ARID OR DESERT LANDFORMS 

1. Types of deserts

• One-fifth of the world’s land is covered with deserts, some rocky, others stony and the rest sandy.
• Deserts are barrens and nothing grows at all is rare, they are called true deserts.
• The deserts are confined within the 15°to 30°parallels of latitude north and south of the equator.
• They are bathed by cold currents which produce a ‘desiccating effect’ so that moisture is not easily condensed by precipitation. Dryness or arbitrary is a keynote. These include the great Sahara desert, Arabian, Iranian and the Thar Desert.
• Gobi and Turkestan are the continental interiors of the mid-latitudes.
• The desert landscape has been divided into five different kinds based on the works of the wind and water in eroding and transporting worn-off materials.

Hamada or rocky deserts: It consists of a stretch of bare rocks, swept clear of sand and dust by wind.  The region is bare and sterile with smooth and polished rocks. The best-known rocky deserts are the Sahara desert, e.g. Hamada le homra in Libya. This covers almost an area of 20,000 square miles.

Rig or stony desert: The winds are not able to blow off due to extensive sheets of angular pebbles and gravel. It is more accessible compared to sandy deserts; the herds of camels are kept in there. In Libya and Egypt, the term Serir is used. In Africa, stony deserts are called reg.

Erg or sandy deserts: This is the sea of sand which represents the popular idea of desert scenery. Winds deposit vast stretches of undulating sand dunes in the heart of the desert. The Calanscio sand sea in Libya is characteristic of the sandy desert. In Turkestan, a sandy desert is also known as koum.

Badlands: Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded. Ravines and gullies are developed by linear fluvial erosion leading to the formation of badlands. Example: Chambal ravines.

Mountain deserts: Some Deserts are found on the plateaus and mountain ranges. Erosion has dissected the desert highland into rough chaotic peaks and uneven ranges. Their Steep slopes are cut by wadis [dry valleys] with sharp and irregular carving due to the action of frost. In the Sahara desert, the Ahaggar and the Tibesti mountains are examples of this.

The Mechanism of Arid Erosion: Arid erosion refers to the process of land degradation and the shaping of landscapes in arid and semi-arid regions, often characterized by low rainfall and limited vegetation cover. The mechanisms of arid erosion involve various natural forces that contribute to the wearing away and transportation of soil and rock materials. 

Weathering: Weathering is the process by which rocks and minerals on the Earth's surface are broken down into smaller particles over time. This natural phenomenon is a key component of geological processes that contribute to the formation and modification of landscapes. Weathering can occur through various physical, chemical, and biological mechanisms. 

2. The action of winds in deserts

Wind erosion carried out in the following ways

• Deflation: This involves lifting and blowing away loose materials from the ground. Deflation results in the lowering of the land surface to form large depressions called deflation hollows. The Qattara Depression of the Sahara desert lies almost 450 feet below sea level.
• Abrasion: The sandblasting of the rock surfaces by the winds that hurl sand particles against them is called abrasion.
• Attrition: Attrition, in the context of geology and geomorphology, refers to a process of erosion where rocks and particles are worn down and reduced in size as a result of mutual abrasion and collision. This process typically occurs in transportation environments, such as rivers, where the movement of sediment particles causes them to collide and wear each other down.

3. Landforms of wind erosion in deserts

Rock pedestals or mushroom rocks: The sandblasting effect of winds against any projecting rock masses wears back the softer layers so that an irregular edge is formed on the alternate bands of hard and soft rocks. Grooves and hollows are cut in the rock surfaces, carving them into fantastic and grotesque-looking pillars called rock pedestals. Such rock pillars will be further eroded near their bases where the friction is greatest.  This process of undercutting produces rocks of mushroom shape called mushroom rocks or gour in the Sahara.

Zeugen: Tabular masses have a layer of soft rocks lying beneath a surface layer of more resistant rocks. The erosional effect of the wind on soft and resistant rock surfaces carves them into a weird-looking ridge and furrow landscape. Mechanical weathering initiates their formation by opening up joints of the surface rocks. Wind abrasion further east into the underlying softer layer so that deep furrows are developed. The hard rock then stands above the furrows as ridges or Zeugen. Zeugen may stand 10 to 100 feet above the sunken furrows. Continuous abrasion by winds gradually lowers the Zeugen and widens the furrow.

Yardangs: Quite similar to Zeugen but instead of lying in horizontal strata upon one another, the hard & soft rocks of Yardangs are vertical bands Rocks are aligned in the direction of prevailing winds. Wind abrasion excavates the bands of softer rocks into long, narrow corridors, separating the steep-sided overhanding ridges of hard rock called Yardangs.

