Sand Dunes and Loess
A sand dune is any hill of loose sand shaped by the wind. Sand dunes form where there is a source of sand—for example, a sandstone formation that weathers easily to release individual grains, or perhaps a beach supplied with abundant sand from a nearby river mouth. Active dunes constantly change shape under wind currents, but they must be free of a vegetation cover in order to form and move. They become inactive when stabilized by a vegetation cover, or when patterns of wind or sand sources change.
Dune sand is usually composed of quartz, which is extremely hard and doesn't easily decay. Dune sand grains are beautifully rounded by abrasion. In strong winds, sand grains move downwind in long, low leaps, bouncing after impact with other grains. This type of hopping, bouncing movement is called saltation.
TYPES OF SAND DUNES
One common type of sand dune is an isolated dune of free sand called a barchan, or crescent dune. This type of dune has the outline of a crescent, and the points of the crescent are directed downwind. Barchan dunes usually rest on a flat, pebble-covered ground surface. They begin as a sand drift in the lee of some obstacle, such as a small hill, rock, or clump of brush. Once a sufficient mass of sand has gathered, it begins to move downwind, becoming a crescent. We usually find barchan dunes arranged in chains extending downwind from the sand source.
Transverse dunes are formed when there is so much sand that it completely covers the solid ground. These dunes are wave-like ridges separated by trough-like furrows. They are called transverse dunes because their crests are at right angles to the wind direction, much like waves on an ocean. The entire area is known as a sand sea because it resembles a storm-tossed sea that is suddenly frozen in motion.
In the Sahara Desert, enormous quantities of reddish dune sand have been weathered from sandstone. This sand makes up a great sand sea, called an erg. Elsewhere, you find a desert pavement of pebbles on top of vast flat-surfaced sheets of sand. This type of surface is called a reg.
Saharan dunes can be elaborately shaped. For example, Arabian star dunes have served for centuries as reliable landmarks for desert travelers because they remain fixed in place. You can also see star dunes in the deserts of the border region between the United States and Mexico.
Parabolic dunes have the opposite curvature, with respect to wind direction, as the barchan dune. A common type of parabolic dune is the coastal blowout dune. These dunes form along shorelines where sand is abundant.
On semiarid plains, where vegetation is sparse and winds are strong, groups of parabolic blowout dunes develop to the lee of shallow deflation hollows. Sand is caught by low bushes and accumulates on a broad, low ridge. These dunes have no steep slip faces and may remain relatively immobile.
In some cases, the dune ridge migrates downwind, forming hairpin dunes with long parallel sides. Another class of dunes consists of long, narrow ridges oriented parallel with the direction of the prevailing wind. These longitudinal dunes may be many kilometers long and cover vast areas of tropical and subtropical deserts in Africa and Australia.
In the United States, longitudinal and parabolic dunes occur on the Colorado Plateau, near the adjacent northern corners of Arizona and New Mexico.
Landward of beaches, there is often a narrow belt of dunes guarding the coastline. These coastal foredunes are irregularly shaped hills and depressions. They are normally covered by beach grass with a few other species of plants that can survive the severe environment. This plant cover traps sand moving landward from the adjacent beach. As a result, the foredune ridge builds upward, becoming a barrier several meters above high-tide level.
Foredunes form a protective barrier for tidal lands on the landward side of a beach ridge or barrier island. In a severe storm, the swash of storm waves cuts away the upper part of the beach. Although the foredune barrier may then be attacked by wave action and partly cut away, it will not usually yield. Between storms, the beach is rebuilt, and, in due time, wind action restores the dune ridge, if plants are maintained.
But if the plant cover of the dune ridge is trampled and reduced by traffic—from vehicles or by foot—deflation will rapidly create a blowout. The new cavity becomes a trench across the dune ridge. When storms bring high water levels and intense wave action, swash is funneled through the gap and spreads out on the tidal marsh or tidal lagoon behind the ridge. Sand swept through the gap is spread over the tidal deposits.
If eroded, the gap can become a new tidal inlet for ocean water to reach the bay beyond the beach. For many coastal communities of the eastern U.S. seaboard, dune ridges protect tidal marshes and estuaries from overwash.
In several large midlatitude areas of the world, the surface is covered by deposits of wind-transported silt that have settled out from dust storms over thousands of years. This material is known as loess. (The pronunciation of this German word is somewhere between “lerse” and “luss.”) Loess is usually a uniform yellowish to buff color and lacks any visible layering. It tends to break away along vertical cliffs wherever it is exposed by the cutting of a stream or grading of a roadway. It is also very easily eroded by running water, and when the vegetation cover that protects it is broken, it is rapidly carved into gullies. Loess has been widely used for cave dwellings both in China and in Central Europe because it's easily excavated.
Figure 16.27 shows the world's major areas of loess. The thickest deposits of loess are in northern China, where layers over 30 m (about 100 ft) thick are common and a maximum thickness of 100 m (about 300 ft) has been measured. This layer covers many hundreds of square kilometers and appears to have been brought as dust from the interior of Asia. In the United States, there are thick loess deposits in the Missouri-Mississippi Valley.
The American and European loess deposits are directly related to the continental glaciers of the Ice Age. At the time when the ice covered much of North America and Europe, the winter climate was generally dry in the land bordering the ice sheets. Strong winds blew southward and eastward over the bare ground, picking up silt from the floodplains of braided streams that discharged the meltwater from the ice. This dust settled on the ground between streams, gradually building up a smooth, level ground surface. The loess is particularly thick along the eastern sides of the valleys because of prevailing westerly winds. It is well exposed along the bluffs of most streams flowing through these regions today.
Loess is an important agricultural resource. It forms rich black soils that are especially suited to cultivation of grains. The highly productive plains of southern Russia, the Argentine pampa, and the rich grain region of north China are underlain by loess. In the United States, corn is extensively cultivated on the loess plains in Kansas, Iowa, and Illinois. Wheat is grown farther west on the loess plains of Kansas and Nebraska and in the Palouse region of eastern Washington.
Induced deflation is a frequent occurrence when shortgrass prairie in a semiarid region is cultivated without irrigation. Plowing disturbs the natural soil surface and grass cover, and in drought years, when vegetation dies out, the unprotected soil is easily eroded by wind action.
That's why much of the Great Plains region of the United States has suffered from dust storms generated by turbulent winds. Strong cold fronts frequently sweep over this area and lift dust high into the atmosphere at times when soil moisture is low. The “Dust Bowl” of the 1930s resulted from induced deflation.
Human activities in very dry, hot deserts have also significantly helped raise high dust clouds. As grazing animals and humans trample the fine-textured soils of the desert of northwest India and Pakistan (the Thar Desert bordering the Indus River), they produce a dust cloud. Such clouds hang over the region for long periods and can extend to heights of 9 km (about 30,000 ft).