Sand trap: confined to a mountain valley, windblown sand has piled up into towering dunes

Natural History, July, 2005 by Robert H. Mohlenbrock

More than a hundred miles long and fifty miles wide, the San Luis Valley lies within the Rocky Mountains of south-central Colorado and northern New Mexico, bordered on the west by the San Juan range and on the east by the Sangre de Cristo range. Dominated by rubber rabbit-brush and grasses, the valley shares its name with San Luis Creek, a stream that flows through its northern half. The more prominent waterway, however, is the Rio Grande, which originates in the San Juans and flows through the southern part of the valley. The configuration of the two mountain ranges and the ceaseless action of wind and water have bestowed upon the valley the tallest sand dunes in North America. Rising as high as 750 feet above the valley floor, they are the centerpiece of the Great Sand Dunes National Park and Preserve.

Through volcanic action the San Juans began to form about 35 million years ago, and the Sangre de Cristos were rapidly uplifted beginning about 19 million years ago. Rain, wind, and--beginning about 1.6 million years ago--ice-age glaciers chiseled away at the peaks. Sand, gravel, and clay were washed into the valley, leaving behind lakes containing sand and silt. During dry periods, these lakes shrank or disappeared, and prevailing southwesterly winds blew clouds of sand from the dry lakebeds across the valley. Sand grains too heavy for the wind to carry beyond the crest of the Sangre de Cristos accumulated near the base of the mountains. Occasional strong northeasterly winds also helped trap the sand in the valley and pile it into dunes.

The dunes cover about thirty square miles on the eastern side of the valley, in a region near its midpoint. One reason they are so concentrated here is that several small streams help capture and return sand blown out of this zone. In the dune field, the sand is continually shifted back and forth by the wind, and vegetation has a hard time taking hold. Plants that do become established often get buried. Blowout grass and lemon scurf pea are two species that can send out roots and rhizomes several feet into the shifting sands, thus ensuring better stabilization. South and west of the dune field is a flatter, more vegetated zone known as the sand sheet, and today that is the main source of additional windblown sand.

Dunes come in various shapes. Barchans are crescent-shaped dunes that creep slowly across the landscape as sand is blown over the top and slides down the leeward side. The windward side is the convex side of the crescent. Barchans can also join end to end to form transverse dunes--rows of sand that run perpendicular to the direction of the wind.

Parabolic dunes are formed when the wind causes a blowout, that is, begins to gouge sand out from around a patch of vegetation that has weakened its grip. The excavated sand is slowly blown forward, but the sand on either side of its path remains stabilized by other vegetation. The forward-moving pile forms the "nose" of a parabola, while the anchored vegetation left in place on either side forms two trailing arms, often stretching behind for long distances. Star dunes, with arms that extend from a central point, form where the winds pummel the sand from several directions.

The dunes were first protected in 1932 by the establishment of Great Sand Dunes National Monument, which embraced fifty-nine square miles. The monument remained essentially the same size until 2000, when Congress authorized an expansion to create the Great Sand Dunes National Park and Preserve. The first major addition was the transfer of about sixty-five square miles of the Rio Grande National Forest to form the national preserve. The preserve extends up the slopes of the Sangre de Cristos, providing numerous niches that enhance the biological diversity in the preserve. Streams and small marshy areas within the park provide additional habitats for plants.

Although the winds are continually reshaping the dunes, they have not wrought substantial changes in living memory. Forest Service scientists E. Durant McArthur and Stewart C. Sanderson compared photographs of the area taken in 1936 with conditions in 1990 and found that the dune masses were relatively stable. They did note, however, that nearby marshlands and scattered ponds had become dramatically smaller.

RELATED ARTICLE: Habitats.

Dune In the dune field, mainly where it grades into the sand sheet, the most common plants are blowout grass, Indian rice grass, and lemon scurf pea. Other species are burr ragweed, crown-leaf evening primrose, hairy bugseed, rush skeleton plant, and Russian thistle (a nonnative invader). The vegetation in the sand sheet is made up primarily of grasses known as sand dropseed and sandhill muhly. Among the other species are broom groundsel, James' catseye, narrow-leaf gromwell, narrow-leaved penstemon, needle and thread, nodding buckwheat, rubber rabbitbrush, and yucca. Prairie sunflower brightens the sand sheet with vivid yellow from late July through early September.

Streambank Large and small creeks are lined with such trees as coyote willow, narrow-leaf cottonwood, quaking aspen, and thinleaf alder. Skunkbush, trumpet gooseberry, and Woods' rose are shrubs that occur here and there along the streams. Among the grasses and herbs are Kentucky bluegrass, nodding brome, slender wheatgrass, starry false Solomon's seal, and western yarrow.