Barchans — Crescent-Shaped Sand Dunes

A Barchan is a crescent-shaped (moon-shaped) sand dune.

If you look at a Barchan from above, it looks exactly like a crescent moon or the letter 'C'.

• The central part of the dune is the highest and thickest.
• The two ends of the crescent are long, sweeping tails called 'horns'.

Barchans form in flat, dry deserts where the wind blows steadily from one constant direction and there is a limited supply of sand.

1. As the wind blows, it pushes sand up the back of the dune (the windward side), which is long and gently sloping.
2. When the sand reaches the peak, it tumbles down the steep front side (the slip face).
3. Because there is less sand at the edges, the wind pushes the edges faster than the heavy center. This causes the edges to stretch forward, creating the two 'horns'.

This is a classic geography exam question: Which way do the horns of a barchan point?

Answer: The horns always point downwind (in the direction the wind is blowing). If you are lost in the desert and see a barchan with its horns pointing South, you know the wind is blowing from North to South.

Barchans are found in almost all major deserts on Earth, including:

• The Thar Desert in India (especially near Jaisalmer).
• The Sahara Desert in Africa.
• The Gobi Desert in Asia.

They are highly mobile dunes. A Barchan can 'walk' across the desert floor as the wind constantly moves sand from its back to its front, migrating several meters a year.