The findings are based on a lucky accident by James Norris and his cousin paleobiologist 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.