"From high in the San Juan Range to the southwest of the dunes, the blowing wind picks up speed as it flows down the slopes and across the broad San Luis Valley, passing over the Rio Grande. Over thousands of years, the Rio Grande has transported weathered sediments from the San Juan Range into the valley where they accumulate next to sluggish sections of the river. As the wind passes over these sediments, the smallest particles are carried aloft. Blocked by the Sangre de Cristo Mountains, the wind velocity drops as it courses through the Mosca, Medano, and Music mountain passes. The weakened wind cannot carry the sand, and it accumulates in a pocket next to the Sangre de Cristo range, creating the Great Sand Dunes. The tallest dune rises almost 700 feet above the floor of the valley."