Landform
·
Small to medium tracts or parcels of the
earth’s surface are called landforms
Landscape
·
Several related landforms together make up
landscapes, (large tracts of earth’s surface)
1.
Each landform has its own physical shape,
size, materials and is a result of the action of certain geomorphic processes
and agent(s)
2.
Actions of most of the geomorphic processes
and agents are slow results take a long time to take shape
3.
Due to continued action of geomorphic
processes Landforms may change in their shape, size and nature slowly or fast.
Reasons for the modification in landforms
1.
Changes in climatic conditions
2.
Vertical or horizontal movements of
landmasses
Evolution
of landforms
1.
Stages of transformation of either a part
of the earth’s surface from one landform into another or transformation
2.
Each and every landform has a history
3.
A landmass passes through stages of
development
a.
Youth
b.
Mature
c.
Old age
4.
Geomorphology deals with the reconstruction
of the history of the surface of the earth
Geomorphic
agents
1.
Running water
2.
Ground-water, glaciers, wind and waves
3.
Acting over long periods of time produce
systematic changes leading to sequential development of landforms.
4.
Each geomorphic agent produces its own
assemblage of landforms
5.
Each geomorphic process and agent leave
their distinct imprints on the landforms
6.
Most of the geomorphic processes are
imperceptible can only be seen and measured through their results
Two sets of landforms are produced
1.
Erosional or destructional
2.
Depositional or constructional
3.
Many varieties of landforms develop by the
action of each of the geomorphic agents depending upon the type and structure
4.
Folds, faults, joints, fractures, hardness
and softness, permeability and impermeability, etc. come under structure of
rocks.
Some
independent controls
(i)
stability of sea level
(ii)
tectonic stability of landmasses
(iii) climate
(iv) Any
disturbance in any of these three controlling factors can upset the systematic
and sequential stages in the development and evolution of landforms.
(v)
A brief discussion is presented as to how
landmasses are reduced in their relief through
erosion and then, development
RUNNING WATER
1.
In humid regions running water is
considered the most important of the geomorphic agents in bringing about the
degradation of the land surface.
2.
There are two components of running water.
3.
One is overland flow on general land
surface as a sheet another is linear flow as streams and rivers in valleys.
4.
Most of the erosional landforms made by
running water are associated with vigorous and youthful rivers flowing along
gradients.
5.
With time, stream channels over steep
gradients turn gentler due to continued erosion lose their velocity,
facilitating active deposition
6.
Depositional forms associated with streams
flowing over steep slopes will be on a small scale compared to those associated
with rivers flowing over medium to gentle slopes.
7.
The gentler the river channels in gradient
or slope, the greater is the deposition.
8.
When the stream beds turn gentler due to
continued erosion, downward cutting becomes less dominant.
9.
Lateral erosion of banks increases and as a
consequence the hills and valleys are reduced to plains.
10. Overland
flow causes sheet erosion depending upon irregularities of the land surface.
11. The
overland flow may concentrate into narrow to wide paths, because of the sheer
friction of the column of flowing water
12. 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.
13. These
rills will gradually develop into long and wide gullies
14. The
gullies will further deepen, widen, lengthen and unite to give rise to a
network of valleys
15. In the
early stages, down-cutting dominates during which irregularities such as
waterfalls and cascades will be removed.
16. In the
middle stages, streams cut their beds slower, and lateral erosion of valley
sides becomes severe.
17. The
valley sides are reduced to lower and lower slopes.
18. The
divides between drainage basins are likewise lowered until they are almost
completely flattened
19. A
lowland of faint relief with some low resistant remnants called monadnocks standing out here and there.
20. This
type of plain forming as a result of stream erosion is called peneplain (an
almost plain) characteristics of each of the stages
Youth
1.
Streams are few during this stage with poor
integration and flow over original slopes showing shallow V-shaped valleys.
2.
With very narrow floodplains along trunk
streams.
3.
Streams divides are broad and flat with
marshes, swamp and lakes.
