On Wednesday, August 25, the 105th birthday of the National Park Service, Sean and I ventured into the dunefield of Great Sand Dunes National Park. We’d gazed on it from varying distances for two days, but now it was time to experience it closely. On this second full day in the Park, we wanted to prioritize the dunes, but we also wanted to hike in them first thing while it was still cool and before the day heated up and made the experience less pleasant.

On the afternoon of Tuesday, August 24, we continued exploring the parts of Great Sand Dunes National Park and Preserve that were not actually the dunes proper. We’d decided to save them for the following morning, when temperatures would be cooler. We toyed with the idea of driving to a trailhead on the other (eastern) side of the Sangre de Cristo Range to hike to a couple of alpine lakes high in the range, but the drive was almost two and a half hours.

So instead we opted for Mosca Pass Trail, which leads from near the Visitor Center complex up into the Sangre de Cristos to a low pass between the San Luis Valley and the Wet Mountains Valley. The hike was 3.5 miles to the crest of the pass, then 3.5 miles back to the trailhead. The Falcon Guide rated it Easy. We figured it would be a nice end to a day of hiking around the foothills zone between the dunefield and the mountains.

We were wrong.

Great Sand Dunes National Park and Preserve protects 149,000 acres of dune field, transition zone, and a portion of the Sangre de Cristo Mountains in south central Colorado. The Park and Preserve was authorized by Congress in the waning days of the Clinton administration in 2000 and established in 2004 under the Bush administration. The dunes proper had received earlier protection in the waning days of the Hoover administration in 1932 after local communities became alarmed that the dunes might be destroyed for industrial use for gold mining or concrete production.

That the tallest sand dunes in North America rise above the enormous, flat, high-elevation (above 7,000 feet) San Luis Valley in Colorado—and not in, say Death Valley, nor the Mojave, Sonoran, or Chihuahuan Deserts, places generally much sandier than the valley—is a unique circumstance of geography. The sand that comprises the dunes comes mainly from the San Juan Mountains dozens of miles to the west across the broad, flat valley. Sediment washed down from the mountains by snowmelt, rain, and the Rio Grande, whose headwaters are in the San Juans and which begins its long journey to the Gulf of Mexico by emerging into the San Luis Valley. Sandy sediment is deposited on the western side of the valley and then blown by the prevailing winds rushing down from the San Juans. The Sangre de Cristo Mountains, the southernmost range in the Rockies, run north-south for 242 miles, but the dunes formed only in a nook just north of Mount Blanca, the range’s highest peak and a sacred mountain to a number of Native American peoples. The dunes formed adjacent to a relatively low saddle in the range where three major ephemeral springs flow down into the valley. The streams, which secure the sand, are important for the stability of the dunes and for the hardy plant communities that grow from the extensive sand flats surrounding the dunes proper. But it is the storm winds rushing down from the low saddle in the Sangre de Cristos that balance the prevailing winds from the west, keeping the dunes themselves remarkably stable over years and decades.

Even with the uniqueness of the region’s wind and geography, the sheer amount of sand cannot be explained just by this process. More recent evidence suggests that the southern end of the San Luis Valley was once covered by a vast lake, remnants of which are visible as wetland complexes west and south of the dunes and a vast aquifer beneath the valley, which makes agriculture possible. It is the sand of this ancient lakebed that comprises the bulk of the dunes.

Tuesday, August 24, we’d spend exploring this singular place by getting to know the transition area between the dunes and the mountains.