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, United States. Originally designated Great Sand Dunes National Monument, Great Sand Dunes National Park and Preserve was created by an act of the United States Congress on September 13, 2004. The park contains approximately 85,000 acres (340 km˛, 130 mi˛).
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 km˛). They are perhaps 12,000 years old. The dunes were formed 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, and the dunes are slowly growing. The wind changes the shape of the dunes daily.
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