This was first published here five years ago today.
The San Luis Valley is said to be the largest mountain valley in the world. It runs north-south for 125 miles between the San Juan and Sangre de Cristo (Blood of Christ) mountain ranges in south central Colorado. Both of these ranges have numerous peaks above 14,000 feet (4300m).
Just south of Poncha Pass, the narrower northern end of the San Luis Valley is an extraordinarily picturesque landscape — even this week without snow on the mountains. Further south the distance between the ranges widens to 65 miles and the Valley becomes broad and flat — and less picturesque. The altitude of the Valley averages near 7,500 feet.
The Rio Grande del Norte rises in the San Juan Mountains and flows generally eastward into the San Luis Valley. East of Alamosa the “Great River of the North” bends south toward New Mexico. Through centuries the river deposited sand and silt from the San Juan Mountains along its meandering, changing course across the Valley. The prevailing wind blew these deposits eastward toward the Sangre de Cristos, where they were trapped at the foot of the mountains. Today the resulting sand pile is known as the Great Sand Dunes National Monument and Preserve.
The dunes tower as high as 750 feet (230m) and cover nearly 40 square miles. They are the tallest dunes in North America. Sufficient rain and snow fall to keep the dunes stable, though the surface dries quickly and the winds sculpt and restructure the surface continuously. Here the expression “leave nothing but footprints” has little meaning as footprints will soon be gone.
Hiking in the dunes is encouraged (with the usual caveats about heat, water, lightning and not getting lost). Showers and changing rooms are provided near the parking lot — just as at a beach. Walking across the broad, sandy space between the parking lot and the first dunes and then up into the dunes I was surprised by the amount of sand stowing away in my socks and shoes. The sand makes walking more strenuous than on more solid surfaces. 
It also makes sliding and rolling appealing.
The Sangre de Cristos loom more than a mile above the dunes, curving around them from the north to the southeast. The Valley land to the west is being acquired by the National Park Service to prevent the mining of ground water from under the dunes. Once the acquisition is complete, the Monument will be come the 57th National Park.
And it did become a national park on September 13, 2004, though the 58th not the 57th.