NCERT Textbook - Landforms and their Evolution | Geography (Prelims) by Valor Academy - UPSC PDF Download

 Page 1


LANDFORMS AND THEIR
EVOLUTION
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?
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
CHAPTER
2024-25
Page 2


LANDFORMS AND THEIR
EVOLUTION
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?
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
CHAPTER
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
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:
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 6.1)
and a canyon is characterised by steep
step-like side slopes (Figure 6.2) 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.
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
2024-25
Page 3


LANDFORMS AND THEIR
EVOLUTION
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?
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
CHAPTER
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
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:
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 6.1)
and a canyon is characterised by steep
step-like side slopes (Figure 6.2) 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.
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
2024-25
LANDFORMS AND THEIR EVOLUTION 49
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
Figure 6.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamil Nadu in the form of gorge
Figure 6.2 : An entrenched meander loop of river
Colorado in USA showing step-like side slopes of
its valley typical of a canyon
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.
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 6.2).
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..
DEPOSITIONAL LANDFORMS
Alluvial Fans
Alluvial fans (Figure 6.3) 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 gradients and gets dumped and
spread as a broad low to high cone shaped
2024-25
Page 4


LANDFORMS AND THEIR
EVOLUTION
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?
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
CHAPTER
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
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:
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 6.1)
and a canyon is characterised by steep
step-like side slopes (Figure 6.2) 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.
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
2024-25
LANDFORMS AND THEIR EVOLUTION 49
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
Figure 6.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamil Nadu in the form of gorge
Figure 6.2 : An entrenched meander loop of river
Colorado in USA showing step-like side slopes of
its valley typical of a canyon
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.
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 6.2).
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..
DEPOSITIONAL LANDFORMS
Alluvial Fans
Alluvial fans (Figure 6.3) 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 gradients and gets dumped and
spread as a broad low to high cone shaped
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 50
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 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 6.4) 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.
Figure 6.3 : An alluvial fan deposited by a hill stream on
the way to Amarnath, Jammu and Kashmir
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 head to toe and they appear
as high cones with steep slope in arid and
semi-arid climates.
Figure 6.4 : A satellite view of part of Krishna river
delta, Andhra Pradesh Figure 6.5 : Natural levee and point bars
2024-25
Page 5


LANDFORMS AND THEIR
EVOLUTION
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?
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
CHAPTER
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 48
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:
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 6.1)
and a canyon is characterised by steep
step-like side slopes (Figure 6.2) 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.
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
2024-25
LANDFORMS AND THEIR EVOLUTION 49
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
Figure 6.1 : The Valley of Kaveri river near Hogenekal,
Dharmapuri district, Tamil Nadu in the form of gorge
Figure 6.2 : An entrenched meander loop of river
Colorado in USA showing step-like side slopes of
its valley typical of a canyon
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.
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 6.2).
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..
DEPOSITIONAL LANDFORMS
Alluvial Fans
Alluvial fans (Figure 6.3) 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 gradients and gets dumped and
spread as a broad low to high cone shaped
2024-25
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 50
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 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 6.4) 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.
Figure 6.3 : An alluvial fan deposited by a hill stream on
the way to Amarnath, Jammu and Kashmir
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 head to toe and they appear
as high cones with steep slope in arid and
semi-arid climates.
Figure 6.4 : A satellite view of part of Krishna river
delta, Andhra Pradesh Figure 6.5 : Natural levee and point bars
2024-25
LANDFORMS AND THEIR EVOLUTION 51
Natural levees and point bars (Figure 6.5)
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. Point bars are also known
as meander bars. They are found on the
concave side of meanders of large rivers and
are sediments deposited in a linear fashion by
flowing waters along the bank. They are almost
uniform in profile and in width and contain mixed
sizes of sediments.
In what way do natural levees differ from
point bars?
Meanders
In large flood and delta plains, rivers rarely
flow in straight courses. Loop-like channel
patterns called meanders develop over flood
and delta plains (Figure 6.6).
the banks slowly get transformed into a small
curvature in the banks; the curvature deepens
due to deposition on the inside of the curve
and erosion along the bank on the outside. If
there is no deposition and no erosion or
undercutting, the tendency to meander is
reduced. Normally, in meanders of large rivers,
there is active deposition along the concave
bank and undercutting along the convex bank.
The concave bank is known as cut-off bank
which shows up as a steep scarp and the
convex bank presents a long, gentle profile
(Figure 6.7). As meanders grow into deep
loops, the same may get cut-off due to erosion
at the inflection points and are left as ox-bow
lakes.
GROUNDWATER
Here the interest is not on groundwater as a
resource. Our focus is on the work of
groundwater in the erosion of landmasses and
evolution of landforms. The surface water
Figure 6.6 : A satellite scene showing meandering
Burhi Gandak river near Muzaffarpur, Bihar, showing
a number of oxbow lakes and cut-offs
Meander is not a landform but is only a
type of channel pattern. This is because of
(i) propensity of water flowing over very gentle
gradients to work laterally on the banks;
(ii) unconsolidated nature of alluvial deposits
making up the banks with many irregularities
which can be used by water exerting pressure
laterally; (iii) coriolis force acting on the fluid
water deflecting it like it deflects the wind.
When the gradient of the channel becomes
extremely low, water flows leisurely and starts
working laterally. Slight irregularities along
Figure 6.7 : Meander growth and cut-off loops and
slip-off and undercut banks
2024-25