COASTAL PLAINS

The coastal plains of southern California are those areas that lie adjacent to the Pacific Ocean and extend inland to the base of the interior foothills. On several occasions in past geological time, these areas have been inundated by advancing seas which deposited thick layers of sediments. Today we recognize these extensive flat deposits of sand and shale as a series of marine terraces.
The flat nature of the coastal plains developed in shallow seas as seafloor depressions became filled with sediments more rapidly than the adjacent level surfaces, producing an almost perfectly flat topography. Then, through the millennia, the seas retreated, exposing the seafloor as a new terrestrial mass - the coastal plain. Subsequent erosion of this surface by streams and rivers has produced a network of valleys and canyons that interrupt the otherwise monotonous expanse of flatness.

MOUNTAIN RANGES

The Peninsular Ranges, which are about 300 miles in length, extend in a northwest to southeast direction from Gorgonio Pass into northern Baja California. The San Jacinto and Santa Rosa Mountains form an eastern ridge that lies adjacent to the Salton Trough. Mt. San Jacinto rises to an elevation of 10,831 feet. The Santa Ana, Agua Tibia, Laguna, Cuyamaca and Vallecitos Mountains form an approximate parallel ridge to the west. The Perris and Anza uplands form a relatively flat plain between these major ridges. The more coastal and southern mountains are of a much lower elevation. However, in northern Baja California, the Sierra de San Pedro Martir again rises to elevations over ten thousand feet. To the west of the mountains is a vast area of coastal foothills, canyons and mesas extending west to Long Beach and south along the Pacific Coast to the Mexican border.

THE DESERT

The Colorado Desert to the east of the Peninsular Ranges is characterized by broad valleys (Coachella, Imperial, Borrego), the Salton Sea, the Algodones dunes and a number of desert mountains of low elevations. These desert mountains include the Indio Hills, the Orocopia and Chocolate Mountains, and the Superstition Hills.
This desert region is bordered on the east by the Colorado River, to the west by the Peninsular Ranges, and to the north by the eastern end of the Transverse Ranges. This area is characterized by numerous varieties of igneous, metamorphic, and sedimentary rocks*

*Extracted Natural History of Southern California: Ancinec, Radford, Schwenkmeyer