Subject: 'Sailing rocks' mystery finally solved

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Scientists have finally solved the mystery of how rocks can move across the flat ground of a dry lake bed in Death Valley, California.

Visitors have long been puzzled by the sight of boulder tracks criss-crossing a dusty bowl known as the Racetrack Playa in Death Valley National Park. But two researchers now say the rocks - which can sometimes be heavy and large - are propelled along by thin, clear sheets of ice on breezy, sunny days. They call it "ice shove". "I'm amazed by the irony of it all," paleobiologist James Norris tells the LA Times. "In a place where rainfall averages two inches a year, rocks are being shoved around by mechanisms typically seen in arctic climes."

The findings are based on a lucky accident by James Norris and his cousin Richard Norris - while they were studying the sliding rock phenomenon. They actually witnessed the boulders moving in December when they went to check their time-lapse cameras in the valley. "There was a pop-pop-crackle all over the place in front of us and I said to my cousin, 'This is it'," Richard Norris says in the science journal Nature. They watched some 60 rocks sail slowly by, leaving the well-known snaking trails in the ground. "A baby can get going a lot faster than your average rock," Norris notes. The rocks also don't slide around very often - scientists estimate only a few minutes out of a million - which is why the event has not been noticed before.

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