NCERT Solutions for Class 11 Geography Chapter 7 Landforms And Their Evolution

Ans : (i) (a) Youth stage
(ii) (d) Canyon
(iii) (b) Limestone region
(iv) (a) A small to medium sized shallow depression
(v) (a) Cirque 

Ans : (i) The incised meanders in rocks and meanders in plains of alluvium indicates the status of original land surfaces over which streams have developed.


(ii) Generally, the surface run-off simply goes down swallow and sinkholes and flow as underground streams and re-emerge at a distance downstream through a cave opening. When sinkholes 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.


(iii) Underground flow of water is more common than surface runoff in limestone areas because
limestone is rich in calcium carbonate, the surface water as well as ground'.water through the chemical process of solution and precipitation deposition, develop varieties of landforms. These 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:
 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) 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. 

Ans : (i) In humid regions, There are 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 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 ground'.vater 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 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 ground'.vater. Many depositional forms develop within the limestone caves. The depositional landforms in limestone areas by the action of groundwater are stalactites, stalagmites and pillars.


(iii) 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
ouüash plains along with other depositional features.