Mesas and buttes: Mesas are typically formed through a combination of uplift and erosion. The horizontal rock layers are uplifted by tectonic forces, and then erosion by wind and water carves away the surrounding softer rock, leaving the harder layers on top. Buttes undergo a similar formation process to mesas. They are often remnants of mesas that have further eroded, leaving behind a smaller, isolated structure.

Isenberg: They have isolated residual hills rising abruptly from the ground level. They are characterized by very steep slopes & rather rounded tops. They are often composed of granite or gneiss and are probably relics of an original plateau, which has been almost entirely eroded.

Ventifacts or dreikanter: These are the pebbles faced by sand-blasting and shaped and thoroughly polished by wind abrasion to shapes resembling Brazil nuts. Rock fragments, mechanically weathered from mountains and upstanding rocks, are moved by wind and smoothed on the windward side. If the wind direction changes another facet is developed. Such rocks have characteristic flat facets with sharp edges. Amongst the Venti facts, those with three wind-faceted surfaces are called dreikanter.  These wind-faceted pebbles form the desert pavement a smooth, mosaic-like region, closely covered by numerous rock fragments and pebbles.

Deflation hallows: Winds lower the ground by blowing away the unconsolidated materials, and small depressions may form.  Similarly, minor faulting can also initiate depressions and the eddying action of oncoming winds will wear off the weaker rocks until the water table is reached. Water then seeps out forming oasis or swamps, in the deflation hollows or depressions. Waves of severe drought and dust storms are called great dust bowls. It occurred in the North American prairies, having devastating consequences for the residents, livestock, and agriculture.

4. Landforms of wind deposition in deserts

Materials eroded & transported by winds must come to rest somewhere. The finest dust travels vast distances in the air sometimes as long as 2300 miles before they settle down. The dust from the Sahara desert is sometimes blown across the Mediterranean to fall as blood rains in Italy or on the glaciers of Switzerland. Dust that settles in the Hwang Ho basin from the Gobi desert has accumulated over the past centuries to a depth of several hundred feet. As windborne materials are shifted according to their coarseness, it can be expected that the coarser sands will be too heavy to be blown out of desert limits. They remain as dunes or other depositional landforms within the desert themselves. Some of the major depositions of the wind depositions are as follows.

• Dunes: They may be active or live dunes, constantly on the move, or inactive fixed dunes, rooted with vegetation. Dunes are most well represented in the ERG desert where a sea of sand is being continuously moved, reshaped, and re-deposited into a variety of features. The two most common types of dunes are Barchan & Seifs.
• Barchan: These are crescentic or moon-shaped dunes that occur individually or in groups. These are initiated probably by a chance accumulation of sand at an obstacle, such as a patch of grass or a heap of rocks. They occur transversely to the wind. The windward side is convex and gently sloping while the leeward side, being sheltered, is concave and steep (the slip-face). The crest of the sand dune moves forward as more sand is accumulated by the prevailing wind. The migration of the barchans may be a threat to desert life for they may encroach on an oasis burying palm trees or houses.
• Seifs or longitudinal dunes: Seif dunes, also known as longitudinal dunes or linear dunes, are a type of sand dune characterized by their elongated, parallel ridges. These dunes typically form in areas with a steady and unidirectional wind flow, leading to the development of linear features aligned with the prevailing wind direction.
• Loess: The fine dust blown beyond the desert limits is deposited on neighbouring lands as loess. It is a yellow, friable material and is usually very fertile. Loess is, fine loam, rich in lime, very coherent and extremely porous. Water sinks readily so that the surface is always dry. The most extensive deposit of loess is found in northwest China in the loess plateau of the Hwang –ho basin.

5. Landforms due to water action deserts

• Few deserts in the world are entirely without rain or water.  The annual precipitation may be small and comes in irregular showers. But thunderstorms do occur and the rain falls in torrential downpours, producing devastating effects. A single rainstorm may bring several inches of rain within a few hours, drowning people who camp in dry desert streams and flooding mud-baked houses in the oasis.
• As deserts have little vegetation to protect the surface soil, large quantities of rock wastes are transported in the sudden raging torrents or flash floods. Loose gravel, sand, and fine dust are swept down the hillsides. They cut deep gullies and ravines forming bad-land topography.
• Subsequent downpours widen and deepen the gullies when they wash down more soft rocks from the surface. There is so much material in the flash floods that the flow becomes liquid mud.
• When the masses of debris are deposited at the foot of the hill or the mouth of the valley, an alluvial cone or fan or ‘dry delta’ is formed, over which the temporary stream discharges through several channels, depositing more material.
• The pasty alluvial deposits are subjected to rapid evaporation by the hot sun and downward percolation of water into the porous ground and soon dry up leaving mounds of debris. Apart from gullies, there are many larger dry channels or valleys.