Introduction to the Geology of Great Sand Dunes

Medano Creek begins to trickle down each spring as temperatures warm. By late May and early June, the creek can cover most of flat sand area at the base of the dunes.

NPS/Ian Knoerl

by Ian Knoerl, Park Ranger

One very common question that is asked is “Why are the dunes here?”

The answer to this question is both simple and complex. In a simplified way, the answer is that there was a large source of sand, and the right winds. In a way, that’s it, we’re done. Or are we? While this technically answers the question, we can add in many more details to fully explain the presence of the dunefield.

Much of the sand present is not actually from the Sangre de Cristo Mountains that surround the dunes, but from the San Juan Mountains across the valley. These mountains slowly eroded over time, with the resulting sediment accumulating in the San Luis Valley. At this point, a large lake, called Lake Alamosa, covered much of the valley floor. Sediment built up on the lakebed for quite some time. Eventually the lake broke free of the valley and drained out, forming the Rio Grande. Some smaller lakes remained, where the sediment buildup continued. As the area became drier, and these lakes slowly dried up, all the deposited sediment was exposed to the wind.

The wind in the San Luis Valley is predominantly from the southwest. This wind blew the newly exposed sand deposits toward the Sangre de Cristo Mountains. If these were the only winds though, the sand would have eventually been blown up onto, and over the mountains, but there are other wind currents once you reach the mountains. During storms, wind blows back through the mountain passes.

These different wind currents, along with the predominant southwest wind, effectively squeezed the sand in to a large, thirty square mile pile of sand, forming the dunes we see today.

The story doesn’t stop there though. Today, water has once again joined in. The dunefield is flanked by multiple creeks, with Sand Creek, and Medano Creek being the most notable. Now, when the prevailing winds blow sand off of the dunes, the sand is captured by one of these two creeks, which then flow to the southern and western edges of the dune field, where wind then blows the sand back onto the dunes. This process maintains the overall amount of sand, meaning that even though the sand does move, and some dunes do migrate, the overall dunefield is kept relatively stable.

While how long this system will remain stable is unknown, its not expected to drastically change anytime soon. Until then, we have plenty of time to get out and explore the dunes, here at Great Sand Dunes National Park and Preserve.

Learn more about the geology of Great Sand Dunes.