Geologically the Nilgiris belong to the Archean continental landmass of the Indian peninsula, composed of pre-Cambrian, mainly metamorphic rocks (gneisses, charnockites, and crystalline schist). Due to continental drift of the “Indian shield” which until late Jurassic times was a part of the ancient Gondwanaland - and coincident with the Himalayan orogenesis during the Cretaceous and Tertiary periods, geotectonic movements in the southern Deccan resulted in its fragmentation and in vertical dislocations along fault lines that are oriented in three main directions, viz. NNW-SSE, NE-SW and W-E, and that recur in the morphological boundaries as well as in the courses of many streams and rivers of the Nilgiris.
Thus, the triangular-shaped mountain block of the Nilgiris was formed by the phase-wise uplifting of a portion of the Deccan. This horst is almost entirely made up of more or less garnetiferous, acid hypersthenic charnockites (Holland 1900) with a general NE-SW to ENE-WSW strike of foliation (Eastern Ghats trend) and traversed by doleritic and quartzite dykes. It is slightly tilted towards the east - like the entire Deccan Plateau - and has a base size of roughly 2400 square kilometers, of which 40 % rises above 1800 meters in the central Nilgiris Plateau (which falls off steeply on all sides). It culminates in Dodabetta, or “big mountain” with 2636 m elevation above mean sea level.
It is the close spatial association of two contrasting morphological zones or relief types - the “mature” landforms of the elevated plateau and the “juvenile” Sigur, Pykara and other, minor rivers and streams, some of which have been dammed up during the past five decades.
While the first, uppermost level has been designated Dodabetta landforms, the second and third together represent the Ootacamund landforms (Pardhasarathi and Vaidyanathan 1974). All three can be identified as fossil, disintegrating relict landforms showing little, if any, morph dynamism of the kind responsible for their original formation and similar to that still active on the Mysore Plateau and on the Coimbatore Plains. Under present climatic conditions and due to upstream migration of nick points of stream profiles as well as the deforestation and tillage of large tracts of land, linear erosion generally prevails over denudation processes. This is increasingly so towards the edges of the plateau where numerous streams and rivers leave the upper Nilgiris in torrential rapids and picturesque cascades, with heights of up to 120 m, which emphasize the youthfulness of the Nilgiri mountain block as a morphological unit.
