Sailing stones (also called sliding rocks, walking rocks, rolling stones, and moving rocks) are part of the geological phenomenon in which rocks move and inscribe long tracks along a smooth valley floor without animal intervention. The movement of the rocks occurs when large, thin sheets of ice floating on an ephemeral winter pond move and break up due to wind. Trails of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa, in Nevada, and most famously at Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable.
Description
The Racetrack's stones speckle the playa floor, predominantly in the southern portion. Historical accounts identify some stones around 100 m (330 ft) from shore, yet most of the stones are found relatively close to their respective originating outcrops. Three lithologic types are identified:
syenite, found most abundant on the west side of the playa; dolomite, subrounded blue-gray stones with white bands; black dolomite, the most common type, found almost always in angular joint blocks or slivers. This dolomite composes nearly all stones found in the northern half of the playa, and originates at a steep promontory, 260 m (850 ft) high, paralleling the east shore at the south end of the playa. Intrusive igneous rock originates from adjacent slopes (most of those being tan-colored feldspar-rich syenite). Tracks are often up to 330 ft (100 m) long, about 8 to 30 cm (3 to 12 in) wide, and typically much less than 2.5 cm (1 in) deep. Most moving stones range from about 15 to 46 cm (6 to 18 in) in diameter. Stones with rough bottoms leave straight striated tracks, while those with smooth bottoms tend to 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 to the direction from which it came. Trail length also varies – two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track. A balance of specific conditions is thought to be needed for stones to move:
A flooded surface A thin layer of clay Wind Ice floes Warming temperatures causing ice breakup
Research history
At Racetrack Playa, these tracks have been studied since the early 1900s, yet the origins of stone movement were not confirmed and remained the subject of research for which several hypotheses existed. However, as of August 2014, timelapse video footage of rocks moving has been published, showing the rocks moving at high wind speeds within the flow of thin, melting sheets of ice. The scientists have thus identified the cause of the moving stones to be ice shove.
Early investigation The first documented account of the sliding rock phenomenon dates to 1915, when a prospector named Joseph Crook from Fallon, Nevada, visited the Racetrack Playa site. In the following years, the Racetrack sparked interest from geologists Jim McAllister and Allen Agnew, who mapped the bedrock of the area in 1948 and published the earliest report about the sliding rocks in a Geologic Society of America Bulletin. Their publication gave a brief description of the playa furrows and scrapers, stating that no exact measurements had been taken and suggesting that furrows were the remnants of scrapers propelled by strong gusts of wind – such as the variable winds that produce dust-devils – over a muddy playa floor. Controversy over the origin of the furrows prompted the search for the occurrence of similar phenomena at other locations. Such a location was found at Little Bonnie Claire Playa in Nye County, Nevada, and the phenomenon was studied there, as well. Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from the Racetrack. In 1952, a National Park Service Ranger named Louis G. Kirk recorded detailed observations of furrow length, width, and general course. He sought simply to investigate and record evidence of the moving rock phenomenon, not to hypothesize or create an extensive scientific report. 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 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, which some researchers, such as geologist George M. Stanley, who published a paper on the topic in 1955, feel is too heavy for the area's winds to move. After extensive track mapping and research on rotation of the tracks in relation to ice floe rotation, Stanley maintained that ice sheets around the stones either help to catch the wind or that ice floes initiate rock movement.
Progress in the 1970s Bob Sharp and Dwight Carey started a Racetrack stone movement monitoring program in May 1968. Eventually, 30 stones with fresh tracks were labeled and stakes were used to mark their locations. Each stone was given a name and changes in the stones' positions were recorded over a seven-year period. Sharp and Carey also tested the ice floe hypothesis by corralling selected stones. A corral 1.7 m (5.5 ft) in diameter was made around a 8 cm (3 in) wide, 0.45 kg (1 lb) track-making stone with seven rebar segments placed 64 to 76 cm (25 to 30 in) apart. If a sheet of ice around the stones either increased wind-catching surface area or helped move the stones by dragging them along in ice floes, then the rebar should at least slow down and deflect the movement. Neither appeared to occur; the stone barely missed a rebar as it moved 8.5 m (28 ft) to the northwest out of the corral in the first winter. Two heavier stones were placed in the corral at the same time; one moved five years later in the same direction as the first, but its companion did not move during the study period. This indicated that if ice played a part in stone movement, then ice collars around stones must be small.
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