GREAT SAND DUNES NATIONAL PARK AND PRESERVE — Our high alpine desert does get water, our yearly amount is 11.2 inches. Most of the water happens during our monsoon season in July, August, and September. After a storm you may see some unusual geological features called sand strips which are an initial stage of dune development.
At Great Sand Dunes, sand strips can develop after it rains, when the dune surface is wet. Wet sand is cohesive and is not moved by moderate winds, but the wind does dry the sand one sand grain at a time. The dry grains are movable, and they often blown along the moist surface coming to rest against other dry grains, and patches of dry sand start to grow. As it gets bigger, it then starts to absorb bouncing sand grain leading to more accumulation of dry sand and the sand strip pattern develops. It’s a good example of a natural, self-organizing system as it produces a somewhat regular pattern. If the process continues, the strips can develop wind ripples that migrate and help the strip get thicker. If the strip persists long enough, it will continue to thicken and begin to migrate leading to its transition into a protodune. If the protodune continues to grow to the point where it creates a turbulent zone, then it becomes a dune. The turbulent zone is very effective at trapping sand making the dune a true sand trap. On sand strips and protodunes, most of the moving sand blows over the feature. Dunes trap most of the sand.
Sand strips are temporary features as the entire dune surface will soon be dry and they are incorporated into the preexisting dune form. The small slipfaces at the top of the ridge are also mostly dry. Slipfaces have high porosity allowing air to move through them and that promotes driving. Also, the coarse sand in the troughs also dries quickly as it has less surface area per volume than the fine sand and that also leads to faster drying. Great Sand Dunes is a natural physics laboratory.
