Sunday, November 04, 2007

Tim Cahill Explains How Rocks Go On A Walkabout in Death Valley

Gathering no moss: No one has ever seen rocks up to 700 pounds go skimming like hockey pucks across the Death Valley playa called the Racetrack when it gets wet, but the tracks bear witness.

Excerpted from the San Francisco Chronicle:
I imagine the sight of Death Valley National Park is something akin to scientific pornography for hard-rock geologists.

There are the obvious soaring mountains and abysmal valleys, of course. But in most other places on Earth, the folding and buckling of rocks, the colliding of crustal plates, the shores of advancing and retreating lakes, the evidence of volcanic activity, the scrape of glaciers across rock, the subtle and not so subtle effects of erosion are covered over in grass or dirt, in snow or ice.

The Earth is a modest mother, but Death Valley is, for the most part, naked.
It is also the only place on Earth where geology itself has made me laugh out loud. I am thinking specifically of an area in the northwest section of Death Valley called the Racetrack, where, inexplicably, rocks as big as microwave ovens go zipping across the desiccated mud for distances of more than half a mile.
The evidence is all there: deep tracks in the surface, with a rock at the end. One concludes, reluctantly, that the rocks somehow traveled a couple of hundred yards, leaving a telltale trail behind. There are over 150 of these roving rocks. But no one has ever seen them move.

The Racetrack proper, about 3 miles long and a mile wide, is what is called a playa, a dry, smooth lake bed. The Racetrack is a mere 2 inches higher on the north end than the south. Flat as a pool table. The surface is sun-baked mud, hard as rock, and patterned in polygons the size of doughnuts. It is an otherworldly sight, and there is a sense on the playa of post-apocalyptic silence, broken only by the whisper and wail of the wind.

This impression is compounded by the Grandstand, a 73-foot-high island of rounded bedrock at the north end that looks like the summit of a mountain buried in a sea of sediment. One supposes that observers - rock-racing fans - might sit on the Grandstand, as at a horse race, and observe stones zooming toward them from the southern end of the playa.

I had plenty of time to contemplate the Grandstand. I'd driven 30 miles from Ubehebe Crater, and the gravel road proved to be less brutal than expected, so I arrived at the Racetrack in the early afternoon. The best time to see the rock raceways is around dawn or dusk, when the slanting rays of the sun show the tracks to their best advantage.

To pass the time, I climbed to a ridge off nearby Ubehebe Peak for one of the most spectacular views in the park. On this February day, the temperature stood in the low 70s and made for pleasant climbing. When I reached the ridge, the slope dropped off like a cliff to the west, where I could see the sands and salt pans of the Saline Valley 4,000 feet below.

And that is one definition of Death Valley: It is a land of intense vertical relief. This is true of most of southeastern California, a region torn by earthquakes, once flooded with vast inland seas and eroded by wind and rain.

Nevertheless, it is primarily rising mountains and falling valley floors that create the astonishing counterpoint of land that lies hundreds of feet below sea level guarded by peaks rising two miles above. Slow tectonic torture corrugates the landscape as two massive crustal plates meet and slide past each other under California: the Pacific and North American plates. The ridge on Ubehebe Peak was a good place to contemplate this ongoing process.

When I turned around, to the east, I was looking directly down onto the Racetrack and the Grandstand. In the desolate wind on the exposed ridge, the Grandstand rocks looked like the very tips of buried buildings, like an undiscovered city swallowed up in silt, like alien and interred skyscrapers.

At the far edge of the Racetrack a multigenerational Japanese family walking on the playa looked as if they were skating across an ice rink. Soon enough, I was walking on the flat, feeling a mild sense of vertigo, a bit of dizziness that suggested I might just fall off. How? Where? I don't know, but try talking sense to your inner ear.

At the south end of the Racetrack, where the playa abuts an 850-foot-high mountain face, rocks had tumbled from elevation out onto the playa. Some were the size of softballs, others suitcases. These rocks did not gather any moss. They were movers. Robert Sharp and Allen Glazner, in their book Geology Underfoot in Death Valley and Owens Valley, explain the process. The playa receives 3 to 4 inches of rain a year during winter storms and summer cloudbursts. Parts of the Racetrack flood. Fine, intensely slippery clay settles, and the winds, which may reach 90 miles an hour, must overcome the forces of friction for the rocks to break free. Once that happens, it takes only about half the wind power to keep the rocks moving.

Some rocks made straight paths, some curved. Some traveled a hundred yards in one direction, stopped in a muddy muddle, apparently thought better of their direction, and made a 180-degree turn to ramble off in another direction.

Some trails were wide for a while, narrow, then wide again. Occasionally, half a dozen rocks took off at once from the base of the mountain and seemed to race straight toward the Grandstand like horses at the derby. The tracks often crossed one another. I followed dozens of them, and when I found the rocks at the terminuses of the tracks, they seemed almost sentient.

Why this made me laugh, I cannot say.

Learn more about Death Valley in National Geographic.

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