[4][5] S. K. Samanta explains in the Indian Journal of Landscape Systems and Ecological Studies,[6] From Pauhunri emanates a large thrust, Dongkiya Range which strides southward for about 50 kilometre.
It is separated from Tsen-gui kang by a fracturing belt of the Lachung chhu valley and leans over the Chola Range just north of Cho la (4435m)....
The stream Rate chhu originates near Cho la which makes the boundary between North and East Sikkim, and ultimately plunges into the river Tista near Dikchu.
The Range Chola extends further south for 20 kilometre along the eastern boundary of East Sikkim and acts as the water-divide between Rangpo-De chhu valleys lying in the district and Ammo chhu in Tibet..... De chhu excavates a narrow valley in south-eastern portion of the area, cuts deeply the last tip of Chola Range and ultimately flows southward through a glen to mark the boundary between Darjiling district and Bhutan.It was established as the border between Sikkim and Tibet's Chumbi Valley by the 1890 Convention of Calcutta reached between British India and Qing China.
However the Doklam plateau at the southern end gives rise to complications and the present day border dispute between Bhutan and China.