the geographical formations produced by the aeolian actions

there are 3 processes of deposition (sedimentation,accretion and encroachment) as it is in case of erosion and transportation.Deposition of sand depends on the variations in wind speed,grain size and type,type of obstacle in the path of the wind(i.e.,trees,bushes,lakes etc).Sand is deposited on both windward and leeward side of the obstruction.Accumulated sand mounds on the either side of the obstruction are called sand shadows .sand drifts are sand accumulations between obstacles.

there are 3 types of depositional landform in arid region.they are:-

A)Sand ripples;

B)Sand dunes;

C)Loess.

A)Sand ripples:-

they are small scale depositional features of sand covering most dry,bare sandy surfaces in the deserts.Heights vary from 1cm. - 3cm.; windward slope angle between 8 degree and 13 degree and in extreme 25 degree.their summits are broadly convex with the lee faces having angles upto 30 degrees.some of them have either sharp or smooth brinks.

there are 3 main hypothesis of it's formation:-

1)Ballistic hypothesis:

Bagnold proposed it,basing his belief on the correspondence between ripple wavelength and saltation jump length.a chance strong edy eformed the bed.More sand is ejected on the windward side of this bump, and more landed one saltation path down wind to form the fort the first ripple. Saltation from this second site landed down wind to form the next and so on.

2)wave hypothesis:-

a)the bed as a fluid:- whereby the sand bed was seen as a dense fluid which deformed beneath the moving fluid above.

b)the saltation curtain as a fluid:- Proposed by Brugmans.Accordingly to this,the discontinuity between air and saltation curtain was deformed into gravity waves and are anchored by chance nuclei,ripples develops.

c)Wave like instability in the boundary layer:- It has little empirical support.It depends on the secondary air motion.It was perhaps formed due to regular ripple-scale longitudinal turbulence in the saltation curtain.

d)secondary motion in the lee of the transverse ripples:- wilson suggested centrifugal instability of the flow in the lee of the transverse ripple ridges.

3)Reptation ripple hypothesis:-

an initial irregularity generates perturbations in the population of reptating grains.Repeated ripples form after about 5000 saltation impacts.the ripples have strong peak in wavelengths in order of 6 mean reptation lengths.

B)Sand dunes:-

a dune is a sub aerial body of sand between about 30 cm. - 400 m. high and between about 1m. - 1Km. wide,whose shape has been adjusted to ambient wind conditions by the piecemeal deflation of sand-size particles. They are built on mineral sand (quartz ,carbonate sand) or sand-size aggregation of clays,salts or ice.dunes of all these materials can be stabilized by plant colonization or soil formation following a change of climate or sand supply.there are 3 main issues in dune genesis:-minimum size,initiation on firm desert surfaces and replication on the sandy surfaces.their formation requires the following favorable conditions or materials:-

a)sand abundance;

b)high velocity wind;

c)obstacles to trap and the fore the particles to settle  down;

d)suitable place for accumulation.

 dunes are classified on the basis of their morphology,location,ground pattern orientation,structure,number of slip faces,internal structure.

They are as follows:-

Loess(german term;meaning fine loam,loose/unconsolidated material):-

It represents thick deposits of unstratified,non-indurated,buff-colored,well sorted,fine grained sediments.there are total absence of lamineae. for the typical loess to develop,the dust must accumulate at more than 0.5mm/yr.A lower rate would lead to dilution.It requires 3 factors for it's formation:-

1)silt source;

2)wind to transport silt;

3)a suitable site for the deposition and accumulation.

most desert loess is silty while some are clayey.Present day dust are finer than Pleistocene desert derived loess. Many desert margin loess deposits have taken place in periods wetter than at present when there was more vegetation.In desert pavements the silt of the surface crust and the Av-horizon beneath the pavements are possibly loess deposits.Example:-'Parna' deposits,Australia.Loess profiles inferences of aridity and stability.