NCERT Textbook: Landforms & Their Evolution - UPSC PDF Download

 Page 1


LANDFORMS AND THEIR
EVOLUTION
CHAPTER
A
fter weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic 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
follows 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 land-
masses, 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
disturbing 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
© NCERT
not to be republished
Page 2


LANDFORMS AND THEIR
EVOLUTION
CHAPTER
A
fter weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic 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
follows 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 land-
masses, 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
disturbing 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
© NCERT
not to be republished
LANDFORMS AND THEIR EVOLUTION 59
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.
Most of the geomorphic processes are
imperceptible. Cite a few processes which
can be seen and a few which can’t be
seen.
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. 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 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 over steep 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.
Is complete reduction of relief of a high
land mass possible?
Overland flow causes sheet erosion.
Depending upon irregularities of the land
surface, the overland flow may concentrate into
narrow to wide paths. Because of the sheer
friction of the column of flowing water, minor
or major quantities of materials from the
surface of the land are removed in the direction
of flow and gradually small and narrow rills
will form. These rills will gradually develop into
long and wide gullies; the gullies will further
deepen, widen, lengthen and unite to give rise
to a network of valleys. 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. Gradually, the
valley sides are reduced to lower and lower
slopes. The divides between drainage basins
are likewise lowered until they are almost
completely flattened leaving finally, a lowland
of faint relief with some low resistant remnants
called monadnocks standing out here and
there. This type of plain forming as a result of
stream erosion is called a peneplain (an almost
plain). The characteristics of each of the stages
of landscapes developing in running water
regimes may be summarised as follows:
© NCERT
not to be republished
Page 3


LANDFORMS AND THEIR
EVOLUTION
CHAPTER
A
fter weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic 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
follows 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 land-
masses, 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
disturbing 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
© NCERT
not to be republished
LANDFORMS AND THEIR EVOLUTION 59
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.
Most of the geomorphic processes are
imperceptible. Cite a few processes which
can be seen and a few which can’t be
seen.
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. 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 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 over steep 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.
Is complete reduction of relief of a high
land mass possible?
Overland flow causes sheet erosion.
Depending upon irregularities of the land
surface, the overland flow may concentrate into
narrow to wide paths. Because of the sheer
friction of the column of flowing water, minor
or major quantities of materials from the
surface of the land are removed in the direction
of flow and gradually small and narrow rills
will form. These rills will gradually develop into
long and wide gullies; the gullies will further
deepen, widen, lengthen and unite to give rise
to a network of valleys. 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. Gradually, the
valley sides are reduced to lower and lower
slopes. The divides between drainage basins
are likewise lowered until they are almost
completely flattened leaving finally, a lowland
of faint relief with some low resistant remnants
called monadnocks standing out here and
there. This type of plain forming as a result of
stream erosion is called a peneplain (an almost
plain). The characteristics of each of the stages
of landscapes developing in running water
regimes may be summarised as follows:
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 60
Youth
Streams are few during this stage with poor
integration and flow over original slopes
showing shallow V-shaped valleys with no
floodplains or with very narrow floodplains
along trunk streams. Streams divides are broad
and flat with marshes, swamp and lakes.
Meanders if present develop over these broad
upland surfaces. These meanders may
eventually entrench themselves into the
uplands. Waterfalls and rapids may exist where
local hard rock bodies are exposed.
Mature
During this stage streams are plenty with good
integration. The valleys are still V-shaped but
deep; trunk streams are broad enough to have
wider floodplains within which streams may
flow in meanders confined within the valley.
The flat and broad inter stream areas and
swamps and marshes of youth disappear and
the stream divides turn sharp. Waterfalls and
rapids disappear.
Old
Smaller tributaries during old age are few with
gentle gradients. Streams meander freely over
vast floodplains showing natural levees, oxbow
lakes, etc. Divides are broad and flat with lakes,
swamps and marshes. Most of the landscape
is at or slightly above sea level.
