Fig. 1: Contextual map of the Gangetic Plains and study area.

a Geology of the Himalayan Mountain range with accompanying main faults (MFT—Main Frontal Thrust, MBT—Main Boundary Thrust, MCT—Main Central Thrust, STDS—South Tibetan Detachment System, ISMZ—Indus-Tsangpo Melange Zone) and the main rivers of the Gangetic Plains digitised and placed on a 90 m Shuttle Radar Topography Mission digital elevation model^71,75,76, with accompanying geological cross section. Geological cross section was adapted from ref. ⁷⁷. b Geology of the Mohand anticline/Dehradun region and the main rivers (Yamuna and Ganga) which drain western Himalayas. Only Middle Siwaliks sandstone and Upper Siwalik conglomerate are exposed along the Mohand Anticline. Published Siwalik sedimentary log data used in study (MS—Mohand Section⁷¹, KS—Karnali Section⁶³, SK—Surai Khola Section⁶³) and Siwalik sedimentary log data collected in this study (*CS—Chakrata Section) are marked by purple lines. The red dashed line represents published data on the current position of the GST in the Gangetic Plains^7,53. White stars represent the approximate location of Holocene Kosi mega-fan core⁸. Upper Siwalik Conglomerate paleoflow data is from ref. ⁷¹, and suggests that rivers which deposited the Siwalik stratigraphy logged in this study (*CS) flowed in the same direction as the rivers which presently occupy the Gangetic Plains (predominantly flowing south).