- Materials eroded & transported by winds must come to rest somewhere.
- The finest dust travels enormous distances in the air sometimes as long as 2300 miles before they settle down.
- The dust from Sahara desert is sometimes blown across the Mediterranean to fall as blood rains in Italy or on the glaciers of Switzerland.
- Dust that settles in Hwang Ho basin (also known as Hwangtu – the yellow earth) from the Gobi desert has been accumulated over past centuries to a depth of several hundred feet
- As wind borne materials are shifted according to their coarseness, it can be expected that the coarser sands will be too heavy to be blown out of desert limits.
- They remain as dunes or other depositional landforms within desert themselves.
- Hills of sand formed by the accumulation of sand & shaped by the movement of winds, a striking characteristic of erg or sandy desert
- Can be classified as active or live dunes, constantly on move or inactive fixed dunes, rooted with vegetation
- Two most common types of dunes are Barchan & Seifs
- Crescent or moon shaped live dunes which advance steadily in the particular direction of prevailing winds.
- Initiated probably by a chance accumulation of sand across an obstacle, such as patch of grass or a heap of rocks
- They occur transversely to the wind, so that their horns thin out & become lower in the direction of the wind
- Mainly due to reduced frictional retardation of the winds around the edges.
- The windward side is convex & gently sloping whiles the leeward side, being sheltered, is concave & steep.
- The crest of sand dunes moves forward as more sand is accumulated by the prevailing wind.
- The sand is driven up the windward side & on reaching the crest slips down the leeward side so that the dune advances
- The migration of Barchans may be a threat to desert life as they may encroach on an oasis burying palm trees & houses.
- Long rooted sand holding trees & grasses are therefore planted to halt the advancement of the dunes to prevent areas of fertile land from being devastated.
Seif or longitudinal dunes
- Long narrow ridges of sand, often over a hundred miles long, lying parallel to the direction of the prevailing winds, with their crustline rises & falls in alternate peaks & saddles in regular successions.
- Dominant wind blows straight along the corridor between the lines of the dunes so that they are swept clear of sand & remain smooth.
- The eddies that are setup in corridors, blow towards the side of the corridor & drops the sand to form the dune.
- In this manner, prevailing winds increases the length of the dunes into tapering linear ridges while occasional cross winds tends to increase their heights & width.
- The fine dust blown beyond the desert limits is deposited on neighbouring lands as loess.
- It is a yellow, friable (softly crumbled) material rich in lime, very coherent, extremely porous & is usually very fertile.
- Water sinks in readily so the surface is always dry, with streams may cut into thick mantle of soft loess to develop badland topography.