Sailing stones

Sailing stones are rocks that move across a flat playa, leaving tracks in the soft surface. Racetrack Playa, a dry lakebed in Death Valley National Park, is a well-known example: rocks there have left trails stretching for hundreds of metres, even though the stones may remain still for years.

A field study using GPS-equipped rocks, cameras, and weather instruments recorded the process at Racetrack Playa. After rain or other water accumulation, a shallow pond can form. Cold nights may produce a thin sheet of ice; as it melts and breaks into floating panels, light winds can push the panels across the water. The moving ice nudges rocks over the wet, soft mud, leaving tracks. Researchers observed rocks moving at a few metres per minute, a speed difficult to notice without fixed reference points.

In the 2013–2014 observations, more than 60 rocks moved during one event, and some instrumented rocks moved repeatedly. The study showed how this combination of water, thin ice, and wind can move rocks at the site; it did not establish that every trail at every location forms in exactly the same way. The National Park Service cautions visitors not to drive on the fragile playa, disturb the rocks, or walk on muddy areas when the surface is wet.