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Millions of years ago, the area of Los Angeles and Rancho La Brea lay beneath the surface of the Pacific Ocean. During this time, marine sedimentary layers formed and in some places these eventually became rich with fossil fuels produced from ancient sea life. When the ocean levels receded some 100,000 years ago, the area of Rancho La Brea became land. New layers of gravel, sand, and clay formed by the erosion of the emergent hills, and settled on top of the much older marine sediments full of oil.
At Rancho La Brea, the crude oil has been seeping out of the ground through conduits and fissures in the coastal plain sediments for the past 40,000 years, the seeps forming pools in low-lying areas.
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Foot Note!
Commonly called the "tar pits," the liquids that seep out of the ground at Rancho La Brea are actually comprised of asphalt, not tar. Tar is a commercial by-product made by the distillation of woody materials, such as coal or peat, while asphalt is a naturally formed substance comprised of hydrocarbon molecules. |
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La
Brea Geology
Geologic Time
Asphalt
Deposits
