Landforms And Their Evolution

A landform is a small to medium-sized part of the Earth’s surface.

Several related landforms together form a landscape.

Each landform is produced by geomorphic processes and geomorphic agents such as:

• Running water
• Groundwater
• Glaciers
• Waves
• Wind

Every landform has its own shape, size and nature. Landforms keep changing over time due to the continued action of geomorphic processes and agents.

Evolution Of Landforms

The evolution of landforms means the transformation of one landform into another after its formation.

A landform may pass through different stages of development, such as:

• Youth
• Maturity
• Old age

This idea is linked with Davis’ cycle of erosion.

Erosion and deposition are two important parts of landform evolution.

Most geomorphic processes are very slow and operate over long geological periods. That is why they are often difficult to observe directly.

Major Geomorphic Agents

The major agents responsible for landform evolution are:

• Running water
• Groundwater
• Glaciers
• Waves
• Wind

These agents reduce landmasses through erosion and also create new landforms through deposition.

Running Water

Running water is one of the most important geomorphic agents.

It may work as:

• Streams and rivers flowing in valleys
• Overland flow or sheet flow over land surface

Youthful rivers flowing over steep slopes mostly produce erosional landforms.

With time, rivers lose velocity because of continuous erosion and reduction in gradient. This increases deposition.

The gentler the river slope, the greater the deposition.

When river beds become gentler, vertical cutting decreases and lateral erosion increases. This process gradually reduces hills and valleys into plains.

Landforms Formed By Running Water

Running water forms both erosional and depositional landforms.

Erosional Landforms Formed By Running Water

Important erosional landforms are:

• Valleys
• Potholes and plunge pools
• Incised or entrenched meanders
• River terraces

Depositional Landforms Formed By Running Water

Important depositional landforms are:

• Alluvial fans
• Deltas
• Flood plains
• Natural levees and point bars
• Meanders
• Oxbow lakes

Valleys

Valleys formed by rivers are erosional landforms.

Small and narrow rills gradually develop into larger gullies.

These gullies deepen, widen and lengthen to form valleys.

Depending on size and shape, valleys may be:

• V-shaped valleys
• Gorges
• Canyons

V-shaped valleys are common in the upper course of rivers where vertical erosion is dominant.

Gorges are deep and narrow valleys with steep sides.

Canyons are deep valleys with very steep sides, often formed in hard rocks.

Potholes And Plunge Pools

Potholes are circular depressions formed in the rocky beds of rivers.

They are formed when pebbles and boulders get trapped in small depressions and rotate due to fast-flowing water.

This grinding action enlarges the depression over time.

Plunge pools form at the base of waterfalls.

Falling water and rotating rock fragments deepen the river bed at the foot of the waterfall.

Incised Or Entrenched Meanders

Meanders are normally found in the middle and lower course of rivers.

When a river cuts deeply into its bed while maintaining its winding course, incised or entrenched meanders are formed.

They are common where uplift or rejuvenation causes renewed vertical erosion.

River Terraces

River terraces are narrow, step-like flat surfaces found on either side of a valley floor.

They represent former floodplain levels.

River terraces may be of two types:

Type	Meaning
Paired terraces	Terraces occur at the same elevation on both sides of the river
Unpaired terraces	Terraces occur only on one side or at different elevations

River terraces are often linked with river rejuvenation and changes in erosion level.

Alluvial Fans

Alluvial fans are commonly found at the foot of mountain ranges.

When streams flow down steep mountainous slopes, they carry coarse sediments.

As the slope becomes gentler, the stream loses velocity and deposits its load.

This forms a broad cone-shaped deposit called an alluvial fan.

An alluvial fan consists of gravel, sand and other sediments.

Streams flowing across the fan may divide into many smaller channels known as distributaries.

Deltas

Deltas are depositional landforms formed near the mouth of a river.

When a river enters a sea, lake or another standing water body, its velocity decreases.

As a result, it deposits sediments near its mouth.

Over time, these deposits form a delta.

Deltas are usually formed where:

• River brings heavy sediment load
• Sea waves and tides are weak
• River velocity decreases near the mouth

Flood Plains

A flood plain is a flat plain formed along the banks of a river.

It is created by repeated flooding and deposition of fine sediments.

During floods, rivers overflow their banks and spread sediments over nearby low-lying areas.

Flood plains are fertile because they receive fresh alluvial deposits.

Natural Levees And Point Bars

Natural levees are raised banks formed along river channels.

During floods, the river deposits coarse materials near its banks.

Repeated deposition builds up natural levees.

Point bars are deposits found on the inner side of meander bends.

They form because water velocity is lower on the inner side of a meander, leading to deposition.

Meanders

Meanders are winding curves or loops formed by rivers.

They are common in the middle and lower course of rivers.

Meanders develop due to:

• Lateral erosion on the outer bank
• Deposition on the inner bank

The outer bank of a meander is known as the cut bank.

