NPS "Great Sand Dunes National Park - Geology" NPS NPS NPS NPS NPS geologists' research NPS NPS geologists' research "Great Sand Dunes National Park - Geology" NPS http://www.nps.gov/grsa/learn/nature/sanddunes.htm dried up more than 400,000 years ago Formation of the San Luis Valley and Great Sand Dunes http://www.nps.gov/grsa/learn/nature/sanddunes.htm