EROSIONAL LANDFORMS
Valleys
Valleys start as small and narrow rills; the rills
will gradually develop into long and wide
gullies; the gullies will further deepen, widen
and lengthen to give rise to valleys. Depending
upon dimensions and shape, many types of
valleys like V-shaped valley, gorge, canyon,
etc. can be recognised. A gorge is a deep valley
with very steep to straight sides (Figure 7.1) and
a canyon is characterised by steep step-like
side slopes (Figure 7.2) and may be as deep as
a gorge. A gorge is almost equal in width at its
top as well as its bottom. In contrast, a canyon
is wider at its top than at its bottom. In fact, a
canyon is a variant of gorge. Valley types depend
upon the type and structure of rocks in which
they form. For example, canyons commonly
form in horizontal bedded sedimentary rocks
and gorges form in hard rocks.
Figure 7.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamilnadu in the form of gorge
Figure 7.2 : An entrenched meander loop of river Colorado
in USA showing step-like side slopes of its valley
typical of a canyon
© NCERT
not to be republished
Page 4


LANDFORMS AND THEIR
EVOLUTION
CHAPTER
A
fter weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic 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
follows 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 land-
masses, 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
disturbing 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
© NCERT
not to be republished
LANDFORMS AND THEIR EVOLUTION 59
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.
Most of the geomorphic processes are
imperceptible. Cite a few processes which
can be seen and a few which can’t be
seen.
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. 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 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 over steep 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.
Is complete reduction of relief of a high
land mass possible?
Overland flow causes sheet erosion.
Depending upon irregularities of the land
surface, the overland flow may concentrate into
narrow to wide paths. Because of the sheer
friction of the column of flowing water, minor
or major quantities of materials from the
surface of the land are removed in the direction
of flow and gradually small and narrow rills
will form. These rills will gradually develop into
long and wide gullies; the gullies will further
deepen, widen, lengthen and unite to give rise
to a network of valleys. 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. Gradually, the
valley sides are reduced to lower and lower
slopes. The divides between drainage basins
are likewise lowered until they are almost
completely flattened leaving finally, a lowland
of faint relief with some low resistant remnants
called monadnocks standing out here and
there. This type of plain forming as a result of
stream erosion is called a peneplain (an almost
plain). The characteristics of each of the stages
of landscapes developing in running water
regimes may be summarised as follows:
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 60
Youth
Streams are few during this stage with poor
integration and flow over original slopes
showing shallow V-shaped valleys with no
floodplains or with very narrow floodplains
along trunk streams. Streams divides are broad
and flat with marshes, swamp and lakes.
Meanders if present develop over these broad
upland surfaces. These meanders may
eventually entrench themselves into the
uplands. Waterfalls and rapids may exist where
local hard rock bodies are exposed.
Mature
During this stage streams are plenty with good
integration. The valleys are still V-shaped but
deep; trunk streams are broad enough to have
wider floodplains within which streams may
flow in meanders confined within the valley.
The flat and broad inter stream areas and
swamps and marshes of youth disappear and
the stream divides turn sharp. Waterfalls and
rapids disappear.
Old
Smaller tributaries during old age are few with
gentle gradients. Streams meander freely over
vast floodplains showing natural levees, oxbow
lakes, etc. Divides are broad and flat with lakes,
swamps and marshes. Most of the landscape
is at or slightly above sea level.
EROSIONAL LANDFORMS
Valleys
Valleys start as small and narrow rills; the rills
will gradually develop into long and wide
gullies; the gullies will further deepen, widen
and lengthen to give rise to valleys. Depending
upon dimensions and shape, many types of
valleys like V-shaped valley, gorge, canyon,
etc. can be recognised. A gorge is a deep valley
with very steep to straight sides (Figure 7.1) and
a canyon is characterised by steep step-like
side slopes (Figure 7.2) and may be as deep as
a gorge. A gorge is almost equal in width at its
top as well as its bottom. In contrast, a canyon
is wider at its top than at its bottom. In fact, a
canyon is a variant of gorge. Valley types depend
upon the type and structure of rocks in which
they form. For example, canyons commonly
form in horizontal bedded sedimentary rocks
and gorges form in hard rocks.
