This is an image I took back in the mid 1960s when I was studying these rocks to try to determine how they moved. It is not a high quality image because I knew little about photography at the time and the image has faded as well. But his image suggests that the rocks move by being blown by the wind. Notice that the rocks which have the greatest height travel the farthest. Notice too, that they all have the same "signature", but not the same path lengths. Although many authors have suggested that ice is necessary to provide sufficient surface to generate the strength to move such large objects this evidence suggests that is not the case. I can attest to the fact that the surface is extremely slippery when it is wet. In fact, it can become so slippery that it is difficult to stand up. I am guessing that when the playa surface is wet that the coeficient of friction is reduced sufficiently to allow the rocks to move fairly easily.