Maarg Manthan · Topic 3.18

Wind Depositional Landforms: Barchans, Seif, Parabolic, Transverse and Longitudinal Dunes

Geography › Geomorphic Processes and Landform Evolution · Topic 3.18

Wind drops its load of sand and dust where its speed falls, and builds depositional landforms. The most important are sand dunes: crescent-shaped barchans, long seif (longitudinal) dunes, parabolic dunes, transverse dunes and longitudinal dunes. Wind-blown silt forms loess plains, and large areas of sand form sand seas called ergs. The Thar Desert of India, with its barchans and longitudinal dunes, is a well-known example, and its dunes move gradually with the monsoon winds.

6 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution

At a Glance

Barchan Crescent dune, horns downwind
Seif Long sharp-crested dune
Parabolic U-shaped, horns upwind
Loess Wind-blown silt
India Thar Desert
Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution

Where Does It Fit?

This topic completes the pair on wind-made landforms with the features of deposition. It follows the erosional forms of the last topic, whose removed sand and silt build the dunes and loess described here. Previous topic: Wind (Aeolian) Erosional Landforms: Pediments, Pediplains, Playas, Deflation Hollows, Mushroom Rocks. Next topic: Running Water Erosional Landforms: V-Shaped Valleys, Gorges, Canyons, Potholes, Incised Meanders, River Terraces.

What Are the Depositional Aeolian Landforms?

Where the wind loses speed or meets an obstacle, it drops its load. Sand accumulates as dunes, and a large area of sand dunes is an erg or sand sea. Common dune types:

Dune	Shape and setting
Barchan	Crescent-shaped, with the horns pointing downwind; forms where wind is steady in one direction and sand is limited; common in the Thar and the Sahara
Transverse dune	Long ridge at right angles to the wind; where sand supply is abundant
Seif (longitudinal) dune	Long, narrow ridge parallel to the wind; can be hundreds of km long, in the Sahara and Arabia
Parabolic dune	U-shaped, with the arms pointing upwind, held by vegetation
Star dune	Pyramid with several arms, where winds blow from many directions

Wind also carries fine dust over long distances. Loess is a fine, yellowish, wind-blown silt deposited in thick layers, as in northern China, where the Loess Plateau was built by dust from the Gobi and Central Asian deserts, and in parts of the Punjab and Haryana plains. Loess forms fertile soils. The Thar desert of Rajasthan has barchans and many other dunes, as does the Great Indian Desert across Rajasthan, Gujarat, Punjab and Haryana, and the Cold Desert of Ladakh has dunes too, for example around Nubra Valley.

How Do the Main Dune Types Compare?

Dune	Shape and direction	Conditions	Example
Barchan	Crescent with the horns pointing downwind; moves forward	Limited sand, steady wind from one direction	Thar, Sahara, Peruvian coast
Transverse	Long ridges at right angles to the wind	Abundant sand, little vegetation	Parts of the Sahara and coasts
Parabolic	U-shaped with the arms pointing upwind; often anchored by plants	Sand with partial vegetation, coastal blowouts	Coastal dunes
Longitudinal (linear)	Long ridges parallel to the wind	Strong, steady wind and moderate sand	Simpson Desert, Australia; parts of the Thar
Seif	Sharp-crested longitudinal dune, which often snakes along	Winds from two directions in a season	Sahara, Arabian deserts
Star	Pyramid with several arms	Winds from many directions	Grand Erg Oriental (Sahara)

Dunes move: a barchan advances by sand being blown up its gentle windward slope and falling down the steep slip face, at a pace of a few metres to tens of metres a year.

What Is Loess and What Are Sand Seas?

Loess: fine, yellowish, windblown silt, which is often laid down on a large scale from dust blown out of deserts and glacial outwash. It is fertile and easily eroded. Thick loess covers northern China, the Great Plains of the USA, parts of Europe and Central Asia, and the Potwar Plateau in Pakistan.
Ergs (sand seas): vast areas of dunes, as in the Sahara, the Arabian Rub al Khali and the Thar. Only about a fifth of the world’s deserts are sandy; most are rocky or gravelly.
Dust: fine dust may be carried across oceans, for example Saharan dust that fertilises the Amazon and falls over the Atlantic.

Why Does the Thar Desert Matter for India?

