Page 396 - Laboratory Manual in Physical Geology

Wind Wind

Blowouts

Wind

A. Barchan dunes
C. Parabolic dunes

B. Transverse dunes

Wind

Wind

D. Longitudinal dunes
E. Barchanoid ridge dunes
100°–150°

FIGURE 14.6 Common types of sand dunes. Note their basic morphology and internal stratification relative to wind direction.

dissolved in playa water become more and more concentrated determined by dating the radioactive carbon of organic
as the water evaporates. Eventually, evaporite minerals (salts) materials that have been covered up by the large dunes.
precipitate from the water, and all that is left is a white, soggy or The large dunes are now covered with grass (short-grass
dry, salt flat (a level patch of land that is encrusted with salt). prairie) that is suitable for limited ranching. About 17,000
people (mostly ranchers) now live in the Sand Hills.
Sand Seas of Nebraska and the
Arabian Peninsula Dryland Lakes
The amount of rain that falls on a particular dryland normally
Deserts can be rocky or sandy. An extensive sandy desert
fluctuates over periods of several months, years, decades, cen-

is called a sand sea, or erg . The largest erg on Earth is the
turies, or even millennia. Therefore, a dryland may actually
Rub’ al Khali (rūb al ka’lē), Arab for “the Empty Quarter,”
switch back and forth between arid and semi-arid conditions,
of the Arabian Peninsula. It covers an area of about 250,000
semi-arid and dry-subhumid conditions, arid and dry-
square kilometers (nearly the size of Oregon). Many kinds
subhumid conditions, and so on. Where lakes persist in the
of active dunes occur there, and some reach heights of more
midst of drylands, their water levels fluctuate up and down in
than 200 meters. Rub’ al Khali is a true desert (supports no
relation to such periodic changes in precipitation and climate.
agriculture) with rainfall less than 35 mm per year.
Periods of higher rainfall (or snow that eventually melts) and
The Sand Hills of Nebraska ( FIGURES 14.5 ) is only

reduced aridity and evaporation create lakes that dry up dur-
one-fifth the size of the Empty Quarter, or about 50,000
ing intervening periods of less rain and greater aridity and
square kilometers of land, but it is the largest erg in
evaporation. Great Salt Lake, Utah, is an example.
the Western Hemisphere. This sand sea was active in
Great Salt Lake is a closed basin, so water can escape
Late Pleistocene and early Holocene time, but it has
from the lake only by evaporation. When it rains, or
been inactive (i.e., the dunes are not actively forming
when snow melts in the surrounding hills, the water
or moving) for about the past 8000 years. This was
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