The following are the main depositional landforms produced by the depositional action of wind

The following are the main depositional landforms produced by the depositional action of wind in the arid regions:

Ripple marks:

Ripple marks are very small features produced in unconsolidated sedi­ments or sand dunes at right angles to the wind direction. They stretch laterally for long distan­ces. Ripple marks are produced where there is some irregularity on the surface.

Sand ripples develop transverse to the wind direction. Their wave-length is hardly 1 meter. The sand ripples are characterized by coarse grains at their crests and finer particles in their troughs. When there are stronger winds, there are no ripple marks on the surface of a sand patch.

Sand shadows and Sand drifts:

Wherever an obstruction fixed in the path of wind checks its velocity, sand particles strike the obstruction and then fall at its windward base or be swept into the lee of the obstruction, and they accumulate there as a streamlined mound.

Both of these landforms are called sand shadows. They are fixed in size and form by the size and shape of the obstruction.

Sand drifts are accumulation of sand in the lee of a local high-velocity zone between obstacles. Another important site of sand drift is at the foot of a downwind cliff or escarpment.

Sand dunes:

The coarser materials are deposited in drifts in the shape of hills or ridges, called dunes. Any mound or ridge of sand with a crest or definite summit is called a dune. An ideal dune has a long windward slope rising to a crest and a much steeper leeward slope.

A sand dune may be defined as a mound or ridge of wind-blown sand, rising to various heights up to 50 m.

It is found in hot deserts and above high-water mark on low-lying coasts where sand is constantly renewed by onshore winds blowing across the sandy beaches. Desert sand dunes are generally characterized by the absence of natural vegetation.

For dune formation certain conditions are necessary such as, (i) a fairly continuous sand supply; (ii) a constant wind strength and direction; and (iii) an obstacle or series of obstacles to trap the sand.

Desert dunes are usually formed as wave patterns developed where the air flow interacts with the ground surface to create turbulence with dunes accumulating between the eddies.

Sand dunes are undoubtedly the most spectacular features of wind deposition. Sand seas in the Sahara desert are called ‘ergs’.

Sand dunes may be classified as live or fixed. Live dunes change their shape and move further under the effects of the wind. Dunes may also change their shape with the changes in wind direction and strength.

Sand dunes also move due to the erosion of their windward slope. Where wind direction and velocity are relatively constant, a dune can move forward while maintaining its form.

When wind is not fully loaded with newly acquired sand, it picks up more sand from the windward slope, and drops it over the crest, where it slips down the ‘slip face’. By subtraction of sand from one side and addition to the other the dune travels forward.

A dune whose shape and position do not change with time is said to be fixed. Dunes are generally fixed by vegetation, by the position of wind-breaking obstacle, or by back-and forth movement of the crest due to opposing winds. Stabilized dunes are found along the southern edge of the Sahara Desert.

Types of sand dunes:

Some basic types of sand dunes are the following:

(i) Barchans (iv) Longitudinal dunes

(ii) Parabolic dunes (v) Blow-out dunes

(iii) Transverse dunes

(i) Barchans or Crescent-shaped dunes:

Barchans are crescent-shaped isolated dunes. Their windward slope is the convex curve of the crescent, a gentle slope up which sand is moved.

The Concave Leeward slope is at the angle of repose-about 30° to 35° for dry sand. This is the steepest slope the dry sand can retain without slipping or falling.

(ii) Parabolic dunes:

Parabolic dunes are similar in shape to a barchan, but with the difference that the horns point upwind instead of downwind. It is usually formed by the process known as a blow-out in which the centre of the dune is partly removed and carried downwind, leaving the horns behind and drawn out in an elongated form.

Parabolic dunes appear along beaches in non-desert environments. The so called tails of these dunes are stabilized by the protective cover of natural vegetation wherever it is present.

(iii) Transverse dunes;

Transverse dunes are asymmetrical in shape, and form where light to moderate winds blow from a constant direc­tion. There must be an abundant supply of sand. These dunes take the shape of a series of crests and troughs whose peaks are perpendicular to the direction of prevailing winds.

These dunes appear like sea waves. The windward slope of trans-verse dunes is gentle, while the leeward slope is at the angle of repose (30° to 35°).

(iv) Longitudinal dunes:

Longitudinal dunes are long, parallel dunes that form where winds come from different directions. These dunes are called seifs in the Sahara Desert. Each dune is diversified by peak after peak ‘in regular succession like teeth of a monstrous saw’.

Where the prevailing wind is interrupted by strong cross-winds which bring in sand from the sides, the conditions are like those of a one-way-traffic which becomes densely crowded.

The crest of longitudinal dunes may reach a height of more than 100 meters. Such dunes are found south of the Quattara depression. There is a long tract of parallel seifs with corridors of bare desert floor between.

Such dunes are found in the deserts of North Africa, Arabia, and Asia. They are huge, sharp-crested dunes, hundreds of kilometers long and their troughs are almost without sand.

(v) Blowout dunes:

Blowout dunes are commonly formed on beaches where the supply of sand is abundant. There winds blow from a constant direction, and the natural vegetation has fixed the sand.

The shape of these dunes is that of an elongated hill with a deflation hollow on the windward side. However, if the vegetation has not sufficiently fixed the dune, its leeward side gradually moves towards the land as a parabolic dune.

Loess:

The term loess refers to the thick deposits of homogeneous un-stratified silt deposi­ted over vast areas of the grassy regions of China. The material has been picked up from the deserts of central Asia and is carried by wind to the south and southeast.

The extremely well-sorted, finely grained sediment is generally known by the German term Loess, meaning loose’ or unconsolidated. Loess has an Aeolian origin derived from the glacial outwash and other alluvium or fixed sand sheets.

The extremely good size- sorting during long-distance transport of windblown silt accounts for most of the significant properties of loess. This material, where thick enough, buries the underlying landscape. In fact, loess is a constructional landform.

Loess comprises silt-size quartz which is highly calcareous. That is the secret of its fertility. The loess deposits vary in thickness from a few centimeters to many meters. In China and the Gobi Desert it is about 30 or more meters thick.

Loess accumulates not only on plains, but it also covers hills and valleys with a blanket of uniform thickness. Though loess is fine and dusty to touch, it maintains vertical walls when dissected by a stream or artificially by a road.

It is easily eroded because of its fine texture and unconsolidated character. On loess-covered plains unprotected by vegetation, gullies are easily formed.

There are extensive loess deposits in northern and western China, the Pampas of Argentina, the north European plains and bordering uplands. Loess deposits are also found in the Ukraine, the Midwestern Plains and the Mississippi Valley of the United States.

Loess is the parent material for many of the most fertile agricultural soils developed in association with prairie in the American Midwest. In the Midwest of the United States there are deposits of loess which are called adobe.

Whalebacks:

The term ‘whaleback’ is used to describe a very large longitudinal dune with a flat top on which smaller barchans or self-dunes may occur.

Bagnold (1941) described such whalebacks in Egyptian desert which are 1-3 km in width and 50 m high and extend for 480 km. There are several smaller longitudinal dunes surmounting these great ridges.