After weathering processes have had their actions on the earth materials making up the surface of the earth, the eomorphic agents like running water, ground water, wind, glaciers, waves perform erosion.It is already known to you that erosion causes changes on the surface of the earth. Deposition ollows erosion and because of deposition too, changes occur on the surface of the earth. As this chapter deals with landforms and their evolution first start with the question, what is a landform? In simple words, small to medium tracts or parcels of the earth’s surface are called landforms. If landform is a small to medium sized part of the surface of the earth, what is a landscape? Several related landforms together make up landscapes, (large tracts of earth’s surface). Each landform has its own physical shape, size, materials and is a result of the action of certain geomorphic processes and agent(s). Actions of most of the geomorphic processes and agents are slow, and hence the results take a long time to take shape. Every landform has a beginning. Landforms once formed may change in their shape, size and nature slowly or fast due to continued action of geomorphic processes and agents.
Due to changes in climatic conditions and vertical or horizontal movements of landmasses, either the intensity of processes or the processes themselves might change leading to new modifications in the landforms. Evolution here implies stages of transformation of either a part of the earth’s surface from one landform into another or transformation of individual landforms after they are once formed. That means, each and every landform has a history of development and changes through time. A landmass passes through stages of development somewhat comparable to the stages of life — youth, mature and old age. What are the two important aspects of the evolution of landforms?
The evolutionary history of the continually changing surface of the earth is essential to be understood in order to use it effectively without isturbing its balance and diminishing its potential for the future. Geomorphology deals with the reconstruction of the history of the surface of the earth through a study of its forms, the materials of which it is made up of and the processes that shape it. Changes on the surface of the earth owe mostly to erosion by various geomorphic agents. Of course, the process of deposition too, by covering the land surfaces and filling the basins, valleys or depressions, brings changes in the surface of the land. Deposition follows
erosion and the depositional surfaces too are ultimately subjected to erosion. Running water, ground-water, glaciers, wind and waves are powerful erosional and depositional agents shaping and changing the surface of the earth aided by weathering and mass wasting processes. These geomorphic agents acting over long periods of time produce systematic changes leading to sequential development of landforms. Each geomorphic agent produces its own assemblage of landforms. Not only this, each geomorphic process and agent leave their distinct imprints on the landforms they produce. You know that most of the geomorphic processes are imperceptible
functions and can only be seen and measured through their results. What are the results? These results are nothing but landforms and their characteristics. Hence, a study of landforms, will reveal to us the process and
agent which has made or has been making those landforms.
As the geomorphic agents are capable of erosion and deposition, two sets — erosional or destructional and depositional or constructional — of landforms are produced by them. Many varieties of landforms develop by the action of each of the geomorphic agents depending upon especially the type and structure i.e. folds, faults, joints, fractures, hardness and softness, permeability and impermeability, etc. come under structure of rocks. There are some other independent controls like (i) stability of sea level; (ii) tectonic stability of landmasses; (iii) climate, which influence the evolution of landforms. Any disturbance in any of these three controlling factors can upset the systematic and sequential stages in the development and evolution of landforms.In the following pages, under each of the geomorphic regimes i.e. running water; groundwater, glaciers, waves, and winds, first
a brief discussion is presented as to how landmasses are reduced in their relief through erosion and then, development of some of the erosional and depositional landforms is dealt with.
RUNNING WATER
In humid regions, which receive heavy rainfall running water is considered the most important of the geomorphic agents in bringing about the degradation of the land surface. There are two components of runningwater. 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. There may be depositional forms associated with streams flowing over steep slopes. But these phenomena will be on a small scale compared to those associated with rivers flowing over medium to gentle slopes. The gentler the river channels in gradient or slope, the greater is the deposition. When the stream beds turn gentler due to continued erosion, downward cutting becomes less dominant and lateral erosion of banks increases and as a consequence the hills and valleys are reduced to plains.
Excercise
1. Multiple choice questions.
(i) In which of the following stages of landform development, downward cutting is dominated?
(a) Youth stage (c) Early mature stage
(b) Late mature stage (d) Old stage
(ii) A deep valley characterised by steep step-like side slopes is known as
(a) U-shaped valley (c) Blind valley
(b) Gorge (d) Canyon
(iii) In which one of the following regions the chemical weathering process is more dominant than the mechanical process?
(a) Humid region (c) Arid region
(b) Limestone region (d) Glacier 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
(c) A landform forms due to dripping water from surface
(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 (c) Lateral Moraine
(b) Glacial valley (d) Esker
Please refer to attached file for NCERT Class 11 Geography Landforms And Their Evolution
