Geology Reference
In-Depth Information
Star dunes are complex dune shapes usually confined to large, sandy deserts such as
the Sahara in Northern Africa. They are the result of wind blowing from many different
directions (at least three) or shifting its direction constantly. A star dune features a high
central point and numerous ridges of sand coming off it, creating a star shape.
Laying down layers of loess
If dunes result from wind depositing its bed load particles, what happens to all the smal-
ler sediments carried as suspended load in the wind? The particles of silt and clay sus-
pended in wind currents are deposited as
loess.
Here's the process: Small-sized sediments are picked up by wind in glacial or desert en-
vironments, carried far away, and deposited as loess whenever and wherever the wind
stops moving, often blanketing whole regions. For example, much of the Midwestern Un-
ited States and significant portions of the Pacific Northwest are covered in loess that
was deposited thousands of years ago when large parts of North America were covered
by ice sheets (see Chapter 13).
Like dust in the wind: The ecological threat of desertific-
ation
Desertiication
is the process of changing productive farm or grazing land into barren desert. The loss
of useful farmland to desertification is one of the major threats of climate change. This shift is devast-
ating to farmers, ranchers, and communities that depend on the land for food and income.
The most common cause of desertification is allowing animals to
overgraze
or eat all the vegetation in
a confined area. When the vegetation is gone, the topsoil — the most valuable nutrient-rich sediment
— dries out and is blown away as dust in the wind. The presence of vegetation provides a physical an-
chor; the roots hold sediments in place. Vegetation also provides increased moisture to the soil. When
the vegetation and topsoil are removed, the land becomes nearly useless: No crops grow, and no plants
are available for animals to graze. The humans dependent on this land for food have to move some-
where else, where they may start the process all over again, spreading the desertification throughout
an entire region.
The spread of desertification particularly threatens regions that are already arid and depend on the
grazing of herd animals for survival, such as southern Africa and northern China. But desertification
also occurs in semi-arid regions, including the Mediterranean and the American Great Plains. Many
countries have begun to attack this problem by creating strategies for smart land use, including rota-
tional grazing, as well as working to reduce the effects of greenhouse gas climate warming.
