California oil seeps created asphalt pools — not tar — that trapped Ice Age animals.
The sticky black pools attracting tourists between Beverly Hills and downtown Los Angeles are actually natural asphalt, also known as bitumen. Although the repetitive tar pits name has stuck, the seeps are part of America’s oil history.
The La Brea site — discovered by a Spanish expedition on August 3, 1769 — originated from naturally produced California oil seeps found onshore and offshore.
Bitumen Swamp
“We proceeded for three hours on a good road; to the right were extensive swamps of bitumen which is called chapapote,” noted Franciscan Friar Juan Crespi, who accompanied the expedition led by Gaspar de Portola.
Outside the Page Museum of Los Angeles, life-size replicas of several extinct mammals are featured at a bitumen pool at Rancho La Brea in Hancock Park. Photo courtesy Page Museum.
The friar reported in his diary that members of the expedition, “debated whether this substance, which flows melted from underneath the earth, could occasion so many earthquakes.”
Friar Crespi, the first person to use the term bitumen, described the intensely black pools where oil had been seeping through fissures in coastal plain sediments for more than 40,000 years, according to geologists. The occupation of California by the Spanish resulted in the name of Rancho La Brea, or “the tar ranch.”
The Southern California pools of petroleum would become a popular tourist attraction and home to a natural history museum.
Pools form when crude oil seeps to the surface through fissures in the earth’s crust.
The Page Museum at the Rancho La Brea Tar Pits west of downtown Los Angeles on Wilshire Boulevard, is one of the world’s most famous sources of Ice Age fossils. Thanks to its local resource, the museum has uncovered and preserved the largest and most diverse assemblage of extinct plants and animals.
Learn more Southern California petroleum history in Discovering Los Angeles Oilfields.
West Coast Asphalt Pits
Native Americans had used the substance for centuries to waterproof baskets and caulk canoes when, in 1828, Antonio de Rocha established Rancho La Brea via a land grant by the Mexican government.
Although commonly called the “tar pits,” the thick liquid that bubbles out of the ground at Rancho La Brea is actually asphalt — not tar. Tar is a 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.
While drilling for oil and mining for asphalt, the Hancock family discovered the scientific value of Rancho La Brea fossils.
After the American Civil War, Captain George Allan Hancock inherited 4,400 acres of land from the original Mexican land grant. The Hancock family owned and operated a refinery at Rancho La Brea between 1870 and 1890, commercially mining and exporting asphalt to local markets.
Research has been conducted at Rancho La Brea since the early 1900s and now continues at the Page Museum. A scientific publication first recorded the fossils in 1875. Professor William Denton ventured to the pits to evaluate oil prospects — and noted the fossilized remains of animals.
Although Denton wrote about his discovery, it took several decades and another geologist interested in oil prospects, William W. Orcutt, to excavate and collect a substantial fossil collection — including the only complete skull of a saber-tooth tiger in the world.
A circa 1910 photograph of asphalt pools in what is today downtown Los Angeles.
“Asphalt is a superb preservative; small and delicate fossils, such as hollow bird bones or paper-thin exoskeletons of beetles are very well-preserved here,” observes the museum. “As a result, our collection of fossil birds is one of the world’s largest.”
In 1916, the Hancock family — wealthy with the onset of the oil boom in southern California — donated the 23 acres of Hancock Park to Los Angeles County to preserve and exhibit the fossils exhumed from Rancho La Brea.
Visit the Page Museum and the Natural History Museum of Los Angeles County to learn more about Los Angeles as it was during the last Ice Age, when mammoths roamed the Los Angeles Basin.
For a brief oil history of the asphalt on America’s roads, see Asphalt Paves the Way.
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Recommended Reading: Monsters Of Old Los Angeles – The Prehistoric Animals Of The La Brea Tar Pits
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The American Oil & Gas Historical Society (AOGHS) preserves U.S. petroleum history. Please become an AOGHS annual supporter today and help maintain this energy education website and expand historical research. For more information, contact bawells@aoghs.org. © 2024 Bruce A. Wells. All rights reserved.
Citation Information – Article Title: Discovering the La Brea “Tar Pits.” Authors: B.A. Wells and K.L. Wells. Website Name: American Oil & Gas Historical Society. URL: https://aoghs.org/energy-education-resources/discovering-oil-seeps. Last Updated: July 26, 2024. Original Published Date: April 29, 2014.