Racetrack Playa

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Satellite image of Racetrack Playa. The Last Chance range is on the left, Cottonwood Mountains to the right of the light-colored playa surface.

Racetrack Playa is a seasonally dry lake (a playa) located in the northern part of the Panamint Mountains in Death Valley National Park, California, U.S.A. that is famous for rocks that mysteriously move across its surface. Coordinates: 36°40′53″N, 117°33′46″W

1 Geography
2 "Sailing stones"
3 The "Sailing Stones" in fiction
4 References
5 External links

[edit] Geography

The playa is nestled between the Cottonwood Mountains to the east and the Last Chance Range to the west. During periods of heavy rain, water washes down from nearby mountain slopes onto the playa, forming a shallow, short-lived lake. Under the hot Death Valley sun, the thin veneer of water quickly evaporates, leaving behind a layer of soft mud. As the mud dries, it shrinks and cracks into a mosaic of interlocking polygons.

The playa is 3708 feet (1130 m) above sea level, 2.8 miles (4.5 km) long north-south, and 1.3 miles (2 km) wide east-west. It is also exceptionally flat with the north end being only 1.5 inches (4 cm) higher than the south. Two islands of bedrock poke above the playa's surface at its north end:

The Grandstand, a 73 feet (22 m) high outcrop;
A small carbonate outcrop.

Racetrack is dry for almost the entire year and has no vegetation on it. When dry, its surface is covered with small but firm hexagonal mud crack saucers that are typically 3 to 4 inches (7.5 to 10 cm) in diameter and about an inch (2.5 cm) thick. During the bimodal rainy season (summer and especially winter) a shallow cover of water deposits a thin layer of fine mud on and between the saucers of Racetrack. Heavier winter precipitation temporarily erases them until spring when the dry conditions cause new mud cracks to form where the old ones had been. Sandblasting wind continually helps to round the edges of exposed saucers. Annual precipitation is 3 to 4 inches (75 to 100 mm) and ice cover can be 1 to 2.5 inches (2.5 to 6.5 cm) thick. Typically only part of the playa during any given year will flood.

The highest point in the immediate area is Ubehebe Peak, itself 5,678 feet (1731 m) above sea level, a full 1970 feet (571 m) above the lakebed, and 0.85 mile (1.37 km) to its west.

[edit] "Sailing stones"

Most of the so-called 'sailing stones' are 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). They have never been seen or filmed in motion and are not unique to The Racetrack. Similar rock travel patterns have been recorded in several other playas in the region but the number and length of travel groves on The Racetrack are notable. Tracks are often tens to hundreds of feet (low to high tens of meters) long, a few to 12 inches (8 to 30 cm) wide, and typically much less than an inch (2.5 cm) deep. Racetrack stones only move once every two or three years and most tracks last for just 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-sized track in the stone's wake.

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 may 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 man, 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 wind to move. They maintain that ice sheets around the stones either help to catch the wind or move in ice flows.

Bob Sharp and Dwight Carey started a Racetrack stone movement monitoring program in May of 1968. Twenty-five then later thirty 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' position were recorded over a seven year period. Sharp and Carey also tested the ice flow hypothesis by corralling selected stones. A corral 5.5 feet (1.7 m) in diameter was made around a 3 inch (7.5 cm) wide, 1 pound (0.5 kg) track-making stone by having seven rebar segments placed 25 to 30 inches (64 to 76 cm) apart. If a sheet of ice around the stones either increase wind-catching surface area or help move the stones by dragging them along in ice flows, then the rebar should at least slow down and deflect movement. Neither appeared to occur; the stone barely missed a rebar as it moved 28 feet (8.5 m) 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.

Close-up of a rock on the Racetrack

Ten of the initial twenty-five stones moved in the first winter with Mary Ann (stone A) covering the longest distance at 212 feet (64.5 m). Two of the next six monitored winters also saw multiple stones move. No stones were confirmed to have moved in the summer and some winters none or only a few stones moved. In the end all but two of the thirty monitored stones moved during the seven year study. At 2.5 inches (6.5 cm) in diameter Nancy (stone H) was the smallest monitored stone. It also moved the longest cumulative distance, 860 feet (262 m), and the greatest single winter movement, 659 feet (201 m). The largest stone to move was 80 pounds (36 kg).

Karen (stone J) is a 29 by 19 by 20 inch (74 by 48 by 51 cm) block of dolomite and weighs an estimated 700 pounds (about 320 kg). Perhaps not surprisingly Karen didn't move during the monitoring period. The stone may have created its 570 straight and old track from momentum gained from its initial fall onto the wet playa. However, Karen disappeared sometime before May 1994, possibly during the unusually wet winter of 1992 to 1993. Removal by artificial means is considered unlikely due to the lack of associated damage to the playa that the needed truck and winch would have done. A possible sighting of Karen was made in 1994 a half mile (800 m) from the playa.

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 flows 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 as well as in conjunction with ice flows are thought to be motive forces.

Rocks on Racetrack Playa

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).

A balance of specific conditions are thought to be needed for stones to move:

A saturated yet non-flooded surface,
Thin layer of clay,
Very strong gusts as initiating force,
Strong sustained wind to keep stones going.

[edit] The "Sailing Stones" in fiction

The plot of Iain M. Banks's science fiction novel Feersum Endjinn includes a set of mysteriously sliding rocks on a flat plain, evidently inspired by the stones on Racetrack Playa.

[edit] References

Messina, P., 1998, The Sliding Rocks of Racetrack Playa, Death Valley National Park, California: Physical and Spatial Influences on Surface Processes . unpublished Ph.D. dissertation, Department of Earth and Environmental Sciences, City University of New York, New York.

Messina, P., Stoffer, P., and Clarke, K. C. Mapping Death Valley's Wandering Rocks. GPS World April, 1997: p. 34-44

Sharp, R.P., and A.F. Glazier, 1997, Geology Underfoot in Death Valley and Owens Valley. Mountain Press Publishing Company, Missoula. ISBN 0-87842-362-1

Stanley, G. M., 1955, Origin of playa stone tracks, Racetrack Playa, Inyo County, California. Geological Society of America Bulletin, v. 66, p. 1329-1350.

Reid, J.B., Jr., Bucklin, E.P., Copenagle, L., Kidder, J., Pack, S. M., Polissar, P.J., and Williams, M. L., 1995, Sliding rocks at the Racetrack, Death Valley: What makes them move?. Geology v. 23, p. 819-822

Sharp, R.P., Carey, D. L., Reid, J.B., Jr., Polissar, P.J., and Williams, M.L., 1996, Sliding rocks at the Racetrack, Death Valley: What makes them move?; Discussion and Reply. Geology, v. 25, p. 766-767