La Brea Tar Pits
The La Brea Tar Pits is 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.
Formation
Illustration of several species getting mired in the tar pits
Tar pits are composed of heavy oil fractions called gilsonite, which seeps 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.[2]
The oil reaches the surface and forms pools, becoming asphalt as the lighter fractions of the petroleum biodegrade or evaporate.[3]
The asphalt then normally hardens into stubby mounds. The pools and mounds 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 would become covered over with water, dust, or leaves. Animals would wander in, become trapped, and die. Predators would enter to eat the trapped animals and would also become stuck. As the bones of a dead animal sink, the asphalt soaks into them, turning them dark-brown or black in color. Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which then encases the bones. Dramatic fossils of large mammals have been extricated but the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and pollen grains.[4] Examples of some 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.