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The Great Sand Dunes originated from large lakes that once covered portions of the San Luis Valley. This brief animation shows the lakes drying up, and the dunefield emerging in a pocket of the Sangre de Cristo Mountains.
How Were the Great Sand Dunes Formed?
The dunes were formed by the right combinations of wind, water, and sediment. Creeks and streams brought in large amounts of sediment and sand into the valley. Wind then blew the sand toward the bend in the Sangre de Cristo Mountains, where opposing storm winds helped squeeze the sand into the tall dunes you see today. While this explanation does answer in the question above, the story of how the Great Sand Dunes were formed is much more complex, and as new research discoveries occur each year, is continually evolving. Below is a summary of what most geologists currently understand to be the broad series of events that took place in the formation of these massive dunes.
Uplift of the San Juan and Sangre de Cristo Mountains
Through the breaking apart and movement (rifting) of large surface plates on Earth's surface, the Sangre de Cristo Mountains were uplifted in the rotation of a large plate. Fossils from the bottom of an ancient sea are now preserved in high layers of rock in the Sangre de Cristos, illustrating the scale of the uplift. The San Juan Mountains were created through extended and dramatic volcanic activity. With these two mountain ranges in place, the San Luis Valley was born, covering an area roughly the size of the state of Connecticut.Sediments from both mountain ranges filled the deep chasm of the valley, along with huge amounts of water from melting glaciers and rain. The presence of larger rocks along Medano Creek at the base of the dunes, elsewhere on the valley floor, and in buried deposits indicates that some of the sediment has been washed down in torrential flash flood events.
In 2002, geologists discovered lake shoreline deposits on hills in the southern part of the valley, confirming theories of a huge lake that once covered much of the San Luis Valley floor. They named this body of water "Lake Alamosa" after the largest town in the valley. Lake Alamosa suddenly receded after its extreme water pressure broke through volcanic deposits in the southern end of the valley. The water then drained through the Rio Grande River, likely forming the steep Rio Grande Gorge near Taos, New Mexico.
Lake Alamosa Remnants
After Lake Alamosa drained away, smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley. Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Mountains. The sediment filling these lakes became the main source of sand for the dunes. Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes are still found today, in the form of sabkha (alkali flat) wetlands and playa lakes.
Winds of the Valley
Sand that was left behind after these lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Mountains. The wind funnels toward three mountain passes here - Mosca, Medano, and Music Passes - and the sand accumulates in this natural pocket. Sand grains are a perfect size for the winds to move. Sediment grains larger than sand are too heavy to be moved by the wind, while grains smaller than sand are light enough to be picked up and held by the wind and carried away. Sand grains on the other hand, are light enough to be picked up by the wind, but heavy enough that wind typically can’t keep them in the air, so the sand bounces along the surface of the valley floor, moving along, and gathering together in locations where the wind is slowed, or an obstacle is in the way. Environments where wind moving sediment is the dominant process are called aeolian environments. The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically.
Playa Lakes and Wetlands
As time went on, the wetlands and playa lakes (lakes that grow and shrink periodically with water availability) played an important role in the formation of the dunes. Much of the sediment that had been brought into the valley from the mountains was unsorted, having a mixture of grain sizes and materials, including muds, silts, clays, and most importantly sands. These smaller bodies of water and seasonal lakes helped sort the sand from the rest of the sediment. When the lakes and wetlands are full, sand is deposited near the edges forming little sandy beaches. As water levels lower, the sand is then exposed to wind, allowing the aeolian processes to take over. This created a continual sand source for the growing dunes for quite some time, allowing the dunes to accumulate enough sand to reach their current massive size.
Even with the opposing storm winds, most sand is still blown toward the mountain ranges. Here, two seasonal mountain streams, Medano Creek and Sand Creek, capture sand from the mountain side of the dunefield and carry it around the dunes and back to the valley floor. The creeks then disappear into the sand sheet, and the sand blows back into the dunefield. Barchan and transverse dunes form near these creeks. Water from these creeks then reappear in near the southwestern edge of the park, feeding into the wetland and playa lake systems. Learn more about the hydrology of Great Sand Dunes.
A Perfect Storm
This combination of opposing winds, a huge supply of sand from the valley floor, and the sand recycling action of the creeks, are all part of the reason that these are the tallest dunes in North America. There are other dunes in Colorado, and in most western states in the US, but none as tall (741feet/ 226 meters) and none as dramatic. Here giant dunes rise in front of the alpine Sangre de Cristo Mountains, while streams flow across the sand seasonally, making for an unusual and unexpected sight.
Are the dunes still growing? How much do they change over time?
Currently, there is enough vegetation on the valley floor that there is little sand blowing into the main dunefield from the valley. However, even today there are still some small parabolic dunes that originate from the playa lakes in the sand sheet and migrate across grasslands, joining the main dunefield. At other times, some of these migrating dunes become covered by grasses and shrubs and stop migrating. This limits the amount of new sand entering the main dunes system. The lack of new sand coming in suggests that the overall size of the dunefield isn’t growing, but the dunes within are constantly moving in complex patterns. It is possible that lots of new sand enters the dunefield when climatic conditions decrease the vegetation on the sand sheet, such as during an ice age, when it’s cold or during extremely dry periods.
How old are the dunes?
Scientists don’t yet know a precise age and estimates have changed over time. At one time they were thought to be very young and the result of glacial outwash. We have since learned that the dune sand extends hundreds of feet into the subsurface, so they are older than initially believed. The dune sand begins just above the clay deposits of Lake Alamosa, so if Lake Alamosa disappeared about 440,000 years ago, then there have been dunes here since then. It is also possible that there is more dune sand under the lake deposits. We haven’t drilled that deep yet. Keep in mind that the dunes we see today are young and may have come from glacial outwash as recently as 12,000 years ago that flooded the modern playa system. The best answer is the dunes on the surface are young as they are constantly forming, but since the sand goes deep into the subsurface, there have been dunes here for a long time.
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Have you ever heard sand sing? Listen to 15 seconds of this bizarre phenomenon! Just as our own voices are made by air moving through vibrating vocal chords, a humming sound is made at Great Sand Dunes as air is pushed through millions of tumbling sand grains during an avalanche. Avalanches occur naturally during storms, but can also be created by people pushing sand down a dune face. In the 1940s, one of Bing Crosby's musical hits was "The Singing Sands of Alamosa" - a love song based on the sounds of Great Sand Dunes. This humming sound continues to inspire people today!
You may also learn about geological components of the Great Sand Dunes system, hydrology of Great Sand Dunes, and the variety of dune types in the park.
Geologic Map of Great Sand Dunes National Park (USGS/Madole, VanSistine, Romig; 2016)
2007 Rocky Mountain Section Friends of the Pleistocene Field Trip: Quaternary Geology of the San Luis Basin of Colorado and New Mexico, September 7–9, 2007 (USGS online publication) Michael N. Machette, Mary-Margaret Coates, and Margo L. Johnson
Geoindicator: Dune Formation and Reactivation (web page, Global Change Research and Information Office)
Last updated: January 9, 2023