Three Himalayan and sub-Himalayan landforms: duns are flat valleys within the Shiwalik foothills, while bhabar and terai are the gravelly and marshy belts where the hills meet the plains.
- Duns (or doons) are longitudinal flat-floored valleys lying between the lesser Himalayas and the Shiwaliks (the outermost Himalayan range); examples include the Dehra Dun, Kotli Dun, and Patli Dun.
- The Shiwaliks are the youngest, lowest, outermost Himalayan range, made of soft, unconsolidated sediments eroded from the higher ranges; the duns formed in structural depressions within them.
- The bhabar is a narrow belt of porous gravel and pebbles at the foot of the Shiwaliks where streams emerging from the hills deposit coarse material and then disappear underground.
- The terai lies just south of the bhabar, where the streams re-emerge to the surface, creating a marshy, damp, thickly forested belt that was historically malarial and is now largely cleared for farming.
- Together these belts form the transition from the Himalayan foothills to the northern plains, important for groundwater (recharge in the bhabar) and agriculture (in the reclaimed terai).
The dun valleys in the Shiwaliks, the porous bhabar where streams sink, and the marshy terai where they re-emerge are recurring physiography facts and the standard relief sequence at the foot of the Himalayas.
Order from the hills outward: Shiwaliks (and duns within them), then bhabar (gravel, streams disappear), then terai (marshy, streams re-emerge), then the alluvial plains. The bhabar is dry and porous; the terai just below it is wet and marshy.
Shiwalik dun valleys, then the porous bhabar where streams sink, then the marshy terai where they re-emerge, marking the Himalaya-to-plains transition.