The minimum wind-velocity needed to start the saltation process is about 20 Km per hour. Panicles carried by saltation causes more abrasion. Laboratory experiments have shown that a high speed grain in saltation can move by impact, a surface grain six times its own diametre or more than two hundred times its own weight.
As we know, the power of the wind to transport the particles which are made available to it by the erosional processes largely depends on its speed. Whenever the velocity of wind is checked due to some or other reasons, a part or whole of the load starts getting deposited. Drifting material carried by the wind tends to be heaped up by the slightest obstacle and eventually leads to the formation of what is known as aeolian-deposits.
Accumulation of rock-particles etc. transported by the wind may begin due to various factors as
(i) Relief of the locality
(ii) Prevailing wind-velocity, and
(iii) Amount and distribution of vegetation cover etc.
The formation of a sand-dune and its shape is controlled to a great extent by the following factors:
(a) Amount of sand supply,
(c) Constancy of wind direction, and
(d) Amount and distribution of vegetation cover.
The dunes which constantly change form under wind currents are know as live-dunes or active dunes, while the inactive dunes, which have become more or less stabilized by vegetation cover, are known as fixed-dunes.
The mineral composition of the sand grains in sand-dunes depends on both the character of the original sand source and the degree of chemical weathering in the region. Most dunes are composed largely of the mineral quartz.
Besides, dunes are also found to contain grains of ferromagnesian minerals, gypsum and calcite grains.