Structure and Relief
Eastern Himalaya has a more complex geographical history than the Central Himalaya and has a more varied relief. In the south west of the Sikkim are the Singalila Ridge, the western end of a group of uplands as in Nepal known as the sub-Himalaya of the Bhutan. Most of the Sub-Himalayas are in Nepal, a small range reaches into Sikkim, and a fragment comprises the southern half of the Bhutan. The Buxa range of Indo-Bhutan belongs to the ancient rocks of the Himalaya. The ancient folds mainly along east-west axes were worn down, during a long period of denudation lasting to cretaceous times, possibly a hundred million years. During this time the carboniferous and Permian rocks disappeared from the surface, except in the north near Hatisar in Bhutan and in the long trench extending from Jaldhaka River to Torsa River, where limestone and coal deposits are preserved in discontinuous basins. Limestones appear also in Bhutan on the southern flanks of lower Himalayas. The rocks of the highlands are mainly sandstones of the Devonian age, with limestones and shales of the same period in places. The core of the mountain is exposed across the centre, where Paleozoic rocks, mainly Cambrian and Silurian slates and Takhstasang gneiss, outcrop in the north-west and north-east, in the later extending into western region of Arunachal Pradesh. In the Mesozoic era the whole of the worn-down plateau was under sea. In this sea, which covered most of Assam, Bhutan, chalk was deposited as a result of oscillations between land and sea levels, during subsequent periods of the tertiary rocks were laid down. Paro metamorphic belt may be found overlying Chasilakha-Soraya gneiss in some places. Silurian metamorphics in others shows that that there must be long denudation of the surface. This was the time of the Alpine mountain building, and much of the movement in the palaeozoic region was probably connected with it. The Chomolhari tourmaline granites of Bhutan stretching westwards from the Paro chu, but at a greater depth below the present surface belong to this period of uplift, fracture and subsidence.
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