How the La Brea Tar Pits Work

Pit 91

Of those 96 pits we discussed on the last page, the most famous and the most actively dissected has to be Pit 91. In fact, for nearly 40 years, it was the only pit under excavation at La Brea. In the late 1960s, researchers at the pits opted to enhance their excavation technique by harvesting all the fossils available in the pit, not just those that belonged to large vertebrates. Having a broader fossil record would offer a more complete picture of the end of the Pleistocene Epoch.

And so on June 13, 1969 -- a day affectionately referred to as "Asphalt Friday" -- excavations recommenced, only this time the remains of amphibians, reptiles, insects, small birds, shells and plants were among the specimens meticulously collected by diggers. And along with those important, if less flashy fossils, Pit 91 has also offered up a whole host of better-known players of the Pleistocene. These include bones from dire wolves, sabertoothed cats, western horses, ground sloths and mammoths -- and the pit is only about 15 feet (4.5 meters) deep!

The vast majority of these remains have been radiocarbon dated to between about 10,000 to 40,000 years old, and Pit 91, like most of the pits, contains fossils from a broad span of time. Thirty-thousand years is a long stretch of time for animals to become entrapped, but fossil figures in the millions can still be a little surprising. However, researchers say the numbers make sense; based on what they've found in the pits, it would only have taken about 10 large animals every 30 years to provide the wealth of fossilized remains found to date. If an entrapment event like that happened once every decade, that would mean the number of specimens found so far is more than explained.

Work on Pit 91 is currently on hiatus, however, and that's all because of the accidental discovery of what has been codenamed Project 23.