Death Valley, Trails of Racing Stones

Almost hundred years passed since the discovery of the stones in dried mud with mud trails behind them. No one has ever seen these rocks to move and how these imprints form. Stones are between 1 and 300 Kg in weight and their trails have chaotic character. Each stones have on one side the trail and the other side a pile of mud. No other traces on this perfectly horizontal plane. Some trails can be as long as ½ km and some have missing stone at the end. Humidity and temperature revealed an interesting way how these trails can form and how simple it is.