The La Brea Tar Pits (or Rancho La Brea Tar Pits) are a cluster of tar pits around which Hancock Park was formed, in the urban heart of Los Angeles. Asphaltum or tar (brea in Spanish) has seeped up from the ground in this area for tens of thousands of years. The tar is often covered with water. Over many centuries, animals that came to drink the water fell in, sank in the tar, and were preserved as bones. The George C. Page Museum is dedicated to researching the tar pits and displaying specimens from the animals that died there. The La Brea Tar Pits are now a registered National Natural Landmark.
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The La Brea Tar Pits and Hancock Park are situated within the Mexican land grant of Rancho La Brea, now a piece of urban Los Angeles, California, near the Miracle Mile district.
Tar pits are composed of heavy oil fractions called asphaltum, which seeped from the earth as oil. In Hancock Park, crude oil seeps up along the 6th Street Fault from the Salt Lake Oil Field, which underlies much of the Fairfax District north of the park.[1] The oil reaches the surface and forms pools at several locations in the park, becoming asphalt as the lighter fractions of the petroleum biodegrade.
This seepage has been happening for tens of thousands of years. From time to time, the asphalt would form a deposit thick enough to trap animals, and the surface would be covered with layers of water, dust, and leaves. Animals would wander in to drink, become trapped, and eventually die. Predators would also enter to eat the trapped animals and become stuck.
The tar pits visible today are actually from human excavation. The lake pit was originally an asphalt mine. The other pits visible today were produced during the 1913-1915 excavations, when over 100 pits were excavated in search of large mammal bones. Various combinations of asphaltum and water have since filled in these holes. Normally, the asphalt appears in vents, hardening as it oozes out, to form stubby mounds. These can be seen in several areas of the park as well.
As the bones of the dead animals sink into the asphalt, it soaks into them, turning them a dark-brown or black color. Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which holds the bones. Apart from the dramatic fossils of large mammals, the asphalt also preserves very small "microfossils": wood and plant remnants, insects, mollusks, dust, seeds, leaves, and even pollen grains.
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 pits still ensnare organisms today, and so, most of the pits are fenced off to keep humans and animals from getting caught and killed.
The Portolá expedition, a group of Spanish explorers led by Gaspar de Portolá, made the first written record of the tar pits in 1769. Father Juan Crespi wrote, "While crossing the basin the scouts reported having seen some geysers of tar issuing from the ground like springs; it boils up molten, and the water runs to one side and the tar to the other. The scouts reported that they had come across many of these springs and had seen large swamps of them, enough, they said, to caulk many vessels. We were not so lucky ourselves as to see these tar geysers, much though we wished it; as it was some distance out of the way we were to take, the Governor [Portola] did not want us to go past them. We christened them Los Volcanes de Brea [the Tar Volcanoes]."[2]
For some years, tar-covered bones were found on the Rancho La Brea property, but were not initially recognized as fossils, because the ranch itself had lost various animals, including horses, cows, dogs, and even camels, whose bones closely resemble several of the fossil species.
Union Oil geologist W.W. Orcutt is credited with first observing fossilized prehistoric animal bones preserved in pools of asphalt on the Hancock Ranch in 1901. These would be the first of many fossils excavated from the La Brea Tar Pits. In commemoration of Orcutt’s initial discovery, paleontologists named the La Brea coyote (Canis orcutti) in W.W. Orcutt’s honor.[3]
Contemporary excavations of the bones started in the early 20th century {1913-1915}. In the 1940s and 1950s, public excitement was generated by the preparation of previously recovered dramatic large mammal bones.[4] Subsequent study demonstrated that the fossil vertebrate material was well preserved, with little evidence of bacterial degradation of bone protein.[5]
By the 1970s, research attention had shifted to smaller specimens, such as preserved insects and plant parts, including microfossils, such as pollen grains. These remains have contributed to an understanding of the Los Angeles basin during the glacial age, with a cooler and more moist climate.
Methane gas also escapes, causing bubbles that make the asphalt appear to boil. Asphalt and methane appear under surrounding buildings, and require special operations for removal to prevent weakening building foundations.
