Racetrack Playa Geology

Geology: The Racetrack Playa is one of the most enigmatic geologic features in the world. Rocks slide across the playa surface and leave tracks. The rocks have been called “playa scrapers”, “sliding stones” and other terms. There has been a great deal of research done on these rocks over the years (see especially Sharp and Carey, 1976; Messina and Stoffer, 2001) but no one has ever seen them move. Five Palomar students even spent the entire winter in the area and every time the weather turned bad, they stopped by to see if anything happened. Nothing did. This phenomenon has been reported from other playas including the Bonnie Claire, north Panamint, and Superior lake beds in California and one in Utah and one in Tunisia. In no case has movement ever been witnessed. General consensus is that the playa must be wet (and therefore slick) and winds give the impetus. The role of ice (or lack thereof) has been hotly debated. Yearly visits between 1969 and 2007 have led to the following hypothesis from Palomar College: Rain occurs and wets the playa. The water on the playa does not remain too long (hours?) otherwise the clay layers will soften to too great a depth and instead of supplying a low friction surface; the softened layers provide no support for the rocks. Then freezing conditions occur and the water freezes from the top down but not all the way to the playa otherwise a locking phenomena would occur. After freezing the top part of the standing water, but before the entire water column freezes, wind comes and using a sail effect on the ice, moves the rocks. Movement will stop when the water column completely freezes or the playa becomes soft enough so the bull-dozing rocks pile up too much silt and clay ahead of them. We have had many discussions about the need for a brief freezing episode prior to the sequence stated above. However, until someone actually observes the movement, no one can be sure. We have noted not only rocks with trails behind them but sticks, piles of mud and wild burro droppings; all with trails behind them. One can camp south of here near the abandoned Lippencott lead mine several miles further south. There is also a horrible road from there down to the Saline Valley to the west. Besides the sliding stones, there are other interesting geologic features in the valley. You will notice that there is a large outcrop of granitic rock sticking out of the playa at its north end. This outcrop, called the Grandstand, is a source for a few of the sliding stones. Bedrock protruding from a playa is quite unusual. So too, is the nearness of the approach of the dolomite ridge at the southeast edge of the playa here where most of the sliding stones occur. The obvious conclusion is that mass wasting from these two sites initially supplies the stones that later slide across the playa. The dolomite rocks break into quite rectangular chunks which no doubt helps their sliding under the conditions required. On the east side of the playa, several hundred feet up, there is a small copper mine.