PART III : AEOLIAN Or WIND GEOMORPHOLOGY & LANDFORMS

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PART III : AEOLIAN or WIND GEOMORPHOLOGY & LANDFORMS

While water is the most important landscape-forming agent in temperate and humid climates, ice fulfils this role in colder climates. Coastal geomorphic processes (wave and tidal action) likewise are the major processes where fluvial systems and oceans meet. The landscaping effects of groundwater action are limited to karst areas, which is about 10% of the earth,s surface where limestone and dolomite formations occur. The remaining landscape creating work is done by wind in all areas where water is in short supply. As South Africa is considered a water scarce country, aeolian, or wind-driven processes are therefore a key player in determining the form and function of our landscapes.

Unique characteristics of arid regions include that chemical weathering occurs at a much slower rate than where water is abundant. Physical weathering is therefore the primary weathering agent and results in rocks and talus being deposited on the land surface. These materials are largely chemically unchanged and are further characterised by a low content of organic material and water, which in turn is responsible for very slow rates of soil formation. Dry regions are also characterised by sparse vegetation, which would normally protect and anchor soil and talus; the conditions in arid regions are therefore especially favourable for wind action to blow away dry talus and to be a major force in landscape genesis. 2

The distribution of Aeolian geomorphic action on earth

While wind occurs everywhere on the earth’s surface, it is only where surface water is scarce that wind action becomes a major landform- creating agent. The main regulators determining if a landscape will be prone to mainly water or wind erosion are therefore climate and more specifically temperature and rainfall. Hot deserts and semi-deserts (BSh & BWh regions according to the Koppen classification system) are the regions most prone to wind as landform creating agent. While wind is the more prevalent force in shaping desert landforms, water is the more powerful. Despite water being so scarce in these environments, it is the primary agent of erosion in the desert, with many features owing their formation to mass wasting and running water as sheet floods. Even the driest deserts receive occasional rainfall, which then usually occurs as downpours which cause landform creation processes over a large scale in a short time period. Mass action and fluvial action are therefore the main landscape-forming processes in deserts with wind being only responsible for secondary, localised and other minor landforms. The wind action, however, occurs over a persistent and ongoing basis and has an enduring presence and signature in arid landscapes. Due to climate changes over time, the landscape bears evidence of former landforms that were created when different climatic conditions controlled the prevailing landforms, such as the sandstone deposits in the Eastern Free State and Lesotho: these date from times when wind action and deposition by water were the chief landforming agents.

In Africa there are many hot, arid regions, such as the Namib, Sahel and Sahara deserts; and also many semi-arid regions including Namaqualand, the Kalahari and parts of Ethiopia and Somalia. These regions are all have many features in their landscape that were created by the wind-cascade system. Other parts of Africa have long, dry seasons and are subject to prolonged droughts, even though they are not classified as semi-arid regions. In these regions, as well as those where dryland cultivation leaves large areas uncovered by vegetation or crops for part of the year, large volumes of surface material is lost annually and wind erosion plays a major role in forming the landscape. Furthermore, landforms created by wind can also occur in dry river valleys, along coasts and along river floodplains where large quantities of talus are present.

Wind is one of the three dominant agents in hot deserts, after mass wasting and occasional water action. The desert floors get heated up rapidly as they are dry and consist of mainly mineral silicas, while being virtually devoid of organic matter and vegetation. These heated floors heat the air directly above them, resulting in upward movements in the hot lighter air with turbulence, and any obstructions in its path sets up eddies, whirlwinds, updrafts and downdrafts. Winds also move along the desert floors with great speed and the obstructions in their path create turbulence, while very destructive storm winds also occur.

Some fluvial processes and landforms found in deserts.

Deserts are defined by their lack of water and while coastal deserts may experience one or two rainfalls a year; those further inland may get rain once or twice a decade only. Although rain is scarce in deserts (the Sahara’s average annual rainfall is only 127mm), it usually comes down torrentially and lasts mere minutes or a few hours. This enhances rainsplash erosion on the bare, unvegetated desert soil, loosening and spattering soil particles in all directions. The desert rock material also undergoes accelerated mechanical and chemical weathering processes due to drastic diurnal temperature changes, decaying fast, with the torrential rains helping to remove the weathered materials speedily. The dislodged soil and talus is carried off with the flowing water, which picks up and transports more sediment as it rushes along. Owing to a lack of organic-rich soil that can absorb the water, and also the absence of vegetation, the desert landscape offers little resistance to water action and erodes rapidly. Water therefore accomplishes within a few days what would take desert winds a year to accomplish, which 3 means that the weathered debris in deserts is moved by not only wind, but also significantly by rain/sheet wash.

Finding natural depressions, such as gullies and canyons, the sediment-laden water gains speed and power as it is confined and flows downward. Increased velocity allows the water to pick up more and larger sediments and rock debris, eroding them and the surface below as it rushes along. Often clogged with so much debris, the water can resemble a mud-flow (a thick mixture of water, mud, and other surface fragments). Arroyos, the dry streambeds created by previous rains, again fill with water. When an arroyo finally opens onto a flat, broad plain, the rushing water flows out and drops its load of sediments, forming a new alluvial fan. In other areas, basins or depressions in the desert floor fill with water, forming playa lakes that soon evaporate, leaving a dry, cracked, salty lake bed that will remain until the next rain.

