The San Diego Formation (Dall, 1898) is middle or late Pliocene in age (Hertllein and Grant, 1944; Cleveland, 1960). It crops out from the lower south-facing slopes of Mount Soledad at Pacific Beach south to near San Diego Civic Center and along the north-facing slopes of Mission Valley from near Old Town to the eastern boundary of the area. These exposures, attaining a maximum thickness of 30 m, are composed of yellowishbrown, fine- to medium-grained, poorly indurated sandstone. Cobble conglomerate, thin beds of bentonite, marl, and brown mudstone further characterize the section. The thickness of the San Diego Formation increases markedly to the south, where it has been reported to attain a maximum thickness of 400 m (Hertlein and Grant, 1939). The lower 200 m of this section correlates to the south with the Miocene-Pliocene Rqsarito Beach Formation in northern Baja California. The cobble conglomerate beds are composed primarily of Poway-type clasts, but some beds contain up to 50 percent clasts of granitic and metavolcanic rocks derived from the local basement complex. The bentonite is light brown, waxy to earthy, expansible, and very soft. The San Diego Formation rests ranconformably on the older pre-Pliocene rocks and is overlain by the Lindavista Formation. It is separated from the overlying Lindavis ta Formation in some places by an unconformity, but in other places the contact is gradational.

Source - Geology of the eastern San Diego metropolitan area, California (Del Mar, La Jolla, Point Loma, La Mesa, Poway, and SW 1/4 Escondido quadrangles), 1975, California Div. Mines and Geol. Bull. 200, Section A, by Michael P. Kennedy

San Diego Formation shell bed with clams and other fossils. Source SDNHM: Clams of Champions	Shark's tooth from the San Diego Formation
San Diego Formation Scallop fossil Source SDNHM: Fossil Mysteries	San Diego Formation exposed at Tourmaline Surfing Park

References

Cleveland, G.B., 1960, Geology of the Otay clay deposit, San Diego County, California: California Div. Mines Spec. Rept. 64, p. 16.
Dall, W.H., 1898, 18th Ann. Rept.: U.S. Geol. Survey, pt. 2, correlation table opp. p. 334.
Hertlein, L.G., and Grant, U.S., IV, 1939, Geology and oil possibilities of southwestern San Diego County: California Jour. Mines and Geol., v. 35, p. 57-78.
Hertlein, L.G., and Grant, U.S., IV, 1944, The geology and paleontology of the marine Pliocene of San Diego, California, pt. 1, Geo.logy: San Diego Soc. Nat. History Mem., v. 2, p. 1-72.