The Great Sand Dunes National Park in Colorado is home to the tallest dunefield in North America. The dunes rise to about 750 feet tall and cover approximately 30 square miles. In geological terms the dunefield is not very old, it is estimated that the dunes started forming less then 450,000 years ago. The dunes were formed by strong winds blowing across the San Luis Valley that picked up sand that was deposited by the Rio Grande River. When the winds reached the Sangre de Cristo Mountain Range they would loss power and drop the sand. Over time the sand built up into dunes and eventually created the dunefield. This process still continues to happen today and thus the dunefield is still growing.