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Mountain Dunes
Fri Apr 08, 2005
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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