Pit 91 Excavations | Natural History Museum of Los Angeles

The La Brea Tar Pits would be more accurately named the Rancho La Brea Asphalt Seeps. The ground isn't at all like quicksand, it's asphalt-saturated sediment. An excavator stands comfortably and safely on the surface of Pit 91.
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Pit 91

Located just west of the Page Museum, in Hancock Park, is the Pit 91 excavation site. Museum staff has been exploring it for approximately 40 years, but stopped in 2007 to focus attention on Project 23. This summer, excavators will resume work in Pit 91, and visitors can see the excavation from the newly revamped viewing station.

Pit 91 was the ninety-first pit in a sequence of over one hundred "pits" explored during the 1913-1915 excavations undertaken by the Los Angeles County Museum. Digging began on June 13, 1915, and by July it was decided that this large cluster of fossils would be left in situ as a "showpiece" for visitors. Unfortunately, after reaching a depth of approximately nine feet the excavation site suffered repeated cave-ins and floods, and it 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 only just large vertebrate fossils but also the smaller fossils and matrix from the site that had been ignored or tossed aside by early excavators. This has allowed us to recover microfossils including shells, plants, insects, fish, amphibians, reptiles, birds, and small mammals. The resulting data from these microfossils have provided us with valuable environmental information about Los Angeles Basin in the late Pleistocene.

Excavation continued year-round in Pit 91 until 1980, when the site was closed due to budget constraints. Then in 1984, for the Summer Olympic Games, it was reopened and since then, until 2007, excavation at the site continued for 10 to 12 weeks every summer. In the 2007 field season, 3,388 specimens were recovered from Pit 91, including:

  • two  saber-toothed cat skulls,
  • six dire wolf skulls,
  • a near-complete horse skull,
  • several Harlan's ground sloth limb bones,
  • a juvenile Shasta ground sloth jaw,
  • the first confirmed piece of a (very young) mammoth from Pit 91.

Pit 91 is currently 15 feet deep; it is estimated that the deposit extends another 3 to 8 feet further below ground. 

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