RAS question
In Rajasthan's desert geomorphology, which description correctly identifies a barchan dune?
Correct answer: (B) A crescent-shaped sand dune formed by wind deposition, usually very mobile.
A barchan is a crescent-shaped sand dune formed by wind-deposited sand and is usually highly mobile.
Explanation
A barchan is identified by its crescent shape, not merely by the direction in which a dune ridge lies. It is a wind-deposited sand form that is usually very mobile, and NASA Photojournal links the term barchan with crescent-shaped dunes. That is why option B is the precise match. The other options use real desert-geomorphology terms, but they point to different dune forms: transverse dunes are described by deposition at right angles to wind direction, longitudinal or linear dunes run parallel to wind direction, and dhora is a local label for wavy dunes rather than the specific crescent-shaped barchan form.
Why the other options are wrong
- (A) This describes a transverse dune, because the feature is defined by sand deposition at right angles to the wind direction rather than by the crescent-shaped barchan form.
- (C) This describes a longitudinal or linear dune, since the defining clue is that the dune lies parallel to the wind direction.
- (D) Dhora is linked with wavy dunes in local usage, but that does not identify the specific crescent-shaped and mobile barchan dune.
Concept
This tests arid geomorphology vocabulary: the ability to distinguish dune forms by shape and wind relation. It recurs in RAS because Rajasthan's desert landscape makes sand-dune terminology a standard part of the state's physical geography syllabus.