The Los Angeles--Orange County coastal plain basin is bounded on the north and east by the Santa Monica Mountains and the Puente Hills, on the south by the San Joaquin Hills, and on the west by the Pacific Ocean (fig. 125). The mountains are underlain by consolidated rocks of igneous, metamorphic, and marine-sedimentary origin. These consolidated rocks surround and underlie thick unconsolidated alluvial deposits. The major drainages in the basin are the Los Angeles, the San Gabriel, and the Santa Ana Rivers, all of which have headwaters outside the basin.
Marine sediments deposited during periodic encroachment of the sea and alluvium derived from weathering and erosion of the rocks in the surrounding mountains have filled the basin with a thick sequence of deposits. The surface of the basin is relatively flat, but upwarping along the Newport--Inglewood Uplift (fig. 125) has formed hills that rise in places as much as 400 feet above the surrounding coastal plain. Also, along the coast from just north of Long Beach southward to the San Joaquin Mountains, resistant sediments of late Pleistocene age underlie several mesas. The mesas are separated by erosional gaps through which the major drainages either now flow or have flowed historically.
Climate in the basin is Mediterranean, characterized by warm summers, cool winters, and markedly seasonal rainfall. Nearly all rain falls from late autumn to early spring; virtually no precipitation falls during the summer. The average annual rainfall in Los Angeles is about 15 inches. Potential evapotranspiration in the coastal plain exceeds precipitation on an annual basis, and, under natural conditions, the lower reaches of rivers that drain the basin are dry in summer.