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.
