WHY ARE THE DUNES HERE?
Are they limited to this section of the valley? Do they migrate? Answers to these questions are in the winds.
Significant seasonal variations in wind patterns affect the dunes.
Winter and summer bring winds from different directions; northerly in winter and southerly in summer. Landforms influence crosswinds and swirling wind currents (eddies). These crosswind and eddy patterns result in decreased wind speed and cause the sand to drop out in the same general area time after time. Seasonal variations in wind direction help keep the sand in one area.
Between individual dunes, white salt deposits are often seen on the surface of an old clay lakebed (playa). Here too, we see how tall the dunes are because the sand is piled entirely on top of this playa. Infrequent rains produce changes in the playa surface, and occasionally gravel is washed into the area from nearby mountains. Because of the clay content of the soil, rain water does not soak in quickly, but lies in pools. As the pools evaporate, fresh deposits of salts and minerals remain on the surface. Similar salt deposits throughout the valley to the south minimize the amount of available blowing sand by cementing it together.
Dunes in Death Valley consist of sand from local materials found in nearby mountains. The primary source of this sand is believed to be the Cottonwood Mountains to the west and northwest. Although the shapes and patterns of individual dunes may vary daily or seasonally, the dune field itself is relatively stable.
Pick up some sand! Is it fine or coarse? Wet or dry? Warm or cold? Notice the different colors. A magnet may help you separate particles, and a magnifying glass will allow you to examine them more closely. Most black particles are magnetite—most white ones are quartz. Other minerals provide still different colors. The sand here is a product of weathering by wind, rain and chemical changes—breaking blocks of rock (primarily granite) from nearby mountains into particles small and light enough to be carried by run-off and tossed around in the frequent desert winds.
