Lithosphere – Ii Exogenetic Processes

Weathering is the disintegration, breaking down and decomposition of components in the earth’s crust through the action of natural or other agents. Weathering can occur due to exposure to the earth’s atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earth’s crust. Weathering can be physical, chemical or biological.

Gradation is the process of levelling up and formation of land surface by the operation of natural agents like air, water, glaciers or sea waves. The action of these agents results in the formation of various geographical features and structures with the passage of time. It is a highly time-consuming process and can result in the formation of sea cliffs, canyons, waterfalls and various other landforms. It occurs because of the combined action of erosion, transportation and deposition.

The lower course marks the journey of river mainly in the plains loaded with sediments, minerals and huge amounts of debris. This debris is deposited and accumulated in the low-lying river beds resulting in the formation of very fertile soil. This deposition and accumulation of components result in the formation of deltas. Typically, deltas will be triangular shaped that is formed at the low-lying mouth of rivers. It will contain huge volumes of mineral and sediment deposits. The Ganga-Brahmaputra delta is the largest in the world.

The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. The Nullarbor in the Great Australian Coast is the world’s largest karst area.

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Cirque is not a depositional feature of the glacier. It is an erosional feature that erodes and moves through the steep areas and cliffs of the mountains to form a bowl-shaped depression in the surface. Thus it is an erosional feature resulting in the movement of glaciers to places in which later they are deposited.

Erosion and deposition are also caused by the actions of wind. Many geographical features are created by the actions of wind. Loess refers to the deposition and accumulation of the fine silt and porous sand over the regions by the action of the blowing winds. It is largely found in the deserts and arid regions. The loess plateau in China has the densest deposit of loess in the world.

Erosion and deposition are also caused by the actions of ocean waves. It is an important agent for the creation of many geographical features. Stacks are an important feature as a result of the erosion by sea waves. Stacks are the remaining parts of the sea arch that has been completely collapsed by the action of the waves. The remaining seaward portion of the arch in the upper part is the stack. The Old Man of Hoy in Scotland is an important sea stack.

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Cirque is not a depositional feature of the glacier. It is an erosional feature that erodes and moves through the steep areas and cliffs of the mountains to form a bowl-shaped depression in the surface. Thus it is an erosional feature resulting in the movement of glaciers to places in which later they are deposited.

Second order landforms are the intermediate and transitional landforms. It includes the continental rises, slopes, ridges, canyons and trenches in the seabeds and continental masses, mountains, plains and plateaus in the in the surface lands. The Deccan plateau is the apt example of second order landform. It is the largest plateau in India.

Explanation

a. The lower course marks the journey of river mainly in the plains loaded with sediments, minerals and huge amounts of debris. This debris is deposited and accumulated in the low-lying river beds resulting in the formation of very fertile soil. This deposition and accumulation of components result in the formation of deltas. Typically, deltas will be triangular shaped that is formed at the low-lying mouth of rivers. It will contain huge volumes of mineral and sediment deposits. Here the main river will get split into a number of different river channels known as distributaries.

b. Erosion and deposition are also caused by the actions of wind. Many geographical features are created by the actions of wind. Mushroom rocks or pedestal rocks are an important feature caused by the erosive action of the winds. It is basically formed in rocks having a very soft base. In such situations, the wind erodes the soft layers of the base resulting in its erosion. The bottom layers are eroded resulting in the formation of a structure having a mushroom or pedestal-like structure in the top leaving its base like a stand to support the top.

c. Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Eskers are the depositional feature of glaciers consisting of boulders, gravel and sand that is deposited by the melting water in the glaciers. The melting glacier deposits the components that it carries leading to the formation of eskers.

d. The underground water is the major reason for the formation of the karst topography. Karst is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. Stalactites are an important form of karst feature. It is formed by the gradual dripping and evaporation of water having calcite components in the caves. The calcite components after evaporation of water hang down from the ceiling forming stalactites.

e. Erosion and deposition are also caused by the actions of ocean waves. It is an important agent for the creation of many geographical features. Cliffs are an important feature as a result of the erosion by sea waves. They are formed by the forceful dashing and blowing off the sea waves against the adjacent rocks. This results in the formation of steep vertical walls called cliffs.

Weathering is the disintegration, breaking down and decomposition of components in the earth's crust through the action of natural or other agents. Weathering can occur due to exposure to the earth's atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earth's crust. Glacier, wind, rivers, ocean waves and groundwater are the major agents of weathering. Weathering can be physical, chemical or biological.

