The Southern Mountain region, by definition here, includes the San Gabriel Mountains of the Transverse Ranges, and the Peninsular Ranges. The northern and eastern boundary of the region is essentially the San Andreas fault zone; the southwestern boundary is the contact of Cretaceous-Tertiary sedimentary rocks on the older crystalline rocks.

The San Gabriel Mountains consist of a complex series of closely related Upper Jurassic (?) plutonic rocks including principally granite, granodiorite, monzonite, diorite, and gabbro. These include

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numerous fragments of older intrusive rocks and an older metasedimentary series (Placerita) of possible Paleozoic age. The range is flanked on the north by a thick series of continental sandstones and conglomerates, interbedded with volcanics, of probable Miocene age. Intensely folded and faulted marine Pliocene sediments and continental lower Pleistocene gravels lie on the crystalline rocks along the southwest border of the range. Marine Paleocene Martinez sandstone and conglomerate occur in slivers in the San Gabriel fault zone.

In the Peninsular Ranges, rocks of the Lower Cretaceous (?) batholith are exposed, including principally granodiorite, tonalite (quartz diorite), and gabbro. In general, these plutonics have intruded Triassic and Jurassic (?) sediments and volcanics along their western border, and Paleozoic sediments along the eastern border. All pre-Cretaceous formations have been metamorphosed to some extent and are widely distributed through the areas of plutonic rocks. Upper Cretaceous sediments overlie the basement crystalline rocks near the northwestern end of the Santa Ana Mountains and in the Southern Coastal region. Marine and brackish-water sediments of the Paleocene Martinez formation are distributed along the Elsinore fault zone. Continental Miocene sediments and volcanics are found along he San Andreas and San Jacinto fault zones and in the bordering Coachella and Imperial valleys at the east.

Evidences of oil and gas have been found only along the margins of the crystalline rock masses, particularly abundant at the western and southwestern borders of the San Gabriel Mountains where there are numerous seepages. A very small amount of high-gravity oil was produced at the turn of the century from several shallow wells in crystalline rocks in the San Gabriel fault zone in the western San Gabriel Mountains 2½ miles east of Placerita oil field. Of a total of more than 70 exploratory wells drilled in the Southern Mountain region, these are the only ones which had any real shows of oil or gas.

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