Alluvial fan

Precipitation that falls in higher elevations in deserts flows rapidly down to flat areas through canyons, valleys, and other narrow, confined channels. Because most desert soil lacks plants and their root systems to help hold the soil together, the flowing water easily picks up any loose material in its path. The faster the water flows, the larger the pieces of material it is able to pick up and carry along.

When the rushing water finally reaches a plain or flat area, it loses power since gravity is no longer helping it flow down a steep slope. As it slows, the water is unable to carry the sediment—gravel, clay, sand, and silt—it picked up on its way downhill. Large rocks and other heavy material are deposited first at the base of the canyon, followed by other material in decreasing size. No longer confined to a narrow channel, the water spreads out the farther it moves away from the base of the canyon. The finest material it carried is deposited at the outer edge. When the water evaporates, the sediments remain behind. Over time, as more water flows onto the plain, more sediment is deposited, and a wide, fan-shaped pile known as an alluvial fan forms. When two or more alluvial fans merge on a plain to create a broad, sloping surface, they form a bajada.

Arroyo

An arroyo is a desert landform sculpted by the action of water. Sudden heavy downpours cut channels in the desert floor, often in canyons or other low-lying areas. These fast-moving but short-lived streams create deeper channels or gullies with steep sides and an almost flat bed or bottom. Just as quickly as the water appears, it disappears in the normally dry desert environment. What remains is an arroyo, a dry watercourse with a floor that is often gravel-strewn. Stream channels in desert areas are broad, smooth and indefinite and flow for a brief time after rains.

Bajada.

When two or more alluvial fans merge on a plain to create a broad, sloping surface, they form a bajada.

THE WIND CASCADE SYSTEM

The work performed by aeolian systems is concentrated in desert regions and therefore desert is will be used as model to describe this wind cascade system. On the basis of its morphological characteristics, the desert landscape can be subdivided into the deflation-corrasion and the aeolian-accumulation subsystems. The processes of deflation and corrasion dominate in the deflation-corrasion subsystem, while the deposition of wind-transported talus is the major process in the aeolian-accumulation subsystem.

Deflation is the process of removing talus on the earth surface by wind, while corrasion is the physical weathering of rock by talus-carrying wind. Corrasion processes include abrasion, chipping and polishing. The deflation-corrasion subsystem consists of the Hamada and the serir. The hamada has 4 exposed rock surfaces and products of mechanical weathering , namely angular rock fragments scattered about. The serir is a stony desert with smaller rock fragments impacted mainly by corrosion processes. The size of the rock fragments decreases in the direction of the aeolian-accumulation subsystem, from boulders down to sand grains. The aeolian-accumulation subsystem is therefore characterised by fairly homogeneous sand-sized particles and landforms due to accumulation – such as extensive sand sheets, seif dunes, transverse dunes and barchans dunes.

The following are the major morphological regulators of the two subsystems:

Morphological characteristics Deflation-corrasion subsystem Aeolian-accumulation subsystem Particle size Particle size Rock composition Presence of obstructions [landforms, vegetation] Rock structure Spacing of obstructions Rock texture Slope of obstructions Depth of soil-water table Height & extent of obstructions Depth of groundwater table

Regulators Deflation-corrasion subsystem Aeolian-accumulation subsystem Wind velocity Wind velocity Talus present or absent Talus present or absent in air current Size & shape of talus particles Size & shape of talus particles

Wind velocity is a threshold regulator: in practice this means that wind needs to attain a velocity of 5.3 to 7.5 m/s (19 km/h to 27 km/h) to be able to blow fine particles up into the air, or to roll grains of sand along the surface. Velocities lower than these will not cause any movement of particles. Therefore, if talus is available, then its particle size will act as regulator of whether the threshold velocity will cause deflation or corrosion ( if velocity > 20 km/h) picking up or moving the talus particle or whether it will remain stationary in the Aeolian-accumulation subsystem.

EROSIONAL LANDFORMS

Overview: An overview of wind-driven processes and landforms in deserts

The wind moves like a fluid and has erosional ability only if it is strong enough. More often, it merely transports material and very small particlesof less than 0.2 mm in diameter, can be picked up easily and carried aloft for hundreds or thousands of miles by desert winds. Suspended on air currents, dust from Africa's Sahara Desert sometimes crosses the Atlantic Ocean before landing in the west Atlantic and Caribbean Sea, while on the other hand, sand particles of 0.2 to.6.4 mm diameter can be carried only by extremely strong winds. Silt and other very small-sized particles fill the air during dust storms, but these and most other wind-borne grains are too small to cause erosion or sandblasting of major landforms that stand high above the desert floor.

Wind-blown sediment causes the most erosion at a height of no more than 250 mm above the surface, where the wind removes fine-grained particles, which causes deflation - the lowering of the land surface due to the removal of particles by the wind. This continued action leads to blowouts and the formation of desert pavements. Another process is abrasion, whereby sand and silt act as effective tools to abrade 5

(scour) the land surface while it is transported by wind. Any stone or part of the bedrock that has been abraded or shaped by the wind is known as a ventifact (artefact of the wind). A yardang is one large desert landform that is sculpted by the wind though deflation and abrasion. A third process caused by wind action is impact, which is simply the sheer force of momentum which occurs when sand is blown into or against a rock surface, similar to a sandblasting operation. Wind action therefore creates a number of interesting erosional and depositional features in the deserts.

