NCERT Solutions for Class 11th: Ch 6 Landforms and their Evolution
Exercises
Page No: 73
1. Multiple choice questions.
(i) In which of the following stages of landform development, downward cutting is dominated?
(a) Youth stage
(b) Late mature stage
1. Multiple choice questions.
(i) In which of the following stages of landform development, downward cutting is dominated?
(a) Youth stage
(b) Late mature stage
(c) Early mature stage
(d) Old stage
► (a) Youth stage
(ii) A deep valley characterised by steep step-like side slopes is known as
(a) U-shaped valley
(b) Gorge
(ii) A deep valley characterised by steep step-like side slopes is known as
(a) U-shaped valley
(b) Gorge
(c) Blind valley
(d) Canyon
► (d) Canyon
(iii) In which one of the following regions the chemical weathering process is more dominant than the mechanical process?
(a) Humid region
(b) Limestone region
(iii) In which one of the following regions the chemical weathering process is more dominant than the mechanical process?
(a) Humid region
(b) Limestone region
(c) Arid region
(d) Glacier region
► (b) Limestone region
Page No. 74
(a) A small to medium sized shallow depression
(b) A landform whose opening is more or less circular at the top and funnel shaped towards bottom (c) A landform forms due to dripping water from surface
(d) An irregular surface with sharp pinnacles, grooves and ridges
► (a) A small to medium sized shallow depression
(v) A deep, long and wide trough or basin with very steep concave high walls at its head as well as in sides is known as:
(a) Cirque
(b) Glacial valley
(c) Lateral Moraine
(d) Esker
► (a) Cirque
(i) What do incised meanders in rocks and meanders in plains of alluvium indicate?
Answer
Answer
The incised meanders in rocks and meanders in plains of alluvium indicates the status of original land surfaces over which streams have developed.
Answer
Generally, the surface run-off simply goes down swallow and sink holes and flow as underground streams and re-emerge at a distance downstream through a cave opening. When sink holes and dolines join together because of slumping of materials along their margins or due to roof collapse of caves, long, narrow to wide trenches called valley sinks or Uvalas form.
Underground flow of water is more common than surface run-off in limestone areas because limestone is rich in calcium carbonate, the surface water as well as groundwater through the chemical process of solution and precipitation deposition, develop varieties of landforms. These two processes of solution and precipitation are active in limestones occurring either exclusively or interbedded with other rocks.
(v) How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts?
Answer
Winds also move along the desert floors with great speed and the obstructions in their path create turbulence. Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of rocks. In the transportation process sand and silt act as effective tools to abrade the land surface. The impact is simply sheer force of momentum which occurs when sand is blown into or against a rock surface. The 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 in the deserts. The rain or sheet wash is also important.
(ii) Limestones behave differently in humid and arid climates. Why? What is the dominant and almost exclusive geomorphic process in limestone areas and what are its results?
Answer
Limestones are permeable, thinly bedded and highly jointed and cracked therefore, the surface water
percolates well. After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves. This downward
and horizontal movement of water which causes the rocks to erode. Physical or mechanical removal of materials by moving groundwater is insignificant in developing landforms.
In arid climates, water table is below the surface therefore, there is less amount of surface water.
The amount of water differ in these two areas, therefore, limestones behave differently in humid and arid climates.
The dominant and almost exclusive geomorphic process in limestone is the processes of solution and deposition by the action of the groundwater. Many depositional forms develop within the
limestone caves. The depositional landforms in limestone areas by the action of ground water are stalctites, stalagmites and pillars.
(iii) How do glaciers accomplish the work of reducing high mountains into low hills and plains?
Answer
Masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough-like valleys are called glaciers. The movement of glaciers is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more. Glaciers move basically because of the force of gravity.
Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice. The material plucked from the land by glaciers get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking. Glaciers can cause significant damage to even un-weathered rocks and can reduce high mountains into low hills and plains.
As glaciers continue to move, debris gets removed, divides get lowered and eventually the slope is reduced to such an extent that glaciers will stop moving leaving only a mass of low hills and vast outwash plains along with other depositional features.
(ii) Explain the evolution of valley sinks or uvalas.
Answer
(iii) Underground flow of water is more common than surface run-off in limestone areas. Why?
Answer
Underground flow of water is more common than surface run-off in limestone areas because limestone is rich in calcium carbonate, the surface water as well as groundwater through the chemical process of solution and precipitation deposition, develop varieties of landforms. These two processes of solution and precipitation are active in limestones occurring either exclusively or interbedded with other rocks.
(iv) Glacial valleys show up many linear depositional forms. Give their locations and names.
