WHAT ARE THE
LA BREA TAR PITS
WHAT IS RANCHO LA BREA?
Rancho La Brea was initially a Mexican land grant given to Antonio Jose Roche in 1828 to provide pitch & crude oil resources to pueblo residents. It was not until 1875 when William Denton published the first documentation of extinct mammal bones from the locality, despite claims that they belonged to livestock. Over the course of the early 1900s, W.W. Orcutt, F.M. Anderson, John C. Merriam, J.Z. Gilbert, and numerous others began excavating & describing fossils from the asphaltic matrix, recognizing the site's significance as a lagerstätte. In 1924, George Allan Hancock donated 24 acres of the ranch to the County of Los Angeles under the premise that the park would be preserved and the fossils displayed for visitors. Philanthropist George C. Page financed further construction around the park, opening a natural history museum to the public in 1975. From this point on, countless paleontological discoveries have been made at Rancho La Brea, with active excavation and research continuing to the current day.
NHMLAC excavation of Pits 61 and 67 in 1915 looking NNE across the Salt Lake Oil Field towards the Santa Monica Mountains. RLB-327. (©Natural History Museum of Los Angeles County).
PALEOENVIRONMENT OF LA BREA
The Los Angeles Basin 25 kya. Illustration by Joschua Knüppe (Twitter.com/JoschuaKnüppe)
The La Brea Tar Pits preserve a timeframe of roughly 50,000 years, providing crucial insight into the environmental conditions during the waning years of the Pleistocene Epoch. The paleoecological record suggests the region had a predominantly Mediterranean climate with greater humidity than southern California today, suggesting a climate comparable to modern day Monterey, California. Preserved pollen suggests numerous shifts in the local vegetation based on the continental giacial and interglacial cycles; oak, laurel and sagebrush scrubland dotted the landscape towards the end of the Sangamon Interglacial 80,000 years ago, while closed-cone pine & live oak-juniper woodland growing greater in number during the Last Glacial Maximum 26,000 years ago. In the wake of the Younger Dryas 12,900 years ago, the warming climate aridified the La Brean landscape, repopulating the region with coastal sage scrub & chaparral flora that continues to cover most of southern California in the present day.
GEOLOGIC HISTORY OF THE TAR PITS
The oil reservoirs that feed the La Brea Tar Pits are found in late Miocene-age rock, located between 1,000 and 10,000 feet below the surface. The asphalt seeps to the surface through pores and faults in the overlying rock layers caused by the shear stress of earthquakes. The substance found in the tar pits is actually the crudest form of oil known as petroleum, often referred to as natural asphalt. This asphalt is a product of the remains of ancient algae dating back to the Miocene Epoch, when southern California was part of the ocean floor. These algae and other planktonic marine organisms accumulated on the ocean floor and were subsequently buried by sediment. Over time, the buried biomatter was subjected to heat and pressure, which transformed it into hydrocarbons, or crude oil. It now forms bubbling seeps of sticky asphalt that trap unsuspecting animals as big as mammoths, as small as insects, and even microscopic diatoms and pollen.
Generalized cross-section showing geologic structure and relationships of formations at Rancho La Brea during period of miring of Pleistocene animals and plants. Character and structure of the sediments containing the oil sands taken from section in Salt Lake Oil field, modified after Arnold.
(U.S. Geol. Surv. Bull. 309, p. 189, 1907)