Great Sand Dunes National Park and Preserve conserves an area of large sand dunes up to 750 feet tall on the eastern edge of the San Luis Valley.  The park contains the tallest sand dunes in North America. The dunes cover an area of about 30 sq mi and are estimated to contain over 1.2 cubic miles of sand. Sediments from the surrounding mountains filled the valley over geologic time. After lakes within the valley receded, exposed sand was blown by the predominant southwest winds toward the Sangre de Cristos, eventually forming the dune field over an estimated tens of thousands of years.