This photo was taken near sunset at the Great Sand Dunes National Monument in southern Colorado. The dunes, covering an area about 36 square miles and rising up to 750 feet, are the tallest in North America. Sand originally deposited by the Rio Grande River and its tributaries flowing through the San Luis Valley were picked up by prevailing westerly winds and deposited at the foot of the Sangre de Cristo Mountains (seen in the background). The winds are corralled into a valley corner, formed by 13,000-foot mountains, where they slow and lose the energy to carry the heavier sand particles. For thousands of years this process has built the dunes.