In almost perfectly flat dry lakebed at the upper end of Death Valley there some strange rocks known as 'sailing stones' are dotted about the otherwise almost smooth lake bed, some of them weigh up to 36kg, but behind them are tracks sometimes tens of metres long as if they were moving. The tracks can be straight or zig zagged and some of the stones have been marked and their positions seem to change relative to stakes put in the ground, but noone has ever seen a stone move.
The movements seem to occur on the rare occasions whern the lake bed is covered with a shallow layer of water, there is a strong wind and the temperature is very low. Both ice bergs and winds which can reach 90mph only a couple of inches above the lake bed have been suggested as ways of moving the stones but neither has been decisive.
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| Picture of two rocks on Racetrack Playa in Death Valley. Notice the mysterious groves leading away from the stones. © Maveric149 |
Ralph Lorenz, a scientist at Johns Hopkins University has come up with a new explantion, a neat combination of both. The lake floods a couple of inches deep, and as it does so a small ice berg forms around the stone, the water level increases slightly and beacuse ice floats this gives the stone lift, reducing the friction with the ground. The high winds are then able to push the stones along forming the long tracks. They have done some exeriments which support their hypothesis, though it will be hard to know what is happening for sure until someone actually sees one moving.
This is a lovely mystery, but studying areas like the racetrack playa may be useful in understanting places like the shallow lakes on Saturn's moon Titan, where there are some hydrocarbon lakes which seem to occasionally dry out.
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