Sand storm, Tunisia Wind storm, Tunisia http://www.nicholas.duke.edu/eos/geo41/win002.gif http://www.nicholas.duke.edu/eos/geo41/win003.gif Figure 3.1: Sandstorms in deserts

The wind transports larger-grained sediments, particularly sand, through the process called saltation.

While light enough to be picked up by strong wind, sand is too heavy to remain suspended in the air. As a result, it is moved along the Earth's surface by the wind through a series of short jumps and bounces. The majority of sand transported in this way travels within 0.6 meter of the ground. As saltating particles crash to the ground, they can dislodge and move slightly larger particles, such as small pebbles. The sliding and rolling movement of these particles is called surface creep.

Figure 3.2: Suspension, saltation and creep

LANDFORMS

Pediments and pediplains

Landscape evolution in deserts is primarily concerned with the formation and extension of pediments. Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris, are called pediments. Such rocky floors form through the erosion of mountain fronts through a combination of lateral erosion by streams and sheet flooding. Erosion starts along the steep margins of the landmass or the steep sides of the tectonically controlled steep incision features over the landmass. Once pediments are formed with a steep wash slope followed by a cliff or free face above it, the steep wash slope and free face retreat backwards. This method of erosion is termed parallel retreat of slopes through backwasting. So, through this process, the pediments extend backwards at the expense of the mountain front, and gradually the mountain gets reduced leaving an inselberg which is a remnant of the mountain. This explains how the high relief in desert areas is reduced to low featureless plains called pediplains. 6

Playas

Plains are by far the most prominent landforms in the deserts. In basins with mountains and hills around and along, the drainage is towards the centre of the basin and due to gradual deposition of sediment from basin margins, a nearly level plain forms at the centre of the basin.. When the water falls on fairly flat areas, it may collect in a basin or other slightly depressed area, forming a small lake that may last for a while before the water evaporates or is absorbed These shallow lakes are called playas where water is retained only for short duration due to evaporation losses. Evaporation causes the gradual concentration of salts in the playa, as only pure water evaporates, leaving behind the sediments, mostly clay, silt, and various dissolved salts, which form a level, broad, cracked surface. The playas therefore usually contain a good deposition of salt and such playa plains covered up by salts are called alkali flats.

.When water is still present, these bodies are called playa lakes. Although they are very rare, permanent desert lakes do exist. Two examples are the Great Salt Lake of Utah and the Dead Sea of Israel and Jordan.

Deflation Hollows and Caves

Following water, wind is a major cause of erosion in the desert. Without plants and their anchoring roots, loose desert soil is moved easily by near-constant blowing winds. Blowouts, also known as deflation basins or hollows, are depressions made in sand or light soil by strong wind action and may range in size from metres to several kilometres in diameter. As the name of the landform indicates, they are formed by the process of deflation. These depressions may extend over several square kilometres. Blowouts can form around desert plants with hardy roots or around rock structures, leaving them perched atop a column as more and more sand or soil is blown away. Deflation also creates numerous small pits or cavities over rock surfaces. The rock faces suffer impact and abrasion of wind-borne sand and first shallow depressions called blow outs are created, and some of the blow outs become deeper and wider, to the extent that they can be called caves.

There are some examples of deflation hollows in the Richtersveld and in Bushmanland in the northwestern extreme of the Northern Cape Province and also near Velddrift in the Western Cape Province. Because the hollows or cavities that form trap surface water, or at least store more water than their immediate surroundings, this promotes some chemical weathering of the underlying rock, which in turn causes further removal of talus by wind action. The depth of the water table regulates the depth to which a depression hollow can erode, because as soon as the water table is intersected, an oasis, swamp or pan is formed.

This moistened landform is then not subject to further wind erosion. Pan forms are abundant in the Kalahari (in the Northern Cape Province ) and in Botswana, with the extensive Makarikari depression and Verneukpan as examples of large-scale landforms, while there are thousands of much smaller pans dotting this region as well. Other examples of deflation hollows include the shallow salt lakes of the Qattara Depression in Eqypt and the Etosha Pan in Namibia. The Etosha Pan is situated on the floor of a deflation hollow that developed on a structural basin and is one of the largest salt pans in the world, and an attraction for a spectacular diversity of wildlife.

