The Great Sand Dunes are a remarkable anomaly. Beneath the 14,000 feet+ snowy alpine peaks of Colorado reside the highest sand dunes in North America. The highest dune raises more than 750 feet from its base. The dunes were formed from deposits left by the Rio Grande River and a giant lake in the San Luis Valley. Over time, the water evaporated and winds picked up the leftover sands and deposited them at the base of the Rockies. A truly unique landscape in our world.