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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. During this time, starting approximately 35,000 years ago, the famous deposits most people call the La Brea Tar Pits were created and began trapping animals. A better name would be the Rancho La Brea Tar Seeps, because they're not pits but shallow rivulets of tar. (That "lake" of tar you see by Wilshire Boulevard, with the three mammoth replicas, was made by humans gathering the asphalt in the 1800s; it's not a natural feature.) The [UC Museum of Paleontology's LaBrea page] has more info. At the end of the Pleistocene, perhaps some 12,000 years ago, the climate changed abruptly to become hotter and drier, and went on getting dryer until the time of what paleoclimatologists call the Altithermal -- the peak of the current interglacial when temperatures were at their hottest. At this time, the basin's ecosystem wasn't sage scrub or chaparral, but Sonoran desert, like what now grows in Arizona. The summer heat pulled monsoon rains out of the Gulf of Mexico into southern California, making the climate and local ecosystem different from the current Mojave Desert. But as the climate cooled, the basin took on the form that the ancestors of the Gabrielino Indians saw when they first entered the Los Angeles plain. Most of this comes from "Rancho La Brea: A Look at Coastal Southern California's Past," by Christopher Shaw and James P. Quinn, in the June 1986 issue of California Geology. Another reference: The Los Angeles Basin -- A Huge Bowl of Sand |