How do rocks move?

Well, they don't of course, not without help. But in Death Valley, rocks slide across the dried out lake bed (playa), leaving distinctive trails. But no one has ever seem them move, and it's not confirmed yet exactly how they do it. Theories include UFO aliens (what are they doing, playing tic-tac-toe?) and prankish college students. Science is now reporting that, in the winter, it can get cold enough for a thin coating of ice to form on the rocks, creating a slippery surface in contact with the ground and allowing wind to gradually nudge them around. Well, it's good to have a puzzle solved with a prosaic, rather boring explanation. Or is that always true?