Desert pavements (also: Reg, Hamada or gibber plain)

The strength of the wind determines how much and what type of material will be removed from the desert floor. With increasing strength, wind is able to move and transport more and larger particles. Initially, very fine particles are removed, but as more and more of these types of particles are removed, the surface of the land is lowered. This action, known as deflation, continues until what remains on the desert floor is a 7 layer of closely packed pebbles and rocks too heavy for the wind to move. Settled and wind-polished, the entire surface is called a desert pavement or reg. The older the pavement, the smoother and flatter it appears, like a highly worn cobblestone street. Desert pavements are gravel covered plains such as the serirs in Libya from which the landform of the one main component of the deflation-corrasion subsystem derives its name. These stony deserts with small rock fragnments are referred to as ‘gibber plains’ in Australia. A similar landform is the Hamada or reg, (e.g the Hamada el Homra in Libya and the Tanezrouft in Algeria), characterised by exposed rock surfaces with scattered rock fragments. In the Sahara an extensive stony desert plain is known as Reg, not to be confused with Erg, which refers to a sandy desert area. The term gibber plain is used in Australia in parts of the Tirari-Sturt stony desert ecoregion where extensive areas are covered in desert pavements. (Aus: pebble = gibber)

Deflation surface Desert pavement, in the Mojave Desert, California

The stony desert known as Reg del’Adrar in Mauritania Tunisia ; pre-Sahara http://www.nicholas.duke.edu/eos/geo41/win008.gif Figure 3.3: Deflation surface and desert pavement

Oasis

Beneath the Earth's surface water fills the pore spaces and openings between rocks. The upper limit of the groundwater is called the water table, which is present even beneath desert regions. Groundwater hardly ever reaches the desert surface, but when it does, it can transform the stark landscape into a fertile haven thriving with many species of plants and animals that otherwise would not exist in such a hot, dry environment. This green area, existing like an island in a sand sea, is an oasis. Many oases are artificial ecosystems, created by people living in the desert using large pipes to tap into the groundwater to bring it to the surface. A few are the result of natural forces and are centred on springs that have been exposed because of blowouts and other erosive actions by the wind, resulting in a lowering of the land surface.

Abrasion acts on free-standing rock outcrops, modifying them into interesting and unique landforms. Yardangs are the first of these landforms primary formed by abrasion. The others are zeugen and then collectively landfoms such as mushroom rocks, rock pedestals and ventifacts.

Yardangs and zeugen

Yardangs are extensively grooved, fluted, pitted and irregular rock ridges or reliefs of one to about ten metres high running parallel to the prevailing winds. They are caused by differential erosion, when the sand-laden wind corrades zones of softer or weaker rock between harder vertical ridges from old lake sediment where soft, poorly consolidated rock and bedrock surfaces are eroded into alternating ridges and furrows. Large- scale yardangs in Africa are to be found in Egypt (near Kom Ombo, north of Lake Aswan), Chad, Libya and Algeria. Local, but smaller scale yardangs are to be seen near the Umtamvuma River mouth in Kwazulu-Natal; these are 4m high and were formed within a 60-year period. Yardangs and allied "ridge and furrow" features are probably the only landforms in deserts that can be ascribed to wind abrasion alone.

Yardang, Egypt © H Kehl

Top: Yardang, Egypt © Urs Möckli

Yardangs up to 180 m tall in the Lut Desert or Dasht-e Lut The Lut desert is extremely barren, having the only region ("Desert of Emptiness") of Iran's southeastern province of devoid any life, not even bacteria, on Earth with annual Kerman. This great sand and stone desert is among the rainfall ranging from 0 to only 30 mm. driest places on the planet’ Figure 3.4 : Yardangs around the world.

Zeugen Hoodoos - Mushroom Rocks - Pedestal Rocks

When differential erosion of horizontal rock layers take place, we refer to the resulting landform as zeugen. Typically zeugen are mushroom-shaped rock that has been eroded by the abrasive action of windblown sand. The undercutting effect is concentrated near ground level, where sand movement is greatest, and is enhanced in areas of near-horizontal strata when the lowest bed is relatively weak and shows distinct signs of basal abrasion on the upwind side only. A smoothly rounded top (as opposed to a top with rough edges) is created by spheroidal weathering into a form resembling a mushroom.

The supporting column of a mushroom or pedestal rock is caused by etching, differential weathering and natural sandblasting of sedimentary deposits. Often an erosion resistant layer will be deposited above softer sediment layers and after burial and lithification, joints and fractures form in the upper layer, allowing faster erosion of the soft strata below. Abrasion near the ground by windblown sand causes the lower portion of a rock mass to be undercut leaving the upper portion relatively unaffected.

Mushroom, Table and Pedestal Rocks

More localised and on a smaller scale than the previous landforms, many rock-outcrops in the deserts are easily susceptible to wind deflation and abrasion. Softer layers in these outcrops are worn out easily, leaving remnants of more resistant rocks in the shape of mushroom with a slender stalk and capped by a broad and rounded pear shape above. Mushroom rocks and similar landforms are actually the products of mainly chemical weathering, but wind abrasion also contributes in that the chemically weakened (weathered) softer layers are easily abraded by wind. The resulting landform is the pedestal-shaped toadstool capped by a more resistant upper layer. As wind-borne talus has bigger-sized particles nearer the land surface compared to higher up, free standing rock outcrops are subject to more active or more concentrated abrasion at their bases relative to higher sections, resulting in the familiar stem or pedestal shape.

The Mushroom Rock near Clarens, Free State.

