Published: 9:19 AM -01-28-11
The sailing stones, also known as sliding rocks and moving rocks, are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention.
They have been recorded and studied in a number of places around Racetrack Playa, Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is the subject of research.
The stones move only every two or three years and most tracks develop over three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander.
Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake.
Trails differ in both direction and length. Rocks that start next to each other may travel parallel for a time, before one abruptly changes direction to the left, right, or even back the direction it came from.
Trail length also varies � two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track.
Tracks are sometimes non-linear. Most of the so-called gliding stones originate from an 850 foot (260 m) high hillside made of dark dolomite on the south end of the playa, but some are intrusive igneous rock from adjacent slopes (most of those being tan-colored feldspar-rich syenite).
Tracks are often tens to hundreds of feet long, a few to 12 inches (8 to 30 cm) wide, and typically much less than an inch (2.54 cm) deep.
A balance of specific conditions are thought to be needed for stones to move:
a saturated yet non-flooded surface, a thin layer of clay, very strong gusts as initiating force, and strong sustained wind to keep stones going.
Geologists Jim McAllister and Allen Agnew mapped the bedrock of the area in 1948 and made note of the tracks.
Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from the Racetrack.
Speculation about how the stones move started at this time. Various and sometimes idiosyncratic possible explanations have been put forward over the years that have ranged from the supernatural to the very complex.
Most hypotheses favored by interested geologists posit that strong winds when the mud is wet are at least in part responsible.
Some stones weigh as much as a human. Some researchers, such as geologist George M. Stanley, who published a paper on the topic in 1955, feel are too heavy for the area's wind to move. They maintain that ice sheets around the stones either help to catch the wind or move in ice floes.
Professor John Reid led six research students from Hampshire College and the University of Massachusetts in a follow-up study in 1995. They found highly congruent trails from stones that moved in the late 1980s and during the winter of 1992-1993.
At least some stones were proved beyond a reasonable doubt to have been moved in ice floes that may be up to half a mile (800 m) wide.
Physical evidence included swaths of lineated areas that could only have been created by moving thin sheets of ice. So wind alone and wind in conjunction with ice floes are thought to be motive forces.
Physicists studying the phenomenon in 1995 found that winds blowing on playa surfaces can be compressed and intensified. They also found that boundary layers (the region just above ground where winds are slower due to ground drag) on these surfaces can be as low as 2 inches (5 cm).
This means that stones just a few inches high feel the full force of ambient winds and their gusts, which can reach 90 mph (145 km/h) in winter storms.
Such gusts are thought to be the initiating force while momentum and sustained winds keep the stones moving, possibly as fast as a moderate run (only half the force required to start a stone sailing is needed to keep it in motion).
Wind and ice both are the favoured hypothesis for these mysterious sliding rocks.
Noted in Don J. Easterbrook's "Surface Processes and Landforms", he mentioned that because of the lack of parallel paths between some rock paths, this could be caused by the breaking up of ice resulting in alternate routes.
Even though the ice breaks up into smaller blocks, it is still necessary for the rocks to slide.
The mystery of the Sliding Stones may never be fully explained.