Broadly speaking, the territory of India is constituted by the southern or external side of the Himalaya- Karakoram, by the Gangetic plain and finally by the large and squat peninsula of the Deccan. According to itypeusa, these three great and fundamental divisions represent as many structural elements, to which the geological evolution of the entire South Asia is connected: on the one hand the great Cenozoic corrugation that emerged from the congenital instability of the Tethys, on the other a rigid and archaeozoic plate, the Deccan precisely, a Gondwanian fragment moved towards N, where today the gangetic fossa closes. In Indian territory only the western section and part of the eastern section of the Himalayan side are included, the rest being within the borders of Nepal and Bhutan; this has an average transverse development of approx. 200 km and is crossed by deep valleys, culminating in the great nodal massifs of the system, the highest in the Indian territory, including those of Nanda Devi (7817 m), of Kāmet (7756 m) and of Shilla (7026 m) in the western section, of Kangto (7089 m) and other minor ones in the eastern one. The western mountainous section has a very complex conformation. It begins with a pre-Himalayan hilly belt consisting of Pliocene deposits raised by the most recent orogenetic movements of the system and known as Siwalik formations.
Further inland, beyond a marked fault, the external sedimentary strata appearing constituting the Little Himalayas, furrowed by well-populated valleys with a flat bottom (dun) and a mainly longitudinal course. Further internally we pass to the central band of the mighty massifs (the Great Himalayas), where the Palaeozoic formations emergeupper, schist and granite rocks alternating with archaeozoic formations and also with Mesozoic sedimentary strips. In the northwestern section of the country, corresponding to Kashmir, the Indian territory includes not only the external side of the chain (furrowed by large and fertile valleys such as that of Srīnagar), but also the internal side overlooking the great longitudinal valley of the upper Indus dominated from the Karakoram massifs, to which is added the Tibetan appendage of the Ladākh Range. This whole inland region, which is markedly mountainous, has been shaped by glacialism and due to its marked altitude it is not very hospitable. The eastern Himalayan section is on average less elevated than the western one, but the relief rises from the plain with immediately harsh shapes, which makes this region difficult to access: the only passage is the narrow corridor opened by the Brahmaputra, which cuts across, sharply, the whole chain. In this eastern section the formations of the upper Paleozoic prevail, which towards the E give way to vast granite and gneissic outcrops of the Archaeozoic, which the Cenozoic rejuvenations have modeled in tormented forms. The mountainous marginal India is completed, to the E, with the reliefs of Nagaland, a series of meridian-oriented chains that continue in Burmese territory, consisting of crystalline formations, welded to the eastern Himalayas with the characteristic elbow that closes the Assam: this region includes the Brahmaputra plain, with inundable territories, and a large archaeozoic outcrop with rocks similar to those that form the Deccan (“Dharwar formations”) rising on average about 1500 m (Khasi-Jaintia Hills). The long mountainous arch that encloses India to the N is a sort of bastion dominating the Gangetic plain. It extends beyond an often pebbly foothills and a resurgence area characterized by marshy soils (terai), now mostly reclaimed, for over 1500 km from the northwestern threshold that divides it from the Indus basin, up to the vast eastern delta plain (Bengal), where the floods of the Brahmaputra converge; in width it exceeds an average of 400 km. It has an area of approx. half a million km² and is one of the largest floodplains on Earth. Its origin is connected to the contributions of the Himalayan rivers and, to a lesser extent, to those of the Deccan: it is in fact a filling pit that began to form in the Eocene and in which the most ancient marine deposits overlap the fluvial deposits, which there are great fans at the mouth of the valleys; the main contributions are those of the Yamuna rivers, with which the Gangetic plain, Ghāghara and Gandak begins in the NW. To the real Gangetic plain, which extends from Uttar Pradesh to Bengal, is connected to the NW that of Punjab, which belongs to the catchment area of the Indus (therefore almost all included in Pakistan) and is essentially formed by the contributions of five Himalayan rivers (Sutlej, Beās, Rāvi, Chenāb, Jhelum): the huge belt of lowlands, between the Himalayas and the Deccan, is therefore commonly referred to as an Indo-Gangetic plain. Among the alluvial deposits from which it is constituted, the oldest ones (bangar), dating back to the middle Pleistocene, are distinguished from the most recent ones (khadar); the latter form lower Bengal, the vast delta region of the Ganges and Brahmaputra, with its tangle of canals and flood areas. This distinction is very important from a human and economic point of view, as the former, higher and therefore sheltered from floods, are more intensely cultivated and host dense stable settlements, while the latter, easily flooded, have a more sparse population. However, this is particularly dense in doabs, the interfluvial areas of Punjab and Uttar Pradesh, in the highest areas sheltered from floods.
