Six exposures of 7.5 minutes at f/8, ISO 200 were combined for an effective exposure length of 45 minutes. The lighting on the rocks is from a truck that drove up the road during one of the exposures. Trona Pinnacles is out of this world.
Update: Here's an 8 minute exposure of me making the image above, courtesy of Troy Paiva. The size of the car helps give you a sense of the scale of Trona Pinnacles.