Weathering is the disintegration, breaking down and decomposition of components in the earth's crust through the action of natural or other agents. Weathering can occur due to exposure to the earth's atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earth's crust. Glacier, wind, rivers, ocean waves and groundwater are the major agents of weathering. Weathering can be physical, chemical or biological.

Biological weathering occurs because of the actions of the flora, fauna and human interferences. It results in the decomposition of the components due to the biological processes involved with the expansion of the activities of the living organisms. The growth of plants, the penetration of their roots, diffusion of rotten plant and animal cells, burrowing of animals and many other human interferences are the major causes of biological weathering.

The path in which a river flows from the point of origin to its destination is called the course. It can be mainly divided into three:

• Upper course- It is the starting point of the journey of the river. In this plane, the river falls down from the steep mountain slopes and cliffs. Higher the steep of the mountain, higher will be the speed and pace of the fall and greater will be the force to broaden and extend the river valley resulting in erosion of many elements. V-shaped valleys, gorges, waterfalls, canyons and rapids are the major features associated with this course.

• Middle course- In the middle course, the river pass through the sloping plains and valleys along with the particles and components like sand, slits, sediments, minerals and other elements. Actions of erosion and transportation take place in the middle course. The river widens and reduces in speed because of the particles that it carries. Alluvial fans, floodplains, meanders and ox-bow lakes are the major features associated with this course.

• Lower course- The lower course marks the journey of river mainly in the plains loaded with sediments, minerals and huge amounts of debris. This debris is deposited and accumulated in the low-lying river beds resulting in the formation of very fertile soil. Here the main river will get split into a number of different river channels known as distributaries. This deposition and accumulation of components result in the formation of deltas and estuary.

The course of the river results in the formation of different geographical features. An oxbow lake is an important feature formed in the middle course of the river. In the middle course, the river passes through the sloping plains and valleys along with the particles and components like sand, slits, sediments, minerals and other elements. Actions of erosion and transportation take place in the middle course resulting in the bending and twisting of the river to form meanders.

With further deposition, erosion and re-deposition, the meanders become a complete circle. The river will become much narrower with a circular surface resulting in the formation of lakes. Such features are known as ox-bow lakes. Lake Chicot in the USA is the world's largest ox-bow lake.

The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. It results in the formation of stalactites, stalagmites, columns and pillars. The major karst areas in India are:

• Guptadham caves in Western Bihar

• Robert caves and Tapkeshwar temples in Uttarakhand

• Pandav caves Pachmarhi in Madhya Pradesh

• Kutumsar in Chattisgarh

• Borra caves of Vishakapatnam

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Hanging valleys are the result of erosional glacial action. It is because of the erosion of the main valley by the tributary glacier. It occurs when the main valley is broadened and extended by the erosion of the glacier. It leaves the other side of the main valley cut-off from its other side resulting in hanging valleys. It results in the formation of the raid and steep waterfalls.

I. Moraine

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Moraines are the result of depositional glacial action. It is formed by the deposition of the continental glaciers or valley glaciers along with the particles and debris eroded by the glacier. There are different types, sizes and shapes of moraine- ground moraine, lateral moraine, terminal moraine, medial moraine and recessional moraine.

II. Drumlin

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Drumlins are the result of depositional glacial action. They are the result of the formation of moraines. They are the debris and particles carried by the moraines. They are the drawn-out hills of rock, sand, boulders and other elements formed with the moving glaciers. It mainly consists of glacial deposits.

III. Esker

Erosion and deposition are also caused by the actions of glaciers. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Eskers are the depositional feature of glaciers consisting of boulders, gravel and sand that is deposited by the melting water in the glaciers. The melting glacier deposits the components that it carries leading to the formation of eskers.

Erosion and deposition are also caused by the actions of the wind that is blown in the earth's surface. It can result in the formation of geographical features though erosion, transportation and deposition. Many geographical features are created by the actions of wind. Some of them are:

EROSIONAL FEATURES

• Mushroom rocks or pedestal rocks- They are an important feature caused by the erosive action of the winds. It is basically formed in rocks having a very soft base. In such situations, the wind erodes the soft layers of the base resulting in its erosion. The bottom layers are eroded resulting in the formation of a structure having a mushroom or pedestal-like structure in the top leaving its base like a stand to support the top. These are found in Jodhpur, Rajasthan.

