NCERT Solutions for Class 11 Geography Chapter 6 - Landforms and their Evolution

Download PDF

91. Multiple choice questions.

(i) In which of the following stages of landform development, downward cutting is dominated?

(a) Youth stage

(b) Late mature stage

(d) Old stage

Ans. (a) Youth stage

(ii) A deep valley characterised by steep step-like side slopes is known as:

(a) U-shaped valley

(b) Gorge

(d) Canyon

Ans. (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

(d) Glacier region

Ans. (b) Limestone region

(iv) Which one of the following sentences best defines the term ‘Lapies’?

(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.

(d) An irregular surface with sharp pinnacles, grooves and ridges.

Ans. (d) An irregular surface with sharp pinnacles, grooves and ridges

(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

(d) Esker

Ans. (a) Cirque

92. Answer the following questions in about 30 words.

(i) What do incised meanders in rocks and meanders in plains of alluvium indicate?

Ans. The incised meanders in rocks and meanders in plains of alluvium indicate the status of original land surface over which the streams have developed.

(ii) Explain the evolution of valley sinks or uvalas.

Ans. When sink holes, comprising of surface run-off as re-emerged streams, and dolines join due to slumping of material or roof collapse of caves, long, narrow to wide trenches, uvalas, form.

(iii) Underground flow of water is more common than surface run-off in limestone areas. Why?

Ans. This is because surface as well as underground water actively work through the chemical processes of solution and precipitation deposition occurring exclusively or interbedded with rocks, forming a variety of landforms.

(iv) Glacial valleys show up many linear depositional forms. Give their locations and names.

Ans. Linear depositional forms of glacial valleys are as follows:

(a) Terminal Moraines: formed at the end of the glaciers.

(b) Lateral Moraines: formed parallel to the glacial valley.

(c) Ground Moraines: formed over valley floors.

(d) Eskers: flow over the ground with ice forming its banks.

(e) Outwash Plains: formed at the foot of the glacial mountains or beyond continental ice sheets.

(f) Drumlins: formed beneath heavily loaded ice through fissures.

(v) How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts?

Ans. Winds move along the desert floors with great speed, and turbulence caused by the obstruction in their path. They cause deflation by lifting and removal of dust and smaller particles of rocks, abrasion through transportation of sand and silt as effective tools and impact by the force of momentum. These actions create numerous erosional and depositional features in the deserts.

Winds are not the only agent responsible for the erosional features in the deserts. The running water sheet wash is as important for crosional in desert.

93. 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.

Ans. In humid regions, the two components of running water are the overland flow as a sheet on the land and linear flow as streams and rivers in valleys. Most of the erosional landforms are associated with vigorous and youthful rivers flowing over steep gradients. With time, stream channels over steep gradients turn gentler due to continued erosion and lose their velocity which facilitating active deposition of sediment and forming depositional forms associated with streams flowing over steep slopes. The friction of the column of flowing water removes the materials from the surface to the direction of flow, forming small and narrow rills developing into long and wide gullies which further deepen, widen, lengthen and unite to give rise to a network of valleys. During the terminal stage, running water makes deltas.

In arid regions, though rain is scarce in deserts but it comes down torrentially in a short span. Deserts are exposed to mechanical and chemical weathering due to drastic diurnal temperature changes as a result material decay faster and the weathered debris is easily removed by the wind as well as the rains/ sheet wash.

Thus, running water is the most dominating geomorphic agent in shaping the earth’s surface in humid as well as arid regions.

(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?

Ans. Limestones are permeable, thinly bedded, highly joined and cracked through which surface water is able to percolate well vertically. The water flows horizontally through the bedding planes, joints and materials. Such movement of water causes erosion in rocks. Physical (mechanical) removal of materials by moving groundwater is insignificant in developing landforms. That is why, the result of the work of groundwater seen in all type of rocks. But in rocks like timestone or dolomites the surface water as well as groundwater through the chemical process of solution develops variety of landform and process of solution and deposition is called karst topography.

In arid climates, water table is below the surface, thus, the amount of water differs in the two areas. Therefore, limestones behave differently in humid and arid climates.

The dominant and almost exclusive geomorphic process in limestone areas is the process of solution and deposition by the action of groundwater which develops many depositional landforms in the limestone areas such as – stalactites, stalagmites and pillars.

(iii) How do glaciers accomplish the work of reducing high mountains into low hills and plains?

Ans. Glaciers are masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough-like valleys which move slowly due to the force of gravity. The movement ranges from a few centimetres to metres a day. The glaciers cause tremendous erosion feature through friction created by the weight of ice. The material plucked from the land gets dragged along the floors of sides of the valleys causing great damage through abrasion and plucking. Glaciers are capable of causing significant damage to un-weathered rocks and can reduce high mountains into low hills and plains. As glaciers continue to move, debris get removed, divides get lowered and eventually the slope is reduced such that only a mass of low hills and vast outwash plains with other depositional features are left.