"The Los Angeles Basin"Los Angeles county covers over 4,081 square miles, although when adding the connecting counties of (Los Angeles, Riverside, Ventura, Orange, and San Bernardino), which make up the entire Los Angeles Metro area the region covers over 34,000 square miles.
The geological center of the Los Angeles Basin is found at the point where the Rio Hondo and the L.A. Rivers merge in the City of South Gate, it parallels the 710 freeway. This is the approximate location where sand, silt and clay extend the deepest and the mixture of that sediment continues more than 30,000 feet before it hits bedrock. To put this into perspective, the sediment is as deep as Mount Everest is high or since most of us have not been to the Himalayas, take a look at a Jetliner high in the sky, they cruise between 30,000 and 36,000 thousand feet. Now fill it with earth so you are standing next to the Airliner.
The Basin is essentially a huge bowl of sediment and it is formed by the San Gabriel, Santa Ana, Santa Monica Mountains and the Palos Verdes Peninsula. The Los Angeles mountain ranges are also known as the Traverse Ranges because of their east-west orientation. These high points acted as walls and formed the basin as we know it. The basin is like a huge washbasin, although instead of water, it is made up of sediment. This process continues today, anytime there is a rock-slide or a deluge of rain which causes the canyons to give way in a torrent of mud and water which in time end up adding to the growth of the basin itself.
The Los Angeles Basin was underwater around 15 million years ago and the surrounding mountain ranges shifted in a clockwise spiral by tectonic shifts and the underlying crust was pulled, stretched, and cracked releasing molten rock from below. As the rock cooled and thinned it became fragile and collapsed forming a huge geologic bowl.
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 the masses believe, but rather it is rising from the ocean and will continue to do so.
Sources: The Paleontological Research Institution
Los Angeles Almanac, http://www.laalmanac.com/geography/index.htm#Geologic
Nasa, http://www.nasa.gov/