Mukorob, the “Finger of God” near Mariental , Namibia, before its collapse in 1988 Figure 3.5: Examples of Zeugen, Hoodoos, Mushroom Rocks or Pedestal Rocks

The Finger of God, or Mukorob, near Asab and Mariental in Namibia (which toppled in December 1988) was an excellent example of this landform (bottom rigt in Figure . ). It consisted mostly of sandstone and stood 12 m high and 4.5 m wide, weighing some 450 tons, but had a base of just 3 m long and 1.5 m wide. The Mushroom Rock near Clarens in the eastern Free State Province (top left) is another local, but much smaller, example. 11

Ventifacts

On a much smaller scale than the mega-landforms of yardangs and zeugen, rock fragments and stones are shaped into highly-polished conical shapes by wind-driven sand. Typically larger rocks too big to be pushed or moved along by the wind, once they are polished, display smooth faces or facets and are then termed ‘ventifacts’, analogous to artefacts.

The German term ‘einkanters’ is used for rocks with one face only polished, but when such a one- sided polished stone falls over, it presents another face to the prevailing wind, and after a second toppling over, three facets are smoothed and polished. Such three-sided ventifacts are known as ‘dreikanters’. The ergs of the Sahara and the stony northern Namib Desert have extensive sheets of ventifacts.

Ventifacts from all over http://www.nicholas.duke.edu/eos/geo41/win011.gi

Ventifact showing facets http://www.nicholas.duke.edu/eos/geo41/win012.gif

Figure 3.6: Ventifacts

DEPOSITIONAL LANDFORMS

Wind is a good sorting agent. Depending upon the velocity of wind, different sizes of grains are moved along the floors by rolling or saltation and carried in suspension and in this process of transportation itself, the materials get sorted. When the wind slows or begins to die down, depending upon sizes of grains and their critical velocities, the grains will begin to settle. Therefore good sorting of grains can be found in depositional landforms made by wind. More formally, wind will deposit its talus load when its supply of kinetic energy is too low to carry the talus mass. Since a desert has an abundant supply of sand and with nearly constant wind directions prevailing, depositional features in arid regions can develop anywhere. The three commonly distinguished landforms formed by wind deposition of talus are draas, dunes and ripples.

Draas and ergs Draas are huge sand accumulations and where these converge, a landform termed erg is formed – such as the star-shaped Great Continental Erg in Algeria. There are approximately twenty ergs in the Sahara, which collectively cover about 15% of the Sahara’s surface area.

The Great Sand sea near Luderitz in the Namib Desert

Ar Rub al Khali Sand Sea, Arabian Peninsula

Figure 3.7: Sand seas, or draas (ergs) 13

These ‘sand seas’ are extensive accumulations of sand up to 300 m thick and are essentially water deposits, while most of them occupy large depressions which are former (paleo) lakes or shallow oceans, which were subsequently filled with water-borne (i.e. fluvial) talus. The surface of the draas are shaped by wind – with dunes typically developing on the wind side. On a smaller scale, individual ripples are formed on the surface of a draa or dune. All three of these wind-shaped landforms, draas, dunes and ripples, are in essence waves – and can therefore be described in terms of their geometric characteristics like wavelength and amplitude.

The photograph on the top right (Figure 3.7) shows a part of The Ar Rub al Khali sand sea or erg - also known as the "Empty Quarter" – in Oman. In the western portion there is a large region of SW-NE oriented transverse sand dunes (reddish brown) alternating with the blue-grey interdune salt flats, known as sabkha.

The dominant winds are Shamal winds, which are northwesterly trade winds originating in Iraq. The formation of secondary barchan and star dunes on the parallel transverse dunes is due to southwesterly Kharif winds that blow during the monsoon season. The long l transverse dunes begin to break up into isolated star dunes to the northeast and east (right), probably as a result of both wind pattern interactions and changes in the sand supply to the dunes.

The Great Sand Sea of the Namib extends northward for 400km from Lüderitz Bay to Walvis Bay and as far as 140km inland from the coast. The sand consists of quartz grains mixed in a ratio of 9:1 with heavy minerals like garnet, ilmenite and magnetite as well as a little mica. The colours darken from pale buff in the west to deep red in the east as the iron oxide content of the sand increases.

Sand Dunes

Dry hot deserts are good places for sand dune formation. Obstacles to initiate dune formation are equally important. Dune form, i.e. their shape and size, is controlled or determined by three factors, namely the strength and direction of wind, the amount of sand available and the amount (if any) of vegetation present. All dunes are mobile to some extent, and can be classified into live dunes and fixed dunes on the basis of their mobility.

Figure 3.8: Sand dune form controls: sand supply, vegetation & wind strength 14

Live dunes or free dunes have no fixed position, but migrate downwind by erosion on the gently inclined windward side and deposition on the leeward side (slip face) in the same way as described for fixed dunes. The smallest free dunes are common wind ripples that measure only a few centimetres in height. Large dunes are found in extensive dune areas in deserts, in sand seas known as `ergs'. Free dunes move frequently and their shapes vary. They are dependent on the speed the wind is depositing or removing sands and the size of the materials being transported

Coastal dunes occur along beaches or sand-flats that form part of a non- erosional sandy or deltaic coast. The source areas of the sand will eventually lose all sand, silt and clay particles; some become wet (groundwater) depressions whereas others acquire a rocky or boulder-strewn surface known as a `desert pavement'. Figure 3.9: Four of the most common dune forms: barchan, transverse, longitudinal (or linear), and blowout (or parabolic).

