Sailing Stones

These rocks are on the move in Death Valley!

Trails etched into the hardened mud of Racetrack Playa in California's Death Valley present a strange conundrum: How do the heavy stones that make them move? Until 2013, no scientists had directly observed it. Then weather cameras placed at the dry lake bed gave the answer: In cold times, thin sheets of ice form around the stones. Sun eventually melts the ice into large floating panels, and if wind strikes, it pushes the frozen rafts forward while the embedded rocks mark the soft mud below. These findings upended theories that hurricane-force winds and dense ice sheets propelled the stones. Instead, it's a set of gentler, precise conditions: light winds of about 10 miles (16 km) an hour and fragile ice a quarter of an inch (6 mm) thick combine to move rocks a few inches (2-6 cm) per minute.

From National Geographic's "Hidden Earth" which you can purchase here.