Twenty-eight of 30 monitored stones on the southern part of Racetrack Playa moved within a seven-yr interval, leaving distinct tracks. Movements occurred principally during the winters of 1968–1969, 1972–1973, and 1973–1974. Some stones moved in all three episodes, some only in one or two, and a few on other occasions. Movement is clearly related to wet stormy weather.
Greatest cumulative movement, 262 m, and greatest single-episode movement, 201 m, were by a small, 250-g stone. Other monitored stones weighing as much as 25 kg moved cumulative distances of 60 to 219 m. Net direction of movement was north-northeasterly with deviations to east and southeast on occasions by some stones. Movement most likely occurs within one to several days after playa wetting, and velocities on the order of 0.5 to 1 m/sec are inferred from track characteristics.
Thin sheets of ice form in winter on this playa, and eyewitness accounts of ice sheets, some with infrozen stones, being driven by wind across other southern California playas indicate that stone tracks may be made in this manner, as earlier advocated. However, movement of stones out of an encirclement of iron stakes, large changes in neighboring stone separation during movement, disproportionate corresponding reaches within contemporaneous tracks of neighboring stones, and other relationships strongly suggest that monitored stone movements occurred without the aid of extensive ice sheets.
Wind acting directly on the individual stones is considered the prime moving force. A critical element promoting movement may be deposition of a thin layer of fine slippery clay, the material that last settles from suspension after playa flooding.
- Geological Society of America