Fixed dunes, by contrast, tend not to move and their shapes are relatively stable and static and they are usually secured down by vegetation, rocks and opposing winds. They are formed when transported sand settles in the lee of an obstacle such as a bush or a rock, causing the obstruction to grow in size, capturing more sand. The transport capacity of the wind decreases as it drives the sand grains to the top of the dune, causing an increasing part of the transported sand to settle before reaching the dune crest. This steepens the angle of the slope, particularly near the crest. Once the slope angle exceeds the angle of repose of the deposited sand (typically 34˚ for dry sand), shearing sets in along a slightly less steep plane. Thus, a slip face is formed on the leeward side of the dune. Vegetation growing on especially the lower part of dunes may eventually keep most of the sand in place. Dunes along coasts are often fixed by vegetation, where shrub-coppice dunes and also parabolic dunes may develop by landward migration of beach sand.

During transport, selection of particles (sorting and winnowing) occurs; the momentary wind speed and the size, shape and density of minerals determine how far a particular grain will be transported. Fine gravel travels by creep and sand-sized particles by saltation. Silt-sized particles can be carried over great distances (Saharan `dust' settles regularly in central Europe and, in the past, loess formations have formed extensive blankets far from the source areas). Fine, plate-shaped clay minerals and micas are blown out and travel even farther (which explains why wind-borne sediments are normally poor in micas). This sorting of grains results in deposits that consist of pure sand with a uniform particle size. Many aeolian sand deposits show characteristic large-scale cross bedding, indicative of sand deposition on the slip faces of dunes. See Figure 3.10.

Common to all dunes is the contrast between the gentle slope of the windward side (the side facing into the wind) and the steep slope of the leeward side (the side facing away from the wind). The leeward side is known as the slip face of the dune.

Figure 3.10 : Schematic dune structure

Longitudinal dunes, also known as linear dunes, form when the supply of sand is poor and the wind direction is constant, or where sand is more abundant and cross winds converge - often along coasts where the winds from the sea and those from the land meet and push the sand into long lines. They appear as long ridges of considerable length but low in height. The wind channels between existing dunes and forms a vortex flow, which then shapes and maintains the dune form. In the central and greater part of the Namib sand sea, linear or longitudinal dunes lie parallel to one another and to the southerly winds, formations which in places are over 100km long and 120m high. They are arranged in nearly straight rows with valleys or so-called "dune streets" between them. Their slipfaces change position with the seasons. Whereas they face north-east for much of the year when southerly winds prevail, they are turned around to face south-west in winter when the easterlies blow.

Longitudinal dune [USGS]

http://www.nicholas.duke.edu/eos/geo41/win028.gif

Longitudinal dunes, Arabian peninsula http://www.nicholas.duke.edu/eos/geo41/win029.gif Longitudinal dune Australia [NASA] http://www.nicholas.duke.edu/eos/geo41/win031.gif [NASA] Figure 3.11 : Longitudinal (Linear) dunes 16

Seif dunes, a sinuous variety of longitudinal sand dunes, or are usually found in very dry deserts. They are large compound forms where smaller, secondary, linear ridges coalesce and are superposed on the main ridges of the underlying longitudinal dunes. The secondary forms on seif dunes originate from barchan- like forms, but have only one wing or point due to shifting wind conditions disturbing the one point. The remaining lone wings of seifs can grow very long and high. Seifs are the dominant dune form in the Sahara and some of them are up to 100 m long and have a local relief of up to 100 m as well. The seifs in the central Namib Desert south of Walvis Bay reach heights between 50 m and 250 m, reputed to be of the highest dunes in the world. These dunes have their origin as longitudinal dunes formed in the prevailing south-westerly winds, which have been changed into seifs as a result of sporadic easterly winds.

Figure 3.12 : Seif dunes of the Namib desert www.nationalgeographic.com

Barchan dunes are crescent shaped and have their points or wings directed downwind, or away from the wind direction. They form where the original surface over which sand is moving is almost uniform and where the wind direction is constant and moderate. Barchan dunes are a very mobile dune type and are popularly called "wandering dunes". The rate of advancement of the sand is roughly inversely proportional to the height of the crest. This causes the flanks of a shifting dune to advance more rapidly than the central part until the flanks become sheltered by the main mass of the dune. Coalescing barchans produce `transverse dunes'.

In the Namib Desert they grow to a height of about 30m where strong south-westerly winds blow mainly from coastal regions with relatively little sand. The barchans of the Skeleton Coast and those near Walvis Bay point to the north-east and also travel in that direction – due to the south- westerly winds shaping and driving them.

Figure 3.13 : The formation of barchan dunes. Source: Bagnold, 1965 17

Barchan, White sands New Mexico http://www.nicholas.duke.edu/eos/geo41/win026.gif

Small barchans, Tunisia http://www.nicholas.duke.edu/eos/geo41/win025.gif Figure 3.14 : Barchan dunes

Transverse dunes When the supply of sand is plentiful, regular shaped dunes like barchans can coalesce and lose their individual characteristics, forming crescent-shaped (or barchanoid) ridges. If the ridges become fairly straight, they are called transverse dunes. The ideal conditions for their formation is when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction. In the Namib Desert transverse dunes are found a short distance inland from the barchans, reaching heights of two and a half times that of the barchans, with their long axes similarly across the path of the wind. They may be very long and low in height.

