Asphalt pools trapped Ice Age animals in what today is Los Angeles.

 

Although commonly called tar pits, the sticky pools between Beverly Hills and downtown L.A. are actually comprised of natural asphalt, also known as bitumen.

The La Brea site, discovered by Spanish explorer Gaspar de Portola on August 3, 1769, originated from naturally produced California oil seeps, onshore and offshore.

oil seeps are tar pits outside Page Museum in Los Angeles

Outside the Page Museum of Los Angeles, life-size replicas of several extinct mammals are featured at the Rancho La Brea in Hancock Park. Photo courtesy Page Museum.

Swamps of Bitumen

“We proceeded for three hours on a good road; to the right were extensive swamps of bitumen which is called chapapote,” Franciscan friar Juan Crespi noted in a diary of the expedition.

oil seeps illustration of pools and fissures

Pools form when crude oil seeps to the surface through fissures in the earth’s crust.

“We debated whether this substance, which flows melted from underneath the earth, could occasion so many earthquakes,” he added.

Crespi – the first person to use the term bitumen – described the sticky pools in southern California where crude oil had been seeping from the ground through fissures in the coastal plain sediments for more than 40,000 years.

Today, 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 fossils, recognized for having the largest and most diverse assemblage of extinct Ice Age plants and animals.

“The Spanish occupation of California about 300 years ago also played a role in the history of Rancho La Brea,” the museum explains. The Spanish used the name of Rancho La Brea, or “the tar ranch.”

West Coast Asphalt Pits

oil seeps

While drilling for oil and mining for asphalt, the Hancock family discovered the scientific value of Rancho La Brea fossils.

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.

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.

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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.

oil seeps

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.

At the Page Museum (Natural History Museum of Los Angeles County) and at the young people can learn about Los Angeles as it was during the last Ice Age, when mammoths roamed the Los Angeles Basin.

For a brief history of the asphalt on America’s roads, see Asphalt Paves the Way. 

The American Oil & Gas Historical Society preserves U.S. petroleum history. Become an AOGHS supporting member and help maintain this energy education website and expand historical research. For more information, contact bawells@aoghs.org. © 2020 Bruce A. Wells.

Citation Information – Article Title: Discovering the La Brea “Tar Pits.” Author: AOGHS.ORG Editors. Website Name: American Oil & Gas Historical Society. URL: https://aoghs.org/energy-education-resources/discovering-oil-seeps. Last Updated: August 14, 2020. Original Published Date: April 29, 2014.

 

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