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Here in southern Colorado, far from any sea or even a major desert, is a startling sight -- a huge expanse of sand, piled nearly 750 feet high. The towering dunes, the tallest on the continent, seem incongruous, out of place in a land best known for the Rocky Mountains. But here they are, some 30 square miles of light brown sand dunes, restlessly grasping at the western edge of the Sangre de Cristo Mountains. (Sangre de Cristo is Spanish for "Blood of Christ"; the name comes from the deep red reflected onto the snowcapped mountains by the setting sun.)

The dunes were created over thousands of years by southwesterly winds blowing across the San Luis Valley. Streams of water from melting glaciers carried rocks, gravel, and silt down the mountains to the valley floor, where it accumulated and eventually became sand. That sand was then picked up by the wind and carried toward the mountains.

Even today the winds are changing the face of the dunes. So-called "reversing winds" from the mountains pile the dunes back upon themselves, building them higher and higher. Though it's physically impossible for sand to be piled steeper than 34 degrees, the dunes often appear that steep or more because of deceptive shadows and colors that change with the light: gold, pink, tan, sometimes even bluish.

Great Sand Dunes became a national monument in 1932, but in recent years concerns over the possible effects on the dunes and their ecosystem from water usage in the surrounding area have increased. In 2000, Congress passed a law approving park status pending the acquisition of "sufficient land having a sufficient diversity of resources." The necessary property was acquired with the help of the nonprofit Nature Conservancy, and on September 13, 2004, the U.S. Secretary of the Interior arrived at Great Sand Dunes to publicly announce the designation of Great Sand Dunes National Park.