Are they limited to this section of the valley? Do they migrate? Answers
to these questions are in the winds.
[Illustration: _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.
[Illustration: {Dunes}]
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.
[Illustration: _Take a close look at sand grains._]