La Brea Tar pits

For the tar pit in La Brea, Trinidad and Tobago, see Pitch Lake.

La Brea Tar PitsLocationCoordinatesCalifornia Historical Landmark

Official nameReference no. U. S. National Natural Landmark

Designated Small tar pitLa Brea Tar Pits comprise an active paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural asphalt (also called asphaltum, bitumen, or pitch; brea in Spanish) has seeped up from the ground for tens of thousands of years. Over many centuries, the bones of trapped animals have been preserved. The George C. Page Museum is dedicated to researching the tar pits and displaying specimens from the animals that died there. "La Brea Tar Pits" is a registered National Natural Landmark.

Illustration of several species getting mired in the tar pitsTar pits are composed of heavy oil fractions, called gilsonite, which seep from the earth as oil. Crude oil seeps up along the 6th Street Fault from the Salt Lake Oil Field, which underlies much of the Fairfax District north of Hancock Park. The oil reaches the surface and forms pools, becoming asphalt as the lighter fractions of the petroleum biodegrade or evaporate. The asphalt then normally hardens into stubby mounds, which can be seen in several areas of the park.

This seepage has been happening for tens of thousands of years, during which the asphalt sometimes formed a deposit thick enough to trap animals. The deposit then became covered with water, dust, and leaves. Animals wandered in, became trapped, and died. Predators entered to eat the trapped animals and then also become stuck, a phenomenon called a "predator trap". As the bones of a dead animal sank, the asphalt would soak into them, turning them dark-brown or black in color. Lighter fractions of petroleum evaporated from the asphalt, leaving a more solid substance, which then encased the bones.

Dramatic fossils of large mammals have been extricated, and the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and pollen grains. Examples of these are on display in the George C. Page Museum. Radiometric dating of preserved wood and bones has given an age of 38,000 years for the oldest known material from the La Brea seeps.

The Tar Pits in 1910. Oil derricks can be seen in the background. The Chumash and Tongva people used tar from the pits to build plank boats by sealing planks of California redwood trunks and pieces of driftwood from the Santa Barbara Channel, which they used to navigate the California coastline and Channel Islands.