Mountain Dunes
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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 above the valley floor. 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 national park 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, 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. To find out, scientists are taking grains of quartz from deep within the dunes into a laboratory to figure out when they were last exposed to light.

The Weather Notebook is funded by Subaru and is a production of the Mount Washington Observatory. To learn more about The Great Sand Dunes, log on to our website a weathernotebook.org. Thanks today to producer Margaret Landsman and executive engineer Sean Doucette.