In general, the regions having precipitation less than 25 c per annum are termed as deserts and between 25 cm to 50 cm are term- as semideserts.
Running water plays the same role in the semideserts as' does in humid regions. However, some typical semi-arid landforms produced in the semi-deserts. In arid or desert regions, streams a; subdued and wind gets increasing importance as a geologic agen Permanent streams are rare in deserts. Streams may originate elsewhe and pass through deserts like the Nile. However, intermittent an ephemeral streams do occur in deserts.
Wind does its work everywhere on the earth's surface, even on th seas and glacial regions. However, its effect is more pronounced in th deserts, and semi deserts, on barren coasts, stream valleys and lands. In arid and semi arid regions, typical landforms are produced by the combin action of weathering, mass - wasting, running water and the wind.
Geological work of Wind: Wind does the same geological work like rivers and glaciers. It erodes the land, transports the eroded material and ultimately deposits them.
Wind acquires the lossened material by sweeping action which is termed as deflation. This blowing out of sand, dirt, and soil may create depressions on the gound. In deserts, this excavation forms blow-outs.
The cutting tools for wind abrasion are the sand particles, which strike against the bed rocks and obstructions that become spotted, grooved and polished. Wind abrasion becomes more effective when the tools are hard, velocity is high and the bed rock is soft. Rocks with hard and soft minerals are affected differentially forming honey comb structure. Attrition by the wind takes place when the blown particles collide against each other. Greater the velocity of the wind and greater the length of transit, greater is the effect of attrition. In the process, the sand particles become finer and spherical.
Transportation by the wind takes place through traction, saltation and suspension. The fine and light dust particles are picked up and carried away by suspension for hundreds of kilometers.
It is believed that the dust particles forming loess of China have been derived from the deserts of central Asia situated thousands of kilometers away. In case of some volcanic explosions, dust has almost travelled throughout the globe.
One such example of volcanic dust from the Krakatoa volcano of East Indies in 1883, in which case the dust remained in suspension in the atmoshphere for nearly three years. Relatively larger sand grains are transported by traction and saltation.
When the wind encounters any obstruction, the velocity is naturally reduced, resulting in deposition of transported sands. Once sand accumulation takes place, they themselves obstruct the wind movement and the load is deposited.
These are aeolian deposits, especially sand mounds. At times, this deposition is temporary. One important feature of the deposits is that the sand particles are arranged according to their size and weight. The finer dust particles are carried away for long distances and are ultimately deposited elsewhere.