Geography [4 Mark Question Answer]

The following two types of sand dunes are seen

1. Barchan: These are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction. They are found in the deserts of the Sahara. These are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks. These occur transversely to the wind so that their horns thin out and become lower in the direction of the wind.
2. Longitudinal Dunes or Seifs: These are long and narrow sand-ridges which grow parallel to the direction of the prevailing wind. They are found in the interior parts of deserts. In the Libyan desert, they are known as Seifs (means ‘sword’ in Arabic). In the Thar Desert, they are confined to the western margin, where the force of south-west monsoon influences their formation. An important feature of such a dune is that in its crest line there are rise and fall patterns. Seifs are found in the Sahara, Iran, Thar Desert (India), and West Australia. These sand dunes are stationary and they remain in a fixed position.

Winds move along the desert floors with great speed and the obstructions in their path create turbulence. Storm winds are formed which are very destructive. Winds cause deflation, abrasion, and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of the rocks. The impact is simply the shear force of momentum which occurs when sand is blown into or against a rock surface. These are the ways in which wind action creates a number of interesting erosional and depositional features in the deserts.
Winds are not the only agent responsible for the erosional features of the deserts. Along with winds, the running water is also important for this.

From the source to its mouth, a river is divided into three main sections – Upper Course, Middle Course, and Lower Course. These three sections are also called the Profile of a River.

• Upper Course – In the upper course, the dominant activity of a river is erosion. Many landforms are shaped by this activity of the river. Along its course, it erodes bed and banks and carries large boulders of rock with it. Under the impact of the rush of water.
• Middle Course – At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountains area and enters the plains. In the middle course, the energy required to transport the materials is just enough to drag large particles. Fine particles remain suspended in water, large particles are just dragged along the bed of the river and smaller ones are rolled down in the center. The river also gets widened as a result of the great volume of water and the impact of the load.
• Lower Course – The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to the deposition of sediments over a large area near the mouth of rivers. Such sedimentation occurs on the sides of the stream, at the mouth, in the front part as well as in the bed of the river. The deposition over large areas results in a triangular-shaped formation called delta. It resembles the Greek letter A The Ganga -Brahmaputra delta is growing towards the sea. It is also one of the largest in the world. Oxbow lakes are also formed in the lower course of a river.

Various factors affect erosion, transportation, and deposition work of a river. Important factors include the following :

1. The velocity of water: Erosion and transportation are both maximum when velocity is high. Deposition, on the other hand, takes place when the velocity of water is low.
2. The volume of water: The larger the volume of water, the greater is the power of erosion and transportation. Deposition, on the other hand, takes place better when there is a reduced volume of water as well as when the load is greater.
3. Load: Load is the material transported by a river. A load of a river does all the work of erosion and deposition. Mass wasting, deepening of river valleys and formation of landforms depend on the load a river carries.