The inner bank is the depositional side.

Oxbow Lake

An oxbow lake is a crescent-shaped lake formed when a river meander is cut off from the main river channel.

It is usually formed in the floodplains of mature rivers.

In a meander:

• Outer bank undergoes strong erosion.
• Inner bank receives deposition.
• The neck of the meander becomes narrow.

During floods or high discharge, the river cuts through the narrow neck.

The abandoned meander loop becomes an oxbow lake.

Groundwater

The effect of groundwater is not visible in all types of rocks.

Mechanical removal of materials by groundwater is not very important in landform development.

Groundwater is most effective in rocks rich in calcium carbonate, such as:

• Limestone
• Dolomite

In these rocks, groundwater and surface water create landforms through solution and precipitation.

Karst Topography

Any limestone, dolomite or gypsum region showing typical landforms produced by groundwater through solution and deposition is called Karst topography.

The name comes from the Karst region in the Balkans near the Adriatic Sea.

Karst topography has both erosional and depositional landforms.

Erosional Landforms Of Karst Region

• Sinkholes
• Doline
• Uvala
• Lapies
• Limestone pavement
• Caves

Depositional Landforms Of Karst Region

• Stalactites
• Stalagmites
• Pillars

Sinkholes

Sinkholes are small to large depressions formed on the surface in limestone areas.

They are produced when soluble rocks are dissolved by groundwater.

Sometimes, the roof of an underground cavity collapses, creating a sinkhole.

Doline

A doline is a larger depression formed due to solution or collapse in limestone regions.

It is common in karst landscapes.

Uvala

An uvala is a larger closed depression formed by the merging of several dolines.

It is wider than a doline.

Lapies

Lapies are irregular grooves, ridges or solution channels found on limestone surfaces.

They are formed by the solution action of water.

Limestone Pavement

Limestone pavement is a bare limestone surface divided into blocks by joints and cracks.

It forms due to solution along joints in limestone.

Caves

Caves are underground openings formed by the solution action of groundwater in limestone areas.

Groundwater dissolves calcium carbonate and creates underground channels and caverns.

Stalactites

Stalactites are depositional features hanging from the roof of limestone caves.

They are formed by deposition of calcium carbonate from dripping water.

Stalagmites

Stalagmites rise upward from the floor of caves.

They are formed when calcium carbonate is deposited from drops falling on the cave floor.

Pillars

When stalactites and stalagmites join together, they form columns known as pillars.

Glaciers

Glaciers are masses of ice moving as sheets over land or as linear flows down mountain slopes through broad trough-like valleys.

Glaciers erode even unweathered rocks.

They can reduce high mountains into low hills and plains.

Glaciers also deposit sediments and form depositional landforms.

Landforms Formed By Glaciers

Glaciers form both erosional and depositional landforms.

Erosional Landforms Formed By Glaciers

• Cirque
• Glacial trough
• Hanging valley
• Arete
• Horn
• Fjord

Depositional Landforms Formed By Glaciers

• Outwash plain
• Esker
• Drumlins
• Moraines

Cirque

A cirque is a bowl-shaped depression formed by glacial erosion at the head of a mountain valley.

It is one of the most common glacial erosional landforms.

After the glacier melts, water may collect in the cirque to form a small lake called a tarn.

Glacial Trough

A glacial trough is a U-shaped valley formed by glacial erosion.

Unlike river valleys, which are usually V-shaped, glacial valleys have broad floors and steep sides.

Hanging Valley

A hanging valley is formed when a smaller tributary glacier joins a larger main glacier.

The main glacier erodes its valley more deeply than the tributary glacier.

After ice melts, the tributary valley remains at a higher level and appears hanging above the main valley.

Waterfalls may occur from hanging valleys.

Arete

An arete is a sharp ridge formed between two glacial valleys.

It is produced when two glaciers erode valleys on opposite sides of a ridge.

Horn

A horn is a sharp peak formed when several cirques erode a mountain from different sides.

It is surrounded by cirques.

Fjord

A fjord is a deep, narrow, steep-sided coastal valley formed when a glacial trough is submerged by seawater.

Outwash Plain

An outwash plain is a depositional landform created by meltwater streams flowing from a glacier.

These streams carry and deposit sand, gravel and other sediments in front of the glacier.

Esker

An esker is a long, winding ridge of sand and gravel deposited by streams flowing beneath a glacier.

Drumlins

Drumlins are smooth, elongated, oval-shaped hills formed by glacial deposition.

They usually occur in groups.

Moraines

Moraines are deposits of unsorted rock debris carried and deposited by glaciers.

They may occur along the sides, middle or end of a glacier.

Terminal moraines mark the farthest advance of a glacier.

Waves

Sea waves are important geomorphic agents along coastal regions.

They erode, transport and deposit materials along the coastline.

Wave action depends on:

• Coastal location
• Rock structure
• Wave energy
• Sea level change

Wave erosion creates many coastal landforms.

