feet) is well exposed near Ventura in southern California. Even here Pleistocene sedimentation is far from complete because this succession is separated into two parts by a profound angular unconformity. The two divisions of the Pleistocene column are (1) an underlying 4700 feet and possibly as much as 5100 feet of Lower Pleistocene terrigenous shales, mudstones, sandstones, and conglomerates, mainly of marine origin, which dip 35° to more than 75° S., and (2) an overlying 300 feet or less of Upper Pleistocene fluviatile gravels and silty sands which dip only 8° to 15° S. Land vertebrate fossils, including Equus cf. occidentalis in both divisions and a large marine invertebrate fauna found in the lower steeply dipping group, demonstrate that both these groups of strata are Pleistocene.
Paleontologic, stratigraphic, and structural evidence clearly indicate that the principal orogeny in the transverse (east-west) Coast Ranges of southern California occurred during the Pleistocene, not at the beginning as has been commonly believed. This evidence suggests that two of our widely held geological concepts need to be modified.
Extensive uplift and erosion may take place within the geologic lifetime of one vertebrate species in tectonically active areas such as California.
A great angular unconformity with differences in dip as high as 60° may occur within a single geologic period instead of only at the beginning or end of a period, as is commonly assumed.