The Sangre de Cristo Mountains of south-central Colorado stretch
dramatically from top left to lower right of this astronaut
photograph. The mountains are outlined by dark green forests at
lower elevations and white, snow-capped peaks at the highest
elevations. Dun-colored dunes, covering an area of 80 square
kilometers, are banked up on the west side of the mountains.
These dunes make up the Great Sand Dunes National Park and
Preserve. Originally established in 1932 as a National Monument,
it was reauthorized as a National Park in 2004. The park
contains dunes over 227 meters (about 750 feet) high—among the
highest in North America. Sand grains that make up the dunes are
small enough to be moved along by the wind (a process known as
saltation), although much of the dune field is now anchored by
vegetation. Winds blow predominately to the east, so that sand
in the San Luis Valley (part of which appears at image lower
left) is driven towards and piled against the Sangre de Cristo
Mountains. The sand of the dun