Click to see full image. This racetrack in Death Valley National Park, California features rocks, not cars! Rocks that tumble onto this dry lake bed creep along and leave clearly visible tracks behind. It was considered a mystery for many years, but scientists now believe they are nudged along by large sheets of ice that break off and slide around after a winter freeze begins to melt. The park requires scientists to bring in their own (comparable) rocks for experiments rather than use the ones that have been cast onto the surface through erosion.