On the eastern side of the remote, high-mountain San Luis Valley,
between the Blanca Massif and Crestone Needle, are the Great Sand Dunes, the
tallest sand dunes in North America. The dunes cover approximately 39 square
miles and rise to almost 750' above the valley floor.
The dunes were formed from sand deposited
by the Rio Grande river and its
tributaries, which flow through the San Luis Valley. For thousands of years,
prevailing westerly winds have come over the Rockies and down over the river
flood plain, picking up sand particles on the way. These are then deposited at
the east edge of the valley before the wind rises to cross the Sangre de Cristo
mountains. The process continues and the dunes are still being gradually
enlarged by the wind, which also changes the shape and sand patterns of the
dunes daily. In some places, patches of black sand are found caused by deposits
of magnetite, a crystalline black oxide of iron.
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