Figure 7.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamilnadu in the form of gorge
Figure 7.2 : An entrenched meander loop of river Colorado
in USA showing step-like side slopes of its valley
typical of a canyon
© NCERT
not to be republished
LANDFORMS AND THEIR EVOLUTION 61
Potholes and Plunge Pools
Over the rocky beds of hill-streams more or less
circular depressions called potholes form
because of stream erosion aided by the abrasion
of rock fragments. Once a small and shallow
depression forms, pebbles and boulders get
collected in those depressions and get rotated
by flowing water and consequently the
depressions grow in dimensions. A series of such
depressions eventually join and the stream
valley gets deepened. At the foot of waterfalls
also, large potholes, quite deep and wide, form
because of the sheer impact of water and
rotation of boulders. Such large and deep holes
at the base of waterfalls are called plunge pools.
These pools also help in the deepening of valleys.
Waterfalls are also transitory like any other
landform and will recede gradually and bring
the floor of the valley above waterfalls to the
level below.
Incised or Entrenched Meanders
In streams that flow rapidly over steep
gradients, normally erosion is concentrated on
the bottom of the stream channel. Also, in the
case of steep gradient streams, lateral erosion
on the sides of the valleys is not much when
compared to the streams flowing on low and
gentle slopes. Because of active lateral erosion,
streams flowing over gentle slopes, develop
sinuous or meandering courses. It is common
to find meandering courses over floodplains
and delta plains where stream gradients are
very gentle. But very deep and wide meanders
can also be found cut in hard rocks. Such
meanders are called incised or entrenched
meanders (Figure 7.2). Meander loops develop
over original gentle surfaces in the initial stages
of development of streams and the same loops
get entrenched into the rocks normally due to
erosion or slow, continued uplift of the land
over which they start.  They widen and deepen
over time and can be found as deep gorges and
canyons in hard rock areas. They give an
indication on the status of original land
surfaces over which streams have developed.
What are the differences between incised
meanders and meanders over flood and
delta plains?
River Terraces
River terraces are surfaces marking old valley
floor or floodplain levels. They may be bedrock
surfaces without any alluvial cover or alluvial
terraces consisting of stream deposits. River
terraces are basically products of erosion as
they result due to vertical erosion by the stream
into its own depositional floodplain. There can
be a number of such terraces at different
heights indicating former river bed levels. The
river terraces may occur at the same elevation
on either side of the rivers in which case they
are called paired terraces (Figure 7.3).
Figure 7.3 : Paired and unpaired river terraces
When a terrace is present only on one side
of the stream and with none on the other side
or one at quite a different elevation on the other
side, the terraces are called unpaired terraces.
Unpaired terraces are typical in areas of slow
uplift of land or where the water column
changes are not uniform along both the banks.
The terraces may result due to (i) receding water
after a peak flow; (ii) change in hydrological
regime due to climatic changes; (iii) tectonic
uplift of land; (iv) sea level changes in case of
rivers closer to the sea.
DEPOSITIONAL LANDFORMS
Alluvial Fans
Alluvial fans (Figure 7.4) are formed when
streams flowing from higher levels break into
foot slope plains of low gradient. Normally very
coarse load is carried by streams flowing over
mountain slopes. This load becomes too heavy
for the streams to be carried over gentler
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Page 5


LANDFORMS AND THEIR
EVOLUTION
CHAPTER
A
fter weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic 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
follows 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 land-
masses, 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
disturbing 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
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LANDFORMS AND THEIR EVOLUTION 59
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.
Most of the geomorphic processes are
imperceptible. Cite a few processes which
can be seen and a few which can’t be
seen.
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. 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 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 over steep 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.
Is complete reduction of relief of a high
land mass possible?
Overland flow causes sheet erosion.
