This is a lecture about the deposition by wind, dunes and forms of dunes

1. Wind Deposition
By
Kamran Shehzad

2. Loess
• Loess is a deposit of wind-blown silt and clay composed of unweathered, angular grains of quartz,
feldspar, and other minerals weakly cemented by calcite
• Loess has a high porosity, typically near 60%
• Deposits of loess may blanket hills and valleys downwind of a source of fine sediment, such as a
desert or a region of glacial outwash.
• Loess can reach thicknesses of 100 meters (300 feet)
• China has extensive loess deposits, more than 100 meters thick in places. Wind from the Gobi
Desert carried the silt and clay that formed these deposits.
• Loess is easy to dig into and has the peculiar ability to stand as a vertical cliff without slumping
(figure), perhaps because of its cement or perhaps because the fine, angular, sediment grains
interlock with one another
• For centuries, the Chinese have dug cavelike homes in loess cliffs. When a large earthquake shook
China in 1920, however, many of these cliffs collapsed, burying alive about 100,000 people
• Soils that have developed from the loess are usually fertile and productive
• The fertile soils of the midwestern and Pacific northwestern United States include loess.

3. • Home built into
a steep cliff of loess
in central China

4. Desert pavement
• Desert pavement is a thin, surface layer of closely packed pebbles
• The pebbles were once thought to be lag deposits, left behind as strong
winds blew away all the fine grains of a rocky soil
• The pebbles are now thought to be brought to the surface by cycles of
wetting and drying, which cause the soil to swell and shrink as water is
absorbed and lost by soil particles.
• Swelling soil lifts pebbles slightly; drying soil cracks, and fine grains fall into
the cracks. In this way pebbles move up, while fine grains move down
• The surface layer of pebbles protects the land from wind erosion and
deflation
• When the desert pavement is disturbed (as in the 1991 and 2003 Gulf
Wars), dust storms and new sand dunes may result

5. Desert varnish
• Many rocks on the surface of deserts are darkened by a chemical coating known as desert
varnish.
• Although the interior of the rocks may be light colored, a hard, often shiny, coating of dark
iron and manganese oxides and clay minerals can build up on the rock surface over long
periods of time
• These paper-thin coatings can be used to obtain a numerical age of the exposed desert
surface by measuring cosmogenic helium-3 isotopes preserved in the desert varnish.
• Although no one is quite certain how this coating develops, it seems to be added to the rocks
from the outside, for even white quartzite pebbles with no internal source of
iron,manganese, or clay minerals can develop desert varnish.
• One hypothesis is that the clay is windblown, perhaps sticking to rocks dampened by dew. A
film of clay on a rock may draw iron and manganese-containing solutions upward from the
soil by capillary action, and the presence of the clay minerals may help deposit the dark
manganese oxide that cements the clay to the rock, or silica from the dissolution of the rock
surfaces may form the coating
• Another hypothesis is that the oxide is deposited biologically by manganese-oxidizing bacteria
• Regardless of how the varnish forms, the longer a rock is exposed on a desert land surface,
the darker it becomes

7. Sand Dunes
• Sand dunes are mounds of loose sand grains piled up by the wind
• Dunes are most likely to develop in areas with strong winds that generally
blow in the same direction
• Extensive dune fields occur on some of the largest deserts of the world,
such as the Sahara Desert of Africa, which contains vast sand seas
• Dunes are also commonly found just landward of beaches, where sand is
blown inland
• Dune material varies in composition and includes sand size grains of
quartz, feldspar, calcite, gypsum, and rock fragments that are well sorted
and well rounded.
• A dune's shape is constantly changing according to the wind direction
• The steeper, downwind slope is called the slip face

8. Sand Dunes
• The loose sand maintains angles up to about 35 degrees (Angle of repose
for loose sands) on the slip face, creating cross-bedded layers
• When found in sandstone, such cross-bedding strongly suggests deposition
as a dune
• Dunes migrate in the direction of the prevailing winds about 12 meters (40
feet) per year, a result of the wind continually eroding the gentle slope and
redepositing the sand on the slip face
• Sand moving over a dune surface typically forms wind ripples —small, low
ridges of sand produced by saltation of the grains
• Because sand moves perpendicularly to the long dimension of the ripples,
a rippled sand surface indicates the direction of sand movement

10. Types of dunes
• On the basis of particle composition the dunes are of two types
• Sand dune:
• The dune composed of sediment particles having the diameter of
0.06 to 2.0 mm i.e., sand is called Sand dune e.g., western arid
Rajasthan, India
• Gypsum dune:
• The dune composed of gypsum particles of sand size is called gypsum
or calcareous or time dune e.g. New Mexico and Death valley
California

11. Types of Sand Dunes
• Dunes tend to develop certain characteristic shapes, depending on
• (1) the wind’s velocity and direction (that is, whether constant or
shifting);
• (2) the sand supply available;
• (3) how the vegetation cover, if any, is distributed
• On the basis of origin and shape, the sand dunes are of the further
following types

12. Types of Sand Dunes
• Barchan dunes are widely separated, crescent-shaped dunes that form in areas of sparse sand
• Often found on bedrock, the ends of the crescents point downwind
• Barchan dunes are usually separated from one another and move across a barren surface
• Transverse dunes are a series of long ridges that form perpendicular to the wind. More sand is available in
this case as compared to barchan dunes
• They typically occur in coastal areas.
• Parabolic dune requires abundant sands usually form around a blowout in vegetated areas
• It is somewhat similar in shape to a barchan dune, except that it is deeply curved and is convex in the
downwind direction. Because they require abundant sand and strong winds, parabolic dunes are typically
found inland from an ocean beach
• A barchanoid dune, an intermediate variety between barchan and transverse dunes, is formed of scalloped
rows of sand perpendicular to the wind
• It resembles a series of side-by-side barchan dunes.
• Star dunes are isolated hills of sand formed by variable winds in the Sahara and Arabian Deserts
• The bases of these dunes resemble multipointed stars.

13. Types of Sand Dunes
• One of the largest and most dramatic dunes is the longitudinal dune, or seif
• A large ridge of sand that parallels the wind direction, it can be over 100 meters
high and over 100 kilometers long
• They are exceptionally straight and regularly spaced and are typically separated
by barren ground or desert pavement
• Numerous hypotheses have been proposed to explain the development of
longitudinal dunes, but none can adequately explain their spectacular size and
regular spacing
• It appears that crosswinds are important in piling up sand, which adds to the
height of longitudinal dunes, whereas the more constant prevailing wind
direction redistributes the sand down the length of the dunes.
• Smoke bomb experiments to analyze airflow have shown that the wind spirals
down the intervening troughs between longitudinal dunes and may control the
regularity of their spacing.

14. Types of sand dunes