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Mountain Dunes
Wed Jun 25, 2003
Listen in RealAudio 
Wind is one of the great shapers of landscape, and one of the best places to appreciate that
is at the Great Sand Dunes National Monument in southern Colorado. Hi, I'm Bryan Yeaton, and
this is the Weather Notebook.
It's unusual to find sand dunes in the mountains, but Colorado boasts some of the tallest
"aeolian," or wind-formed dunes in North America. Some rise more than 700 feet. Most of the
sand in the dunes was blown there from the other side of the San Luis valley, about fifty
miles away. And, as geologist Andrew Valdez explains, the dunes are a result of the way wind
interacts with the Sangre de Cristo mountain range:
"Out in the valley there's basically one wind direction from the SW, so the sand out there
migrates towards the NE. But there's a zone a few miles from the mountain front where the wind
pattern changes and we get more complex wind directions. We get winds coming over the
mountains from the NE and the turbulent flow makes the wind direction vary in just about every
direction. Once the sand gets into this complicated zone of wind, then the sand stops
migrating because it's not always blown in the same direction, it's blown back and forth in
lots of different directions, so that's why these dunes have gotten so big. They've grown
vertically instead of migrating."
Valdez says the dunes have been here at least since the end of the last ice age about 10,000
years. But no one really knows how old they are or how long it took the winds to make them.
One way scientists may find out, will be by taking quartz grains from deep within the dunes to
figure out when they were last exposed to light.
The Weather Notebook is funded by Subaru of America and by the National Science Foundation.
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