|Credits as above. Picture grabbed from ScienceDaily.com.|
|Stationary rocks (blue arrows) and rock moving from left to right (red arrow)|
|The team used quarried rocks,|
one shown here with its
Floating ice moves around the playa under the influence of winds. When it encounters rocks, it may pile up on the upstream side, increasing the effective cross-sectional area of the rocks to both upstream ice and water and thus facilitating movement. On the other hand, sometimes the ice fragments upon encountering a rock. Norris and Norris suspect that this phenomenon might explain the Sharp and Carey observation of the corral behavior: the rock that didn't move out of the corral was just downstream of a stake that may have shattered the ice. Stones with low profiles might be submerged beneath the ice, some rocks may be too big for the available forces under some wind conditions, and others may not totally or partially encounter ice.
But, the researchers concluded, the mystery may not be completely solved: they didn't get to see the really big ones move.
The authors also point out that the sliding rocks are not unique to Racetrack Playa or even the U.S. Ice-driven rock trails are observed on the bottom of Great Slave Lake in northern Canada and on the shores of the Baltic Sea. The mechanism may apply to rock trails on dry lake surfaces in Spain and South Africa where the lakes are at high elevation and exposed to cold winters.
*Robert P. Sharp and Dwight L. Carey, Sliding stones, Racetrack Playa, California, GSA Bulletin, 87(12), 1704-1717.
**Richard D. Norris, James M. Norris, Ralph D. Lorenz, Jib Ray, Brian Jackson, Sliding rocks on Racetrack Playa, Death Valley National Park: First observation of rocks in motion. PloS ONE, 2014; 9(8) e105948 DOI:10.1371/journal.pone.0105948 link to article is here