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Exploring Colorado’s Great Sand Dunes: A Towering Red Planet Analog

Posted on Mon, July 25, 2016
  • by: Samuel F. A. Cartwright, CEPS Intern
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As the National Air and Space Museum’s annual Mars Day! celebration approaches, we look to a recent research trip taken by a Smithsonian Summer Intern to investigate the similarities between some of Earth’s most amazing dunes and those found on the ruddy surface of Mars.


Tucked away in a corner of Colorado’s San Luis Valley, the Great Sand Dunes have certainly earned their superlative title. The tallest dune rises 230 meters (750 feet) from the base of the 77 square kilometer (30 square mile) dune field, making these the tallest dunes in North America. Add in the base elevation of over 2,440 meters (8,000 feet) above sea level and they might just be the highest, too.

While their exact age remains unknown, the Great Sand Dunes began forming in the last 440,000 years after an ancient lake that once filled the San Luis Valley dried up and left behind extensive deposits of sandy sediment. Over time, predominant winds have pushed the sand towards the Sangre de Cristo Mountains where opposing storm winds blowing through three mountain passes have pushed it back, sculpting the towering dunes to this day. The dune field and the surrounding alpine environment has been protected by the Great Sand Dunes National Park and Preserve since 2000.

Sign welcoming visitors to the Great Sand Dunes National Park and Preserve with the dune field and Sangre de Cristo Mountains in the background; photo by the author.

Though thousands travel to the park each year to admire the dunes, planetary geologists like Dr. Jim Zimbelman with the National Air and Space Museum’s Center for Earth and Planetary Studies see a unique opportunity afforded by such monumental feats of geologic forces. Dr. Zimbelman has been visiting the Great Sand Dunes for over a decade to track their movement in an effort to better understand the nature of dunes on Mars, one of three other bodies in the solar system where such formations have been found. When he returned last month to update his research, he brought along two eager summer interns, me and Mike Sierleja, to help collect data.

After a five-hour drive down from Denver and a night’s stay in the nearby town of Alamosa (the eponym of the ancient lake whose demise seeded the dunes), we arrived at the park offices to meet up with our guide, park geologist Andrew Valdez, a man equally as knowledgeable of the immense complexity of the park ecosystem as he is of how to cook a steak to perfection for road-weary researchers. It was a short drive to the main parking lot where we applied liberal amounts of sunscreen and started towards the looming dune field.

Five of the Great Sand Dunes are over 700 feet tall, including High Dune (far right); photo by the author.

We first had to cross Medano Creek, a broad, shallow stream with unique, periodic surges that help it carry sediments down from the mountains where they are then blown back onto the dunes; the cycling of sand through this alpine artery is one of the reasons the dunes grow so big. After getting used to trekking over the shifting grains on our way to the first dune on our list, we got out Andrew’s GeoXH unit, a souped-up GPS that uses satellites to accurately pinpoint the location of a handheld receiver. With the help of ample water breaks and trail mix, we walked along the crests and troughs of seven dunes on the southern edge of the field to map out their positions in three dimensions.

Visitors to the Great Sand Dunes must cross Medano Creek (center) before scaling the sculpted mounds of sand; photo by the author.

With preliminary comparisons of data showing an unexpected shift in the geometry of the dunes from the previous year, we excitedly made our way out for a second day of research. Our focus this time was on megaripples, a feature formed by the accumulation of larger grains on wind-swept surfaces between dunes. We found a field of relatively undisturbed megaripples and defined a survey area before marking out crests with the GeoXH. Dr. Zimbelman then took a series of pictures that could be used to reconstruct one of the megaripples in three dimensions (a technique known as stereophotogrammetry) and we measured a topographic profile and cross section with the aid of a laser level and a spare spray bottle.

Planetary geologist Dr. Jim Zimbelman uses a ruler to delicately cut a cross section through a wetted sand megaripple; photo by the author.

The two days of research and exploration of the park went by quickly and just as I was starting to get used to the idea of sand being an ever-present companion in my pockets and shoes, it was time for us to leave. Before driving up to Denver for the flight back to Washington, Dr. Zimbelman gave a talk at the visitor center where he described his research and the parallels that can be drawn between our own great dunes and those on Mars. Given that there are more stars in the universe than grains of sand on Earth (and believe me, there are a lot just in that corner of Colorado), there’s no telling how many more dunes are out there waiting to be explored.

A delicate balance of opposing winds around the Sangre de Cristo Mountains have made the Great Sand Dunes the tallest of their kind in North America; photo by the author.