The Edinburgh Geologist
by Bill Baird
Although nobody has actually seen these rocks in motion at Racetrack Playa there have been several theories to account for their movement. Unconfirmed eyowitness reports from other playa lakes in the area speak of sheets of ice with stones frozen to their bases being driven across the playa surface by the wind. Strong winds combined with special low-friction conditions of the lake have been suggested and even the local Indians have been regarded with some suspicion, though there are no footprints along the trackways. Until confirmed and verified by observation of actual movement, and hopefully filmed, we cannot be certain exactly what mechanism moves the stones.
In May 1968, Dr Robert Sharp, Professor of Geology at California Institute of Technology in Pasadena marked 25 of the rocks with numbered stakes, recorded their locations, and settled down for a long wait. He was surprised to be told the very next spring that some of his rocks had moved. The heaviest of these was a rock, weighing 39 pounds, which had travelled 71 feet on a curving track. The furthest travelled was 15 pound rock which had moved 212 feet. Sharp believes that the motive force for these movements is the wind, allied to very special conditions on the lake surface.
The mechanism that he envisages requires that the top layer of the lake bed would have to be water-saturated but the underlying layers must remain firm. Winds in the area of the lake, which lies at 3,708 feet above sea level, can be strong enough to blow out car windows and force hillwalkers to their knees. Katabatic winds have been recorded elsewhere at speeds in excess of one hundred miles per hour which would seem sufficient to move even large rocks under very low friction conditions. Perhaps only by carrying out experiments in a wind tunnel with rocks sitting on low friction clay, could we find out whether this is the correct solution to the roving stones of Racetrack Playa, California.
Stanley, G.M., 1955. Origin of playa stone tracks, Racetrack Playa, Inyo County, California. Geological Society of America Bulletin, 66, p. 1329-1350.
National Geographic Magazine, 1970. Vol. 137, p. 77 & 100-101.
Norris,R.M. and Webb, R.W., 1976. Geology of California p. 119-121. John Wiley & Sons, New York.
Sharp, R . and Carey, D.L.,1976. Sliding stones, Racetrack Playa California. Geological Society of America Bulletin, 87, p. 1704-1717.
[News] [About] [Lectures]