Sand dunes are a special feature of the desert regions.
They are of different types and have a variety of shapes.
The major factors affecting their formation are
(a) amount of sand available
(b) direction and force of wind
(c) an obstruction in the path of the wind e.g. a bush, a stone or a dead animal.
The sand dunes are mobile and they keep on shifting from one place to another.
If vegetation or a line of trees starts growing on the dunes they become fixed.
They also become stationary when they are blocked by a hillock.
In case there is no such obstruction, sand dunes may bury agricultural land, plains and settlements.
There are two main types of sand dunes:
One common type of sand dune is an isolated heap of free sand called a barchan, or crescentic dune.
This type of dune has the outline of a crescent, and the points of the crescent are directed downwind.
On the upwind side of the crest, the sand slope is gentle and smoothly rounded.
They are found in large numbers in the Sahara Desert.
These are long, narrow ridges of sand that lie parallel to the direction of the prevailing winds.
Seif is similar to barchan with a small difference.
Seif has only one wing or point.
This happens when there is shift in wind conditions.
The lone wings of seifs can grow very long and high.
Seif dunes are common in the western part of the Thar Desert of India.
In several large areas of the world, the surface is covered by deposits of wind transported silt that has settled out from dust storms over many thousands of years.
This material is known as loess.
It is very easily eroded by running water and is subject to rapid gullying when the vegetation cover that protects it is broken.
The thickest deposits of loess are in northeast china, where a layer over 30m deep is common and a maximum thickness of 100m has been measured.
Besides China, deposits of loess occur in Mississippi Valley of North America and north of Central European Upland in Germany, Belgium and France.
Loess deposits are found in Australia also.
Mushroom Rocks (Or Rock Pedestals)
When rocks, consisting of alternate hard and soft layers are subjected to wind abrasion, differential erosion results.
The soft layers are easily eroded but the hard layers resist erosion.
As a result of undercutting near the base (due to greater amount of sand and rock particles being transported close to the ground), the resulting feature resembles a rock pillar shaped like a mushroom.
Such formations are common in the Sahara Desert, and are also seen near Jodhpur.