Great Sand Dunes National Park and Preserve
is a United States National Park located in the easternmost parts of Alamosa County and Saguache County, Colorado. Originally designated Great Sand Dunes National Monument, Great Sand Dunes National Park and Preserve was created on September 13, 2004. The park contains the tallest sand dunes in North America, rising about 750 feet (230 m) from the floor of the San Luis Valley on the western base of the Sangre de Cristo Range, covering about 19,000 acres (77 km2). Researchers say that the dunes started forming less than 440,000 years ago.