Sailing stones!

The sailing stones are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention. They have been recorded and studied in a number of playas around Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is subject to research.
Racetrack stones only move every two or three years and most tracks develop over three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake.
Sliding rock trails fluctuate in direction and length. Some rocks which start next to each other start out traveling parallel, but one may abruptly change direction to the left, right, or even back the direction it came from. Length also varies because two similarly size and shaped rocks could travel uniform, then one could burst ahead or stop dead in its track.
Speed is an unknown variable. Since these stones are rarely transported and nobody has witnessed the movement, the speeds the rocks travel at are not known.