Landforms - Evolution - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

Landforms - Evolution

CHAPTER 9 

LANDFORMS AND THEIR EVOLUTION

LANDFORMS MADE BY RUNNING WATER

Denudation:-It is the general lowering of the earth’s surface. This takes place because the agents of erosion wear away the rocks and transport the eroded debris to lower land or right down to the sea.

• Running Water is the most important single agent of denudation.

Consequent Streams:-The initial stream that exist as a consequence of the slope.

Insequent Stream:-The tributary stream which joins the main stream (consequent Stream) obliquely.

Drainage Pattern

1. Dendritic Pattern:-which the tributaries joins the main river obliquely and the pattern so evolved is tree like in appearance.
2. Trellised Drainage/Rectangular Pattern:-When tributaries join the main stream at right angles as subsequent streams.

THE PROCESS OF RIVER ACTION

When a river flow it carries with it eroded materials. These comprise the river’s load and may be divided into three distinct types:-

1. Material in Solution:-These are minerals dissolved in water.
2. Material in Suspension:-Sand, silt and mud are carried along suspended in the water.
3. The Traction load:-this includes coarser materials which are rolled along the river bed.

The ability of a river to move the various grades of materials depends greatly upon the volume or the water, the velocity of the flow and lastly the size, shape and weight of the load. It is said that doubling the velocity of a river, its transporting power is increased more than 10 times.

RIVER EROSION AND TRANSPOTATION

1. Corrasion or Abrasion:-This is the mechanical grinding of the river’s traction load against the banks and bed of the river. Corrasion takes place two distinct ways:

• Lateral Corrasion: This is the sideward erosion which widens the V-shaped valley.
• Vertical Corrasion: This is the downward action which deepens the river channel.

1. Corrosion or Solution: This is the chemical or solvent action of water on soluble or partly-soluble rocks with which river comes into contact.
2. Hydraulic action: This is the mechanical loosening and sweeping away of materials by the river water itself.
3. Attrition: This is the wear and tear of the transported materials themselves when they roll and collide into one another.

The course of a river may be divided into three distinct parts:

1. The Upper or Mountain course (Youth stage).
2. Middle or Valley course (Mature stage).
3. Lower or Plain course (Old stage).
4. The Upper or Mountain Course:
   This begins at the source of the river near the watershed. Here the predominant action of the river is vertical corrasion. The valley so developed is thus deep, narrow and distinctively V-shaped.

The features that are often best developed in the upper course of a river include:

1. River Capture.
2. Rapids, cataracts and waterfalls.

Due to the unequal resistance of hard and soft rocks traversed by a river, the outcrop of a band of hard rock may cause a river to ‘jump’ or ‘fall’ downstream. Thus Rapids are formed. Similar  

falls of greater dimension are also referred to as Cataracts. When river plunge down in a sudden fall of some height, they are called waterfalls.

1. The Middle or Valley Course:

In the middle course lateral corrasion takes place. The volume of water increases with the confluence of many tributaries and this increases the river’s load. The word of the river is predominantly transportation with some deposition.

The features associated with this course are:

a. Meanders

b. River Cliffs

c. Interlocking Spurs.

Meanders: - Caused by the irregularities of the ground which force the river to swing in loop, forming meanders.

1. The Lower or Plain Course:
   Vertical corrasion has almost ceased through lateral corrasion still goes on to erode its bank further. The work of the river is mainly deposition, building up its bed and forming extensive floodplains.
   Large sheets of materials are deposited on the level plain and may split the river into several complicated channels, so that it can be described as braided stream.

Some of the major Plain Course features are:

1. Flood Plains
2. Ex- bow lakes/Dead lake/Mort lake/Bayous.
3. Deltas.

Levees: raised bank of a river formed by the deposition of material.

Delta: fan shaped alluvial area at the mouth of a river formed by the deposition of fine materials.

FAVOURABLE CONDITIONS FOR THE FORMATION OF DELTAS

1. Vertical and lateral erosion in the upper course of the river to provide extensive sediments.
2. The coast should be sheltered preferably tide less.
3. The sea adjoining the delta should be shallow.
4. There should be no large lake in the course of river.
5. There should be no strong current running at right angles to the river mouth, washing away the sediments.

RIVER REJUVENATION

Due to tectonic movement when there is an uplift of land or a fall in the sea level or vice-versa, the three course of a river come into existence once again, which is called rejuvenation of the river.

Negative Movement: it occurs when there is an uplift of land or a fall in sea- level. This will steepen the slope so that down cutting is removed.

Positive Movement: it occurs when there is depression of land or a rise in sea level. This will submerge the lands along the coast, drown the valleys and weaken the erosive power of the river.

