Great Sand Dunes National Park and Preserve is a United States National Park located in the San Luis Valley, in the easternmost parts of Alamosa County and Saguache County, Colorado, United States. Originally created as Great Sand Dunes National Monument on March 17, 1932, Great Sand Dunes National Park and Preserve was established by an act of the United States Congress on September 13, 2004. The park includes 44,246 acres (17,906 ha), and the preserve protects an additional 41,686 acres.

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, glaciers feeding the river and the vast lake that existed upon the valley melted, and the waters evaporated. Westerly winds picked up sand particles from the lake and 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.

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.

Digging a couple inches into the dunes even at their peaks reveals wet sand. Part of the motivation of turning the Monument into a National Park was the extra protection of the water, which Colorado's cities and agriculture covet.

It is very easy to experience the dune-building process. This is a very windy region, as hikers on the Sand Dunes will attest, as on many days they will be pelted by sand and even small rocks when hiking on the dunes. The wind carries sand and rocks from many miles away. While the dunes don't change location or size that often, there are still parabolic dunes that start in the sand sheet, the outer area around the dunes, and migrate towards the main dune field. Sometimes they join the main dune field, and sometimes they will get covered with grass and vegetation and remain where they are. The dunes are relatively stable, however their morphology changes slightly with the seasons. The direction of the wind greatly affects the dune type. The winds normally go from southwest to northeast, however during the late summer months, the wind direction reverses causing reversing dunes. This wind regime is part of the reason why the dunes are so tall.

The dunes contain areas of black sand which are deposits of magnetite, a crystalline black oxide of iron.