For thousands of years prevailing westerlies have blown sand into this natural trap on the west edge of the Sangre de Cristo Range, creating approximately 39 square miles of dunes that rise 700 feet above the valley floor. Although the sands look white or tan, under a microscope they are multi-colored. Some grains are volcanic, from the San Juan Mountains across the valley; others have been formed by streams tumbling out of the mountains. The high water table helps to keep the sand moist and stable. Legend populates this giant sandbox with lost sheepherders, Spanish suits of armor, and web-footed horses.
Archaeological exploration has found evidence of habitation by hunters of the prehistoric Folsom culture, named for archaeological discoveries in Folsom, New Mexico. More recently the Utes occupied the area. An astonished Zebulon Pike described these dunes in the journal of his 1806–1807 expedition as looking “like the sea in a storm.” The dunes were designated as a national monument in 1932, but not until the 1960s were a visitors' center, campground, and amphitheater added for tourists visiting North America's largest inland dunes.