Racetrack Playa under water last December, with visible movement trails. Richard Norris/Scripps Oceanography
The question of how big heavy stones skim across the dry lake bed known as Racetrack Playa in Death Valley National Park is one of those enduring California legends. Revisited by scientists, re-discovered by new generations of journalists, but never really explained. Now a team led by paleobiologist Richard Norris of the Scripps Institution of Oceanography in San Diego proposes an answer — and the researchers think they were present when some of the "sailing rocks" moved this last winter. The solution to the mystery, Norris explains in the following video, is that the rocks slide when the playa is covered in a shallow layer of water and a thin veneer of windowpane ice. Light winds push the ice sheets around, moving even heavy rocks — too slowly to be easily observed. The evidence is hidden until the lake bed dries out, leaving a visible trail on the surface.
From the research team's website:
After several decades of study and observation, the sailing stones of Racetrack Playa have been observed in motion!
During winter 2013-2014, members of our group were present while the playa was flooded and frozen. A steady wind drag moved the thin ice sheets covering the playa’s south end, and these sheets pushed the rocks along, creating the characteristic furrows in the playa’s muddy surface.
“Science sometimes has an element of luck,” Norris said. “Only two years into the project, we just happened to be there at the right time to see it happen in person.” The results of the study are reported today in the online journal Plos One.
The Racetrack, located the Cottonwood and Last Chance mountain ranges at the north end of the park around the Ubehebe Crater, is about three miles long and two miles wide. "At least 10,000 years ago this region underwent climatic changes resulting in cycles of hot, cold and wet periods," Death Valley's website says. "As the climate changed, the lake evaporated and left behind beige colored mud, at least 1,000 feet thick."
Caution: if you go out there, be prepared for about 25 tough miles on backcountry roads in the desert — we don't have to tell you how hot Death Valley gets, or how far away you are from help if something happens. The name is not a promotional slogan. It was earned. And for god's sake, behold the rocks from a respectful distance. Out there your foot prints and scuff marks can last for a long time, and get in the way of the rocks' mysterious time travel. "Muddy footprints will disrupt the rocks' movements and leave unsightly scars for years," the National Park Service says. "The surface of the playa is very fragile."