Depending upon irregularities of the land
surface, the overland flow may concentrate into
narrow to wide paths. Because of the sheer
friction of the column of flowing water, minor
or major quantities of materials from the
surface of the land are removed in the direction
of flow and gradually small and narrow rills
will form. These rills will gradually develop into
long and wide gullies; the gullies will further
deepen, widen, lengthen and unite to give rise
to a network of valleys. 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. Gradually, the
valley sides are reduced to lower and lower
slopes. The divides between drainage basins
are likewise lowered until they are almost
completely flattened leaving finally, a lowland
of faint relief with some low resistant remnants
called monadnocks standing out here and
there. This type of plain forming as a result of
stream erosion is called a peneplain (an almost
plain). The characteristics of each of the stages
of landscapes developing in running water
regimes may be summarised as follows:
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 60
Youth
Streams are few during this stage with poor
integration and flow over original slopes
showing shallow V-shaped valleys with no
floodplains or with very narrow floodplains
along trunk streams. Streams divides are broad
and flat with marshes, swamp and lakes.
Meanders if present develop over these broad
upland surfaces. These meanders may
eventually entrench themselves into the
uplands. Waterfalls and rapids may exist where
local hard rock bodies are exposed.
Mature
During this stage streams are plenty with good
integration. The valleys are still V-shaped but
deep; trunk streams are broad enough to have
wider floodplains within which streams may
flow in meanders confined within the valley.
The flat and broad inter stream areas and
swamps and marshes of youth disappear and
the stream divides turn sharp. Waterfalls and
rapids disappear.
Old
Smaller tributaries during old age are few with
gentle gradients. Streams meander freely over
vast floodplains showing natural levees, oxbow
lakes, etc. Divides are broad and flat with lakes,
swamps and marshes. Most of the landscape
is at or slightly above sea level.
EROSIONAL LANDFORMS
Valleys
Valleys start as small and narrow rills; the rills
will gradually develop into long and wide
gullies; the gullies will further deepen, widen
and lengthen to give rise to valleys. Depending
upon dimensions and shape, many types of
valleys like V-shaped valley, gorge, canyon,
etc. can be recognised. A gorge is a deep valley
with very steep to straight sides (Figure 7.1) and
a canyon is characterised by steep step-like
side slopes (Figure 7.2) and may be as deep as
a gorge. A gorge is almost equal in width at its
top as well as its bottom. In contrast, a canyon
is wider at its top than at its bottom. In fact, a
canyon is a variant of gorge. Valley types depend
upon the type and structure of rocks in which
they form. For example, canyons commonly
form in horizontal bedded sedimentary rocks
and gorges form in hard rocks.
Figure 7.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamilnadu in the form of gorge
Figure 7.2 : An entrenched meander loop of river Colorado
in USA showing step-like side slopes of its valley
typical of a canyon
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LANDFORMS AND THEIR EVOLUTION 61
Potholes and Plunge Pools
Over the rocky beds of hill-streams more or less
circular depressions called potholes form
because of stream erosion aided by the abrasion
of rock fragments. Once a small and shallow
depression forms, pebbles and boulders get
collected in those depressions and get rotated
by flowing water and consequently the
depressions grow in dimensions. A series of such
depressions eventually join and the stream
valley gets deepened. At the foot of waterfalls
also, large potholes, quite deep and wide, form
because of the sheer impact of water and
rotation of boulders. Such large and deep holes
at the base of waterfalls are called plunge pools.
These pools also help in the deepening of valleys.
Waterfalls are also transitory like any other
landform and will recede gradually and bring
the floor of the valley above waterfalls to the
level below.
Incised or Entrenched Meanders
In streams that flow rapidly over steep
gradients, normally erosion is concentrated on
the bottom of the stream channel. Also, in the
case of steep gradient streams, lateral erosion
on the sides of the valleys is not much when
compared to the streams flowing on low and
gentle slopes. Because of active lateral erosion,
streams flowing over gentle slopes, develop
sinuous or meandering courses. It is common
to find meandering courses over floodplains
and delta plains where stream gradients are
very gentle. But very deep and wide meanders
can also be found cut in hard rocks. Such
meanders are called incised or entrenched
meanders (Figure 7.2). Meander loops develop
over original gentle surfaces in the initial stages
of development of streams and the same loops
get entrenched into the rocks normally due to
erosion or slow, continued uplift of the land
over which they start.  They widen and deepen
over time and can be found as deep gorges and
canyons in hard rock areas. They give an
indication on the status of original land
surfaces over which streams have developed.
