Fossils of Anza-Borrego Desert State Park

George T. Jefferson
Associate State Archaeologist

Brian W. Cahill
Park Interpretive Specialist

When most visitors think of Anza-Borrego Desert State Park, located in Southern California's Colorado Desert, they usually envision wild flowers, bighorn sheep, or vast arid landscapes framed by rugged mountains. Few realize that the eroded badlands of Anza-Borrego provide a contrasting view, a window into the region's vanished past. The Park was not always the arid desert we experience today.

The key to understanding this prehistoric scene is paleontology, the study of the fossilized remains of ancient life. And, Anza-Borrego has an exceptional fossil record. Over 500 different types of organisms have been identified, ranging from microscopic plant pollen and spores to the largest of mammoth elephants. Not only are the bones and teeth of long extinct animals preserved, but in some places, also their tracks.

The Salton Trough, a geographically active rift valley, which bounds the eastern edge of the Park, once held a northward extension of the Sea of Cortez. Sediments laid down 5 million years ago in these warm clear tropical waters now yield the preserved shells of a variety of clams, snails, crabs and corals. These organisms have ties with the Caribbean Sea, and record a time before the Isthmus of Panama had formed. The remains of fish, walrus, baleen whales and even sea cows help us to more fully picture this long marine ecosystem.

About 4 million years ago, the ancestral Colorado River began cutting through the Colorado Plateau of Arizona and Utah. The sediments eroded during the formation of the Grand Canyon and spilled into the Salton Trough, creating a vast delta. These brackish marine deposits are recognized in the Park today by their extensive fossil oyster shell reefs and fossilized wood. The types of trees represented now live along the pacific coast of southern California suggesting that any mountains west of the Trough must have been low.

As the Salton Trough was filled with sediment carried by the ancestral Colorado River, what was to become Anza-Borrego gradually changed from a predominately marine environment to a system of interrelated terrestrial habitats. By about 3 million years ago, most of the area once covered by the Sea of Cortez held a large, inland lake. Here, the remains of fresh water clams, snails and fish are not uncommon. Streams and rivers draining the new uplifted Peninsular Range Mountains, which border the western side of Anza-Borrego supported a rich diversity of wild life. Herds of mammoth elephants, tapirs, zebra-like horses, several species of camel and llamas ranged across a landscape of steam border woodlands and savannah-like grassy scrublands. And, had we been there, we may have glimpsed foraging giant ground sloth, beaver, or even a saber-toothed cat or American cheetah on the hunt.

Paleontologic studies first began in the Anza-Borrego Desert during the mid 1930s, and continue today. A dedicated volunteer crew assists State paleontologists in collection, conservation, curation and study of Anza-Borrgeo's rich fossil heritage.

To learn more about Anza-Borrego Desert State Park visit: Anza-Borrego Desert SP

Image of vast and arid landscapes  Image of eroded badlands  Image of mammoth elephant tusk  Image of clams  Image of corals  Image of oyster shell reefs  Image of fossilized wood

Downs, T., and J.A. White, 1968.
A vertebrate faunal succession in superposed sediments from late Pliocene to middle Pleistocene in California.
23rd International Geologic Congress 10:47-47.

Kurten, B., and E. Anderson, 1980.
Pleistocent mammals of North America.
Columbia University Press, New York, NY, 442.

Remeika, P., and L. Lindsay, 1993.
Geology of Anza-Borrego: Edge of Creation.
Sunbelt Publications, Kendall/Hunt Publishing Company, Dubuque, IA, 208.