Spend enough time walking around in the desert and you will eventually find broad, flat expanses of ground completely covered with a blanket of small rounded rocks. At first glance, the ground cover in these spots may seem unremarkable, as if someone had just spread a layer of gravel over a dead surface.
Look a little closer, though, and surprises emerge.
The stones don't look randomly scattered, for one thing. Their corners are often tucked neatly into hollows of the stones adjacent to them, fitted together so closely that sometimes you might have trouble seeing a space between them.
Pick up one of these stones and more surprises come to light. Though there are deposits of gravel hundreds of feet thick in the desert, more often than not the stony "pavement" you're standing on will turn out to be no more than one layer of stones thick, a brittle, protective armor on a thick deposit of sand and silt.
I put "pavement" in quotes above as if I had come up with the metaphor on my own, but that's what desert soil scientists actually call these layers of stones: desert pavement. Desert pavement covers hundreds of miles of the California deserts. Dry lakes and sand dunes capture the popular imagination far more, but they cover only a fraction of the deserts that desert pavement does.
And once you start noticing desert pavement, you'll see it almost everywhere you go in the desert, at least in areas where large amounts of sediment have tumbled down off the slopes of stony mountains. But this odd arrangement of soil -- heavy stones in a thin, nearly complete layer atop deep, fine sediment -- clearly didn't just fall into place intact, any more than flour, sugar, and cocoa powder dumped into a bowl land in the form of a layer cake. If it was up to gravity alone, desert soil would generally be composed of a random mixture of particles ranging in size from dust specks to boulders the size of a 1967 VW Microbus, sorted somewhat by the action of flash floods. Something clearly arranges the stones, sand and silt into a desert pavement soil profile. But what?
For a long time, it was assumed that desert pavement was what was left over after incessant wind -- of which there is certainly a great deal in the desert -- blew off the silt and sand and very fine gravel. It takes a strong wind indeed to move rocks larger than an inch in diameter, so rocks that size and larger would remain, jostled around a little with each gust until they nestled down into a stable, interlocking pattern. Geologists call wind-powered erosion "deflation," and you can still find references to desert pavement here and there as "deflation armor."
"Sheet floods," in which water does much the same thing, have also been proposed as creative forces behind desert pavement. Both wind and water certainly play a role in creation of similar soil pavements in former glacial till in the Midwest, or coastal prairies farther west in California. And geologists are satisfied that in certain spots in the desert, wind and water are all we need to explain desert pavement's existence.
But wind and water do not work as explanations everywhere. There are many places in the desert where desert pavement coexists with cryptobiotic soil crusts, which hold soil against the ravages of wind and gentle rain, thus keeping the dust and sand from blowing or washing away.
Often the soil beneath a desert pavement will have no stones in it at all, which means the pavement wasn't solely formed by removing finer sediments from the very top layer: some process must have sifted stones to the top. Frost heave, salt crystal formation, and the action of soil fungi have all been suggested as mechanisms for moving stones to the surface, all with some plausibility.
As California geologist Andrew Alden reports on his excellent desert pavement page, an swath of desert pavement in the Mojave Preserve raises the explanatory stakes even higher. West of Cima Dome in the Preserve are some of the youngest lava flows in the lower 48. You can find desert pavements made entirely of lava clinkers. Beneath the thin lava cover are thick deposits of fine-grained material, then more lava of the same age.
The most plausible explanation? It starts with a pile of gravel. At the Cima volcanic field that gravel is lava; elsewhere it might be something else: a cobble bar deposited by a flash flood or a shallow slope of scree around the base of a mountain. Whatever the origin of the gravel, it's there first. Wind picks up finer sediment elsewhere and drops it onto the gravel. That finer sediment fills the gaps between the stones. In the winter, ice crystals form in the sand and push the stones upward. In the summer, salt crystals do the same as do, some scientists think, the thin thready mycelia of soil fungi. Stones get pushed upward a millimeter at a time, making more room for silt and sand below. A windstorm happens by with another load of dust: lather, rinse, repeat. Eventually you have a thin layer of stones resistant to wind and water covering a thick reservoir of windblown sediment: desert pavement. At least that's how geologists are starting to figure it works in some places.
However desert pavement forms, it's a slow process. Ancient geoglyphs created by the ancestors of today's desert native people were made by simply moving the desert pavement's cobbles, sometimes through the simple expedient of raking them away to expose the lighter soil beneath. You can see the result in this photo of the Topock Maze, near Needles:
The artwork's age is a matter of some dispute, but it's certainly at least several centuries old. This suggests that however desert pavement forms, it's a rather slow process, and thus repair of damage done to desert pavement would necessarily be slow as well.
The state of Arizona was reminded of the importance of all this earlier this year when a monsoon storm traveled over a hundred miles or so of desert between Tucson and Phoenix. Though that desert had once been protected by desert pavement, bulldozers and grazing cattle and plows and off-road vehicles had ground much of that armor into dust. The result? Centuries' worth of desert dust ended up in the skies, air filters, and lungs of people in Maricopa County (Watch a video of the massive dust storm here).
Desert pavement doesn't just act as a trap for windblown particulate matter. It's been found to sequester surprising amounts of soil nitrates as well, doing us all a favor by keeping that fertilizer out of the groundwater supply. Where you have stable soils with significant amounts of nutrients in them, you're likely to find living organisms. It isn't too surprising that desert pavements often hold significant wildflower seedbanks in them, and a good wet winter will transform the next spring's pavement to a garden. But the soil beneath the pavement can have less-friendly organisms living in it as well. Desert pavements are thought to be a significant reservoir, in some parts of the desert, for the fungus that causes Valley Fever. Leave the pavement intact and the fungi will remain out of our way. Bulldoze the pavement and we may find ourselves exposed to once-sequestered pathogens for a few centuries to come.
This is the kind of thing I love finding out about the desert: a feature of the landscape about as innocuous as you can imagine -- a flat expanse of boring gravel turns out to clean our air of asthma-inducing dust, help keep us from catching Valley Fever, and gives us flowers every now and then besides.
As long as we keep the bulldozers off it.
Chris Clarke is an environmental writer of two decades standing. Author of Walking With Zeke, he writes regularly at his acclaimed blog Coyote Crossing and comments on desert issues here. He lives in Palm Springs.
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