Colorado
The Great Sand Dunes are blown up against the foothills of the Sangre de Cristo Mountains in southern Colorado.
The tallest sand dunes in North America are nowhere near an ocean, nor are they in a true desert. Instead, they are piled up against the edge of the Rocky Mountains in southern Colorado. Great Sand Dunes National Park and Preserve near Alamosa boasts sand dunes higher than 750 feet, blown against the west side of the Sangre de Cristo Mountains, a southern range of the Rockies.
From the air, the dune field stands out between the San Juan Mountains to the northwest and the Sangre de Cristos to the east, appearing as light-colored ripples in the shadow of the mountains. If you had flown over this area a hundred years ago, the dunes would have appeared very much the same. The effects of opposing winds and stream erosion are well balanced and the dune system is quite stable. Ongoing dune-building activity occurs in only the top few inches of the main dune field; the deeper sand is well cemented by moisture.
The sand making up these eighty square miles of dunes was left behind by the once enormous ancient Lake Alamosa that filled what’s now the San Luis Valley, before drying out around 440,000 years ago. Strong winds from the southwest blow the sediment out of the valley toward the mountains, where it piles up against the steep western slope. Occasional storm winds blow out of the northeast and the many streams that drain the mountains also carry sand back into the valley, which is then blown back into the dune field again, in a cycle that has persisted for millennia.
Six types of dunes form in Great Sand Dunes National Park and Preserve. The most common is the reversing dune, formed by the area’s periodically reversing wind conditions. Star dunes are found in the northeast corner of the dune field, formed by more chaotic wind patterns that create branching arms in many directions—the highest dune in the park, 750-foot Star Dune, is, not surprisingly, a star dune. Parabolic dunes are found in the sand sheet on the edge of the dune field itself. Approximately 90 percent of valley’s sand still resides in the sand sheet; only 10 percent makes up the dunes themselves. Barchan dunes and transverse dunes are rare in the park, but can be seen in areas where the wind blows only in one direction. Nebkha or coppice dunes are small sand hills that form around clumps of rooted vegetation.
Six types of dunes are created in Great Sand Dunes National Park and Preserve; the formation of each type is dependent on the amount of sand available and the prevailing wind directions.
Medano Creek carries sediments blown into the mountains back to the dune field, playing an important role in continuing the cycle of dune building.
Great Sand Dunes National Park and Preserve is located in south central Colorado, on the west side of the Sangre de Cristo Mountains. You may fly over the dunes en route to Denver or Colorado Springs, Colorado.