• Inselberg- They are an important feature caused by the erosive action of the winds. These are formed by the action of wind in rocks like igneous rocks that are more resistant to the action of wind. Since these rocks have more resistance against the winds, they stand rising above the surroundings forming abrupt hills and cliffs. Uluru rocks in Australia is a prominent inselberg.

• Yardang- They are an important feature caused by the erosive action of the winds. It is basically formed in rocks having a vertical soft base. In such situations, the wind erodes the vertical soft layers of the base resulting in its erosion. These soft layers get eroded leaving crests and summits. Xinjiang is a significant yardang in China.

DEPOSITIONAL FEATURES

• Loess- Loess refers to the deposition and accumulation of the fine silt and porous sand over the regions by the action of the blowing winds. It is largely found in the deserts and arid regions. The loess plateau in China has the densest deposit of loess in the world.

• Sand dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. With the actions of wind huge amounts of sand gets carried and deposited in different places. These mounds and piles of sand deposited in different places are called sand dunes. Dune 7 in Namibia is the world’s largest sand dune.

• Barchan- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are the hemispherical and crescent-shaped dunes that have a gentle upward slope on the windward side and sharp vertical slope on the other leeward side. It is a common feature in the Sahara desert in Egypt.

• Transverse dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are asymmetrical, lopsided and disjoint in nature. They are formed by the action of the subsequent blowing of fast and slow winds of the same direction. They are largely found in Medano Creek in the U.S.

• Longitudinal dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are symmetrically formed dunes in the same direction as the wind blow. They are mostly even, regular and proportional. They are most common in Sahara desert.

Erosion and deposition are also caused by the actions of ocean waves. It is an important agent for the creation of many geographical features. Wave-cut platforms are an important feature as a result of the erosion by sea waves. The flat and level surface formed at the base of the sea cliffs by the action of waves are wave cut platforms. They are also called beach, shoreline, ledge and coastline.

Weathering is the disintegration, breaking down and decomposition of components in the earth’s crust through the action of natural or other agents. Weathering can occur due to exposure to the earth’s atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earth’s crust. Weathering can be physical, chemical or biological.

Chemical weathering is the decomposition of components in the earth’s crust through chemical reactions. It changes the chemical composition of rocks. Oxygen, carbon dioxide and hydrogen are the agents of decomposition. Oxidation, carbonation, solution and hydration are the different types. Chemical weathering is usually common in places with very hot and humid climates. It occurs in the climatic conditions of the hot and humid regions of the equators, tropics and sub-tropics. The action of the constant availability of water and the rising temperature is responsible for the faster weathering of rocks compared to other regions. The solution and hydration methods of chemical weathering will be most common in such climates. The reaction between the atmospheric humidity and other elements are responsible for the higher rate of weathering.

Delta is the flat, low-lying area formed at the mouth of the river. The deposition and accumulation of components, sediments, minerals and huge amounts of debris results in the formation of deltas. The result is the formation of fertile lands. They are formed in areas having low tides and plain surface. Thus, the absence of rapid tidal actions and the topography of the land makes the deposition and accumulation of silt in the delta.

An estuary is a water body that is formed when the river meets the sea. It does not carry, accumulate or deposit any components because of the constant sea waves. They do not create any fertile lands. Rather the area is more suitable for fishing activities. They are formed in areas with high tides and valleys. Thus the slit and other components they carry are not deposited at its mouth because of the constant actions of the sea waves. The constant lashing of the high tide and the resultant environment does not make the conditions apt for the deposition of silt.

Snowline is the line demarcating between the snow-covered and snow-free areas. It is the line up to which the level of snow extends given a particular point of time. It is the amount of permanent snow. The quantity of snowfall, steepness of the topography, exposure of the area to sun, winds, their speed and velocity are some of the factors that affect the snowline.

The distance of the region from the equator also affects the determination of snow line. It is approximately 4500 m above the sea level at the equator. As one move towards the poles, the snow line at first increases and them falls steadily while approaching the polar region. This is because of the area being constantly covered with snow and with much lesser exposure to the sunlight. This makes the level of snow line near the sea level at the poles.

Erosion can also be caused by the actions of the wind that is blown in the earth’s surface. It can result in the formation of geographical features though erosion, transportation and deposition. Wind can erode the rocks from nearly all the sides and create various types of geographical features.