LANDFORMS OF GLACIATION

Landforms of Highland Glaciations

A glacier erodes its valley by two processes:

a. plucking

b. abrasion

1. Plucking:

      By plucking the glacier freezes the joints and beds of the underlying rocks, tear out individual blocks and drag them away.

1. Abrasion: 

By abrasion, the Glacier scratches, scrapes, polishes and scours the valley floor with the debris frozen into it.

Features of Glaciated highlands:

1. Corrie, cirque or cwm:

A depression which is converted into a steep, horse-shoe shaped basin by   plucking and abrasion, is called cirque. It is called corrie in Scottland and Cwn in Whales.

1. Arete and Pyramidal Peak:

Where two corries cut back on opposite side of a mountain, knife edged ridges are formed called Aretes.

When three or more cirques cut back together, their ultimate recession will form an angular -------or Pyramidal Peak.

1. U-Shaped Glacier Trough: a valley which has been glaciated takes characteristics U-Shape, with a wide, flat floor and very steep sides.
2. Hanging Valley: after the ice has melted a tributary valley hangs above the main valley called Hanging Valley.
3. Moraines: moraines are made up of pieces of rock that are shattered by frost action, imbedded in the glaciers and brought down the valley.

Those that fall on the sides of the Glacier form lateral moraines. When two glacier converge, their inside lateral moraines unite to form a medial moraine.

The glacier eventually melts on reaching the foot of the valley and the ---of transported materials left behind is the terminal moraine.

LANDFORMS OF GLACIATED LOWLANDS

They are mainly depositional in nature, brought by both valley glaciers and continental ice sheets.

1. Roche Moutonnee: 

This is a resistant residual rock hummock. The surface is scratched by ice movement. Its upstream side is smoothed by abrasion and its downstream side is roughened by plucking and is much steeper.

1. Crag and Tail: the crag is a mass of hard rock with a precipitous slope an the upstream side, which protects the softer leeward slope from being completely worn down by the on-coming ice. It therefore has a gentle tart, strewn with the eroded rock debris.

The remaining glaciated lowland features are of a depositional nature:-

1. Boulder Clay or Glacial till: this is an unsorted glacial deposited comprising a range of eroded materials. It is spread out in sheets and form gently undulating till or drift plains.
2. Erratic: - these are boulders of varying sizes that were transported by ice. They came with the advancing glaciers or ice sheets but when the icemelted, they were stranded in the regions of deposition. They are so called because they are composed of materials entirely different from those of the regions in which they are found.
3. Drumlins: - these are group of oval, long hummocks composed wholly of boulder clay, with their elongation in the direction of the ice flow that is on the downstream side.
4. Eskers: - these are long, narrow,sinous(curvy) ridges composed of sand and gravel which mark the former sites of sub-glacial melt- water stream.

ARID OR DESERT LANDFORMS

• Definition: An area with little/no vegetation. Characterised by aridity (low rainfall and high evaporation usually <250mm p.a.)
• About a fifth of the world’s land is made up of deserts.
• Almost all the deserts are confined within the 15°- 30° N as well as S of Equator. They lie in the trade wind belt on the western parts of the continents
• Where Trade winds are offshore. They are bathed by cold currents which produce a desiccating effect so that moisture is not easily condensed into precipitation.
• In the continental interiors of the mid latitudes, the deserts such as gobi are characterized by extremes of temperatures.

Types of Desert Landscape:-

1. Hamada or Rocky Desert :

This consists of a large stretches of bare rocks, swept clear f sand and dust by the wind e.g. Hamada el Homia (Libya).

1. Reg or Stony desert:

This is composed of extensive sheets of angular pebbles and gravels which the winds are not able to blow off. They are called serir in Libya and Egypt.

Location of Deserts

1. Deflation: When wind blows away rock waste leaving a depression in deserts. An example is the Qattara depression in Egypt.
2. Atrrition: The breaking down of the load by particles hitting against each other.

Features Produced by Wind Erosion

1. Rock Pedestals:
   • A tower-like mass of rock made up of alternating layers of resistant and less resistant rock.
   • The less resistant layers are eroded away faster than the more resistant layers leaving a mass of rock with projections protruding from it.
2. Zeugens: 
   • Occur in areas with resistant and less resistant rocks occurring in layers perpendicular to the prevailing wind.
   • The less resistant rock is more eroded than the less resistant rock producing a ridge and furrow landscape.
3. Yardangs: 
   • Occur in areas with resistant and less resistant rocks occurring in layers parallel to the prevailing wind.
   • The less resistant rock is more eroded than the less resistant rock producing a ridge and furrow landscape.
   • An example is near Komombo in Egypt and near InSalah in Egypt
4. Deflation Hollows: 
   • These are depressions produced by the deflation in weak rocks.
   • When the water table is reached, a swamp/oasis may be formed in these depressions.
   • The largest is the Qattara depression in Egypt.
   • Flat floored depressions are called pans.

