A reasonable interpretation of the geologic history of the Colorado River in the Basin and Range province can be put together by considering the work done by several geologists in the Imperial Valley, California, the Parker–Blythe–Cibola area, California and Arizona, and, the Lake Mead and Hualapai Plateau areas, Arizona. In the Imperial Valley, the Imperial Formation, whose age has been interpreted variously as late Miocene or early Pleistocene but most commonly Pliocene, records a transgression of the marine waters of the Gulf of California and deposition of sediments very probably of Colorado River origin. These sediments contain reworked Late Cretaceous foraminifers derived from the Mancos Shale of the Colorado...