4.
Meanders
develop
over these broad upland surfaces.
5.
These meanders entrench themselves into the
uplands.
6.
Waterfalls and rapids may exist where local
hard rock bodies are exposed.
Mature
1.
During this stage streams are plenty with
good integration.
2.
The valleys are still V-shaped but deep
3.
Trunk
streams are broad enough to have wider floodplains
4.
Streams may flow in meanders confined
within the valley
5.
The flat and broad inter stream areas and
swamps and marshes of youth disappear
6.
The stream divides turn sharp. Waterfalls
and rapids disappear.
Old
1.
Smaller tributaries during old age are few
with gentle gradients
2.
Streams meander freely over vast
floodplains showing natural levees, oxbow lakes, etc.
3.
Divides are broad and flat with lakes, swamps and marshes.
4.
Most of the landscape is at or slightly
above sea level.
EROSIONAL LANDFORMS
Valleys
1.
Valleys start as small and narrow rills.
2.
The rills will gradually develop into long
and wide gullies.
3.
The gullies will further deepen, widen and
lengthen to give rise to valleys.
4.
Depending upon dimensions and shape, many
types of valleys like V-shaped valley, gorge, canyon, etc. can be recognised.
5.
A gorge is a
deep valley with very steep to straight sides.
6.
A canyon is
characterised by steep step-like side slopes may be as deep as a gorge.
7.
A gorge is almost equal in width at its top
as well as its bottom.
8.
A canyon is wider at its top than at its
bottom a canyon is a variant of gorge.
9.
Valley types depend upon the type and
structure of rocks in which they form.
10.
Canyons commonly form in horizontal bedded
sedimentary rocks and gorges form in hard rocks.
11.
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.
12.
Once a small and shallow depression forms,
pebbles and boulders get collected in those depressions
13.
Get rotated by flowing water and
consequently the depressions grow in dimensions.
14.
A series of such depressions eventually
join and the stream valley gets deepened.
15.
At the foot of waterfalls large potholes,
quite deep and wide, form because of the sheer impact of water and rotation of
boulders.
16.
Such large and deep holes at the base of
waterfalls are called plunge pools
17.
These pools also help in the deepening of
valleys.
INCISED OR ENTRENCHED MEANDERS
1.
In streams that flow rapidly over steep
gradients, normally erosion is concentrated on the bottom of the stream channel.
2.
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.
3.
Because of active lateral erosion, streams
flowing over gentle slopes develop sinuous or meandering courses.
4.
It is common to find meandering courses
over floodplains and delta plains where stream gradients are very gentle.
5.
Very deep and wide meanders can also be
found cut in hard rocks.
6.
Such meanders are called incised or
entrenched meanders
7.
Meander loops develop over original gentle
surfaces in the initial stages of development of streams
8.
Same loops get entrenched into the rocks
normally due to erosion or slow, continued uplift of the land over which they
start.
9.
They widen and deepen over time and can be
found as deep gorges and canyons in hard rock areas.
River
Terraces
1. River
terraces are surfaces marking old valley floor or floodplain levels.
2. They
may be bedrock surfaces without any alluvial cover or alluvial terraces
consisting of stream deposits.
3. River
terraces are basically products of erosion as they result due to vertical
erosion by the stream into its own depositional floodplain.
4. There
can be a number of such terraces at different heights indicating former river
bed levels
5. The
river terraces may occur at the same elevation on either side of the rivers in
which case they are called paired
terraces.
6. 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 non-paired terraces.
7. Unpaired
terraces are typical in areas of slow uplift of land or where the water column
changes are not uniform along both the banks.
8. 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
LANDFORM
Alluvial
Fans
1. Formed
when streams flowing from higher levels break into foot slope plains of low
gradient.
2. Normally
very coarse load is carried by streams flowing over mountain slopes.
3. 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 deposit called
alluvial fan.
4. 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.
5. Alluvial
fans in humid areas show normally low cones with gentle slope.
