SIKHIM AND BHUTAN
(3) The Mo-chhu or Sankos, which with its numerous tributaries, drains the valley of Poonakha; one branch taking its rise on the southern slopes of Chomolhari and another in the snows to the east of Kala-tsho;
(4) The Monass, by far the largest river in this part of the world, with two main branches of which the Lobrak or Kuru-chhu rises in Tibet, its main source being in glaciers on the northern slopes of the Kulu-Kangri and its adjacent snows, while the second, the Dongma-chhu, rises in the snowy range to the east of Tawang. Other tributaries are the Madu-chhu, running past Tongsa and the Pumthang, draining the Bya-gha Valley.
I can best describe the Mountain System as a series of parallel ranges running in a general direction from north to south, springing from the vast snow-range which forms the southern buttress of the great Tibetan Plateau. These parallel ranges are again cut into innumerable smaller ranges forming a vast labyrinth of valleys running in every direction, while the main ranges, running down to the plains, divide the river systems I have already mentioned, and, as they have no distinctive local names, I have called them after certain peaks or passes.
Beginning from the extreme west there is—
(A) The Singli-la range, the crest of which forms the boundary between Sikhim and Nepal. This range runs from Kangchenjunga and on it are the well-known and often visited peaks of Sandukphu and Phallut, as well as the hill stations of Darjeeling and Kurseong. The principal pass between Sikhim and Nepal is the Chiabhanjan-la.
(B) Further to the east is the Chola range descending from Powhunri to the east of the Donkia-la and forming the water parting between the Teesta and the Am-mo-chhu. There are numerous passes on this range, the most generally used being the Jeylap-la and Natu-la, although, before the present roads were made, the Cho-la was much used, and I have crossed it myself as well by four others, the Yak-la, the Sibu-la, the Thanka-la and another.
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