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The Great Sand Dunes National Park and Preserve in Colorado is an enormous area. Its history of human habitation goes back some 11,000 years, and many of those so-called primitive communities coined some of the most poetic names for this vast tract of dune-filled land, from the Sand that Moves to Sandy Place Lake. But how did this extraordinary place come to be?
Geologists have explained it as a product of the moving surface plates in the earth’s structure, which could have happened due to major earthquakes, millions of years ago, for instance. So what they think happened was the who range of the Sandre de Cristo mountains, which borders one side of the area, was moved – and this is proved by the presence of marine fossils high up in the rocks of these mountains. Millions of years ago, these rocks were under the sea.
The San Juan Mountains, which is on the other side of the area, came about due to major volcanic activity, also several millions of years ago. The San Luis Valley, in between these two mountain ranges extends over an area of roughly 5,000 square miles, and with the shifting mountains, it became like a giant, enclosed basin. Many flash flood events and the like over time made this a very full and pressurized basin, a lake, if you will, which gradually, as the years passed, began to challenge its own walls.
Extreme water pressure broke through the water-filled valley on its southern edge, allowing for the flow of rivers which most likely formed what is today the Rio Grande Gorge near New Mexico. As the lake in that basin between the mountains receded so did a vast area of sand sheet remain in its wake. So what is a sand sheet? In simple terms, it’s an area defined by the fine sheet of sand which covers it. And while the thickness of the sandbank is not necessarily flimsy, it is the texture of the sand that is fine, like beach sand. In fact, it’s not very different from the kind of sediment you would find at the bottom of a lake, and the wind, funneled through the mountain ranges around it, shifted it, patterned it and molded it within the confines of the valley.