Stahler: Mysterious moving rocks
Special to The Union
The forces of nature sometimes come together in just the right place, at just the right time, to assemble rainbows and mirages, waterfalls and geysers, shooting stars and lunar eclipses.
Twenty thousand years ago, glaciers flowed down the valleys of the Yuba, the American, the Feather: Rivers of ice, fed by snow from the intense winter storms of the ice age.
Intense winter storms hit the desert southwest then, too, but rather than snow, they dropped rain. The deserts of the southwest experienced, not an ice age, but a pluvial – an age of rain.
The desert southwest is today threaded by dry river valleys … not just dry washes, that flood every few years, but the valleys of real rivers, that once carried significant water.
Some pluvial rivers filled, others drained, pluvial lakes. After the end of the pluvial/ice age, roughly 10,000 years ago, the lakes dried up; the dry lake-beds consisted of sediments and salts, white expanses that are super-flat and super-smooth. They’re called “playas” (Spanish: “beaches”).
Racetrack Playa lies at just over 3,800 feet in the Panamint Range, the mountains that form the western wall of Death Valley.
The playa is well-named, in part because a rocky “grandstand” sits at one end. But more than that, Racetrack Playa is the site of a race … run by rocks.
Racetrack Playa is bordered, in part, by cliffs. From these cliffs, rocks fall onto the playa. But the rocks on the playa don’t always match those of the nearest cliff.
The rocks have moved. Further evidence, easier to see, betrays their movement.
Behind each rock is a track — a groove in the salty silt — that shows where the rock has been, and which way it’s going.
The tracks of nearby rocks parallel one another, revealing that neighboring rocks often move in lockstep — when one rock moved straight, or jogged to the left or right, so did others nearby.
Lest you conclude that these are small, inch-wide rocks, easily blown by a strong wind — some are, indeed, small. But others are roughly a foot across; these are heavy rocks.
Racetrack Playa has occasioned all sorts of explanations. Perhaps the rocks are simply sliding downhill when winter rain wets the playa surface.
Problem is, the playa drops only an inch or two per mile — not much of a slope.
Death Valley, its floor below sea level, bakes in the summer, but gets quite cold in the winter; higher elevations get colder yet.
After a winter storm, rainwater can freeze on the surface of the playa.
Perhaps a sheet of ice, attached to a rock, creates a sail, catching the wind and propelling the rock — and its neighbors — forward.
Scientists tested this hypothesis some years ago, by building “corrals”: they pounded lengths of rebar vertically into the playa, the bars just far enough apart that a rock could slip through, but close enough together to break up any ice that might be attached. Rocks escaped, and continued sliding across the playa.
Another suggestion, of course, was aliens (I made this suggestion myself, when I ran into a geology class at the Racetrack one day. The suggestion was made in jest, but also, in frustration.).
A report in the online journal PLOS One now hypothesizes that rocks and ice are buoyed up by a thin layer of water that coats the playa; large panels of floating ice, along with wind-blown water, then push the rocks forward.
I have to admit, I don’t see how this would allow a large rock to escape a “corral.”
I’ll be revisiting the Racetrack this fall; I’m now out-of-town, but will contact the authors when I return, and will report back to our readers.
It’s saddening to follow a track at the Racetrack, for dozens, even hundreds, of feet, but find no rock at the end. It’s become a souvenir in someone’s closet
Some years ago, the park service had to dig trenches around the playa to stop off-roaders doing donuts through the rock trails. This place is close to magic; it deserves better.
Al Stahler’s science programs can be heard on alternate Tuesdays at noon on KVMR (89.5 FM). He teaches classes to students of all ages, visits classrooms, and may be reached at email@example.com
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