Not Exactly a Day at the Beach
No two dune destinations are the same; each has its own surprises. The view on arrival at Great Sand Dunes National Monument in south central Colorado is awesome, and not just because the 700-foot-high dune formations are North America's highest. Nestled in the curving ridge of the snow-peaked Sangre de Cristo Mountains, with the glittering shallow waters of Medano Creek between you and the beginning of the dunes, the scene is a photographer's dream landscape. The creek is seasonal, usually skirting the dunes from the time snowmelt begins in April and lasting for about four months, though it varies yearly depending on the summer rains. In a rare year, the creek forms the foreground for autumn foliage colors.
For centuries the Rio Grande flowed through the San Luis Valley, carrying eroded bits of sediment that were fed to the river primarily via tributary streams coming down from the volcanic San Juan Mountains (where the headwaters of the Rio Grande are located). Some sediments were also deposited by glaciers during the Ice Age. As often happens, the Rio Grande changed course, leaving behind a great amount of exposed sediments and sand. For thousands of years, the wind moved this sand, producing 39 square miles of dunes in the area.
Through most of the year, southwest winds move across the San Luis Valley, pushing and bouncing sediments and sand grains to the northeast. The 14,000-foot Sangre de Cristo Mountains, however, quickly put a stop to any further sand movement, except for fine dust particles that make it over some of the mountain passes. The result is a spectacular pileup of dune formations, intensified when brief storm winds roar down the peaks from the northeast. These gales cancel the slow work done by weaker winds. As a result, the windward and lee faces suddenly reverse, and the dunes fold back on themselves. Unable to migrate due to space restraints, these dunes grow higher and higher. Fair amounts of moisture also inhibit dune movement, though surface sands move freely.
Wind from one direction produces crescent-shaped barchans as well as linear transverse dune formations, which require a larger sand supply than barchans. Reversing dunes result from two wind directions, and slip faces can form on either side of the existing formation. Multiple wind directions result in star dunes that grow vertically and have three or more slip faces. A complex network of star dunes lies in the northwest lobe of the dunefieldformed at the base of Mount Herard, which separates the wind-fluctuating Music and Medano passes.
A variety of materials compose the dunes, with 51.7 percent of the dune matter consisting of volcanic rock fragments, 19.1 percent being quartz, 8.9 percent feldspar, 2.5 percent sandstone, and 5.2 percent various other minerals. Sand grains in the park are quite small, lying between 0.2 and 0.3 mm in diameter.
Few plants grow on the dunes in this high, parched valley; blowout grass, Indian ricegrass, scurfpea, yucca, and bright yellow prairie sunflowers, however, are a few species that have adapted to the sand. Around the dunes are grasslands, as well as pinyon and juniper trees. Cottonwoods frequently line creeks. The Pinyon Flats Trail to the dunes is a good place to look for plants and wildflowers. Along the Montville Nature Trail are Rocky Mountain maple, quaking aspen, white fir, mountain spray, thinleaf alder, and several edible fruitschokeberry, gooseberry, currant, wild rose, and skunkbush (which early white settlers used to make a lemonade-like drink).
The dunes themselves are not very hospitable to animals, but two well-adapted species of insects are endemic: the Great Sand Dunes tiger beetle and the giant sand treader camel cricket, both roamers over sand. On the fringes of the dunes, small rodents (including the kangaroo rat), bobcats, coyotes, and pronghorn antelope find suitable habitat, though many animals are nocturnal; watch for tracks. Mule deer are not skittish at all and often come to the flats near the campground. Birdsespecially warblers, kinglets, and siskinsfind hospitable habitat in the vegetation near Mosca Creek. Ravens and blackbilled magpies fly the skies.
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