Materials that have been eroded and carried by winds must come to a place where it rest. The fine dust particles can be carried by winds to far away places and may be completely eradicated out of the desert. It has been observed that some dust grains can travel even more than 2.300 miles before they finally settle on land or water.
Italy and glaciers or Switzerland witness ‘Blood rains’ due to the dust that comes from the Sahara Desert across the Mediterranean sea. Similarly, the Hwang-Ho basin has accumulated a lot of dust (of more than a hundred feet), which comes from the Gobi Desert.
Windborne particles are filtered according to their roughness (coarseness). The coarser the dust particle, the harder it is for the wind to carry the particles with it. The heavy particles that cannot travel long distances remain as dunes within the desert. The location of these particles are not static, and thus even dunes keep changing their position. If you saw a dune one day, it will most likely won’t be there the next day.
The migration pattern depends on some factors:
● The size of the dust particles they carry.
● The direction of the wind.
● The velocity of the wind.
● The nature and location of the surface.
● The absence or presence of water and vegetation
Some major features of wind deposition
These are the small hills like structures formed by the deposition of sand and sculpted by the movement of winds. The dune can be active or live which constantly change its position or inactive and fixed which are often rooted with vegetation. The most proper dunes are mostly found in the erg desert. In these desert, the dunes are most active and continuously reshape, move and redeposit forming various desert features.
The dunes vary in size, alignment, and shape and hence have been given so many names. Example – ridges or attached dunes, Ergs, tail dune, head dune, lateral dune, advanced dune, pyramidal dune, wake dune, star dune, sword dune, hairpin dune, parabolic blowout dune, and transverse dune.
Let’s discuss some major types of dunes in detail:
The shape of these dunes is crescentic or moon-shaped which can both be seen individually and in groups. The barchans are active dune which advances unwaveringly before the winds that come from a certain direction. These are most commonly found in the deserts of Turkestan and the deserts of Sahara. The initiation of formation of Barchans occurs due to the accumulation of sand particles at a barrier which could be as small as grass or as big as a heap of rocks.
The formation occurs in the transverse direction to the wind, which leads to the thinning of the horns and lowering in the direction of wind due to the decreased frictional retardation of the blowing winds at a certain speed. The convex shape and gentle slope are formed in the windward side while the concave and steep slope is formed on the leeward side. As the sand moves forward the crest of the dunes also moves forward. The sand is directed to the windward side and slips down the leeward side when it reaches the crest of the dune and thus helps in advancing of the dune. The dunes advance at the rate of almost 25 feet per year (high dunes). The height of the high dunes can go up to 50 feet to 100 feet a year.
Though, the lower dunes may advance up to just a few dozens of feet of height. Erd du Djourab in the south of Tibesti mountains is a good example where the formation of Barchans is quite common. Similarly, Arabian Sea, Djado Plateau, and the Namib Desert also witness the formation of barchans.
b) Seifs or Longitudinal dunes – The long and narrow ridges of sand that are seen lying in the direction of the strong winds are called Seifs or Longitudinal dunes. The word Seif is an Arabic word which means ‘sword.’ The high indented ridges may reach a height of more than 200 feet. The rising and falling of the Crestline occurs during the peaks and weigh down in regular successions like the teeth of a saw. The winds swept the sand and remain smooth because these winds blow straight along the corridor between the lines of dunes.
The eddies tend to drop the sand to form the dunes due to low power, and it blows towards the sides of the corridor. In this way, the strong winds increase the length of the dunes into long, tapering linear ridges while the intermittent crosswinds tend to raise their height and width. Qattara Depression in the Sahara Desert is known for extensive seif dunes. Similarly, the Thar Desert and the Western Australian Desert also have these kinds of dunes.
Loess – The sand particles that are blown away by winds at far away places outside the limits of desert form loess. These particles are yellow, crumbly material and are usually very productive. This land generally consists of fine loam, lime, very potent and extremely penetrable (porous). This land absorbs water easily, and hence the surface always seems dry. These also give rise to badland topography due to the cutting of deep valleys through the broad mantle of soft loess. It is even difficult to develop roads on this land because the surface is so soft that the road sinks in this region leading to the formation of steep walls.
One of the largest loess deposit is found in China in the loess plateau (Hwang Ho).