In which of the following districts of Rajasthan are the ‘Barchans’ found?

Correct — A. A barchan is a crescent-shaped sand dune whose horns point downwind. It forms only where there is loose sand, a fairly steady wind and too little vegetation to hold the surface down.

In Rajasthan that means the Thar, and Jaisalmer lies in the heart of it — the driest and sandiest of the four districts named.

The other three sit in the Aravalli belt or on the country east of it, semi-arid or hilly ground that carries no active dune field → option (a).

Why the others are wrong

• (b)
  Alwar — Alwar lies in the Aravalli belt of north-eastern Rajasthan, well outside the sand sea. It is hill-and-forest country, better known for the Sariska reserve than for dunes.
• (c)
  Udaipur — Udaipur is hill and lake country in the southern Aravallis. Its surface is rock and soil, not the deep loose sand a barchan has to be built from.
• (d)
  Bhilwara — Bhilwara lies east of the Aravallis, in the semi-arid south-east of the state. The range stands between it and the desert, so no dune field forms there.

The Aravalli range cuts Rajasthan in two, and almost every physical-geography question about the state turns on which side of it a district falls.

West of the Aravallis is the Thar, the great Indian desert — Jaisalmer, Barmer, Bikaner — with low and erratic rainfall and landforms built by wind. East and south lie the Aravalli hills themselves and the wetter, rockier plateau and plains beyond.

Wind builds dunes in several shapes. A barchan is crescent-shaped with horns pointing downwind and forms where the sand supply is limited. A longitudinal or seif dune runs parallel to the wind, and a transverse dune lies across it.

Key facts

• A barchan is a crescent-shaped sand dune with its horns pointing downwind.
• Barchans occur in the Thar Desert of western Rajasthan, which includes Jaisalmer district.
• The Aravalli range separates the desert west of Rajasthan from the wetter country to its east.
• Longitudinal or seif dunes run parallel to the prevailing wind, unlike transverse dunes, which lie across it.

Common traps

• Reading 'district of Rajasthan' as 'anywhere in Rajasthan' and picking a district outside the Thar.
• Confusing a barchan with a longitudinal dune, which runs parallel to the wind rather than across it.

No directly related past PYQ was found.