Transverse dunes {USGS] http://www.nicholas.duke.edu/eos/geo41/win027.gif Figure 3.15 : Transverse dunes

Star dunes

Mountainous piles of sand known as star dunes dominate the inland margin of the sand sea where high winds blow from all directions. They are reputed to be among the highest dunes in the world, as high as 220 m or even higher when they rest on a raised surface. Star dunes are named for their shape as seen from above, a lot of sharp ridges winding outwards and downwards from a central crest.

Figure 3.16 : Star dunes

Shrub-coppice dunes

Shrub-coppice dunes, (also known as Nebkha dunes) usually only a metre or two high, are common at the sea-shore, where clumps of grass or shrubs such as mesquite form obstructions to windblown sand. The sand starts to collect around the vegetation, gradually forming a small mound. As the mound grows, the roots lengthen to keep the plant above the surface, thereby anchoring the dune, while it also grows taller. These dunes are frequently found on sand sheets and on and around larger parabolic dunes. 19

Figure 3.17: Coppice dune

Parabolic dunes (blowout dunes) Parabolic dunes form when sandy surfaces are partially covered with vegetation. They are also crescent shaped, but unlike the barchan dunes, their points or wings (horns) point into the wind, while the arch is downwind. They typically grow from the horns or trailing arms that are anchored around an obstruction like vegetation or rock blocking the wind, while its central part migrates with the wind after a blowout or depression hollow has formed. Parabolic dunes reach heights of up to 20 or 30 m except at their crescent, where more sand piles up as it is halted or slowed by surrounding vegetation.

Figure 3.18 : Parabolic dunes

These dunes are usually less regularly shaped than barchans and in plan view can be either U-shaped or V-shaped and are more elongated. They consist of mounds of well-sorted, very fine to medium sand and usually occur on beaches, downwind from blowouts in partially vegetated sand sheets. They also occur in semiarid areas, i.e. where some precipitation is common and near-surface moisture is available in the lower parts of the dune and underlying soils. Parabolic dunes can also extend inland into vegetated areas in coastal zones and on shores of large lakes. Most parabolic dunes do not grow to heights greater than a few tens of meters except at their forward portions, where sand piles up as its advance is halted or slowed by surrounding vegetation. There are two principal sub-types of parabolic dunes: long walled and elliptical types.

Loess

This is the lightest material carried by the winds which form a so-called blanket covering the existing land. This blanket is easily eroded and rain penetrates through them rapidly. A large portion of the world’s loess has its origin from deserts. Loess is a predominantly silt-sized sediment originating from broken-down rock fragments, which is formed by the accumulation of wind-blown dust. Loess is fairly even- sized sediment, pale yellow or buff in colour, typically non-stratified and often calcerous – i.e. of calcium carbonate origin. Loess grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar , mica and other minerals. Loess can be described as a rich, very fertile, dust-like soil, which erodes very easily. The word loess, with connotations of origin by wind-deposited accumulation, can probably be traced back to Alemannic dialects of German.

Loess profile in Vicksburg, Mississippi. Loess near Hunyuan, in China’s Shanxi A loess ioutcrop in Patagonia, USA. province. Southern Argentina. Figure 3.19 : Loess as found in different continents

Loess deposits may become very thick; more than a hundred meters in areas of China and the Midwestern United States. It generally occurs as blanket deposit that covers areas of hundreds of square kilometres and tens of metres thick. Loess often stands in either steep or vertical faces. Because the grains are angular, loess will often stand in banks for many years without slumping. This soil is easily excavated to form cave dwellings, a popular method of making human habitations in some parts of China. In several areas of the world, loess ridges have formed that are aligned with the prevailing winbds during the last glacial maximum. These are called paha ridges in America and greda ridges in Europe. The form of these loess dunes has been explained by a combination of wind and tundra conditions. Glacial or peri-glacial loess is derived from the floodplains of glacial braided rivers that carried large volumes of glacial meltwater and sediments from the annual melting of continental ice sheets and mountain icecaps during the summer, while non-glacial loess can originate from deserts, dune fields, playa lakes and volcanic ash. The thick Chinese loess deposits are non-glacial loess having been blown in from deserts in northern China. The loess covering the Great Plains of Nebraska, Kansas, and Colorado is non-glacial desert loess. Non-glacial desert loess is also found in Africa and Australia. Sandy parent materials are also abundant in areas where sand accumulates after selective transportation of weathering material by wind or water. Aeolian (wind-borne) sands will be discussed in this paragraph.

Dune and other desert features: Words to Know

Abrasion: The erosion or wearing away of bedrock by continuous friction caused by sand or rock fragments in water, wind, and ice. Aeolian: Formed or deposited by the action of the wind. Bajada: Several alluvial fans that have joined together. Basin: A hollow or depression in Earth's surface with no outlet for water. Crescent: The sickle-moon shape of a barchan dune and also the shape of a parabolic dune. Crest: The highest point or level; summit. Deflation: The lowering of the land surface due to the removal of fine-grained particles by the wind.