6. They
appear as high cones with steep slope in arid and semi-arid climates.
Deltas
1. Deltas
are like alluvial fans but develop at a different location.
2. The
load carried by the rivers is dumped and spread into the sea.
3. If
this load is not carried away far into the sea or distributed along the coast,
it spreads and accumulates as a low cone.
4. Unlike
in alluvial fans, the deposits making up deltas are very well sorted with clear
stratification.
5. The
coarsest materials settle out first and the finer fractions like silts and clays
are carried out into the sea.
6. As
the delta grows, the river distributaries continue to increase in length delta continues to build up into the sea.
Floodplains,
Natural Levees and Point Bars
1. Deposition
develops a floodplain just as erosion makes valleys.
2. Floodplain
is a major landform of river deposition.
3. Large
sized materials are deposited first when stream channel breaks into a gentle
slope.
4. Fine
sized materials like sand, silt and clay are carried by relatively slow moving
waters in gentler channels.
5. Found
in the plains and deposited over the bed and when the waters spill over the
banks during flooding above the bed.
6. A
river bed made of river deposits is the active floodplain.
7. The
floodplain above the bank is inactive floodplain.
8. Inactive
floodplain above the banks basically contain two types of deposits
a. Flood
deposits
b. Channel
deposits
9. In
plains, channels shift laterally and change their courses occasionally leaving
cut-off courses
10. get
filled up gradually
11. Such
areas over flood plains built up by abandoned or cut-off channels contain
coarse deposits.
12. The
flood deposits of spilled waters carry relatively finer materials like silt and
clay. The flood plains in a delta are called delta plains.
13. Natural levees
and point bars are some of the important landforms found associated with
floodplains.
14. Natural
levees are found along the banks of large rivers.
15. They
are low, linear and parallel ridges of coarse deposits along the banks of
rivers,
16. 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.
17. They
are high nearer the banks and slope gently away from the river.
18. The
levee deposits are coarser than the deposits spread by flood waters away from
the river.
19. When
rivers shift laterally, a series of natural levees can form.
20. Point bars
are also known as meander bars.
21. They
are found on the convex side of meanders of large rivers and are sediments
deposited in a linear fashion by flowing waters along the bank.
22. They
are almost uniform in profile and in width and contain mixed sizes of
sediments.
23. If
there more than one ridge, narrow and elongated depressions are found in
between the point bars.
24. Rivers
build a series of them depending upon the water flow and supply of sediment.
25. As
the rivers build the point bars on the convex side, the bank on the concave
side will erode actively.
Meanders
1. In
large flood and delta plains, rivers rarely flow in straight courses.
2. Loop-like
channel patterns called meanders develop over flood and delta
3. Meander
is not a landform but is only a type of channel pattern
4. This
is because
(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
5. When
the gradient of the channel becomes extremely low, water flows leisurely and
starts working laterally
6. Slight
irregularities along the banks slowly get transformed into a small curvature in
the banks
7. the
curvature deepens due to deposition on the inside of the curve and erosion
along the bank on the outside
8. If
there is no deposition and no erosion or undercutting, the tendency to meander
is reduced.
9. Normally,
in meanders of large rivers, there is active deposition along the convex bank
and undercutting along the concave bank
10. 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 and is known as slip-off bank
11. 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.
Braided
Channels
1. When
rivers carry coarse material, there can be selective deposition of coarser materials
causing formation of a central bar which diverts the flow towards the banks
2. This
flow increases lateral erosion on the banks
3. The
valley widens, the water column is reduced and more and more materials get
deposited as islands
4. Lateral
bars developing a number of separate channels of water flow.
5. Deposition
and lateral erosion of banks are essential for the formation of braided pattern
6. Channel
bars and islands of sand, gravel and pebbles develop on the floor of the
channel and the water flow is divided into multiple threads.
7. These
thread-like streams of water rejoin and subdivide repeatedly to give a typical
braided pattern.
