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The wind erosion is most predominant in arid regions. Since there is little vegetation or moisture to bind the loose surface materials, the effects of wind erosion are more pronounced in the desert region.

1. Delta : The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to depsoition 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 a large area 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. 

2. Oxbow lakes are also formed in the lower course of a river.

The cheif landforms resulting from the deposition of a river are the following : 

1. V-shaped Valleys : It is formed from a stream eroding downward, through a process, called down cutting. These valleys are formed in the initial stages of rivers and have steep slopes. 

2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I-shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

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 centre. The river also gets windened as a result of great volume of water and the impact of the load.

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 the 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.

Denudation is the wearing away of landmass by various processes like weathering, erosion mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levelling the peaks.

From the source to its mouth, a river is divided into three main sections –

(a) Upper Course, 

(b) Middle Course

(c) Lower Course. 

These three sections are also called the Profile of a River. 

 (a) Upper Course – In the upper course the dominant activity of a river is erosion. Many land forms 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 rush of water. 

(b) 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 centre. The river also gets windened as a result of great volume of water and the impact of the load. 

(c) Lower Course – The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to depsoition 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 a large area 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.

Waterfall : A waterfall is generally formed due to large differences in the rate of erosion. The smaller differences lead to formation of step-like features known as rapids. The water that falls down the edge of a hard resistant rock may have at the bottom a soft rock. It is this soft rock that gets eroded fast and creates a hollow basin called plunge pool. Some of the world’s highest and best known falls are Angel Falls (986 m) in Venezuela and Yosemite fall (778 m), California, USA.

The importance of weathering is explained as follows: 

1. In weathering, the rocks are broken up into small fragments, this facilitates soil formation. 

2. Weathering is responsible for erosion and mass movement. 

3. Depending on the depth of weathered materials, the growth of vegetation and bio-diversity are determined. 

4. Weathering is helpful in massive erosion and degradation of high reliefs. 

5. Topographies are the result of weathering and erosion. 

6. By the weathering and deposition of rocks, the concentration of valuable minerals like iron, manganese, aluminium, copper, etc takes place, by which their exploitation, processing and refinement become easy.

7. Weathering is an important process of soil formation.

Weathering is a process of breaking down of rocks, in which due to disintegration and decomposition, rocks become weak and start getting separated. 

Under weathering, the weakening of rocks takes place in two ways:

1. By mechanical fragmentation. 

2. By chemical decomposition.

The process of weakening down of rocks by physical factors like temperature, rainfall, air-pressure, etc. is called disintegration. This type of weathering is called Physical Weathering. The process of weakening down of rocks by chemical processes like oxidation, carbonation, hydration, etc. is called Chemical decomposition. This type of weathering is also called chemical Weathering.

The processes of physical and chemical weathering are different from each other, but both types of weathering are not independent of each other. The effects of factors that participate in physical and chemical weathering cannot be separated from each other. For example, temperature plays an important role in physical weathering, due to which the rocks undergo expansion and contraction and they become weak.

Weathering is completely influenced by chemical composition of rocks. On the basis of chemical composition, the ability to absorb heat in rocks is also affected. Similarly, water will not react with any rock, unless it receives heat by temperature or pressure. The processes of chemical weathering are not the same in all the heating boards. Chemical weathering is more active in tropical climatic regions where the temperature is high throughout the year.

Thus, it is clear that the physical and chemical weathering are not independent of each other and they are controlled by atmospheric conditions.

Weathering is a spatial process in which rocks disintegrate at their own place, but during this process many factors or components affect it. In these controlling factors, the structure and composition of rocks, slope of land, diversity of climate and vegetation are mainly included. 

The short description of all these factors/components is as follows:

1. Structure and Composition of Rocks: Chemical weathering is more in the rocks with higher porosity and soluble minerals. Mechanical weathering is more in the rocks with vertical base layers, and chemical weathering is more in the rocks with horizontal layers.

