Death Valley National Park is a desert valley situated in eastern California. It lies within the boundaries of the Mojave Desert, and is noted for being a region of extremes. At 282 feet below sea level, it boasts the lowest elevation in North America. Only 85 miles away is Mount Whitney, which stands at an ear-popping 14,505 feet above sea level, making it the highest elevation in the contiguous United States. The climate is also extreme. Extrememly hot and dry, with an average annual rainfall of only about 2 inches. At 134 degrees Fahrenheit, Death Valley also lays claim to the highest recorded temperature in the Western Hemisphere.
Death Valley is full of breath-taking, and varied, landscapes. It is gloriously erie, full of mysteries, abandoned mines, ghost towns, and strange geology. This brings us to Death Valley's most famous mystery: The Sliding Stones of Racetrack Playa.
Racetrack Playa is a flat, dry lake bed that has been so scorched by the desert climate, that its clay floor has dried and cracked into what looks like a mosaic of small, clay polygons. Stones, ranging in size from a few ounces, to as much as 700 pounds, mysteriously slide across the playa upon their own accord, leaving a grooved trail behind. Some of the trails are quite long, measuring hundreds of feet in length. The stones are largely made of dolomite, which break off or fall from surrounding cliffs and highlands on the southern end of the playa. Most of the stones travel in a southwest to northeast direction. While no one has ever directly observed a stone moving, there have been numerous studies which have tracked their movement - some of which have spanned over a time perioed of several years. Movement, seems to occur only in the cold winter months, possibly providing a clue to this mystery.
The stones' tracks vary in length and have been known to go in all directions. Tracks have been observed following a fairly strait path, then will suddenly turn, or even reverse directions. Stones, side by side, may even follow different paths, or one may move while the other shows no movement at all. Some stones have also been known sit stationary for long periods of time, then suddenly start moving. The reverse also occurs, as a stone will show consistent movement over an extended period of time, then suddenly stop. Even more puzzling, though less frequent, some tracks terminate with no stone present. Essentially, the stone disappears.
The Sliding Stones of Racetrack Playa have been the subject of numerous studies, some of them spanning over the course of several years. The list of researchers have included National Park Service Park Rangers, NASA interns, geologists, scientists and experts in various disciplines, and and many who are just plain fascinated with the phenomenon. One noteable body of research is that of Paula Messina, as part of a dissertation. Messina's work includes a thorough review of past research, detailed GPS-tracked charts showing stone movement, and other well-documented information and analysis. Various theories have been proposed, in an attmept to explain the stones mysterious movements.
High sustained wind speeds, and gusts up to 70 miles-per-hour have been recorded on the playa, particulary in the winter months. While high wind gusts could possibly topple over or push some of the smaller stones, the sustained wind speeds are not high enough to move the larger stones. However, one proposed theory suggests that if the surface of the playa is wet, it may reduce friction of the stone enough for strong sustained, or gusts of wind, to push the stones along.
The bottom of the playa is extremely flat, and rises only about one inch in elevation over it's 4.5 mile span. Because Racetrack Playa is so flat, and low in elevation, storm runoff or melting snow from the surrounding highlands can collect in the bottom of the playa, very quickly turning it into a broad, but very shallow lake. The floor of the playa, being clay, does become extremely slick when wet, making the wind theory seem very plausible. Particularly when the general stone movement follows the prevailing wind direction: southwest to northeast.
Rainfall and runoff from melting snow occurs almost exclusely in the winter months. This again lends credibility to the theory that high wind speeds, and reduced ground friction may cause the rocks to slide, as stone movement only seems to occur during the winter months as well. However, the fact that stones in very close proximity to one another can move in different or opposite directions, or one stone may move while the other stays stationary, presents some opposition to this theory.
Very similar to the theory of fast wind speeds pushing the stones across the slick, wet clay surface, is the theory that ice sheets are aiding the stones' movements across the playa. In this theory, sheets of ice form in the shallow water of the playa, during the winter months. The water freezes around the stone, forming an 'ice collar', through which the bottom of the stone protrudes. The ice sheet floats in the shallow lake, slightly reducing the weight of the stone, allowing the wind to push the whole sheet of ice, while the protruding stone drags the bottom of the playa, creating its signature tracks.
If this were the case, one would expect stones in close proximity to be captured within the same ice sheet, leaving behind identical track patterns. And in some cases... they do.
In a seven year study of the stones that began in 1968, and continued until 1975, geologists Bob Sharp and Dwight Carey tested this hypotheis by driving rebar stakes into the ground around a stone, effectively forming a corral. The corral was about 5 and a half feet in diameter, consisting of 7 pieces of rebar, set about 25 to 30 inches apart. The corraled stone weighed about a pound, and was roughly 3 inches across - small enough to slide through the rebar stakes. However, its movement should be hindered, or even halted, if the ice sheet theory was correct, as the ice sheet would be too wide to fit through the stakes.
The stone moved out of corral during its first winter of observation, narrowly missing a rebar stake, casting some doubt on the sliding ice sheet theory.
As part of an investigation conducted by a group of NASA interns, measuresments were taken to see if magnetism, radiation, or the slope of the playa might play a part in the movement of the stones. Instrumentation showed no unusual readings or fluctuations in magnetism, or radioactivity. Levels were used to measure slope, to see if slight elevation differences might cause the rocks to slide downhill, when the surface of the playa is slick. However, the slope is essentially flat. In fact, as mentioned previously, the north end of the playa is about one inch higher in elevation than the south end, making the general movement of the stones slightly uphill.
Despite the numerous investigations by various researchers, no single theory seems to fully explain the mysterious movement of the stones of Racetrack Playa. While some of the theories have some supporting evidence, and certainly warrant further investigation, each theory also seems to carry inconsistencies that casts doubt upon them as a definative explanation. Add to this, the desolate, moon-like setting of the playa and its surroundings, plus the fact that no one has ever witnessed a stone move, we are left with yet another fascinating unexplained mystery.