The Sailing Stones...of Death Valley

The sailing stones, also referred to as sliding rocks or moving rocks, are a geological phenomenon found in Racetrack Playa, Death Valley. The stones are assumed to slowly move across the surface of the playa, inferred from the long tracks behind them, without human or animal intervention. They have neither been seen nor 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 grooves on The Racetrack are notable.

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.

Description

Tracks are sometimes not rectilinear.Most of the so-called 'sailing 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 (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.

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.

Geography.



The Racetrack Basin lies within the boundary of Death Valley National ParkWithin the basin lies an almost perfectly flat dry lake, known as the Racetrack Playa. Resting on its flat dry lakebed are pebble- to boulder-sized rocks, with masses estimated at up to 320 kilograms (Sharp and Carey, 1976)

This in itself could be explained by simple mass wasting processes, yet the position of the rocks, often hundreds of meters from a source area, and the presence of furrows in the clay playa surface leading toward the rocks suggests that the rocks are moving by traction.


Trails created by the rocks vary in length and direction. Some trails show gradual (curving) or abrupt (angular) changes in direction; most trails indicate a general south/southwest to north/northeast motion (Kirk, 1952; Sharp and Carey, 1976). Distances traveled of as much as 3.2 kilometers are inferred by the presence of distinctive lithologies far from possible areas of origin. Many rocks appear to break off dolomite cliffs at the south end of the playa, and are ultimately "deposited" where the playa meets an alluvial fan, about 2 kilometers to the north (Sharp and Carey, 1976; Messina, Stoffer and Clarke, 1997). Some trails are parallel to others that are in close proximity and generally strike from south to north, consistent with prevailing wind patterns, thereby suggesting wind as the motive force.

Actively studied for 50 years, the rocks that mysteriously move around the dried lake bed playa in Death Valley, called the Racetrack, are yet to have an unquestionable explanation for their movement.

In 1955 George M. Stanley first proposed the theory that the rocks move with the assistance of ice sheets forming after the playa surface is flooded.

There have been times when the playa is flooded with standing water up to 7 cm deep and temperatures commonly drop below freezing at the Racetrack (elevation is 3,708 feet so it has cooler temperatures than the valley floor which has the lowest elevation and the highest recorded temperature in the western hemisphere) in the winter and early spring.

In 1976 Robert Sharp and Dwight Carey diputed the ice-sheet theory. They analyzed the tracks and concluded becasuse of track characteristics and the geometries of the tracks relative to each other that ice sheets could not have been involved in forming the tracks and moving the rocks.