Wind can easily erode rocks having a very soft base resulting in the formation of mushroom/pedestal rocks. In such situations, the wind erodes the soft layers of the base resulting in its erosion. The bottom layers are eroded resulting in the formation of a structure having a mushroom or pedestal-like structure in the top leaving its base like a stand to support the top. Inselberg is another important feature caused by the erosive action of the winds. These are formed by the action of wind in rocks like igneous rocks that are more resistant to the action of wind. Since these rocks have more resistance against the winds, they stand rising above the surroundings forming abrupt hills and cliffs. The feature yardang is basically formed in rocks having a vertical soft base. In such situations, the wind erodes the vertical soft layers of the base resulting in its erosion. These soft layers get eroded leaving crests and summits.

In regions rich in limestone, the erosive actions of the underground water with the limestone may result in the development of karst regions. The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. Thus the surface run-off is much lesser in the regions.

Weathering is the disintegration, breaking down and decomposition of components in the earth's crust through the action of natural or other agents. Weathering can occur due to exposure to the earth's atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earth's crust. The main types of weathering can be classified into three. They are:

• Physical weathering- It is the disintegration and breaking down of components in the earth’s crust through the action of physical forces. It does not change the chemical composition of rocks. It occurs due to the changes in temperature during the day and the night in cold and dry places. Water, wing, pressure and other physical factors are the agents of disintegration. It can be again classified into different types. They are:

 Exfoliation- It is caused by the friction developing between the rocks when they are exposed alternately to heat and cold temperatures. The rocks disintegrate layer-by-layer with the constant changes in the heat resulting in the weathering of rocks. It can either happen through sheeting or shattering.

 Granular disintegration- It occurs when the changes in temperature and humidity react on crystalline rocks. This action causes the particles of the rocks to fall out from the surface due to the pressure exerted by the constant reaction with the changes in temperature and frost.

 Block disintegration- It is caused by the friction developing between the rocks when they are expanding and contracting during the day and the night. This causes strain and pressure in the joints and hinges of the resulting in their weathering and disintegration.

• Chemical weathering- It is the decomposition of components in the earth's crust through chemical reactions. It changes the chemical composition of rocks. Oxygen, carbon dioxide and hydrogen are the agents of decomposition. Chemical weathering is usually common in places with very hot and humid climates. It occurs in the climatic conditions of the hot and humid regions of the equators, tropics and sub-tropics. The action of the constant availability of water and the rising temperature is responsible for the faster weathering of rocks compared to other regions. The reaction between the atmospheric humidity and other elements are responsible for the higher rate of weathering. It can be further classified into different types. They are:

 Oxidation- This majorly occurs in ferrous rocks containing iron. The iron in these rocks reacts with the oxygen in the atmosphere resulting in the formation of iron oxide. This chemical reaction decomposes the rocks from within and results in its weathering.

 Carbonation- This mainly occurs in carbonate rocks. It is the reaction between the water molecules and the carbon dioxide in the atmosphere resulting in the formation of carbonic acids. The reaction of the carbonic acid with the carbonate rocks decomposes the rocks from within and results in its weathering. It basically occurs in the limestone region in the formation of caves.

 Solution- This is the reaction of water with the particles and substances found in the rocks. The action of water in rocks results in breaking down of its particles from within and results in its weathering.

 Hydration- This is the reaction some chemical compounds of the rocks with water. When these chemicals are exposed to water, they result in the swelling and enlarging of the minerals in the rocks. This decomposes the rocks from within and results in its weathering.

• Biological weathering- Biological weathering occurs because of the actions of the flora, fauna and human interferences. It results in the decomposition of the components due to the biological processes involved with the expansion of the activities of the living organisms. The growth of plants, the penetration of their roots, diffusion of rotten plant and animal cells, burrowing of animals and many other human interferences are the major causes of biological weathering.

The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. The Nullarbor in the Great Australian Coast is the world’s largest karst area.

Most of the weathering and erosion occurs through chemical weathering. The solution is the main method of decomposition. Water reacts with the particles and substances found in the rocks, resulting in the breaking down of its particles from within. The reaction between the water molecules and the carbon dioxide in the atmosphere resulting in the formation of carbonic acids. The reaction of the carbonic acid with the carbonate rocks decomposes the rocks and results in its weathering. It basically occurs in the limestone region in the formation of caves.

