Q: The Great Sand Dunes National Park and Preserve in the San Luis Valley always has baffled me. How did the dunes get there? Was there an ocean there at one time?
A: There’s no shortage of colorful explanations about why the sand dunes exist, but, as is often the case with geology, the truth is not nearly as exciting. While much of Colorado was at one time under water, the Great Sand Dunes did not serve as a beach. Rather, the answer lies in the surrounding mountains. Water and wind scour the nearby hills and break the rock down into sand, which is then blown or carried by nearby rivers down to San Luis Lake in the northern part of the valley. According to Andrew Valdez, a geologist at the park, the lake varies in size depending on climatic conditions. As the lake dries, the sand is exposed to the wind and is blown northeast to the base of the Sangre de Cristo Mountains, where it stacks as high as 750 feet after meeting opposing winds flowing over the mountains.
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