Features Produced by Wind Deposition

• Dunes: deposits of sand by wind in deserts.
  1. Barchans: Crescent shaped deposits made when sand being blown by wind encounters an obstruction such as a rock or a dead camel.
  2. Seifs: Ridge-shaped deposits of sand with steep sides lying parallel to the prevailing winds.

Wind Blown Deposits in Deserts

• Wind blows fine particles out of deserts each year.
• Some are deposited in the sea.
• Others accumulated on land to form fertile land known as loess.

Features Produced by Water in Deserts

• Rain in deserts is infrequent and unpredictable. When it does occur, it does so in torrents.
• Torrential run-off occurs in small narrow valleys called rills.
• These rills may enlarge to form gullies.
• The gullies may further enlarge to form deep steep-sided valleys with wide flat floors and rocky walls called wadis.
• A torrential rain carries large quantities of materials which may turn into mudflows.
• The deposited material is called alluvial flans.
• This may be deposited into wadises. If irrigated, they form very fertile alluvial plains.
• Some valleys form good drainage basins. They may form temporary lakes known as playas or sebkhas.
• These soon dry up forming the playas into salt-flats.
• Some basins may be rimmed by uplands. The basins may join together to form a continuous feature called a bajada.
• Between the playa and bajada is a gently sloping platform called a pediment

The Action of Waves

• Coast: The strip of land where it meets the sea
• Coastline: The margin of land. The limit to which wave action takes place.
• Shore: The strip of land lying between the high and low water levels.
• Shoreline: The limit of the shore. The line where the shore and the water meet.
• Beach: A shore covered by a deposit of sand and/or pebbles.

• Factors Determining the Nature of Coasts

• Wave action
• Tidal currents
• Nature of the rocks forming the coast
• Height of the coast
• Nature of the climate
• Work of man

• Formation of Waves

• Wind blows over the sea surface.
• The surface of the sea exerts frictional drag on the lower layer of the wind.
• The top layer (with the least drag) moves faster than the lower layer and hence tumbles over it.
• This causes a circular motion of wind energy that acts on the sea to create waves.
• Wave Erosion
• Wave erosion like river erosion involves 4 processes:
  1. Abrasion/Corrasion: The wearing away of the sides and the bed of a river by the impact of the load.
  2. Hydraulic Action: Erosion by the force of moving water.
  3. Atrrition: The breaking down of the load by particles hitting against each other.
  4. Solution/Corrossion: When minerals dissolve in water.

Features Produced by Wave Erosion:

Cliffs, Wave Cut Platforms and Offshore Terraces

• A notch is cut by waves at high tide level and developed further.
• As this notch is developed, a cliff is formed.
• The cliff steepens as weathering attacks the base further.
• As the cliff retreats, the rock debris is swept by the backwash creating a wave-cut platform.
• Some of the debris collects along the seaward edge of the wave-cut platform forming and off-shore terrace.

Caves, Geos, Arches and Stacks

• Holes in the cliff face are enlarged by wave action
• A tunnel like opening called a cave is formed.
• The cave may develop further forming a long narrow inlet known as a geo.
• An arch is created when a cave in a headland is eroded right through i.e. the inlet has two openings
• When the arch collapses, the end of a headland stands up as a stack.

Headlands and Bays

• These are formed in areas of alternating resistant and less resistant rocks.
• Erosion/wave action acts less on the more resistant rock creating headlands and more on the less resistant rock creating bays.

Factors Affecting the Rate of Wave Erosion

1. Breaking point of the wave.
2. Wave steepness
3. Configuration of the coastline
4. Depth of the sea
5. Supply of beach material
6. Beach width
7. Nature of the rock

Wave Transport

• Sources of the load include:
  • Rivers entering the sea
  • Landslide on cliffs
  • Wave erosion
• Types of material transported include:
  • Sand
  • Shingles
  • Mud
• Process: Swash (forward moving waves) and backwash push and drag material up and down the shore resulting in longshore drift.

Wave Depositional Features

1. Beach: Formed by deposition of mud, sand or pebbles along the coast.
2. Barrier Beach: A long ridge of sand parallel to but separated from the coast ridge by a lagoon.
3. Spit: A narrow ridge of sand joined to the mainland with the other end terminating in the sea
4. Bar: A ridge of material (usually sand) lying parallel to the coast
5. Tombolo: A ridge joining an island to the mainland
6. Offshore Bar: Developed on the gently sloping seabed. Occurs when sand is thrown up by waves breaking close to the coast.

Mudflat: Developed when tides/waves deposit fine sand along gently sloping coasts particularly in bars and estuaries.