File:Racetrack Playa in Death Valley National Park.jpg

English: Landscape image; Location: Racetrack Playa in Death Valley National Park; orientation of this view is south-to-north from near the southern edge of the playa. The Grandstand is visible on the horizon a little to the right of the center of the image, about 2 miles distant from the camera. Note the track through the dried mud behind the boulder. These tracks are left when winter storms deposit an inch or two of rain water on the surface of the playa, which consists of extremely fine grained, impermeable clay that becomes very slippery when wet. This water can freeze overnight into sheets of ice with the rocks locked into them, with a thin layer of liquid water trapped between the ice and the ground surface; if a windstorm then occurs, with speeds above about 50 mph, the ice can be moved by the wind, dragging the rocks along with it, which then gouge the tracks through the mud. The curvature, and abrupt changes in direction of the tracks, reflects changes in speed and direction of the wind, and collisions between different slabs of ice that deflect them, while the ice sheet was being pushed along. The rocks originate in the mountains surrounding The Racetrack, most of them from a specific steep bluff on the south edge of the playa, and there are usually several hundred scattered around at any given time, ranging in size from gravel to boulders a half-meter across that can weigh 300 kg or more. The cause of the movement of these rocks was a mystery for many decades until it was finally observed and photographed by eyewitnesses Richard and James Norris on December 21, 2013 [see http://www.latimes.com/local/lanow/la-me-ln-rocks-move-death-valley-lake-bed-20140827-story.html].