Some of the erosional features of underground water are:

• Terra Rossa- This majorly occurs in ferrous rocks containing iron. The iron in these rocks reacts with the oxygen in the atmosphere resulting in the formation of iron oxide. This gives a red colour to the clay soil. When the limestone in the rocks dissolves in the water, it results in the deposition and accumulation of red clay in the surface resulting in the formation of clayey to filthy soil known as terra rossas. It is mostly found in the Mediterranean regions.

• Lappies- When the limestone in the rocks dissolves and decomposes in the water, it results in the formation of huge and big troughs and furrows. These channels of furrows are known as lappies.

• Sinkhole- When the limestone in the rocks dissolves and decomposes in the water, it results in the formation of huge and big funnels and shafts in the surface. They can be very deep with depth ranging up to nine meters.

• Caves and caverns- The reaction between the water molecules and the carbon dioxide in the atmosphere resulting in the formation of carbonic acids. When the carbonic acids react with limestone, huge hollows and caves called caves are formed. Caverns are also caves having irregular and asymmetrical floors.

Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Erosion and deposition are also caused by the actions of glaciers. It is also known as the river of ice. Glaciers can be divided into two types. They are:

• Continental glacier- It is formed in regions having a cold climate over years. It covers broad and vast areas. They are formed because of their longitudinal location. The pace of erosional and depositional activities are much lesser.

E.g. the Himalayas

• Valley glacier- It originates from snow-capped mountain ranges. It covers only narrow and small areas. They are formed due to altitudinal and longitudinal reasons. Erosional and depositional activities occur more rapidly resulting in the formation of a diverse environment.

E.g. Greenland

Erosion and deposition are also caused by the actions of the wind that is blown in the earth's surface. It can result in the formation of geographical features though erosion, transportation and deposition. Many geographical features are created by the actions of wind. Some of the depositional works of the wind are:

• Loess- Loess refers to the deposition and accumulation of the fine silt and porous sand over the regions by the action of the blowing winds. It is largely found in the deserts and arid regions. The loess plateau in China has the densest deposit of loess in the world.

• Sand dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. With the actions of wind huge amounts of sand gets carried and deposited in different places. These mounds and piles of sand deposited in different places are called sand dunes. Dune 7 in Namibia is the world's largest sand dune.

• Barchan- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are the hemispherical and crescent-shaped dunes that have a gentle upward slope on the windward side and sharp vertical slope on the other leeward side. It is a common feature in the Sahara desert in Egypt.

• Transverse dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are asymmetrical, lopsided and disjoint in nature. They are formed by the action of the subsequent blowing of fast and slow winds of the same direction. They are largely found in Medano Creek in the U.S.

• Longitudinal dunes- It is the deposition action of the wind. It is formed mainly from the sands carried during sandstorms in deserts. They are symmetrically formed dunes in the same direction as the wind blow. They are mostly even, regular and proportional. They are most common in Sahara desert.

• First order landforms- They are the coarsest and the roughest level of landforms. It includes the continents and the ocean basins that comprise that largest area and volume in the earth’s surface.

• Second order landforms- They are the intermediate and transitional landforms. They are associated with their tectonic structures by the changes in the endogenic forces. It includes the continental rises, slopes, ridges, canyons and trenches in the seabeds and continental masses, mountains, plains and plateaus in the in the surface lands.

• Third order landforms- They are the most detailed, thorough and the newest order of landforms. They are mostly created by the erosive and depositional activities on the earth's surface. They are associated with the exogenic forces. It includes islands, coastlands, beaches, coral reefs, sand dunes and caves.

The river falls down from the steep mountain slopes and cliffs. Higher the steep of the mountain, higher will be the speed and pace of the fall and greater will be the force to broaden and extend the river valley resulting in erosion of many elements. V-shaped valleys, gorges, waterfalls, canyons and rapids are the major features associated with this course. Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Erosion and deposition are also caused by the actions of glaciers. It is also known as the river of ice. U-shaped valley is the valley that is formed by the erosive action of glaciers. It occurs when the glaciers move down a river valley.

Gradation is the process of levelling up and formation of land surface by the operation of natural agents like air, water, glaciers or sea waves. Running water and rives are very important agents of gradation. The extent of the action of rivers depends upon many factors. The volume of water in the river, its velocity and speed, the gradient and slope of the land, the number of sediments and particles carried by the river are some of the factors determining the flow of the river.

