The region surveyed was in Tibet from the Tsangpo in the north down to the Nepalese border. It included the basin of the Arun River, the region north and east of Everest, and in the west the headwaters of the Bhote Koshi flowing into Nepal. Heron considered that a river had once flowed east-west just north of Himalaya, possibly joining the Tsangpo as a tributary. The Arun River flows southwards between Everest and Kangchenjunga and he thought that its headwaters had cut backwards and to the north until the east-west river had been captured in the middle of its course. The Arun Gorge is deep as it cuts completely through the Himalayan ridge and for the origin of its upper part Heron was unable to give an explanation.