Great Sand Dunes National Park and Preserve is located in Colorado and was created by the U.S. Congress in 2004. The park contains approximately 85,000 acres, of which about 19,000 acres are covered by sand dunes, including the tallest dunes in North America, which rise over 750 feet. Researchers say the dunes started forming less than 440,000 years ago from sand and soil deposits of the Rio Grande and its tributaries flowing through the San Luis Valley.
Over the ages, westerly winds picked up sand particles from the river flood plain. As the wind lost power before crossing the Sangre de Cristo Range, the sand was deposited on the east edge of the valley. This process continues today, and the dunes are slowly growing. The wind changes the shape of the dunes daily. There are several streams flowing on the perimeter of the dunes. The streams erode the edge of the dune field, and sand is carried downstream. The water disappears into the ground, depositing sand on the surface. Winds pick up the deposits of sand, and blow them up onto the dune field once again.
(Source: Wikipedia, Image: Ed Siasoco)