Maharashtra Board Class 9 Geography Chapter 4 Exogenetic Processes Part II PDF Download

Exogenetic Processes Part II

Exogenetic Processes

We have learnt in the previous lesson that the earth's surface wears away because of erosion. The eroded material gets carried away by agents. When the speed of the agent reduces, the materials get deposited.

The agents like running water (river), glaciers, wind, sea waves and groundwater do the work of erosion, transportation and deposition. Because of these agents, the earth's surface keeps on undergoing changes and new landforms are formed. We will study some of these landforms in this lesson.

Work of Rivers and Landforms

How will you differentiate between a rill, gully, stream and a river?

What is a river?

Geographical Explanation

Running water flows naturally in a direction according to gravity along the slope and makes its own way. This is called a flow of water. When many such flows of water come together, a river is formed.

Factors Affecting River Work

The slope of the land, the type of rock, volume of water flowing in the river and the length of the flow, volume of sediments in the river, etc., are the factors on which the erosional, transportational and depositional work of the rivers depend.

Erosional Work of Rivers

The rivers originate at a much higher altitude from the sea level. Here, the river flows at a great speed and therefore, its power to erode is great. The riverbed and the river banks get eroded because of the fast flow of water, sand particles, pebbles, etc. and the various tributaries joining the main river. All these lead to the formation of gorges (canyons), V-shaped valleys and waterfalls.

Teacher's Note

Rivers are like giant cutters that shape our land. In India, the Ganges River creates many such shapes as it flows from mountains to the sea.

Exam Trick

Remember: Fast river = Erosion. Slow river = Deposition. Just like a fast car cuts through mud, fast water cuts through rocks.

Points to Remember

Rivers flow from high to low places.
Fast-flowing rivers erode rocks and soil.
Rivers create V-shaped valleys and gorges.
Tributaries join the main river and help erode more.
Erosion happens more in hilly areas.

Transportation and Deposition by Rivers

A river flows down the slope from a hilly region. At the foothills, the change in the slope causes deposition of coarse sediments. As these are deposited in a triangular shape, they form an alluvial fan. As the steepness of the slope decreases and the transporting capacity of the river reduces, it starts flowing slowly. It bends (meanders) often in its way in an effort to cross even small obstacles. By the time the river reaches the sea, its riverbed becomes very wide and its speed becomes very slow. The sediments of the river get deposited in its bed and on its banks.

The factors that determine the deposition of sediments are the length of the rivers, volume of water, amount of sediments, and the slope of the river and the earth's surface. Thus, landforms like flood levees, flood plains and deltas are formed because of deposition of sediments.

Try to understand the formation of landforms like gorges, V-shaped valleys, waterfalls, alluvial fans, meanders, flood levees, flood plains and deltas with the help of teachers.

Teacher's Note

In the monsoon season, rivers carry lots of mud and sand. This is why flood plains near rivers get very rich soil for farming.

Exam Trick

Remember: Delta = river mouth = triangular shape. The Ganges delta is the biggest delta in India and gives very fertile land.

Points to Remember

Slow rivers drop heavy sediments as alluvial fans.
Meanders are S-shaped bends in slow rivers.
Flood plains form next to rivers when floods happen.
Deltas form where rivers meet the sea.
All these landforms are from deposition work.

Work of Glaciers and Landforms

In regions where the temperatures are generally below freezing point, precipitation is in the form of snowfall. Layers of snow accumulate on the earth's surface because of snowfall. The heavy weight of these overlying layers makes the snow move along the slope. At the base of the layer, the snow starts melting because of the friction and the pressure from above. Glacier starts moving slowly along the slope.

Like the river, a glacier too carries out the work of erosion, transportation and deposition.

Erosion Work by Glaciers

Though its velocity is less, the mass of the ice is more and hence the glacier erodes its own banks and its bed on a large scale. The erosional work of glaciers produces landforms like cirques, arêtes, horns, U-shaped valleys, hanging valleys and rôche moutonnées (or sheepbacks).

Transportation and Deposition by Glaciers

The glaciers carry sediments with them. These sediments are called moraines. Depending on the location of the deposits, moraines can be divided into 4 types: ground moraines, lateral moraines, medial moraines and terminal moraines. The depositional work of glacier produces landforms like drumlins, eskers, etc.

Teacher's Note

Glaciers are like slow rivers of ice. In the Himalayas in India, glaciers are melting because of climate change and this affects water supply to many cities.

Exam Trick

Remember: Four types of moraines = Four sides. Terminal at the end, lateral on sides, medial in middle, ground at bottom. Like a sandwich with layers!

Points to Remember

Glaciers move very slowly, even slower than you can see.
Glaciers are heaviest erosion agents because of their huge mass.
U-shaped valleys are made by glaciers.
Moraines are piles of rock carried by glaciers.
Glaciers work only in very cold places.

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