racetrack playa, 3.23.05.
Rocks like this at the racetrack playa are deposited at the playa edge by adjacent slopes and cliffs, and then they raft around for quite a while leaving winding trails along their path. No one has ever witnessed the rocks in motion, and surprisingly, the general trend of movement is slightly uphill.
One theory on what makes them move such great distances is that they are blown by wind and move when the playa is wet and slippery. Another theory is that their movement is aided by an ice surface created when the wet playa freezes on cold nights. These are now challenged by an "ice-raft" theory, see: http://blogs.agu.org/landslideblog/2013/05/17/was-the-bingham-canyon-landslide-the-largest-ever-non-volcanic-landslide-in-north-america/