"Happy Birthday, Hiking the Great Sand Dunes, Colorado" Greeting Card by Catherine Sherman. People look as small as ants as they climb the giant dunes of Great Sand Dunes National Park and Preserve in Colorado. Their foot prints cover the dunes, but will soon disappear as the sand shifts and blows. Great Sand Dunes National Park and Preserve is in the San Luis Valley. Originally created as Great Sand Dunes National Monument in 1932, Great Sand Dunes National Park and Preserve was established by an act of the United States Congress in 2004. The park includes 44,246 acres, and the preserve protects an additional 41,686 acres. 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. Researchers say that the dunes started forming less than 440,000 years ago. The dunes were formed from sand and soil deposits of the Rio Grande River and its tributaries that collect against the Sangre de Cristo Mountains to the east. A photograph of Mount Herard is on the back of the card.