In The Pits

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You’re a mastodon.
(Bear with us here!) You’re wandering around western Kern County—granted, it wasn’t called Kern County at that point but that’s neither here nor there. You’re enjoying the sunny weather and decide to stop for a leisurely drink. When your thirst has been quenched at a shallow pool of water you go to move, only to find your front legs stuck
in the ground.

Photo Courtesy of Tim Elam

Photo Courtesy of Tim Elam

How did that happen?
Why can’t you pull your giant feet out of this mud?
Well, my hairy friend, you’ve just gotten yourself stuck in a tar pit. Prepare to die of exposure!
Okay, that might be a bit of a downer, but the little scene we just painted for you was a very common occurrence tens of thousands of years ago. Animals often found themselves trapped in what are known today as the McKittrick tar pits (located in southwest Kern) and, contrary to popular thought, they did not sink into the tar (also known as asphaltum) but rather died from the elements, including weather and scavenging carnivores.
“Of the larger animal remains found at McKittrick, there are wolf, deer, sloth, mastodon, horse, camel, saber-tooth cat, fox, bear, and others,” explained Tim Elam, a retired petroleum geologist who serves on the Buena Vista Museum of Natural History and Science Board of Directors.
“And the species of birds are dominated by water birds like ducks, geese, stork, heron, and other shore birds,” he added. And just how old are these fossils?
Using radioisotope dating (isotopes of carbon 14), researchers have dated the recovered fossils at McKittrick. The oldest dated about 38,000 years old while the youngest are roughly 12,000 years old. Seems silly to say that something 12,000 years old is young, but we humans haven’t been here very long when you look at the history of the planet.

Photo courtesy sjvgeology.org

Photo courtesy sjvgeology.org

But we might be getting ahead of ourselves. We should probably explain what a tar pit is, right? It’s not like it’s just a big vat of Acme tar set out by Mother Earth in a Wile E. Coyote-esque attempt to catch a bunch of animals. There’s a lot more to the evolution of a tar pit!
According to Elam, who is a past president of the San Joaquin Geological Society, “oil and gas were generated thousands of feet beneath the San Joaquin Valley.” Both those elements—oil and gas—weigh significantly less than the water and rocks they were surrounding and would then raise to the surface of the Earth. “The oil and gas tried to escape,” he explained. “At McKittrick, some of that oil made it to the surface. That is because a major fault line, the McKittrick thrust fault, crushed rocks and created fractures in rocks all the way to the surface…the oil rose through that fractured rock and seeped into slightly lower elevation land areas, only a few feet deep, forming tar seep ponds.”

But another process was happening at the same time: while the oil rose to the surface, bacteria feasted on the lighter-weight, more liquid parts of the oil, so by the time it reached the crust, the oil was a very viscous, gooey tar, which can seep across the land, collecting sediments.
In the case of McKittrick, this process happened over the course of a three-mile long stretch.
“Animals and plant parts such as leaves and seeds either wandered into the tar areas, were carried in by water, or were blown in,” Elam said. “Often animals visited them because the tar seeps collected not only tar, but water from storms. Larger animals may have come to drink water, not realizing there was tar below the water.” And that’s where they met their end. But it’s good that they did! Their bones were fossilized and preserved so that years later, scientists could discover them—giving the world we know today insight on what animals roamed the planet before modern day humans.

McKittrick Paleoseep shows the filled in pits and the roadcut along Highway 33, including the 10-12’ thick layer of fossil tar seep.

McKittrick Paleoseep shows the filled in pits and the roadcut along Highway 33, including the 10-12’ thick layer of fossil tar seep.

“At McKittrick, we have both active seeps and fossil seeps,” Elam explained. “Active seeps are areas where tar/oil is seeping out of the ground today, while fossil seeps are areas where tar more than ten thousand years old has been preserved…often preserving evidence of life from that older time.”
It’s actually the composition of the tar that allows the fine detail of teeth, bones, and other hard parts of the animal to be preserved in great detail; the tar actually prevents oxidation of the hard parts.

Elam elaborated that fossil seeps like McKittrick (and those found close by at Maricopa) are very rare. “There are only five or six in the entire western Hemisphere and two are in Kern County!” The reason they are so rare is that natural erosion of the Earth’s crust almost always destroys seeps soon after they form. So it’s very random that we have these small patches of earth that allow us to study the history of our planet. It’s another amazing thing about Kern County.
“One of the reasons the McKittrick tar seeps were investigated in the first place and subsequently excavated was because a road [Highway 33] was going to be built between Taft and McKittrick in the early Twenties,” Elam added. “There was great concern that the competency of the road was always going to be compromised because of too much tar at the surface. Discovery of large mammal bones caused paleontology experts such as dinosaur expert Charles Sternberg to be brought in to help excavate and identify finds.” That was between 1925 and 1927.

Tim Elam serves on the Buena Vista Museum of Natural History and Science Board of Directors.

Tim Elam serves on the Buena Vista Museum of Natural History and Science Board of Directors.

Excavations were done during two major periods. The first occurred from 1921 to roughly 1930, spearheaded by the University of California and the second took place from 1949 to 1950, headed by the Los Angeles County Museum of Natural History and the Kern County Museum.
“The excavation pits, on either side of Highway 33, were filled in,” Elam said. “However, some unexcavated areas remain.”
These areas allow scientists to put together a story of what the landscape was like during these specific years of our planet’s history.
“At McKittrick, the types of plants and animals suggest that area had more rainfall at that time, more inland surface water, and thus more plants for herbivores to feed on; there was not a shortage of plants for grazers,” he said. “You can decipher who was feeding on whom, just from looking at the relative number of carnivores, herbivores, omnivores, and comparing them to how they live in the present day. Trapped animals attracted more animals, which became trapped themselves.”
If you take the short drive to visit the tar pits, a geological and paleontological experience awaits you. Of course, you can take an even shorter drive to see some of the fossils found at McKittrick: the Buena Vista Museum of Natural History and Science has fossils of a coyote foot, a hawk foot, and three types of beetles (and the museum is also working with a researcher from Washington State University who is extracting pollen and spores from McKittrick tar). There are also replicas of a saber tooth cat skull, a mammoth tooth, and other creatures that roamed Kern County; creatures that had the unfortunate luck to wander into a small patch of asphaltum tens of thousands of years ago.
Visit www.sharktoothhill.org for more information!

Article appeared in our 30-6 Issue – February 2014

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