The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. Most of the weathering and erosion occurs through chemical weathering. The solution is the main method of decomposition. Water reacts with the particles and substances found in the rocks, resulting in the breaking down of its particles from within. The reaction between the water molecules and the carbon dioxide in the atmosphere resulting in the formation of carbonic acids. The reaction of the carbonic acid with the carbonate rocks decomposes the rocks and results in its weathering. It basically occurs in the limestone region in the formation of caves. Thus, limestone regions have less surface run-off and underground water because of these reactions.

Yes, the wind is the most prominent gradient agent in the deserts. Erosion and deposition are also caused by the actions of wind. Many geographical features are created by the actions of wind. Besides winds, underground and running water can be considered as another agent. But it is rare for water to be an important agent. Since the land is highly arid and the presence of other agents like water, ice is very sporadic, wind can be considered as the only agent causing erosion and deposition in deserts.

In regions rich in limestone, the erosive actions of the underground water with the limestone may result in the development of karst regions. The underground water is the major reason for the formation of the karst topography. It is formed due to the dissolution of limestone, dolomite and gypsum with the groundwater. It is created because of the gradation, erosion and deposition of these soluble rocks with the groundwater. Thus the surface run-off is much lesser in the regions.

The river falls down from the steep mountain slopes and cliffs. Higher the steep of the mountain, higher will be the speed and pace of the fall and greater will be the force to broaden and extend the river valley resulting in erosion of many elements. V-shaped valleys, gorges, waterfalls, canyons and rapids are the major features associated with this course. In the middle course, the river passes through the sloping plains and valleys along with the particles and components like sand, slits, sediments, minerals and other elements. Actions of erosion and transportation take place in the middle course. The river widens and reduces in speed because of the particles that it carries. Alluvial fans, floodplains, meanders and ox-bow lakes are the major features associated with this course. The lower course marks the journey of river mainly in the plains loaded with sediments, minerals and huge amounts of debris. This debris is deposited and accumulated in the low-lying river beds resulting in the formation of very fertile soil. Here the main river will get split into a number of different river channels known as distributaries. This deposition and accumulation of components result in the formation of deltas and estuary. Thus, the silt, sediments and other particles carried by the river is responsible for its widening in the lower course.

i. Ganga-Brahmaputra delta (Asia) and Delta of Nile (Egypt)

ii. Nullarbor in the Great Australian Coast

iii. Hot desert- Thar Desert (Asia) and Sahara desert (Africa)

Cold desert- Ladakh (Asia) and Gobi desert (Asia)

iv. Greenland (North America)

Carbonation

Explanation.

This mainly occurs in carbonate rocks. It is the reaction between the water molecules and the carbon dioxide in the atmosphere resulting in the formation of carbonic acids. The reaction of the carbonic acid with the carbonate rocks decomposes the rocks from within and results in its weathering. It basically occurs in the limestone region in the formation of caves.

Wave-cut platforms

Explanation

Erosion and deposition are also caused by the actions of ocean waves. It is an important agent for the creation of many geographical features. Wave-cut platforms are an important feature as a result of the erosion by sea waves. The flat and level surface formed at the base of the sea cliffs by the action of waves are wave cut platforms. They are also called beach, shoreline, ledge and coastline.

Gradation

Explanation

Gradation is the process of levelling up and formation of land surface by the operation of natural agents like air, water, glaciers or sea waves. The action of these agents results in the formation of various geographical features and structures with the passage of time. It is a highly time-consuming process and can result in the formation of sea cliffs, canyons, waterfalls and various other landforms. It occurs because of the combined action of erosion, transportation and deposition. It is the combined action of erosion, transportation and deposition.

Snowline

Explanation

Snowline is the line demarcating between the snow-covered and snow-free areas. It is the line up to which the level of snow extends given a particular point of time. It is the amount of permanent snow. The quantity of snowfall, steepness of the topography, exposure of the area to sun, winds, their speed and velocity are some of the factors that affect the snowline.

U-shaped valley

Explanation

Glaciers are huge masses of snow that is transported, accumulated and deposited in other regions due to the action of natural agents. Erosion and deposition are also caused by the actions of glaciers. It is also known as the river of ice. U-shaped valley is the valley that is formed by the erosive action of glaciers. It occurs when the glaciers move down a river valley.