CDS (I) 2022 General Knowledge · Question 56 of 120

Correct — C, Bajada. At the foot of a desert mountain front the ground falls away in two belts. The inner belt is a gently sloping rock surface cut by sheet floods and lateral stream erosion — the pediment proper. Beyond it lies a wedge of sediment washed off the mountains, deposited as alluvial fans at each valley mouth which spread and merge into a continuous apron. That apron of coalescing alluvial fans is the bajada, and in the older pediment vocabulary the alluvium-mantled outer zone of the same footslope is what is termed the peripediment. Of the four options only the bajada names a mountain-foot depositional surface at all: an inselberg is a hill, a zeugen is a wind-carved ridge and a playa is the floor of the basin, not its margin.

• (a)Inselberg — An inselberg is the opposite kind of feature — a residual hill of resistant rock left standing above a plain as the mountain front retreats by parallel slope recession. It rises out of the pediment surface rather than forming any part of it.
• (b)Zeugen — A zeugen is a wind-erosion landform, a tabular mass in which a hard cap protects a softer bed beneath so that abrasion undercuts it into a mushroom-like ridge. It is made by wind rather than by running water, and it has nothing to do with the footslope of a mountain.
• (d)Playa — A playa is the flat centre of an enclosed desert basin, where drainage converges and a shallow lake stands briefly after rain before evaporating and leaving salts. It sits at the lowest point, downslope of the bajada, not at the mountain foot.

Desert landscape evolution turns on the formation and extension of pediments. Gently inclined rocky floors close to mountains at their foot, with or without a thin cover of debris, are called pediments; they form through the erosion of the mountain front by a combination of lateral erosion by streams and sheet flooding. Once formed, the steep wash slope and the free face above it retreat backwards, and by this parallel retreat of slopes the pediment extends at the expense of the mountain, which is gradually reduced to an inselberg and finally to a low featureless plain called a pediplain.

The stem's word peripediment is not the term Indian school textbooks use, and it is worth saying so plainly rather than pretending otherwise. It belongs to the older geomorphological literature on pediments, where the footslope is divided into an inner rock-cut pediment and an outer zone mantled with alluvium; that outer zone is the peripediment, and it is the same ground the depositional term bajada describes. What the candidate can settle from first principles is the elimination. Ask of each option whether it is a slope at the base of a mountain built of washed-out sediment. An inselberg is a residual hill, a zeugen is wind-cut, a playa is a basin floor. Only the bajada is a mountain-foot apron, which is why the key lands where it does even for a candidate who has never met the word peripediment.

• A bajada is a series of coalescing alluvial fans along a mountain front, and it grades back into the pediment upslope.
• A pediment is a very gently sloping bedrock surface, cut into rock and at most thinly veneered with alluvium; the bajada below it is aggradational.
• Pediments extend backwards by the parallel retreat of slopes, reducing the mountain to an inselberg and eventually to a pediplain.
• Playas are shallow lakes on the level floor at the centre of a desert basin; when the salt crust dominates, the surface is called an alkali flat.
• Bajadas frequently contain playa lakes at their lower margin, where the fan apron meets the basin floor.

• Placing the playa at the mountain foot; it lies at the centre of the basin, downslope of the bajada.
• Treating the pediment and the bajada as the same surface — the first is cut into rock, the second is built of sediment.
• Reading zeugen as a water-formed feature when it is a product of wind abrasion.

As a term-to-definition item on desert landforms, or as a matching question pairing geomorphic agents with the features they produce.

• practice — not a real PYQ

  The residual hill of resistant rock left standing above a pediplain in a desert region is called a

  • (a)bajada
  • (b)playa
  • (c)inselberg
  • (d)zeugen

  Answer(c) Inselberg — it survives as the mountain front retreats and the pediment extends backwards.

• practice — not a real PYQ

  A shallow lake occupying the level floor at the centre of an enclosed desert basin is known as a

  • (a)playa
  • (b)bajada
  • (c)pediment
  • (d)loess plain

  Answer(a) Playa — it holds water only briefly after rain and often leaves a crust of salts, the alkali flat.