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Lecture Notes of Geography, T.Y.B.A.B.Ed. (Sem- V,2023-24 )Dr. Sagar P. Mali Aeolian Erosional Landforms: o Deflation Hollows (Blow outs) o Desert Pavements o Ventififacts,driekanter and z...

Lecture Notes of Geography, T.Y.B.A.B.Ed. (Sem- V,2023-24 )Dr. Sagar P. Mali Aeolian Erosional Landforms: o Deflation Hollows (Blow outs) o Desert Pavements o Ventififacts,driekanter and zweikanter o Rock Pedestal o Zeugens o Yardangs 1. Blow Out or Depletion Hollows It is a type of Aeolian landform, which is saucer shaped or bowl shaped depression, generally created by blowing away or deflation of the sand by strong wind. It is an erosional feature. It is generally formed sue to lack of vegetation. The wind finds it easier to blow away the sand from a patch of land without any vegetation.They remain small in size extending to few kilometers and around 70 meters of depth. The depth is limited to the water table or lag gravel as wet sands deters deflation and favours the growth of plants. They may develop over several days or a couple of seasons. Many have suggested that blowouts evolve into parabolic dunes when a scientists prevailing wind advancesa sectionof the rim and vegetation stabilizes thearms. 2.Sand Deposition orSand dunes Heaps or mounds of sands are generally called sand dunes or simply dunes. Though sand dunes are significant depositionalfeaturesof desert areas but they are also formed in all those areas where sands are available in profusion and wind is capable of transporting and depositing them in suitableareas.This dunes are also formed in coastal areas (riverinedunes) etc. There is a wide range of variation in the Vidya Prabodhini College of Commerce, Education, Computer& Management, Parvari-Goa 29  Lecture Notes of Geography, T.Y.B.A.B.Ed. (Sem-V,2023-24 )Dr. Sagar P. Mali shape, size, structure, length and height in different types of dunes. On an average, their height ranges between a few meters and 20 meters but sometimes they are several hundred meters in height and 5-6 km in length. The wind ward slope is generallygentle (50-150)while reward slope in steep (200-300). Formation of sand dunes begins with the accumulation of sands in theform of low sand mounds due to obstructions (eg. trees, bushes grasses, rocks, hills etc.) Formation of sand dunes requires (i) abundance of sands (ii) high velocity winds, so that huge quantity of sands may be blown and transported to form dunes (ii) obstacles of trees, bushes, etc boulders, wall etc. (iv) suitable places for accumulation of sand. Dunes are generally formed in groups and, such dune areas are called dune complex, dune colony and dune chain. ENecthve wind drecion Slplace slope (stoss) Previous siotacees Wedward Loowand slope Successe siplaces created as dune Drection dune o migatos movement 3. Barchan Dunes: Barchan dunes have a crescentic shape with two horns. The windward side in convex while the leeward is concave and steep.This face maintains an angle of about 35° sides as concave ends of each ridge are curved in the direction, thewinds is blowing and they develop horns.Such barchans are quite common in Turkestan. When these barchans get coalesced together,they appear like sinuous strings; these are the elementary forms of dunes and result from a combination of moderate sand supply and moderate wind velocity. Barchans do not need any obstruction to develop. The maximum height of barchans in about 30 meter (100 ft) and their maximum width and length about 400 meter (1300 ft). The Barchan is formed when the wind isnearly unidirectional; it is oriented with the horns downward. An additional evidence for the association of barchans with unidirectional winds is provided by Kharga Oasis, one of the only deserts stations with wind records;at this Vidya Prabodhini College of Commerce,Education, Computer& Management, Parvari-Goa 30  Lecture Notes of Geography, T. Y.B.A.B.Ed. (Sem- V,2023-24 ) Dr. SagarP. Mali locally the winds are almost unidirectional and the barchans is the only form of dune present. Transverse dunes are transformed into barchans when sand supply becomes more limited downward. They are formed in groups when there is ample supply of sand. Isolated barchans are formed when the supply of sands is inadequate. A barchans dune will tend to form a mound of sand, as the wind, will be felt more on the windward slope of a mound of sand, they will tend to sand, they will tend to accumulate on the crest, for the effect on; it is felt less here. Accumulation at the top of the lee slope leads to the steepening of that slope and hence to a decrease of the wind effect on it. The steeping of the lee slope by accumulation at the top goes on until the angle of rest of the material is exceeded, when shearing takes place along a slightly less steep surface.The flanks of the original mound of sand advance more rapidly are inversely proportional to the height of the slip face. In this way the crescentic Barchan form is developed. The rate of migration is very fast. The can move upto 25 ft. a year. sbp fac Wind direction -sand avalaches Barchan Dune dune mavenent 1m, 4. Longitudinal Dunes: Longitudinal Dunes are formed parallel to the wind movement. In other words, these dunes are formed by a modification the shape of the transverse in dunes. They are huge aeolian landforms extending hundreds of kilometers in length with average width of a kilometer or more and average height of several hundred meters. Wind ward slope of these dunes is gentle while the leeward slope is steep. These dunes are formed in the inner parts of the great deserts where high velocity winds are constant in direction or they change their direction seasonally.Thus, longitudinaldunes are generally formed in the heart of trade wind deserts.Great Vidya Prabodhini College of Commerce, Education, Computer& Management, Parvari-Goa 31  ) Lecture Notes ofGeography, T.Y.B.A.B.Ed. (Sem- V ,2023-24 Dr. SagarP. Mali systems of longitudinal sand dunes are found in Australia, Libyan, Saharan,South African, Thar desert (India). If the wind and moves violently, it carves out a channel through the is faster minor hollows and concavities on the leeward. The sand is trailed out along them and the transverse dunes are divided into two separate ridges parallel to the prevalent wind direction. They remain nearly fixed in their positions. In the Egyptians desert many seifs are about 100 meter high but in southern Iran they rise to more than 200 meter. Longitudinal dunes are separated by reg or hammada sand-free bare surfaces. The corridors so formed are called caravans. Wind 5.Transverse Dunes: Transverse Dunes are dunes deposited transverse to the prevailing wind direction.They are formed due to ineffective winds blowing along the coast and margin of the desert. They are not very common depositional features in extensive deserts. They appear as wave-like features. Their shape is asymmetrical and the gentler slope is on the windward side. On the leeward side there is usually a hollow or a concavity. They are big sized but small multitudes of them. They can't be formed by less sand. They are found in extensive desert areas for eg. Sahara, Kalahari or Australian desert. The entire area where transverse dunes are formned may be called as 'sand sea', because it resembles a storm tossed sea suddenly frozen to immobility. Deep depressions lie between the dune ridges. Transverse dune belts also form adjacent to beaches that supply abundant sand and have strong on shore winds.