The La Brea Tar Pit Fossils of Los Angeles California

Among the most complete asphaltic fossil records stretching back over the last 50,000 years during the Late Pleistocene ice age is the Lagerstätte of the La Brea Tar Pits found in Hancock Park off Wilshire Boulevard in the heart of Los Angeles, California.

La Brae Tar Pit Mammoth - Image by Natural History Museums of Los Angeles County

The asphalt seeps at La Brea Tar Pits are the only consistently-active and urban Ice Age excavation site in the world and should make your Top 100 bucket list of ideas for fossil sites to visit.

In 1964 the 24 acre site of the La Brea Tar Pits was designated a Natural Landmark of the USA and in 2022 the La Brea Tar Pits (34° 03' 50'' N118° 21' 20'' W) that is part of the Natural History Museums of Los Angeles County was named among the “First 100 Geological Heritage Sites” designated by the International Union of Geological Sciences (IUGS).

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There are a number of tar pits or asphalt seeps that remain active today around the world. They are also one of the Earths natural occurring repositories for capturing environmental conditions and recording the impact of climate change on species adaptations.

Entrance to La Brae Tar Pits Museum - Image by the Natural History Museums of Los Angeles County

The Natural History Museums of Los Angeles County (NHMLAC) is L.A.’s oldest museum building and is widely regarded unlike any museum organisation in the world.

Its collection has over 35 million specimens and artefacts and is the second largest in the United States. Discoveries from the tar pits still take place every day by visitors, scientists, and historians alike.

NHMLAC is best known for its extensive La Brea Tar Pits collection of megafauna of fossilised mammal carnivores including saber-toothed cats (Smilodon fatalis), giant sloths (Paramylodon harlani), dire wolves (Canis dirus), coyotes (Canis latrans), mammoths, mastodons along with birds, plants, insects, fish and amphibians.

Exhibit of a saber-toothed cat (Smilodon fatalis) - Image by the Natural History Museums of Los Angeles County

Tar pits are deceptively dangerous places and are often described as a "death trap". They are an active hazard to animals that venture too close and become mired and stranded in the tar unable to escape.

Animals would do not die immediately and for long periods of time they would struggle on the surface eventually succumbing to exhaustion and die from thirst or starvation.

In turn a combination of predators and scavenger animals, birds and insects would be attracted to what initially would appear to be an opportune meal on the dying of dead animal. However, very often they themselves would become stuck and experience a similar fate.

Where oil fields have experienced periodic tectonic movements the underlying rock formations become fractured and the vents and fissures allow under pressure the release to the Earth’s surface of natural petroleum or crude oil creating a tar pit.

As the crude oil seeps upwards through often a limestone of sandstone sedimentary rock it experiences a fundamental change upon reaching the Earth surface.

Once the petroleum is exposed to the atmosphere it will begin to immediately evaporate and can lose up to 75% of its volume over time.

A combination of the lighter volatile compounds and any water will evaporate leaving the heavier compounds to accumulate leaving a thick, black, oily, viscous and smelly form of petroleum called bitumen also known interchangeably as asphalt, tar or pitch.

Once on the surface the force of gravity and local topography may allow the bitumen to flow over the surface of the ground until it pools in a natural depression. Alternatively, petroleum can seep under pressure through porous sedimentary rocks to saturate the rock and again pool in depressions.

Below the surface of Los Angeles sits on one of the richest oil and gas basins in the United States. Los Angeles is on the eastern edge of the boundary zone between the Pacific and North American Tectonic Plates and near the big bend of the San Andreas fault.

The occurrence of the asphalt seeps or tar pits and fossil record at Rancho La Brea (Spanish for “the tar ranch") has been significantly influenced by the tectonic history of Southern California.

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The source of La Brea Tar Pits petroleum is the Salt Lake Oil Field found several hundred meters below surface of Hancock Park and pools within the present day lowlands of the larger Los Angeles basin that covers an area of over 2,500Km2.

The Los Angeles Basin is bounded by both the Transverse and Peninsular ranges of northern low relief alluvial plain deposited from the Santa Monica Mountains and hills. These ranges expose Mesozoic and older basement, sedimentary and igneous rocks from the Late Cretaceous to late Pleistocene age.

The City of Los Angeles and Mountains - Image by Larry Gibson

The petroleum of the La Brea Tar Pits pools within Palos Verdes Sands of the Los Angeles alluvial plain. This is a shallow marine sandstone gravel dated from the Irvingtonian North American Land Mammal Stage between 1.806 +/- 0.005 to 0.3 million years ago.

This area has also experienced periodic intervals of uplift and faulting allowing the crude petroleum to seep to the Earths surface over the last ~60,000 years forming shallow pools.

During the Pleistocene Epoch, dated between 2.588 - 0.0118 million years ago the geography of the Rancho La Brea area was one of a fairly open semi-desert environment with a rich and diverse habitat for many species.

During the Miocene Epoch (dated between 23.03 - 5.333 million years ago) the tectonic activity of Southern California formed a deep structural basin several kilometres filled with as many 13 successive marine platforms of deposited marine sediments of plants, algae, and plankton.

This created the geological conditions of time, temperature and pressure to produce petroleum by transforming organic matter into kerogen and by cracking or converting of organic into hydrocarbons by the process of catagenesis.

Reconstruction of a mammoth becoming mired - Image by the Natural History Museums of Los Angeles County

Before the first mention of the tar pits of Los Angeles or "springs of pitch" as referred to in 1796 by the Franciscan friar called Juan Crespi who recorded the pioneering Portolá expedition into California by the Spanish military officer and first Spanish Governor of the Californias Gaspar de Portolá y Rovira (1716 – 1786).

The American Indian or Indigenous American, frontiers people and early settlers dismissed the fossils found at the tar pits as the remains of domestic animals.

They used the tar as a caulking material to seal joints or seams against leakage in baskets, canoes, fuel and a waterproofing building for roofs.

La Brae Tar Pit Excavation - Image the Natural History Museums of Los Angeles County

It was not until 1875 after the purchase of the land by Major Henry Hancock that the geologist William Denton visited the tar pits and identified the canine tooth of a sabre-toothed cat.

In 1901 William W. Orcutt, The Head Petroleum geologist from the Union Oil Company of California whilst studying the oil resources of the region noted that the bones in the asphalt seeps belonged to many extinct species. His account would be written up by is wife Mary Logan Orcutt in the publication, "The Discovery in 1901 of the La Brea Fossil Beds" following his death.

Finally, my thanks to Claire and Whitney from the Natural History Museums of Los Angeles County Communications Department for the wonderful images.