Evolution of the Los Angeles Basin
Until less than 100,000 years ago, the whole of what is now the Los Angeles basin was under the Pacific Ocean. During the late Miocene and early Pliocene Epoch, the San Gabriel and Santa Monica Mountains began to rise, at a spot where the Pacific and North American tectonic plates crushed together. Rock from the hills began eroding into the sea, forming deposits on the bottom of the bay.
At the same time, the sea bottom where the basin is now had long been sinking -- subsiding, as geologists call it. The combination of rapid erosion and subsidence led to the formation of very thick Pliocene deposits -- as much as 1,500 feet thick in Hancock Park.
The mountains are still rising today, but eventually the ocean bottom stopped subsiding. The deposits from the Santa Monicas' erosion piled up, making the bay shallower and shallower. Sometime after 100,000 years ago, they were high enough to reach the surface, and the Los Angeles basin had been formed. The process still continues today, with massive slides of mud, boulders and debris flowing into the nearby canyons and sometimes burying houses -- and people. Huge "catch-basins" capture the vast flows of boulders and debris, but sometimes they fail.
By this time, it was no longer the Pliocene but the Pleistocene -- what non-scientists call the Ice Ages. At this time, the climate in the L.A. basin was different from what we have today; for one thing, there was probably twice as much rain. It was more like the way Monterey Bay is now, 200 miles north -- cooler and foggier, with stands of oak woodland, Monterey cypress, and Monterey and Bishop pine, nibbled on by mastodons. There were even sequoia trees in the mountains.
The mastodons and other prehistoric creatures have died out, and so have the Monterey cypress and pines and the Bishop pines. So have the sequoias, although the incense cedars that grow in the mountains today look much like them. The ancestors of today's Native Americans entered the basin But the Los Angeles basin kept growing, fed by debris sliding down from the Santa Monica and San Gabriel mountain ranges. It would still be growing today, if we had not put up the catchment basins that hold back the mudslides and rockslides.