The Pit 91 excavation site is located about 100 yards west of the La Brea Tar Pits Museum in Hancock Park. It was the ninety-first pit in a sequence of 96 sites explored during the 1913-1915 excavations undertaken by the Los Angeles County Museum. Digging began on June 13, 1915 and by July, after reaching a depth of approximately nine feet, it was decided that this large deposit of fossils would be left in situ as a "showpiece" for visitors. It subsequently suffered repeated cave-ins and floods, and was abandoned with thousands of fossils still awaiting excavation.
On June 13, 1969 (or "Asphalt Friday" as it has come to be known), Pit 91 was reopened with the idea of collecting everything; not just large vertebrate fossils but also the smaller fossils that had been ignored by early excavation efforts. Processing of the matrix surrounding the larger fossils has allowed us to recover microfossils including freshwater molluscs, plants, insects, fish, amphibians, reptiles, birds, and small mammals. These in turn have provided us with valuable environmental information about the Los Angeles Basin in the late Pleistocene. You can watch footage from the opening ceremony on "Asphalt Friday"
The main deposit in Pit 91 is currently exposed at 15 feet. It is estimated that the deposit extends another 3 to 5 feet further below ground.