form three notes: WIND ACTIONS

WIND ACTION AND THE FEATURES IT PRODUCES

Wind refers to the air in motion from high pressure to low pressure belt. Wind action is very powerful in arid and semi–arid regions. Examples of deserts include the Sahara, in North Africa, Namib, Kalahari and Gobi deserts.

Types of desert surfaces

Sandy Desert (Erg): This is undulating plain of sand whose surface is blown into sand dunes and nipples. The Sand Sea of Egypt and Libya is a good example of an erg.

 Stony Desert (Reg): This is a desert surface covered with boulders and stones produced by daily temperature changes. Most of stony deserts are formed in Algeria, Libya and Egypt.

 Rocky Desert (Hamada): This consists of extensive areas of bare rock from which all fine materials have been removed by deflation. Abrasion by the fine materials polishes and smooths the rock surfaces. One of the largest hamada is Hamada el Hamra, in the Sahara of Libya.

 Badlands: This is a land broken by extensive gullies, separated by steep-sided ridges. This type of desert is quite different from the three deserts explained above, in that it develops in semi-arid regions which experience sudden violent rainstorms.

 The action of winds in a desert

The mechanism of erosion by wind involves deflation, abrasion and attrition.

 Deflation: Is the blowing away of rock waste thus lowering the desert surface and producing depression, some of which are very extensive.

Abrasion: This is the breaking up of the rock when small particles are hurled against rock surfaces by wind, helping to produce such features as rock pedestals, zeugens and yardangs.

Attrition: This is the process by which rock particles collide and rub against each other, as they are transported by wind, and wear away. This is the source of sandy deserts.

Features produced by wind erosion

Rock pedestal: This is a tower-like feature made of alternate horizontal layers of hard and soft rocks. The rocks in a pedestal are made in alternate horizontal layers of hard and soft rocks. Soft layers are eroded away faster than the hard layers. Most of them are found in Saudi Arabia and Niger. 

Zeugen: This is a ‘ridge and furrow’ landscape produced by wind abrasion in a desert surface which has a layer of resistant rock underlain by a layer of weak rock. Mechanical weathering open up the joints on the surface rocks, thus enabling wind abrasion to attack the underlying soft layers. The ridges are called zeugens and these may be as high as 30 metres.  Examples of Zeugens include those in Sahara Desert.

  Yardangs:  Thes are elongated rock ridges of vertical or nearly vertical layers of resistant rock separated by softer rocks. The belts of resistant rock stand up as sloping ridges, varying in height from 5 m to 15 m but having lengths of up to 1000 m. Good examples of yardangs occur near In Salah (Central Algeria) and near to Kom Ombo (Egypt).

 Depression hollows: Some hollows produced by wind deflation reach down to water-bearing rocks. When this happens, a swamp, or an oasis, develops. An example is Qattara Depression, in Egypt, which is over 120 m below sea level. Some deflation hollows are probably produced in part by faulted rocks.

Inselberg: This is a residual hill consisting of hard, resistant rock, left up standing on the earth’s surface after the less resistant rock has been worn away by denudation process.  When inselbergs are characterized with rectangular rocks are called Kopjes.

Ventifacts: These are heavier rocks or pebbles left behind after wind has sorted and carried away all materials. Most of them are sharpened and flattened

Features produced by wind deposition

Barchans /bark ham: Barchans are crescent-shaped sand dunes, lying at right angles to the prevailing wind. Barchans may occur individually or in groups.  The windward side of a bark ham is gently slopping and lee ward side is steep and slightly concave.  Examples of bark hams are found in Western Libya, Eastern Chad and Northern Niger.

  Seifs: These are long, narrow ridges of sand which lie straight along the depressions between small hills (dunes). Extensive lines of seif dunes are found in the Sahara desert, south of the Qattara Depression, southern Persia and Tharp desert.

 Loess: Loess is an accumulation of fine particles carried and deposited by wind beyond the desert limits. It occurs extensively in the loess plateau of North West China and is known as Hangtag and in USA where is called Adobe.