The Southern Coastal region includes a narrow coastal strip of sedimentary deposits extending from the vicinity of San Onofre on the north to the Mexican border on the south. This strip of sedimentary rocks of Cretaceous, Tertiary, and Quaternary age varies in width from 4 to 20 miles. Exposures of pre-Upper Cretaceous crystalline rocks are adjacent on the east in the Peninsular Ranges and extend westward, forming the basement below the marginal sedimentary belt. The area of sedimentary rocks is about 949 square miles.

The northern part of the region, in the vicinity of San Onofre, is a southeastward extension of the Los Angeles Basin; it is the only part of the province in which marine Miocene sedimentary rocks are known. The southern part of the region consists of the San Diego Pliocene-Pleistocene basin. These two basin areas are underlain by Eocene and Cretaceous sediments, and the strip between them is occupied by an Eocene sedimentary series overlying Cretaceous sediments at shallow depths. In the vicinity of San Onofre, the Upper Cretaceous Chico formation lies directly on the pre-Upper Cretaceous granitic rocks with their inclusions of Triassic and older slates and schists. The Chico formation is very largely conglomerate and breccia. A boulder conglomerate near the base of the Chico may be part of the Trabuco formation of the Santa Ana Mountains. The overlying Eocene consists of massive conglomerate, yellow and brown sandstones, and shale of upper Eocene age, probably, in part at least, the equivalent of the Poway conglomerate