The trench of the great plains is bounded to the S by the escarpment of the so-called Central Highlands, a region that structurally represents the northern edge of the Deccan, corrugated in the Paleozoic era and then subjected to tectonic disturbances in the Mesozoic and Cenozoic; the basaltic expansions that cover a large part of the Deccan and a part of the Central Highlands themselves are connected to the latter era. This region is bordered to the NW by the Arāvalli mountains, rejuvenations of an ancient Algonquian chain (Precambrian), while to the SW it has its characteristic orographic component in the alignments of the Vindhya and Sātpura, divided between them by a marked structural element, the Narmada valley; in the center it includes the Gondwana plateau and to the E that of Chota Nagpur. On average, the altitude fluctuates around 500-600 m and only the Arāvalli take on steep forms in correspondence with some granite masses, which in Guru Sikhar reach 1722 m; in general the senile forms predominate and the region presents itself in an advanced stage of peneplanation. However, following the Cenozoic rejuvenations, the hydrographic network is markedly engraved, especially on the gangetic side. Outside the area affected by the basaltic coverings, such as the Malwa plateau, N of the Vindhya mountains, the rocks of the lower Paleozoic emerge (for example the gneisses granite and schistosis of the Vindhya) and there is no shortage of areas where those archaeozoic formations that represent the substratum of the Deccan come to light. In relation to the different geological structures, the morphological lines vary, which do not always have a plateau trend. Thus in Gondwana the sandstones have determined the formation of successive escarpments in the wide fluvial slopes, while in Chota Nagpur, due to the prevalence of granite gneisses, there is a more dynamic morphology and a decidedly hilly aspect. The morphology of the Thar desert is determined by the arid climate, with rocky outcrops of the Paleozoic that emerge above Neozoic surfaces, largely represented by dune alignments., a vast penepiano located W of the Arāvalli; it yields to the S to an equally arid region, scattered with marshy and brackish basins (rann), including the broad Rann of Kutch (Kachchh quagmire) followed by the squat peninsula of Kāthiāwār, an area of Cenozoic basalt formations engraved by large radial valleys. AS of the Central Highlands you enter peninsular India; the structural limit is indicated by a depression line that from the Tāpi valley continues towards E with the groove of the Mahānadi river. The structure of the Deccan is that of rigid planks; the basal element is constituted by the archaeozoic granite gneisses that emerge all over the eastern part of the peninsula, while in the north-western section the ancient strata are covered by the aforementioned Cenozoic basaltic expansions, which give rise to a tabular morphology of the region. But as a whole, the Deccan landscapes are relatively varied, also due to the presence of the two peripheral mountain alignments, the Eastern and Western Ghats., which border the peninsula. The Western Ghats are much higher, also due to the general slight inclination towards the E of the whole Deccan plateau; they form a continuous whole, high on average approx. 1000 m, and descend towards the coast with an often steep escarpment, at the foot of which runs a short flat selvedge in which the materials transported by the short streams that cut into the escarpment settle. In the southern part, the Western Ghats reach their maximum heights in some Horst, which form the massifs of Doda Betta (2637 m), Ānai Mudi (2695 m) and Pālayankottai (1654 m), a relief that dominates the apex southern part of the peninsula, whose extremity is in the granite cape Comorin. The Eastern Ghats, besides being lower, have a discontinuous orography; in them open the wide valleys of the rivers that drain the peninsula, coming from the NW and drawing on the internal slope of the Western Ghats. The hydrographic network, whose dissymmetrical development is connected to the general inclination of the peninsular plateau, moves the entire internal part of the Deccan, where there are paleozoic “islands” superimposed on the archaeozoic substrate. The eastern coastal selvedge is varied, alternating large plains with smaller flat areas; in correspondence with the river outlets there are in fact large delta conoids; on vast stretches, finally, the coast has sandy edges that close off lagoon spaces.
