Each wind produces changes to the surface of the sand. It takes a 10 mph
(16 kph) wind to move fine, dry sand. During a sandstorm, it may seem as
if the sand is blowing thousands of feet into the air. Your view of the
distant mountains may be obscured. Don’t be fooled! The dunes are not
entirely airborne! What you see are dust particles being carried aloft
because they are smaller and lighter than sand grains. Seldom does
dune-sized sand itself rise more than 6 ft. (2 m) above the ground, it
is too heavy.
[Illustration: {Ripples}]
[Illustration: {Movement of sand grains}]
Actually, most sand travels within about 18 in. (46 cm) of the ground in
a jumping movement called saltation. A windborne grain bounces a short
distance before crashing into the sandy surface sending other grains
into flight. The process continues as the wind carries the new grains a
little farther, crashing them, in turn, into the surface and other
grains along the way. Sand grains too heavy to blow in the wind creep
along the surface of the sand, while smaller ones are truly airborne
above the dunes. As a consequence, the entire sand surface flows or
creeps along.
Have you been comparing the texture, size and color of sand in various
sections of the dunes? Some differences will be more obvious if you have
the chance to visit both the tall dunes by Highway 190 and the low ones
by the picnic area. The soft, fine sand by the picnic area is carried
farther by the wind than the coarser material found in the taller dunes.
Smaller grains move farther than large ones. As a result of this sorting
of sand grains by variable winds, the larger grains accumulate in small
ridges forming ripples. Size and shape of these formations are related
to specific interactions between sand and wind. The windward side of a
dune generally has more dramatic sculpturing. Examine an area with
interesting ripples. Wind speed, direction, duration, and the amount of
sand are important factors that have been measured and used to determine
the size and spacing of dune ripples. Just as these ripples form, the
dunes themselves respond to changes in each of these factors.
{ Speed { Size
WIND { Duration + SAND { Uniformity = RIPPLE FORMATION
{ Direction { Amount
[Illustration: {Cross-section of dune}]
SUMMIT
ANGLE OF REPOSE
SLIPFACE
WINDWARD
ANGLE OF REPOSE
FOC
34°
All dunes may be described by the above traits. Variations in shape and
size of individual dunes are determined by the mathematical values
applied to each of the traits mentioned in the discussion of ripple
formation. Another feature of interest is the dune shape. Among these
dunes you may find examples of this classic textbook model and its
variations.
[Illustration: _Plants on the playa in between the dunes_]
Note that the downwind side of the dune, the slipface, is the steeper
one. Fine dry sand can only pile up so high before it breaks away and
slides down the slipface as an avalanche to rest at the more stable
angle of repose, about 34 degrees. We create miniature avalanches as we
walk along the crest of a steep dune.