Great Sand Dunes National Park is home to the largest sand dunes on the continent. Located in the San Luis Valley of southern Colorado, Great Sand Dunes sits at the base of the snowcapped Sangre de Cristo Range. The dunes are thought to be about 500,000 years old, and are composed of sediment washed away by the Rio Grande. Medano Creek runs along the side of the 30 square-mile dune-field. Great Sand Dunes is a major summer tourist draw, and the creek-bed is often packed with tourists basking in the mid-mountain sands.