The Thar occupies about 200,000 sq km of Rajasthan and extends into Gujarat, Haryana and Punjab, with rainfall mostly under 250 mm in the west. It is the most densely populated desert in the world. The Aravalli range lies to its east, and the Indira Gandhi Canal brings Himalayan water to western Rajasthan, which has made farming possible in parts of the desert. Shelterbelts and stabilisation of dunes are used to fight encroaching sand. The monsoon winds do not bring much rain to the Thar, as in Monsoon Mechanism: Traditional and Contemporary Theories (Air Mass Theory, Jet Stream Theory). As an analytical point, desert landforms show how a landscape records both the dominant process and the rare event: the dunes show the wind of centuries, but the wadis show flash floods of a few hours.

What Is the Significance for India and the Environment?

Dunes shift under the monsoon wind and may bury farmland, canals and roads, a process called desertification, which is a global concern. In Rajasthan, the Indira Gandhi canal and shelterbelts of trees have been used to stabilise sand and bring irrigation. The wind erosion that supplies the sand is described in Wind (Aeolian) Erosional Landforms: Pediments, Pediplains, Playas, Deflation Hollows, Mushroom Rocks. As an analytical point, wind deposits show the direction of the formative winds, and ancient dune fields in India show that the climate of the past was drier.

Why Is This Topic Important?

Terms Dune types are asked
India Thar and desertification
Climate Past winds recorded

What Came Next?

The next topic moves to landforms made by running water, beginning with erosional forms. Next topic: Running Water Erosional Landforms: V-Shaped Valleys, Gorges, Canyons, Potholes, Incised Meanders, River Terraces.

Key People and Terms

Barchan Crescent-shaped dune
Seif Sharp-crested longitudinal dune
Erg Sand sea
Loess Wind-blown silt deposit
Slip face Steep leeward side of a dune
Desertification Spread of desert-like conditions

Exam Corner

Points to Remember

Barchan horns point downwind; parabolic arms point upwind.
Loess is silt, not sand.
Longitudinal dunes lie parallel to the wind.
Transverse dunes lie across the wind.
The Thar lies mainly in Rajasthan.

Do Not Confuse With

Barchan and parabolic dune Horns downwind versus arms upwind
Transverse and longitudinal dunes Across the wind versus along it
Loess and sand dune Silt sheet versus sand ridge

Memory Hook

Barchan horns downwind, parabola arms to the wind; loess is silt, sheets that thinned.

Mains Angle

Questions ask for dune types and desertification. These points add depth.

Forms Dunes and loess
Movement Dune migration
Control Shelterbelts and canals

A Question You May Face

An original practice question, not a past paper question.

“Describe the depositional landforms made by wind and discuss the spread of desertification in India.”

How to Answer

Dunes Types.
Loess Origin and extent.
Thar Features.
Control Canal and shelterbelts.

GS Relevance

Prelims: types of dunes, loess, Thar Desert. GS Paper 1: aeolian landforms; desertification.

Frequently Asked Questions

What is a barchan?

A barchan is a crescent-shaped sand dune with its horns pointing downwind. It forms where the wind blows steadily from one direction over a hard surface with a limited supply of sand, and it can slowly move forward. Barchans are common in the Thar and the Sahara.

What is loess?

Loess is a fine, yellowish, wind-blown silt deposit that forms thick, fertile layers. The best-known loess deposit is the Loess Plateau of northern China, built up by dust from the deserts of Central Asia, and there are also loess areas in the plains of north India.

Where is the Thar desert located?

The Thar desert, or the Great Indian Desert, lies mainly in western Rajasthan and extends into Gujarat, Haryana and Punjab, with part in Pakistan. It covers about 200,000 sq km in India, receives under 250 mm of rain in the west, and has many barchan dunes.

What is a seif dune?

A seif dune is a long, sharp-crested sand dune that lies parallel to the prevailing wind, often with a snaking shape, and is formed where the wind comes from two directions in different seasons. Such dunes are common in the Sahara and Arabian deserts and can be tens of kilometres long.

What is an erg?

An erg is a large area of wind-blown sand with dunes, also called a sand sea. Examples include the Grand Erg Oriental in the Sahara and the Rub al Khali in Arabia. Ergs form only a part of the world's deserts, since most deserts are rocky or gravelly.

PYQ Practice — Statement Analysis

1 Barchan dunes are crescent-shaped with horns pointing downwind.

True

They form under steady one-directional wind.

2 Loess is a deposit of coarse sand.

False

It is fine wind-blown silt.

3 Parabolic dunes have their arms pointing upwind.