“I have always loved the desert. One sits down on a desert sand dune, sees nothing, hears nothing. Yet through the silence something throbs, and gleams." — Antoine de Saint-Exupery

By DR. CAMERON BARROWS

For many who have not explored our deserts, their mental images of deserts have become intertwined with sand dunes. There are places where that is true; the Sahara in northern Africa or the Namib in southwestern Africa are dominated by thousands of square miles of sand dunes. However, North American deserts are dominated by alluvial fans, bajadas and blocks of granite, metamorphic rocks, or sandstone jutting up through those sandy crusts. Sand dunes are here, but they comprise a small fraction of our deserts’ landscapes. The reason for their rarity, and for their isolated, insular, island-like distribution is that the topographic ingredients necessary for creating sand dunes are uncommon. First you need an on-going source of sand, lots and lots of sand. But not just any sand, like a Goldilocks’ choice it needs to be just the right size (0.5-0.25 mm in diameter). Any smaller and it blows away in the wind as dust. Any larger and the wind won’t be able to push it into dune shapes. Then you need aridity to limit plant growth that would otherwise stabilize that sand. Then you need wind, strong winds that keep the sand grains moving, keeping them from becoming stabilized.

Rivers are the most common sand suppliers. The Nile in north Africa, the Orange River in southwestern Africa. For the North American deserts, the Colorado River is the origin for most sand dunes. Five or six million years ago as the Colorado Plateau was uplifted, the Colorado River was created. The Colorado River cut through the layers of sandstone of what would eventually become the Grand Canyon and carried those grains of quartz downstream. During flood events, the Colorado River jumped its banks and spread out over a landscape that was just beginning to feel the increasing aridity caused by the uplifting Sierra Nevada and Peninsular Mountain ranges. With the flooding water came lots and lots of orange-brown sand grains that had been eroded from the walls of the incipient Grand Canyon. The Algodones dunes at the south end of the Salton Sink, and the Gran Desierto in northwestern Mexico are among the dune systems that owe their existence to the creation of the Grand Canyon. The Mojave River, flowing east across the Mojave Desert supplied the sand that would become the Kelso Dunes. Lake Manley, which once filled Death Valley, created the sand source for the sand dunes that dot that valley’s floor. In all these cases, the dunes are built from grains of quartz, eroded from relatively soft sandstone canyon walls.

There are very rare exceptions to the need for sandstone for supplying quartz grains that then create sand dunes. White Sands National Park’s sand dunes in southern New Mexico are comprised of grains of brilliantly white gypsum, gypsum that precipitated out of the shallow waters of nearby Lake Lucero. The area has a prolific source of calcium sulfate (gypsum) from Permian rocks found in nearby mountains, especially to the north, as well as from equivalent rocks present beneath the Tularosa Basin. The other exception are the sand dunes of the Coachella Valley, built from grains of eroded granite, with quartz, as well as feldspar and mica, giving these dunes a distinct salt and pepper color. No other dunes in North America, or perhaps anywhere else on earth are formed from grains that were once solid blocks of granite.

Granite is not rare. It forms the batholith core of the Peninsular and Sierra Nevada Mountain ranges, and granite is not uncommon in the hundreds of isolated mountains across the North American Deserts. So why aren’t there more granite-sourced sand dunes? The answer is that granite is exceedingly hard, and so unlike sandstone, is resistant to erosion. But then why have dunes formed in the Coachella Valley? I have searched for an answer to this enigma since I arrived here almost 40 years ago. The primary source of the Coachella Valley’s sand dune sand has always been readily apparent. On any clear day you can see an eroded scar near San Gorgonio Peak in the San Bernardino Mountains. That scar is at the headwaters of the Whitewater River and Mission Creek. During flood events, water brings sand grains down their courses and spreads that fresh sand onto the valley floor. Strong west and northwesterly winds then move that sand throughout the valley, much to the annoyance of the current cohort of humans living in its path. First inhabitants, the Cahuilla, smartly selected village sites outside the aeolian sand flow (the name aeolian, literally “of the wind'’ derives from the Greek god’s name Aeolus). Current residents take no responsibility for their own role in home site selection and so find it difficult to understand why they are subjected to these forces of nature, and why those forces have not been subdued.

Knowing where the sand comes from doesn’t answer the question of why, with the abundance of granite exposures across the desert southwest, why is there just one place where granite feeds sand dunes? Given how resistant to erosion granite is, a better question might be why are there sand dunes in the Coachella Valley at all? Yet, prior to the 1950s, there was a 100 square mile expanse of sand dunes here, one of the largest dune systems in the California deserts. 

Only very recently have I formulated a hypothesis that might explain this quandary. We all know that the San Andreas earthquake fault zone skirts across the northern edge of the Coachella Valley. Perhaps slightly lesser known is that the San Andreas fault splits here, with the southern “Banning branch” continuing through the San Gorgonio Pass, through Los Angeles, and on up to San Francisco. The northern, “Mission Creek branch” moves through Desert Hot Springs, through the Wildlands Conservancy’s Mission Creek Preserve, but then abruptly stops under the San Bernardino Mountains. On a recent hike with our California Naturalists, we climbed a ridge that gave us an excellent view of the fault’s path. The fault formed a valley that was aimed precisely at that scar below Mount San Gorgonio, that same scar that is the sand source for the Coachella Valley’s dunes. It isn’t hard to imagine the moving plates along the San Andreas fault could be acting as a “battering ram”, slamming into and shattering the granitic batholith, making the granite more easily eroded. That unique juxtaposition of that earthquake fault branch shattering otherwise solid rock, could explain why there is such an abundant source of granitic sand here, while granite mountains elsewhere, without a powerful, active earthquake fault, do not supply enough sand to create dunes.

Regardless of their origins, isolated sand dunes throughout the North American deserts were created very recently on a geologic time scale, probably within the Pleistocene Epoch. These dunes created a new albeit challenging habitat for animals and plants to colonize. The primary challenge was a habitat surface that was in motion with every gust of the wind. How could plants become established, and without plants how could animals survive? As impossible as it seems, nature repeatedly devises solutions regardless of the challenge. Today, every one of those isolated dune systems is inhabited by beetles, spiders, crickets, lizards, and plants found nowhere else on earth, each endemic to its own dune system. With modern genetic analyses, even species that look like those found on dunes elsewhere are being found to be unique to the dunes upon which they live.

The granitic sand dune system of the Coachella Valley is no exception. There is the Coachella Valley milkvetch, the Coachella Valley giant sand treader cricket, Barrows’ dune beetle, and the Coachella Valley fringe-toed lizard, each found nowhere else on earth. Except that those original one hundred square miles have been reduced to less than a mere 4-5 square miles, fragmented into small patches around the valley. Those remaining fragments are largely protected; however, they still require regular inputs of new sand to keep from becoming stabilized. The adaptations dune animals and plants have developed to thrive on active sand dunes are of no value once a dune becomes stabilized. Time and time again we have watched as small dune fragments have become stabilized. Within just a few years, the dune lizards, beetles, crickets, and plants are all gone. The corridors supplying new sand critical to sustaining an active dune habitat are much less protected and are under siege by people expecting nature to be subdued, to ease their frustration with annoying blowing sand and occasional flooding. The task of ensuring that we continue to be able to share this planet with all of nature, is an on-going, never-ending task.

Nullius in verba

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