The point where the Los Angeles and Rio Hondo Rivers merge in the City of South Gate is the geologic center for the Los Angeles Basin. It is the approximate location of where the sand, silt and clay of the Los Angeles Basin extend the deepest. In fact, the mixture of sediment beneath this location extends more than 30,000 feet downward before hitting solid rock (about as deep as Mount Everest is high). It is the center of a tremendous sand-filled hole whose walls are formed by the San Gabriel, Santa Monica and Santa Ana Mountains and the Palos Verdes Peninsula (which was once an island). When earthquakes occur, this huge "bowl of sediment" amplifies the motion in unpredictable ways (which is why one city block is hard-hit by a temblor and a neighboring block escapes serious damage). It would be similar to shaking a bowl of jelly.
About 15 million years ago, the Los Angeles Basin was underwater. As surrounding mountain ranges (including the San Gabriel and Santa Monica ranges) shifted in a clockwise spiral, the underlying crust stretched and cracked and released molten rock from below. The crust thinned and "collapsed," forming an immense geologic "bowl." Sand, silt and clay from the sea and ancient rivers poured into the bowl. Microorganisms also poured into this hole, piling high in huge layers. These layers would eventually become the oilfields of Los Angeles.
About 5 million years ago, the crust ceased to stretch and the bowl began to shrink. The hole filled in and seismic activity started pushing the contents upward. Rock that once lay at the ocean floor was being forced to the surface. Sediment also continued to flow from the mountains onto this growing mound. As it rose above sea level, this pile of sediment began forming what we now call the Los Angeles Basin. In effect, Los Angeles has not been "falling into the sea," as popularly believed, but rather rising from the ocean.
The Los Angeles Basin is the largest flat basin opening onto the Pacific Ocean.