Why Does Colorado Have Sand Dunes?

When thinking of towering sand dunes in North America, one usually thinks of the Outer Banks of North Carolina or the white sands in New Mexico. Interestingly enough, though, some of the most prominent sand dunes on the continent are right here in Colorado. 

The Great Sand Dunes National Park and Preserve in the San Luis Valley protects some of the most interesting geological features in the country. Tucked at the feet of the Sangre do Cristo Range in the south-central portion of Colorado are nearly 150,000 acres of massive sand dunes. The ocean of undulating waves seemingly frozen in time are the remains of an ancient lake that once covered most of the valley nearly a million years ago. 

These sand dunes, seemingly so out of place in the shadows of such prominent mountain ranges, are the tallest sand dunes in North America. By looking at both their ancient and not-so-ancient history, we can catch a glimpse of what Colorado looked like for the wildlife and humans who occupied the area so long ago.

Geological history of the Colorado sand dunes

To grasp the existence of the sand dunes and how their presence has defined the area, we have to look at what natural processes had to occur in order for them to exist in the first place. 

First, the San Luis Valley is defined by the mountain ranges around it: the Sangre de Cristo and the San Juan Mountains. Both these mountain ranges were formed through different geological processes that occurred millions of years ago.

Geological evidence suggests that early southern Colorado and much of the surrounding area was mostly highlands. Early, as in hundreds of millions of years ago. 

By the middle and late Cretaceous Period (a cool 100 million years ago), most of North America’s interior was covered by the Western Interior Seaway, which connected the Arctic and Atlantic Oceans. At its deepest, this sea was only about 3,000 feet deep, meaning that sand and silt had plenty of opportunity to move across the seafloor and deposit into the rock below. This shallow inland sea, which split the continent into two halves, deposited sand and silt over central North America.

The sea slowly split in two and receded, leaving a sediment-laden land in its wake. At this point, the Rocky Mountains did not even exist yet, but by the late Cretaceous period tectonic plates were starting to shift all over western North America. This period of shifting was known as the Laramide orogeny.

The Laramide orogeny lasted from about 80 to 55 million years ago, during which tectonic and volcanic activity bent and folded the Earth’s crust dramatically. The result of this plate movement and volcanic activity was that the Earth effectively “burst” up through the cracks, creating the Rocky Mountain ranges we know today.

So, what does all this have to do with sand dunes? Well, it was the combination of tectonic activity, the receding inland sea and subsequent glacial activity that created a smooth, silt-laden plain that we know as the San Luis Valley today. The Sangre de Cristo Mountains (which were formed by folding) and the San Juan Mountains (formed mainly by volcanic activity) border the San Luis Valley and continue to deposit loose silt and sediment into the valley.

When the seawater and glacial lakes finally receded and dried up, the smaller lakes, streams and creeks that cut through the valley continued to erode the already-loose surface. The formation of the mountains surrounding the valley naturally funnels wind from the southwest to the northeast, depositing any loose sand in a curved pocket of the towering Sangre de Cristo mountains. 

These sand deposits built up over time, and today they stand as massive sand dunes in the heart of Colorado’s Rocky Mountains. They, of course, make up the Great Sand Dunes National Park and Preserve we have today! 

A sight millions of years in the making

Now we can see that the sand dunes in southern Colorado are, indeed, a uniquely special geographical and geological piece of North America’s history. They are a visual representation of the ancient geological activities that created the world we are in today, and they are still changing! 

Sand dunes are constantly changing because of wind and rain, but the evolution depends on the weather and climate on both a large and small scale. Periods of great drought mean that sand is looser, vegetation is thinner and dunes can change more quickly. If there is more rain, then vegetation can hold the dunes in place and water can deposit sand in more specific areas. Smaller sand dunes continue to migrate toward the main dune field even today, but the larger sand dunes are stable enough that they can sustain a more consistent size. The largest sand dune in the park and, subsequently, in North America, is over 750 feet tall! 

These sand dunes are ancient and more than a little eerie. This sea of sand sits directly below jagged mountain peaks capped with snow, and the wind blowing over the dunes can cause impromptu avalanches of liquid-looking sand. If the wind blows just right, you can hear it whistling through the loose sand creating a low humming noise under the shimmering surface.

If you live in our luxury Denver apartments and are looking to explore this unique place, make sure to read up on more of our Rocky Mountain research on dinosaurs around Denver and Colorado! 

Featured photo courtesy Pixabay/zzzn10