Been meaning to get to this one for awhile...
I was just in Death and Owens valley for a week this past March and bought a couple of great books on the local geology. One excellent book on the geology of Death and nearby Owens Valley is the following: Geology Underfoot: In Death and Owens Valley by, Robert Sharp and Allen Glazner. In general, the geology underfoot series is excellent-- you don't have to be a geologist or even scientist to understand the basics of what's going on.
Here's a rather long description from a chapter of the book called, "The Sailing Stones of Racetrack Playa":
"The playa's name comes from its oval shape and the unusual bedrock islands near its north end, The Grandstand (73 feet high) and a smaller neighboring carbonate-rock knob. By a stretch of the imagination, these islands could be a grandstand and judges' stand. The playa's name also fits for the rocks that mysteriously sail across the playa surface when conditions are just right, leaving tracks that record their travels in the soft, wet mud. Such tracks are not unique to The Racetrack-- they have been reported in at least eight other playas in southern California and Nevada-- but the numerous stones on The Racetrack's surface make tracks most abundant and spectacular here...
Stones have moved across the playa surface, leaving distinct tracks in the wet mud tens to hundreds of feet long, a few inches to 12 or more inches wide, and a small fraction of an inch deep. The longest continuous track we [the authors] have measured is 1,982 feet, though others have measured even longer tracks. Unless abnormally deep, tracks are discernable for no more than three or four years...
So, just how do the stones move?
Geologist George M. Stanley investigated the tracks in detail and published his research in 1955. He carefully surveyed many stone tracks and concluded that wind transported the stones while they sat frozen in thin sheets of ice. His evidence that ice sheets existed included scrape marks up to 200 feet wide on the playa and small, iceshoved, pebble ramparts along the south shore.
Bob Sharp [one of the authors] and Dwight Carey began monitoring The Racetrack's stones, with Park Service permission, in May 1968. They labelled 25 stones that had recently made trails and marked their positions with steel stakes. They eventually increased the number of monitored stones to thirty. The stones ranged from cobbles as small as 2.5 inches to boulders 14 inches across and weighing up to 56 pounds. The researchers marked each stone with an erasable identifying letter and assigned each stone a name. They recorded the stones' changes in position over a seven year interaval.
During the first winter, ten of the initial twenty-five stones moved, in two stages separated by a sitzmark and change in direction. One stone, Mary Ann (A) moved 212 feet. The researchers recorded similar major episodes during two of the following six winters, but in some winters no stones, or only one or two, moved. Insofar as we know, the stones did not move during the summers, although a park ranger once informally reported a trail-making stone that he felt had moved during the summer. In all, twenty-eight of the thirty monitored stones moved during the seven-year study, but only six moved during each of the three major episodes. A stone called Nancy (H), a modest-sized cobble 2.5 inches in diameter and weighing half a pound, recorded the greatest cumulative monitored movement, 860 feet. Nancy moved in all three major episodes and also experienced the greatest single-episode movement-- 659 feet! [Go Nancy!]...
Most geologists have long favored wind as the force that moves the stones. Even the ice-transport hypothesis relies on wind. Ice provides a greater tractive surface for the wind than that afforded by a single stone. Researchers Sharp and Carey evaluated the role ice plays in moving The Racetrack's stones by constructing a circular corral... around a small, track-making stone... during the first winter, 1968-69, the stone moved out of the corral 28 feet to the northeast, leaving a clear track and just missing a stake. The researchers then placed two heavier stones in the corral, one of which moved five years later in a direction and distance similar to the initial stone. Its partner did not move. A sheet of ice could hardly be so selective. Stones that escaped from the corral could have had only a small collar of residual ice at best. Sharp and Carey concluded that stones do not need to be frozen into a sheet or floe of ice to move.
Tracks close to the south shore of The Racetrack show that small, equidimensional cobbles move both by sliding and by rolling; the rollers leave tracks distinctly different from those left by sliders. A stone may employ both modes during a single episode of movement. Rolling seems an unlikely behavior for stones frozen into ice. At this locality, closely associated small stones create patterns of crossing tracks and changes in separation distance that are impossible for stones frozen into the same ice floe. These relationships further suggest that stones move on The Racetrack without the aid of ice.
However, in 1995, Prof. John Reid... reported on precise surveys of highly congruent trails on The Racetrack made in the late 1980s and during the winter of 1992-93. Their study demonstrates beyond reasoable doubt that some track-making stones were carried by a large sheet of wind-driven ice. Reid and his students envisioned huge ice floes as much as 0.5 mile across... however, many relationships and configurations exist within the pattern of tracks on The Racetrack that are not compatabile with ice transport... under the right conditions, wind alone can move rocks...
How much wind does it take to start the rocks moving? In 1995 three physicists analyzed conditions whithin the boundary layer of wind blowing across any flat, smooth playa surface..."
And so on... I'll stop here, but this gives you a sense of the depth of the article. Apparently, a bimodal rainfall pattern and a fine veneer of clay provide lubrication that allows rocks to move across the surface when the wind is strong enough. The stones can "literally sail across the playa surface" when conditions are right.
Best,
Kitten
