The Mysterious Stones of Death Valley

No one sees them move.  No one knows what makes them move.  Clearly, they do move.  Giant stones sliding across Death Valley which leave paths in the cracked hard Earth.  All sliding in different directions and sometimes chancing course abruptly, these are not pebbles, but massive boulders, and here is what they look like...

The mysterious stones of Death Valley are magnificent.  They usually all take different paths, even turning around each other. 

No one has ever seen them move and nobody knows the speed they move with. The mysterious sailing stones of Death Valley often turn while they are sliding through the flat leveled valley and this leaves different tracks behind them. Some stones move further than others over two to five years.

What's your theory?