The Racetrack is a playa, a dry lake bed and one of the flattest natural surfaces on earth, with the north end just two inches higher in elevation than the south end. When it rains, mountain runoff carries fine mud and silt to the playa, collecting in a shallow lake. As the water evaporates, these particles settle to form the flat playa surface. The Grandstand is a 73 foot high outcrop of bedrock surrounded by a sea of sediment. Rocks sometimes tumble from the mountains, landing on the playa and moving around, leaving mysterious tracks on the surface at the southern end of the playa, about a half-mile away.