RAS question
Sand dunes that are crescent-shaped with horns pointing downwind are called:
Correct answer: (B) Barchans.
Crescent-shaped sand dunes whose horns point downwind are called barchans.
Explanation
Barchans are the dune form described in the question because their defining visual clue is a crescent shape with horns directed downwind. They occur in the Thar Desert and are mobile dunes, shifting about 2-3 m per year in the wind direction. NASA Science describes barchan dunes as crescent-shaped sand dunes whose horns point downwind. That makes the orientation of the horns more than a decorative detail; it is the key field marker that links dune shape to the wind environment in which the dune formed and moved.
Why the other options are wrong
- (A) Longitudinal dunes are elongated forms, so they do not match the crescent-shaped dune with horns pointing downwind.
- (C) Star dunes have multiple arms, whereas the question asks for a crescent-shaped form with two downwind horns.
- (D) Seif dunes are sword-shaped, not crescent-shaped with horns pointing in the wind direction.
Concept
This tests aeolian landforms in the Rajasthan geography syllabus, especially how dune shape records wind direction. It recurs in RAS because the Thar Desert makes sand-dune morphology a high-yield physical geography theme.