2. Slope of Land: Weathering is less in the land with gentle and low slope relatively to the land with steep slope. 

3. Diversity in Climate: Chemical weathering is more in humid tropics, while mechanical weathering is more in arid and tropical regions.

4. Influence of Vegetation: Vegetation partially is a factor of weathering and partially is a constraint too. The amount of weathering is also more in vegetation less tropical regions due to more insolation.

The fragmentation of rocks by the biotic community (vegetation, living – organisms and humans) found on the earth’s crust is called Biological weathering.

A type of erosive action in which pebbles, stones, boulders, rock-particles, etc. along with any factor of erosion, erode the land-surface by rubbing or scraping, is called Corrasion.

Bio – diversity represents the presence of various biological species found in a region on the earth’s surface. Weathering has a significant impact on bio – diversity. Bio – diversity is completely affected by vegetation. Rocks and minerals are trans – located by weathering and new surfaces are formed. This helps in the penetration of moisture and air in the surface by chemical process. Through this, the humus, carbonic and acidic materials enter into the soil, by which bio – diversity is affected. In this way, it can be said that weathering is responsible for bio – diversity on the earth’s surface.

In Denudation, the following processes are included: 

1. Weathering: It is a spatial process, in which the rocks break down by dis-integration and decomposition at their own place. This process is called Weathering.

2. Erosion: It is a dynamic process in which rocks keep on dis-integrating by being rubbed, eroded, trans – located or transported through glaciers, underground water, waves, wind and rivers. This process is called Erosion. 

3. Mass Translocation: Movement and transfer of rock debris in massive amount along the slope by gravitational force is called Mass Translocation.

The theory related to the Cycle of Erosion of Penck has the following drawbacks: 

1. Penck presented the theory of the Cycle of Erosion in German language. When it was translated from German to other languages (mainly in English), the meaning became distorted. 

2. The theory becomes non-interesting due to being obscure. 

3. Scholars do not agree on the concept of continuous movement in crust and the parallel withdrawal of slopes as presented by Penck.

The factors responsible for erosion include glacier, underground water, sea waves, wind and flowing water.

German scholar, Penck, classified his concept related to erosion into the following stages:

1. First Stage: According to Penck, in this stage, both the processes of upliftment and erosion move together. But the upliftment is more than the erosion. 

2. Second stage: In this stage, uplift and erosion remain equally active. As a result, the valleys start to become wide and deep. 

3. Third Stage: In this stage, due to competitive rate, the dorsal difference of the upper and lower curves of the uplift and erosion processes remains the same. 

4. Fourth Stage: In this stage, the rate of uplift gets inactive and weak, and the erosion is effective at the same rate. As a result, the valleys are deep and move down. 

5. Fifth Stage: In this stage, along with the uplift, the rate of erosion also gets inactive and weak. The dorsal difference of both the curves decreases. The concept of Penck related to erosion is being presented by the following diagram.

The two landforms resulting from erosion in the upper course of a river are the following : 

1. V-shaped Valleys : It is formed from a stream eroding downward, through a process, called down cutting. These valleys are formed in the initial stages of rivers and have steep slopes. 

2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I-shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

In the upper course the dominant activity of a river is erosion. Many land forms 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 rush of water.

The process of the erosion of rocks by the heavy pressure of river water or whirlpool is called Hydraulic Action.

Plunge Pool: Deep trough built in a base rock by the erosion caused by pebbles and stones present in the whirlpool, which fall down from a height with the waterfalls.

When the sand, dust or other unorganized rocks – flour are carried away by the winds from one place to another, this process is called Deflation.

Deflation Hollows : Deflation involves the lifting as well as the blowing away of loose materials from the ground. The sands and pebbles are carried in the air or driven along the ground. The finer dust and sands may be deposited even outside the desert margins. Deflation results in the lowering of the land surface to create depressions called deflation hollows.