For years people have been mystified by the apparent movement of rocks, some weighing over 100 pounds, over hundreds of feet along the flat surface of Racetrack Playa in Death Valley leaving long tracks etched in the ground as they move. ... Using time-lapse photography they caught on camera the rocks sliding across the playa at the surprisingly fast pace of up to 15 feet (3-5 meters) per minute on December 4th and December 20th, 2013 as well as other occasions. In all cases rainfall preceded the events, which froze just beneath the playa surface.
http://www.wunderground.com/blog/weatherhistorian/comment.html?entrynum=300
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