When winds cross the desert, they carry vast quantities of dust either to another part of the desert or to some other areas. Smaller movements known as Eddies lead to formation of ripples which develop into dunes.

Sand dunes form where the wind deposits sand. A Desert sand dunes. B Sand dunes line many beaches like this one in Australia.

Ripples

These are very small features rarely more than a few centimetres high and they often occur on the slopes of dunes as well as on sandy surfaces.

Dunes:

These vary in size from a few hundred metres in height. They are mainly in two types namely, barchans and seifs.

Barchans dunes:

These are crescent shaped and lie at right angles to prevailing winds with horn like features point down wards. It results from a small rock obstruction of sand particles.

As it develops, edges are moved down to form a crescent shaped feature. The wind ward face is gently slopping but the leeward side is steep and slightly concave.

A barchan moves from one place to another by sand grains carried away and slopping over the concave side. They also vary in height from 30m to 400m. Barchans occur in Erg di Djourals, south of Tibesti Mountains, Djado plateau in Northern Nigeria.

Seif dunes
The word seif is an Arabic word meaning a curved sword. A seif dune is a ridge shaped with steep sides lying parallel to prevailing wind and to each other. The crest of a seif is sharp and may be over 100m high and 150km long. They are separated by flat corridors, 25-400m wide and corridors are swept clear of sand by prevailing wind.

Eddies blow up against sides of dunes. It develops from a small sand ridge and as it moves, it slowly moves forward in the direction of the prevailing wind. Examples are Egypt and Libya as well as Algeria.