The mountainous rim to the North and NE of the Indian territory is formed by the southern slopes of the Himalayan range proper and affects, to the west of Nepal, the federated states of Himachal Pradesh and Uttar Pradesh, and to the east of Sikkim and Arunachal Pradesh (former territory of the NE border). With Jammu and Kashmir the India penetrates the mountain system until it reaches the southern slopes of Karakoram, including the Ladakh Mountains and part of the upper Indus valley, which, with that of the Tsang-po (Brahmaputra), constitutes the conventional northern limit of the range. The western section of the Himalaya includes the Zaskar range, the Upper Himalayas (which continues into Nepal with peaks over 8000m) and the Lower Himalayas with lower ranges. The heights increase from W to East and from S to N, reaching the maximum altitudes, in Indian territory, with Kamet (7756 m) and Nanda Devi (7816 m). Of Cenozoic orogeny, the Himalayan ranges are engraved by deep valleys and by numerous and elevated passes. The Lower Himalaya, with its asymmetrical structure, made up of highly altered Precambrian and Paleozoic sedimentary rocks, is furrowed on the lower edge by long valleys with a flat bottom (dun), densely cultivated and populated, such as the Dehra Dun, at 900 meters. The Upper Himalaya, which is also highly asymmetrical, presents more harsh forms due to the outcrop of granite rocks of the Palaeozoic, alternating with more ancient metamorphites (gneisses, schists) and recent intrusive formations. There are numerous glaciers, remnants of a larger Pleistocene glaciation that shaped the reliefs above 2000-3000 m.
According to directoryaah, the crystalline chains are separated from the plain by the Siwalik, a series of foothills between 450 and 2000 m, consisting of thick Plio-Pleistocene deposits accumulated at the base of the mountains and corrugated or interrupted by faults during the last orogenic movements (Sub-Himalayas). These movements are responsible for the seismicity of the subhimalayan regions, the scene of sometimes very serious events, such as the earthquake of magnitude 7.9 on the Richter scale which in January 2001 struck the north-western section of the country and in particular Gujarat, causing about 100,000 victims and destroying cities and rural settlements, with very serious damage to infrastructures and the production system, in one of the key regions of the country’s economy.
The mountain slopes are rich in forests and pastures thanks to the abundance of waters, which flow into the numerous tributaries of the Indus and Ganges, as well as into the Ganges itself. The eastern section of the Himalayas, conventionally marked E by the groove of the Brahmaputra, features the same series of geological formations and crosses the Federated States of Nagaland and Manipur. Generally less elevated than the western and central Himalayas, this section has harsh shapes, emerges abruptly from the plain and is less known and inhabited, also because it is covered by dense vegetation.
The northern mountainous rim is followed by the Gangetic Plain, which together with that of the Indus occupies over 700,000 km 2, mostly in Indian territory (the middle and lower Indus valley belong to Pakistan; most of the area delta of Brahmaputra to Bangladesh); heights vary between 320 m in the high plain of the Ganges and less than 150 m in Bengal. The Gangetic plain is made up of alluvial material deposited by the rivers in a filling trench (foreland basin) between the Deccan plateau and the Himalayan reliefs. It is connected to it, in Indian territory, by a part of the Punjab plain, made up of the floods of the Indus tributary rivers, and, towards the E, the plain of the Assam, crossed by the Brahmaputra between the slopes of the Himalayas and the mountains on the border with Burma. In the foothills, a maximum of about fifty kilometers wide, coarse, permeable materials prevail, so the waters re-emerge further downstream in the form of springs, forming a swampy and unhealthy area, with dense tropical vegetation and characteristic fauna (Terai). Towards the valley, the river network delimits more or less wide interfluvial shelves, with densely populated Pleistocene terraces (because they are higher and safe from floods), and others more recent (khadar) often flooded; the latter, widespread in Bengal and generally in the eastern regions, are only temporarily inhabited.
The great plain ends in the S with the central plateaus (Malwa and Bundelkhand), the northern edges of the Deccan plateau. This, conventionally delimited by the Tapti-Mahanadi line, is a large peninsular region (1.6 million km 2) in the shape of a triangular cusp, which with some offshoots actually extends almost to Delhi. Remnant of an ancient southern continent (Land of Gondwana), the Deccan has undergone corrugations and peneplanations that have given rise to complex tabular forms, different in type of rock and form of erosion, often rejuvenated, edged by asymmetrical reliefs and interrupted by faults; the average height is about 600-700 m. At the base, granite-gneissic masses prevail, with intrusion of ancient rocks and sediments deformed by Precambrian and Paleozoic corrugations. The crystalline formations emerge to the South and E, while on the north-western side they are covered by extensive basaltic sheets (500,000 km 2) which have leaked concurrently with the Himalayan orogeny. From the alteration of these lavas the black earths (regur), particularly suitable for the cultivation of cotton, while reddish lateritic soils not very fertile derive from gneisses and granites.
The central plateaus end to the West with the Aravalli Mountains, a relic of a Precambrian mountain system; to the South they are truncated by the Vindhya and Kaimur Mountains, which follow one another from W to East dominating the 880 km long trench into which the Narmada, a tributary of the Arabian Sea, and the Son, a tributary of the Ganges, flow in the opposite direction. AO degli Aravalli extends up to the Indus, the desert basin of the Thar, and to the south a low coastal region that collects the waters coming from the Aravalli in brackish and periodically marshy basins, sometimes very large. Between the Gulf of Kutch and the Bay of Cambay, the classic way of penetration of Europeans towards the interior, develops the squat peninsula of Kathiawar, whose tabular relief can be considered the continuation of the Satpura Mountains.
While to the north of the Tapti-Mahanadi line more or less parallel formations prevail and with an E direction, the Deccan peninsula consists of a series of internal plateaus bordered towards the sea by longitudinal reliefs (the Ghats) converging in the S of the peninsula at the node of the Nilgiri Mountains. From the pit of Tapti to Capo Comorin, the Western Ghats, of granite-gneissic structure, form a 1500 km long ridge that plunges towards the sea with 500-600 m steps, engraved by ravines and ravines. High on average over 1000 m, the Western Ghats reach greater heights in the Nilgiri Mountains and in the Cardamom range (Anaimudi Mountains, 2695 m, the highest peak south of the Himalayas). Towards the inside, the relief slopes down to a platform covered with lava and with still rugged and stepped forms, and further to the South in the plateau of Karnataka, at 700 m, excavated in archaic formations (gneiss, schists) with granite domes (drug) and wide, senile valleys, interrupted by waterfalls. As you continue towards the E, the shapes become sweeter in the Chhattisgarh plainand in that of Tamoul. In some places these plains directly overlook the Bay of Bengal, in others they are separated from it by the Eastern Ghats, not very high (on average about 600 m) and fragmented into isolated massifs. The eastern coastal strip is much wider, more discontinuous and varied than the western one, bordered by coastal strips that delimit coastal lakes and ponds, and in constant evolution.