In 2007, researchers from UC Riverside discovered that the bubbles were caused by hardy forms of bacteria embedded in the natural asphalt. After consuming petroleum, the bacteria release methane. Of the bacteria sampled so far, about 200 to 300 are previously unknown species.[6]
The George C. Page Museum, part of the Natural History Museum of Los Angeles County, was built next to the tar pits in Hancock Park on Wilshire Boulevard. Construction began in 1975 and the museum opened to the public in 1977.[7] It tells the story of the tar pits and presents specimens from them. Visitors can walk around the park and see the tar pits. On the grounds of the park are life-size models of prehistoric animals in or near the tar pits.[8] Of more than a hundred pits, only Pit 91 is still regularly excavated by researchers, and can be seen at the Pit 91 viewing station, which is outside the museum, and free to enter. Currently, Pit 91 excavations have been put on hiatus so that excavators can operate year-round on material from Project 23. Paleontologists supervise and direct the work of volunteers.[7]
La Brea is a famous and accessible paleontological site because it is in a large city, with dramatic exhibits well presented at the Page Museum.
On February 18, 2009, George C. Page Museum formally announced the 2006 discovery of 16 fossil deposits which had been removed from the ground during the construction of an underground parking garage for the Los Angeles County Museum of Art next to the tar pits.[9] Among the finds are remains of a saber-toothed cat, six dire wolves, bison, horses, a giant ground sloth, turtles, snails, clams, millipedes, fish, gophers, and an American lion.[9][10] Also discovered is a near-intact mammoth skeleton, nicknamed Zed; the only pieces missing are a rear leg, a vertebra and the top of his skull, which was shaved off by construction equipment in preparation to build the parking structure.[10][11][12]
These fossils were packaged in tree boxes at the construction site and moved to a compound behind Pit 91, on Page museum property so that construction could continue. Twenty-three large accumulations of tar and specimens were taken to be separated. As work for the public transit Metro Purple Line is extended, museum researchers know that more tar pits will be uncovered, for example near the intersection of Wilshire and Curson.[9]
Among the prehistoric species associated with the La Brea Tar Pits are mammoths, dire wolves, short-faced bears, ground sloths, and the state fossil of California, the saber-toothed cat, Smilodon fatalis. Only one human has ever been found, a partial skeleton of a woman[13] dated to approximately 10,000 calendar years (~9,000 radiocarbon years) BP[14], who was 17 to 25 years old at death[15] and found associated with remains of a domestic dog, and so interpreted to have been ceremonially interred[16]. John C. Merriam of the University of California led much of the early work in identifying species in the early 20th century.
The park is known for producing myriad mammal fossils dating from the last glacial period. While mammal fossils generate significant interest, other fossils, including fossilized insects and plants, and even pollen grains, are also valued. These fossils help define a picture of what is thought to have been a cooler, moister climate in the Los Angeles basin during the glacial age. Among these fossils are microfossils. Microfossils are retrieved from a matrix of asphalt and sandy clay by washing with a solvent to remove the petroleum, then picking through the remains under a high-powered lens.
Tar pits around the world are unusual in accumulating more predators than prey. The reason for this is unknown, but one theory is that a large prey animal (say, a mastodon) would die or become stuck in a tar pit, attracting predators across long distances. This predator trap would catch predators along with their prey. Another theory is that dire wolves and their prey may have been trapped during a hunt. Since modern wolves hunt in packs, each prey animal could take several wolves with it.
Below is a partial list of extinct and extant mammals with their scientific names included on the right side. This is a selection from the complete catalogue.[17]
A partial list of extinct and extant birds found as fossils at La Brea.
Brea is Spanish for "tar." The "tar" pits were used as a source of asphalt (for use as low-grade fuel and for waterproofing and insulation) by early settlers of the Los Angeles area. The original Rancho La Brea land grant stipulated that the tar pits be open to the public for the use of the local Pueblo. Initially, they mistook the bones in the pits for the remains of pronghorn antelope or cattle that had become mired.
Rancho La Brea is the most famous, but there are two other asphalt pits with fossils in southern California: the Carpinteria Tar Pits in Carpinteria, Santa Barbara County and the McKittrick Tar Pits in McKittrick, in Kern County. There are other fossil-bearing asphalt deposits in Texas, Peru, Trinidad, Iran, Russia, Venezuela, and Poland.
For other rich deposits, fossilized where they occurred, see Lagerstätten.
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