“The desert tells a different story every time one ventures on it.” — Robert Edison Fulton Jr.

By DR. CAMERON BARROWS

If time travel was possible, how thrilling would it be to be able to watch the passage of time, to see the rise and fall of mountains, the intrusion and retreat of oceans across continents, and the changes in climate — and how those changes are reflected by the plants and animals that occupy every corner of our planet? You would be part of a landscape that is changing, slowly, but always changing.

Without time travel, looking across our desert today it might be tempting to assume that it has always been so, rather than just a snapshot of a long process of change. However, any assumption of long-term stability would ignore the evidence.

In lieu of a time machine, we have geology. Geology is the study of the processes that form the landscapes we see today. The evidence of those processes has been written in stone. The challenge is learning to interpret what the rocks tell us. During the Paleozoic Era, 570-250 million years ago, much of what is now North America was covered by a shallow sea. There are hints of that past landscape, fossil trilobite beds in the Mojave Desert date to that era. Dolomite and marble exposures in the Santa Rosa Mountains, metamorphic remnants of limestone deposition, likely harken to that era. Next came the Mesozoic Era, 250-66 million years ago, the “age of dinosaurs.” A period of mountain building and erosion, the youngest, upper most layer exposed in the Grand Canyon, the Kaibab limestone was deposited during the early days of this era. On certain trails that take you down from the canyon rim, you can see vertebrate trackways embossed in the exposed limestone. The still younger layers, exposed in Zion and Bryce Canyon National Parks, and Dinosaur National Monument, reveal ongoing erosion and rising mountains.

Our current Era, the Cenozoic, dates from the impact of the asteroid that marked the end of the age of dinosaurs, 66 million years ago. The Cenozoic Era, like the previous eras, has been subdivided into Periods, in this case the older Tertiary and more recent and current period, the Quaternary Period. Unlike the earlier Eras, these periods are further divided into Epochs, with the oldest within the Cenozoic being the Paleocene Epoch, followed by the Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and the current being the Holocene, although there is a movement to re-name our current Epoch the Anthropocene, the age of humans. The Quaternary Period encompasses both the Pleistocene and Holocene/Anthropocene Epochs.

The Peninsular Range has as its core Mesozoic granites, formed as the cooled batholiths that once “fed” molten lava to a chain of volcanoes extending from Baja California Sur up into British Columbia. The southern volcanoes are of course long extinct and eroded away. During the Miocene and Pliocene Epochs, roughly 20-25 million years ago, the cooled granitic batholiths began to be uplifted, forming what is now the Peninsular Mountain Range, north through the Sierra Nevada Range. That slow uplifting resulted in a growing rain shadow, incrementally increasing the aridity to the east of the mountain ranges and creating the deserts we know today. That increasing aridity was gradual enough to allow plants and animals to move, shift and/or evolve to survive and thrive within the deserts. Still, even with the building rain shadow there were periods of cooler-wetter conditions, such as the glacial maxima during the Pleistocene Epoch.

There were also periods of ocean incursions when the Gulf of California extended north into the Coachella Valley. Evidence of those incursions are marine fossils, dated to the Pliocene Epoch, or about 5-6 million years ago. These fossils are found in what has been named the Imperial Formation (named after Imperial County). The fossils include oyster beds, such as those near the Willis Palms Palm Oasis. Mixed within the oysters are shark’s teeth and fish otoliths, ossified fish ear bones. However, it takes an otolith expert to distinguish an otolith from a piece of gravel or sand. Another exposure of the Imperial Formation can be found in an unnamed canyon east of Whitewater canyon. There a whale skull was once excavated, as well as walrus and manatee bones. The fish species, as well as the manatees, are thought to be closely associated with species now restricted to the Caribbean Sea, and so are from a time before the Isthmus of Panama closed the connection to the Pacific Ocean.

Above the Imperial Formation, and so younger, more recent, Pleistocene Epoch deposits, are called the Palm Springs Formation. There fossils are rare, but Douglas fir, willow, oak, and cattail fossils have been found, along with mammoth, horse, and camel bone fragments. These all point to a cooler-wetter landscape. This is when the Colorado River periodically flooded into the Mojave Desert and Salton Sink, and with those flood waters were tons of sediment. Those sediments created sand dunes and alluvial fans.

Geology creates a foundation for both understanding the past and interpreting the present. It is the backdrop upon which we can understand current patterns of our deserts’ rich biodiversity, and for those with some imagination, it creates a time machine to see the changes that compose the world we see today.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.