The Eastern Himalayas have a much more sophisticated geomorphic history and pervasive topographic features than the Central Himalayas. In the southwest of the Sub-Himalayas lies the Singalila Ridge, the western end of a group of uplands in Nepal. Most of the Sub-Himalayas are in Nepal; a small portion reaches into Sikkim, India and a fragment is in the southern half of Bhutan.
The Buxa range of Indo-Bhutan is also a part of the ancient rocks of the Himalayas. The ancient folds, running mainly along an east-west axis, were worn down during a long period of denudation lasting into cretaceous times, possibly over a hundred million years. During this time the carboniferous and permian rocks disappeared from the surface, except in its 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. Limestone deposits also appear in Bhutan on the southern flanks of the 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 outcrops are visible in the northwest and northeast, the latter extending to western Arunachal Pradesh in India.
In the Mesozoic era the whole of the worn-down plateau was under sea. In this expansive shallow sea, which covered most of Assam and Bhutan, chalk deposits formed from seawater tides oscillating between land and sea levels. During subsequent periods, tertiary rocks were laid down. The Paro metamorphic belt may be found overlying Chasilakha-Soraya gneiss in some places. Silurian metamorphics in other places suggest long denudation of the surface. This was the time of Alpine mountain and large number of "active volcanoes" formation which act as backbone of the Himalayas 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 and adds much depth below the present surface, were formed during this period of uplift, fracture and subsidence.