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Badland, pediments, bajadas and playas are special types of landforms of arid and semi-arid regions caused by mechanical disintegration of rocks and water action.
1. Badland Topography:
The regions of weak sedimentary formations are extensively eroded by numerous rills and channels which are occasionally developed due to occasional rainstorms. The linear fluvial erosion results in the formation of ravines and gullies which are divided by numerous undulating narrow ridges. Thus, the ground surface becomes so uneven and corrugated due to numerous gullies and ravines that it becomes entirely difficult to walk on it. Such type of landscape is called badland topography.
2. Bolsons and Playas:
The intermontane basins in arid or semiarid areas are generally called bolsons. Such basins are characterized by 3 unique landforms which from the mountain front downward are pediments, bajadas and playas (fig. 21.10). Numerous ephemeral streams after originating from the surrounding mountain fronts drain into the bolsons. Some water collects in the centre of is a playa in the centre of the Tarim Basin.
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The playas range in areal extent from a few square metres to tens of square kilometres. These are called ‘khabari’ and ‘mamlaha’ in Arabian deserts while they are known as ‘shafts’ in Sahara. Playa lakes may last for days, weeks or even longer before they are completely dried up by evaporation. Evaporation of playa lake water results in the formation of encrustation of alkaline materials (sodium bicarbonate or sodium carbonate), salt deposits such as borax. Such salt-covered playa beds are called salinas.
3. Bajada:
Gently sloping depositional plain between pediments and playa is called bajada. Bajada is formed due to coalescence of several alluvial fans. Thus, bajada is a wholly depositional feature. The slope in its upper part ranges between 8° and 10° but it becomes 1° tozero in its lowermost portion touching the playa (fig. 21.10).
4. Pediments:
Pediments situated between mountain front and bajada in intermontane basin are broad, extensive, and gently sloping areas of rockcut surfaces which spread as aprons around the bases of mountains. The term (pediment) was first used by G.K. Gilbert (1882) for the surfaces of the alluvial fans that encircle mountains in deserts.
According to Kirk Bryan (1922) ‘pediments are slopes of transportation cut on bedrock, usually covered with a veneer of alluvium in transit from high to lower levels. In form and function, a pediment is similar to an alluvial fan, the difference being that a pediment is an erosional landform and a fan is constructional’ (A. Bloom).
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It may be pointed out that pediments are now not considered to be confined only to the arid and semiarid regions rather they are found widely in tropical and temperate climate. A true pediment represents rockcut surfaces of mountain side.
This feature is, in fact, a slope of derivation or transportation as thin veneer of debris also moves through this surface down the slope. These bare rock surfaces extend for several kilometres in length. The general gradient ranges between 1° – 7°. Several theories have been postulated to account for the development of desert pediments.