Answer
Glacial valleys show up many linear depositional forms:
• Terminal moraines: formed at the end (toe) of the glaciers.
• Lateral moraines - formed along the sides parallel to the glacial valleys
• Ground moraines - many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors.
• Eskers - flow over the ground with ice forming its banks.
• Outwash Plains - The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets.
• Drumlins - form beneath heavily loaded ice through fissures in the glacier.
Glacial valleys show up many linear depositional forms:
• Terminal moraines: formed at the end (toe) of the glaciers.
• Lateral moraines - formed along the sides parallel to the glacial valleys
• Ground moraines - many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors.
• Eskers - flow over the ground with ice forming its banks.
• Outwash Plains - The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets.
• Drumlins - form beneath heavily loaded ice through fissures in the glacier.
(v) How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts?
Answer
Winds also move along the desert floors with great speed and the obstructions in their path create turbulence. Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of rocks. In the transportation process sand and silt act as effective tools to abrade the land surface. The impact is simply sheer force of momentum which occurs when sand is blown into or against a rock surface. The 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 in the deserts. The rain or sheet wash is also important.
3. Answer the following questions in about 150 words.
(i) Running water is by far the most dominating geomorphic agent in shaping the earth’s surface in humid as well as in arid climates. Explain.
Answer
In humid regions, There are two components of running water. One is overland flow on general land surface as a sheet. Another is linear flow as streams and rivers in valleys. Most of the erosional landforms made by running water are associated with vigorous and youthful rivers flowing along gradients. With time, stream channels over steep gradients turn gentler due to continued erosion, and as a consequence, lose their velocity, facilitating active deposition. Overland flow causes sheet erosion. Depending upon irregularities of the land surface, the overland flow may concentrate into narrow to wide paths. In the early stages, down-cutting dominates during which irregularities such as waterfalls and cascades will be removed. In the middle stages, streams cut their beds slower, and lateral erosion of valley sides becomes severe. During their terminal stages, the running water makes deltas.
(i) Running water is by far the most dominating geomorphic agent in shaping the earth’s surface in humid as well as in arid climates. Explain.
Answer
In humid regions, There are two components of running water. One is overland flow on general land surface as a sheet. Another is linear flow as streams and rivers in valleys. Most of the erosional landforms made by running water are associated with vigorous and youthful rivers flowing along gradients. With time, stream channels over steep gradients turn gentler due to continued erosion, and as a consequence, lose their velocity, facilitating active deposition. Overland flow causes sheet erosion. Depending upon irregularities of the land surface, the overland flow may concentrate into narrow to wide paths. In the early stages, down-cutting dominates during which irregularities such as waterfalls and cascades will be removed. In the middle stages, streams cut their beds slower, and lateral erosion of valley sides becomes severe. During their terminal stages, the running water makes deltas.
In arid regions, though rain is scarce in deserts, it comes down torrentially in a short period of
time. The desert rocks devoid of vegetation, exposed to mechanical and chemical weathering processes due to drastic diurnal temperature changes, decay faster and the torrential rains help in removing the weathered materials easily. The weathered debris in deserts is moved by not only wind
but also by rain/sheet wash.
Thus, Running water is by far the most dominating geomorphic agent in shaping the earth’s surface in humid as well as in arid climates.
(ii) Limestones behave differently in humid and arid climates. Why? What is the dominant and almost exclusive geomorphic process in limestone areas and what are its results?
Answer
Limestones are permeable, thinly bedded and highly jointed and cracked therefore, the surface water
percolates well. After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves. This downward
and horizontal movement of water which causes the rocks to erode. Physical or mechanical removal of materials by moving groundwater is insignificant in developing landforms.
In arid climates, water table is below the surface therefore, there is less amount of surface water.
The amount of water differ in these two areas, therefore, limestones behave differently in humid and arid climates.
The dominant and almost exclusive geomorphic process in limestone is the processes of solution and deposition by the action of the groundwater. Many depositional forms develop within the
limestone caves. The depositional landforms in limestone areas by the action of ground water are stalctites, stalagmites and pillars.
(iii) How do glaciers accomplish the work of reducing high mountains into low hills and plains?
Answer
Masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough-like valleys are called glaciers. The movement of glaciers is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more. Glaciers move basically because of the force of gravity.
Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice. The material plucked from the land by glaciers get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking. Glaciers can cause significant damage to even un-weathered rocks and can reduce high mountains into low hills and plains.
As glaciers continue to move, debris gets removed, divides get lowered and eventually the slope is reduced to such an extent that glaciers will stop moving leaving only a mass of low hills and vast outwash plains along with other depositional features.