WIND ACTION AND THE FEATURES IT PRODUCES

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. 

GLACIATION

 Glaciation refers to the process whereby a certain area on the earth’s surface is affected by glaciers (moving ice). Glaciation also refers to the process that takes place due to the influence of moving ice.

Glacial erosion

Glacial erosion, which predominates in the highlands, consists of the following mechanism or processes:

Sapping: This refers to the breaking up of rocks by alternate freezing and thawing of water at the bottom of cracks between a mass of ice and the side and floor of a valley, or the side of a mountain.

Plucking: This is the tearing away of the blocks of rock which have been frozen into the sides or bottom of a glacier. 

Abrasion: This is the wearing away of rocks beneath a glacier by the scouring (scrapping) action of the rocks embedded in the glacier. 

Feature produced by glacial erosion

i. Cirque (corrie): A semi-circular, steep-sided basin cut into the side of a mountain, or at the head of a valley. It is formed by the process of plucking, which steepens the basin, and abrasion, which deepens the valley. Some corries contain glaciers, but in others the glaciers have melted and they now contain lakes (sometimes called tarns). Examples of tarns are Lake Tana in Ethiopia and Teleki tarn on Mount Kenya.

ii. Arete: A steep-sided, knife-edged ridge separating two cirques. It is formed by the cutting back of the walls of cirques by plucking. Examples of Artes are found on Mount Kenya.

iii. Pyramidal peak: A jagged peak with a steep sided, angular horn. It is formed by the steepening of the back walls of several cirques which lie on the sides of a mountain. Examples of pyramidal peaks are found on Mount Elgon.

iv. U-shaped valley: A steep-sided, flat-bottomed, wide valley which contains features formed by both glacial erosion and deposition on the foot of the glaciated highland. It is formed by vertical and lateral erosion of moving ice. Most U-shaped valleys were originally river valleys.

v. Hanging valley: A tributary valley of a U-shaped valley which ends abruptly, high above the floor of the U-shaped valley and separated from it by an almost vertical slope. It is formed due to unequal down-cutting on the tributary valley.

vi. Rock basin: An irregular depression in the floor of a U-shaped valley formed by unequal glacial erosion of the bedrock. It develops when the thickness and weight of a glacier increase, e.g. at the junction of two glaciers. Sometimes a rock basin becomes a lake when the glacier melts.

vii. Truncated spurs: These are blunt-ended rock ridges which descend from the steep sides of a U-shaped valley or glacial trough. They are often separated by hanging valleys.

viii. Ice-eroded plain: An extensive area once covered by an ice sheet which smoothed off the original landforms to give a rounded topography, with large areas of bare rock scratched by boulders embedded in the base of the ice, and rock basins in areas of weak rock, and the whole swept almost clean of the original weathered rock.

ix. Roche Moutonnee: An outcrop of resistant rock smoothed by a glacier on the upstream side into a gentle slope. On the downstream side, the glacier erodes by plucking to give steep and jagged slope. It is formed where resistant rocks rise above the surrounding land surface. The upstream side of the rock is plucked to a steep slope.

x. Crag and tail: A head of resistant rock which protected a weaker rock from ice erosion on the downstream side.

Depositional features of glaciations

Moraine: These are unsorted rock fragments of all sizes, from sand to boulders, formed partly by frost action and partly by glacial abrasion, transported by a glacier and dumped in ridges or sheets.

Types of moraines

A moraine that forms along the sides of a glacier is called lateral moraine; that along the front of the glacier is called terminal moraine, and that at the bottom of the glacier is called ground moraine. When two glaciers join, their inner lateral moraines join together and give a medial moraine.

Boulder clay plain: A plain made of clay and boulders, deposited by ice sheets and glaciers over a surface. 

Drumlin: Elongated, oval-shaped hill made of boulder clay and about 1 km long and 25 to 100 m wide. Each drumlin is a small hill, tending towards an egg shape, with its steepest slopes and summit at the up-ice end.  

Eskers: An esker is a long, narrow, winding ridge of stratified sand and gravel. The materials that form an esker are deposited by sub-glacier streams which retreat their way in the channel under the ice. Eskers reach up to 40 metres high. They are mostly found in Scandinavian countries.

Kame: Is an irregular-shaped mass of stratified material formed as a delta on the surface of a stationary glacier or at its margin. It is a mound-like hill of poorly sorted material mostly sand and gravel, deposited at or near the terminus of a glacier.  

Erratic: A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. Most erratics can be found at Kimberlay (South Africa), North East USA and Wales in Britain.