Driving the road into this national park, your first look at the landscape might leave you wondering how North America’s tallest sand dunes formed and came to rest smack dab against the Sangre de Cristo Mountains. Where did all that sand come from, and why is it isolated against the mountainsides? To learn the answers to these questions, it’s important to first understand that formation of these dunes took just the right amount of water, wind, and sand.

Mountains, dunes, and sand sheet, Great Sand Dunes National Park / Rebecca Latson

According to park staff:

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.

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 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.

Lake Alamosa

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. Streams from both mountain ranges have continued to bring sand to the depressions where the lakes form. In the past, this included the Rio Grande.

Lake Alamosa, the subsequent smaller lakes, and Rio Grande have fed sand to Great Sand Dunes for the past 400,000 years. The dune sand extends to depths of 300 feet (91 m) below the valley floor and just above Lake Alamosa deposits, indicating that there have been dunes here since the time of Lake Alamosa.

Remnants of these large lakes are still here today, in the form of sabkha (alkali flat) wetlands and playa lakes.

Winds of the Valley

When the lakes periodically dry, sand left behind blows 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 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, and make it difficult for the dunes to migrate into the mountains.

Fun Fact: Did you know the dune sand is magnetic? As you enter the park and walk toward the dunefield, you’ll probably notice streaks or patches of black in the dunes. The black consists of sand-sized particles of magnetite, a magnetic mineral. A magnet placed on the black portions of the sand will pick up those magnetite grains.

Sand System Components

There are four components to the Great Sand Dunes geological system: watershed, dunefield, sand sheet, and sabkha.

The geologic components to the sand system of Great Sand Dunes National Park and Preserve / Rebecca Latson

According to park staff:

The mountain watershed of Great Sand Dunes receives heavy snow and rain each year. Creeks flow from alpine tundra and lakes, down through subalpine and montane woodlands, and finally around the main dunefield. Sand that has blown from the valley floor is captured and carried back toward the valley. When creeks disappear into the valley floor, sand is again picked up and carried into the main dunefield. This recycling action of water and wind contributes to the great height of this dunefield. 

The 30 square mile (78 sq. km) active dunefield is where the tallest dunes reside. It is stabilized by opposing wind directions (southwest and northeasterly), creeks that recycle sand back into it, and a 7% moisture content below the dry surface. The dunefield is composed of reversing dunes, transverse dunes, star dunes, and a few barchan dunes. It is estimated to contain 6.5 billion cubic meters (1.5 cubic miles) of sand.

The sand sheet is the largest component of the Great Sand Dunes geological system, made up of sandy grasslands that extend around three sides of the main dunefield. Almost 90% of the sand deposit is found here, while only about 10% is found in the main dunefield. The sand sheet is the primary source of sand for the Great Sand Dunes. Small parabolic dunes form here, then migrate into the main dunefield. Nebkha dunes form around vegetation.

The sabkha forms where sand is seasonally saturated by rising ground water. When the water evaporates away in late summer, minerals similar to baking soda cement sand grains together into a white crust. Areas of sabkha can be found throughout western portions of the sand sheet, wherever the water table meets the surface. In this area are both deeper stream or spring-fed wetlands with rich plant and animal life, and shallower, salty evaporation groundwater. Sabkha deposits can accumulate into thick crusts. In the 19th century, a tiny town appeared nearby called Soda City. Residents collected and packaged the powder for use as washing soda.

To get a great view of this entire sand system, drive south on Highway 150 from the visitor center, then west on Lane 6 North for a total trip of 15 miles (24 km)  to the San Luis Lakes State Wildlife Area. Stand on the concrete boat ramp and look back toward the park.

A view of the Great Sand Dunes sand system from San Luis Lakes State Wildlife Refuge / Rebecca Latson

For a more detailed explanation of the park’s geology, including the “singing sand” phenomenon, check out this page.