The Dehra Dun (intermontane valley) located in the NW Himalaya has been considered to develop in response to folding of the Mohand anticline over a ramp in the Himalayan Frontal Thrust (HFT). The Mohand anticline is an upright, asymmetric fault-bend fold composed of Middle and Upper Siwalik sandstones and conglomerates of Miocene to Pleistocene (<780 ka) age. Total displacement on the underlying HFT is ~4 km. Sediments in the Dehra Dun consist of two main packages:

(1) cobble and boulder conglomerates that form low, rounded hills in the hangingwall of the Santaugarh fault. These are interpreted as eroded, basal remnants of early fill, now only locally preserved

(2) pebble, cobble and boulder conglomerate fans that have been deposited by both streamflow and debris-flow processes. These fans form a coalescing bajada, with multiple sources that roughly coincide with the modern drainage network.

At the northern edge of the Dun, Middle Siwalik rocks are folded and thrust over older fan deposits, while the youngest generation of fan deposits have backfilled paleovalleys eroded into the Santaugarh fault hanging wall. These relationships indicate that fan gravels were derived from erosion of the MBT and Santaugarh hangingwall blocks, and that the final stage of gravel aggradation was likely due to shutdown of the Santaugarh fault, which does not offset the bajada surface.

The bajada across the Dehra Dun is incised by the active drainage network, with maximum incision of ~150 m near the MBT. Incision of the bajada appears to have begun by 10-12 ka.

Location map of the Dehra Dun area. The Yamuna and Ganga Rivers drain the Lesser Himalaya  and cross the Dehra Dun before exiting into the foreland. White areas show remnants of the bajada surface, now incised by up to 150 m.

Generalized geology of the Dehra Dun region. Modified from Thakur et al. (2007)  MBT- Main Boundary Thrust