LANDFORMS OF WIND EROSION IN DESERTS

In the combined processes of abrasion, deflation and attrition, a wealth of characteristic desert landforms emerge.

1) Rock pedestals or mushroom rocks: The sand-blasting effect of winds against any projecting rock masses wear back the softer layer 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, craving 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 under-cutting produces rocks of mushroom shape called mushroom rocks or gour in the Sahara.

 

2) Zeugen: These are tabular masses which have a layer of soft rocks lying beneath a surface layer of more resistant rocks. The sculpturing effects of wind abrasion wear them into a weirdlooking ‘ridge and furrow’ landscape. Mechanical weathering initiates their formation by opening up joints of the surface rocks. Wind abrasion further ‘eats’ into the underlying softer layer so that deep furrows are developed. The hard rocks then stand above the furrows as ridges or zeugen, and many even overhang. Such tabular blocks of zeugen may stand 3 to 30 m (10 to 100 feet) above the sunken furrows. Continuous abrasion by wind gradually lowers the zeugen and widens the furrows.

 

3) Yardangs: Quite similar to the ‘ridge and furrow’ landscape of zeugen are the steep-sided yardangs. Instead of lying in horizontal strata upon one another the hard and soft rocks of yardangs are vertical bands and are aligned in the direction of the prevailing winds. Wind abrasion excavates the band of softer rocks into long, narrow corridors, separating the steep-sided overhanging ridges of hard rocks, called yardangs. They are commonly found in the Atacama Desert, Chile, but the more spectacular ones with yardangs rising to 8-15m (25-50 feet) are best developed in the interior deserts of Central Asia where the name originated.

 

4) Ventifacts or dreikanter: These are pebbles faceted by sand-blasting. They are shaped and thoroughly polished by wind abrasion to shapes resembling Brazil nuts. Rock fragments, mechanically weathered from mountains and upstanding rocks, are removed by wind and smoothed on the windward side. If wind direction changes another facet is developed. Such rocks have characteristic flat facets with sharp edges. Amongst the ventifacts those with three wind-faceted surfaces are called dreinkanter.

These wind-faceted pebbles form the desert pavement, a smooth, mosaic-like region,closely covered by the numerous rock fragments and pebbles.

 

5) Depressions or Deflation hollows: Desert depressions are variously called playas, pans, flats, dayas and deflation basins. Their surface is either of fine debris (clay and silt) or evaporite (sodium chloride or sodium carbonate). They range in size from a few square metres to hundreds of square metres. Some depressions are probably of Quaternary age such as Tovat (136 m), Bodele (155 m) and Qattara (-133 m) in Egypt. Some depressions are occupied by oasis so that they are habitable, such as Tovat , Toaodeni and El Golea in the Sahara; Jalo and Al kufrah in Libya; Farafra, Dakhla, El Kharga and Suira in Egypt.

 

Depressions may have different origins other than by wind deflation:

1. They may be produced by faulting as in the Basin and Range province in the U.S.A.
2. They may be relicts of channels blocked by dunes.
3. They may be formed by solution in limestone areas.