Erg: A vast area deeply covered with sand and topped with dunes. Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Gully: A channel cut into the Earth's surface by running water, especially after a heavy rain. Leeward: On or toward the side facing away from the wind. Saltation: The jumping movement of sand caused by the wind. Silt: Fine earthy particles smaller than sand carried by moving water and deposited as a sediment. Slip face: The steeply sloped side of a dune that faces away from the wind. Surface creep: The rolling and pushing of sand and slightly larger particles by the wind. Ventifact: A stone or bedrock surface that has been shaped or eroded by the wind. Windward: On or toward the side facing into the wind.

Bibliography

Books

Gallant, Roy A. Sand on the Move: The Story of Dunes. New York: Franklin Watts, 1997. Lancaster, Nicholas. The Geomorphology of Desert Dunes. New York: Routledge, 1995.

Websites http://www2.agc.army.mil/research/products/desert_guide/lsmsheet/lspara.htm dunes http://www.nps.gov/grsa/naturescience/dune-types.htm http://geog-leics.blogspot.com/2012/08/desert-depositional-features.html 13 april 2013 http://www.indiana.edu/~geol116/Week11/wk11.htm . http://hrsbstaff.ednet.ns.ca/mcallip/Deserts/desert_deposit_features.htm http://www.ux1.eiu.edu/~cfjps/1300/deserts.html http://www.google.co.za/url?sa=i&source=images&cd=&docid=1Gk1J6g1KgticM&tbnid=5T83zLSc- aKezM:&ved=0CAgQjRwwAA&url=http%3A%2F%2Fwww.handsontheland.org%2Fgrsa%2Fresources%2Fc urriculum%2Fmid%2Fdunes%2Fphoto_files%2Flongitudinal.htm&ei=eQppUc_OBJSThgeE9IHYCQ&psig=AF QjCNF6A1YoY3Cy3nbL8vAnK7FrILGpxw&ust=1365924857233144 http://www.scienceclarified.com/landforms/Basins-to-Dunes/Dune-and-Other-Desert- Features.html#b#ixzz2QKPl4to5 http://www.desertusa.com/mag99/sep/papr/desfeatures.html http://pubs.usgs.gov/gip/deserts/ http://www.eos.ubc.ca/courses/eosc110/fletcher/slideshow/deserts/deserts.html http://alic.arid.arizona.edu/sonoran/Physical/geomorphology.html http://www.aqd.nps.gov/grd/tour/sanddune.html http://www.desertusa.com/geofacts/sanddune.html http://www.fao.org/docrep/003/Y1899E/y1899e06.html http://www.scienceclarified.com/landforms/Basins-to-Dunes/Dune-and-Other-Desert-Features.html#b

APPLIED GEOMORPHOLOGY

Was the Sphinx Originally a Landform?

In 2001 Farouk El-Baz, Boston University professor and director of the university's Center for Remote Sensing, published a paper in which he suggested that the pyramids and the Great Sphinx located on Egypt's Giza Plateau were based on natural landforms found in the eastern Sahara Desert. El-Baz pointed out that the landscape of the Nile River valley features coneshaped hills that have lasted many years because their shape forces the strong winds in the area upward, preventing the wind from eroding them or wearing them down. The pyramid builders, El-Baz believes, would have looked to these landforms in their quest to build lasting structures.

Extending his theory even further, El-Baz asserted that the Great Sphinx, the enormous sculpture with the head of a man and the body of a reclining lion, might even have been carved by ancient Egyptians in 2500 B.C.E. from an existing desert landform. He cited the works of early twentieth-century explorers and geologists that described wind-eroded yardangs in northwestern China and southwestern Egypt as "sphinx- like" or "lionlike." El-Baz believes the head of the sphinx was an existing yardang (a wind-sculpted ridge) the Egyptians reshaped. They then formed the body, which sits in a hollow or depression, by digging out the naturally occurring limestone on the plateau around it. due to compression caused by atmospheric pressure (atmospheric pressure increases closer to the planet's surface).

Great Sphinx, Egypt. Photograph reproduced by permission of the Corbis Corporation

During its descent, the warming air pushes the air below it back toward the equator, since air flows always move toward areas of low pressure. Passing over land on its way back to the equator, the now heated, dry air evaporates any moisture in the air, creating dry regions or deserts.

These wind patterns explain the formation of many of the world's deserts in the area between 15° and 35° latitude north and south of the equator. Although much weaker, similar atmospheric circulation (with heated, dry air evaporating moisture as it moves over Earth's surface) occurs over both poles, creating polar deserts. While heated because of compression by the atmosphere, the air over the polar regions is not as warm as in equatorial regions simply because the Sun is farther away from Earth in these areas. Polar deserts, however, are similar to hot deserts because they have very low humidity and precipitation levels.

The topography (physical features) of Earth in combination with atmospheric pressure creates other deserts around the planet. Rain-shadow deserts are those that lie on the leeward side of mountain ranges located near coasts. As moisture-laden air flows inland from an ocean and encounters a mountain range, it is pushed upward. Cooling as it rises, the air begins to lose its moisture on the windward side of the mountain range through rainfall. Once on the leeward side, in the "shadow" of the mountain range, the air has little moisture left. Heated by compression as it descends, the warm, dry air forms deserts in the slope of the range. All deserts in North America are formed by this action.