What are the differences between incised
meanders and meanders over flood and
delta plains?
River Terraces
River terraces are surfaces marking old valley
floor or floodplain levels. They may be bedrock
surfaces without any alluvial cover or alluvial
terraces consisting of stream deposits. River
terraces are basically products of erosion as
they result due to vertical erosion by the stream
into its own depositional floodplain. There can
be a number of such terraces at different
heights indicating former river bed levels. The
river terraces may occur at the same elevation
on either side of the rivers in which case they
are called paired terraces (Figure 7.3).
Figure 7.3 : Paired and unpaired river terraces
When a terrace is present only on one side
of the stream and with none on the other side
or one at quite a different elevation on the other
side, the terraces are called unpaired terraces.
Unpaired terraces are typical in areas of slow
uplift of land or where the water column
changes are not uniform along both the banks.
The terraces may result due to (i) receding water
after a peak flow; (ii) change in hydrological
regime due to climatic changes; (iii) tectonic
uplift of land; (iv) sea level changes in case of
rivers closer to the sea.
DEPOSITIONAL LANDFORMS
Alluvial Fans
Alluvial fans (Figure 7.4) are formed when
streams flowing from higher levels break into
foot slope plains of low gradient. Normally very
coarse load is carried by streams flowing over
mountain slopes. This load becomes too heavy
for the streams to be carried over gentler
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 62
gradients and gets dumped and spread as a
broad low to high cone shaped deposit called
alluvial fan. Usually, the streams which flow
over fans are not confined to their original
channels for long and shift their position across
the fan forming many channels called
distributaries. Alluvial fans in humid areas
show normally low cones with gentle slope from
as a low cone. Unlike in alluvial fans, the
deposits making up deltas are very well sorted
with clear stratification. The coarsest materials
settle out first and the finer fractions like silts
and clays are carried out into the sea. As the
delta grows, the river distributaries continue
to increase in length (Figure 7.5) and delta
continues to build up into the sea.
Floodplains, Natural Levees and Point Bars
Deposition develops a floodplain just as
erosion makes valleys. Floodplain is a major
landform of river deposition. Large sized
materials are deposited first when stream
channel breaks into a gentle slope. Thus,
normally, fine sized materials like sand, silt and
clay are carried by relatively slow moving
waters in gentler channels usually found in the
plains and deposited over the bed and when
the waters spill over the banks during flooding
above the bed. A river bed made of river
deposits is the active floodplain. The floodplain
above the bank is inactive floodplain.  Inactive
floodplain above the banks basically contain
two types of deposits — flood deposits and
channel deposits. In plains, channels shift
laterally and change their courses occasionally
leaving cut-off courses which get filled up
gradually. Such areas over flood plains built
up by abandoned or cut-off channels contain
coarse deposits. The flood deposits of spilled
waters carry relatively finer materials like silt
and clay. The flood plains in a delta are called
delta plains.
Natural levees and point bars (Figure 7.6)
are some of the important landforms found
associated with floodplains. Natural levees are
found along the banks of large rivers. They are
low, linear and parallel ridges of coarse deposits
along the banks of rivers, quite often cut into
individual mounds. During flooding as the
water spills over the bank, the velocity of the
water comes down and large sized and high
specific gravity materials get dumped in the
immediate vicinity of the bank as ridges. They
are high nearer the banks and slope gently
away from the river. The levee deposits are
coarser than the deposits spread by flood
waters away from the river. When rivers shift
laterally, a series of natural levees can form.
Figure 7.4 : An alluvial fan deposited by a hill stream
on the way to Amarnath, Jammu and Kashmir
head to toe and they appear as high cones with
steep slope in arid and semi-arid climates.
Deltas
Deltas are like alluvial fans but develop at a
different location. The load carried by the rivers
is dumped and spread into the sea. If this load
is not carried away far into the sea or distributed
along the coast, it spreads and accumulates
Figure 7.5 : A satellite view of part of Krishna river
delta, Andhra Pradesh
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