mystery of Death Valley Moving Stone

There where two idea of explanation for the Racetrack moving stones. 1) Ice Skating when the clay lake bed is frozen pushed by strong wind, 2) Lake bed collects water from both side hills and the surface of the clay becomes mud/soft and slippery. Here is third understanding of theremal expantion every nigh&day, where the Death Valley has the highest daily climate difference 40degree. This idea explains all of the facts observed.

http://www.nicovideo.jp/watch/sm22257310