Southwesterly prevailing winds have deposited sand in the San Luis Valley of southern Colorado and have deposited the greatest mass of sand near the western flank of the Sangre de Cristo Mountains.  This area is preserved as the Great Sand Dunes National Monument, covering approximately 104 km² and containing the tallest dunes in North America.  The tallest dune rises 230 m from the floor of the San Luis Valley to an altitude of 2600 m above sea level.  The dunes are composed of up to 70% volcanic rock fragments derived from the surface of the Santa Fe and Alamosa formations along with more recent fluvial and lacustrine deposits carried from the San Juan Mountains by the Rio Grande [Johnson, 1967].