Erosional Landforms Formed By Waves

Important erosional landforms formed by waves include:

• Sea cliffs
• Wave-cut platforms
• Sea caves
• Sea arches
• Sea stacks
• Stumps
• Sea terraces
• Geos and gloups

Sea Cliffs

Sea cliffs are steep rock faces formed by the erosive action of waves along the coast.

Waves attack the base of the coast and create a notch.

With continued erosion, the upper part collapses, forming a steep cliff.

Wave-Cut Platform

A wave-cut platform is a flat rocky surface formed at the base of a sea cliff.

It is created as waves cut back the cliff over time.

Sea Cave

Sea caves are formed when waves exploit cracks, joints and weak zones in coastal rocks.

Continuous wave erosion enlarges these openings into caves.

Sea Arch

A sea arch is formed when waves erode a cave through a headland.

If erosion continues from both sides, the cave may open through the rock and form an arch.

Sea Stack

A sea stack is an isolated pillar of rock standing in the sea.

It forms when the roof of a sea arch collapses.

Stump

A stump is a smaller remnant of a sea stack.

It forms when waves further erode a stack.

Sea Terraces

A sea terrace or marine terrace is a rock terrace formed when a wave-cut platform and associated sea cliff are raised above sea level.

This may happen due to tectonic uplift or sea-level change.

Geos And Gloups

Waves repeatedly strike joints in the roof of a cave.

This compresses and releases air, enlarging cracks and forming a natural shaft.

When waves break into the cave, water or air may be forced upward through the shaft.

Such a shaft is called a gloup or blowhole.

Continued enlargement of blowholes and wave action can weaken the cave roof.

When the roof collapses, a long narrow inlet may develop. This is known as a geo.

Depositional Landforms Formed By Waves

Important depositional landforms formed by waves include:

• Beaches
• Bars
• Barrier bars
• Spits
• Lagoons

Beaches

Beaches are deposits of loose sediments such as sand, gravel or pebbles along the shore of a water body.

Bars

A bar is a ridge of sand and shingle formed in the offshore zone.

It usually lies almost parallel to the coast.

Barrier Bars

When an offshore bar becomes exposed above sea level due to further accumulation of sand, it is called a barrier bar.

Spits

When a ridge of sand is attached to land at one end and extends into the sea, it is called a spit.

Spits may form across the mouth of a river or entrance of a bay.

Lagoons

Bars may enclose shallow water bodies behind them.

These enclosed water bodies are called lagoons.

Over time, lagoons may fill with sediments.

Wind

Wind is an important geomorphic agent in arid and semi-arid regions.

Its work is most effective where:

• Vegetation is sparse
• Surface material is dry and loose
• Wind velocity is high

Wind forms both erosional and depositional landforms.

Erosional Landforms Formed By Wind

Important erosional landforms formed by wind include:

• Deflation hollows
• Mushroom rocks
• Yardangs
• Inselbergs

Deflation Hollows

Deflation hollows are depressions formed when wind removes loose, dry and fine particles from the surface.

They are common in desert regions.

Mushroom Rocks

Mushroom rocks are formed by wind abrasion.

Wind carrying sand particles erodes the lower part of rocks more strongly than the upper part.

As a result, rocks develop a narrow base and broad top, resembling a mushroom.

Yardangs

Yardangs are streamlined ridges formed by wind erosion.

They are usually elongated in the direction of the prevailing wind.

Inselbergs

Inselbergs are isolated residual hills rising abruptly from surrounding plains.

They are common in arid regions and represent resistant rock masses left after erosion.

Depositional Landforms Formed By Wind

Important depositional landforms formed by wind include:

• Ripples
• Loess
• Dunes

Ripples

Ripples are small, regular, wave-like ridges of sand.

They form at right angles to the prevailing wind direction.

Loess

Loess is a deposit of wind-blown silt-sized particles.

It is mainly composed of quartz.

The formation of loess requires:

• A source of silt
• Wind to transport the silt
• A suitable area for deposition

Dunes

Dunes are accumulations of loose sand formed by wind action.

They are usually made of quartz grains because quartz is highly resistant to weathering.

Common Types Of Dunes

Important dune types include:

• Barchans
• Parabolic dunes
• Seif dunes
• Longitudinal dunes

Barchans

Barchans are crescent-shaped dunes.

Their horns point downwind.

They form where:

• Wind direction is constant
• Sand supply is limited

Parabolic Dunes

Parabolic dunes form when sandy surfaces are partly covered with vegetation.

They are reversed barchans with the same wind direction.

Seif Dunes

Seif dunes are long, narrow dunes formed due to changes in wind direction.

They often have a single extended ridge.

They may also be similar to barchans but with only one wing or point.

Longitudinal Dunes

Longitudinal dunes are long ridges of sand formed parallel to the prevailing wind direction.

They form where sand supply is limited.

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