The field is near the northern edge of the Los Angeles Basin, about two miles (3 km) south of the Hollywood Hills, the nearest portion of the Santa Monica Mountains. The Santa Monica Fault, not known to be active, demarcates the boundary between the basin and the mountains. Several other faults cut through the field, including the 3rd Street Fault and the 6th Street Fault; the latter of these is presumed to be the conduit through which crude oil has emerged on the surface as the La Brea Tar Pits, which are at Hancock Park at the southern boundary of the oil field, near Wilshire Boulevard. A layer of sediments of Quaternary age, both alluvial and shallow marine, forms a cap of approximately thickness on the underlying formations, several of which are oil-bearing. First is the Upper Pliocene Pico Formation, which is not petroleum-bearing in the Salt Lake field. Underneath the Pico are the late Miocene and Pliocene Repetto and Puente Formations. The Repetto Formation is a sandstone and conglomerate unit probably deposited in a submarine fan environment, and is a prolific petroleum reservoir throughout the Los Angeles Basin. Underneath the Repetto is the late Miocene Puente Formation. All of these rock units are faulted and folded, forming structural traps, with oil trapped in anticlinal folds and along fault blocks.