File:Sliding rocks by John Fowler.jpg

English: These roaming rocks on the Racetrack Playa in the northern part of Death Valley National Park have been known for years, but until recently no satisfactory explanation existed for their remarkable behavior. Nobody has ever witnessed their motion, yet the tracks they leave in the hard surface attest to their mobility.

The playa (“beach” or “dry lake” in Spanish) is located down 27 miles of dirt road south from the Ubehebe Crater. It is a couple of miles long and a mile wide and very flat. The most recent explanation of how the rocks move involves this flatness and the elevation of about 3800 feet. When it rains, the southern half of the playa forms a broad lake about an inch deep. On cold nights, a thin layer of ice can form on the surface. This sheet traps the rocks. If the wind blows hard enough, the sheet can glide over the underlying cushion of water, dragging the trapped rocks along.

Although hard when dry, the surface of the playa becomes very slippery and soft when wet. This lets the rocks form their trails, which become visible when the playa dries and hardens again.

It rained the night before I got there, and the south end of the playa was covered with water and some pockets of ice. It was drying fast, though, which was lucky for me. I got to walk out and get some photos the next day. The view here is to the north from near the southern end.

The playa darkens when wet, and this explains the splotchy appearance in this photograph.