The Eastern Ghats form the eastern boundary of the Deccan Plateau. These are a series of detached hills of heterogeneous composition which are called by various local names.
Their average elevation is 1100 m. They depict true mountain characteristics between the Mahanadi and Godavari (width 200 km in the north and 100 km in the south). Here the average height is 920 m but some peaks rise above 1500 m (Nimalgiri 1515 m in Koraput district Mahendragiri 1500 m in Ganjam district).
The predominant rocks are khondalites and charnokites; the former is metamorphosed sedimentaries (garnetiferous sillimanite gneisses), while the latter are intrusive rocks resembling granites. These ranges from the watershed between the west-flowing (Machkund, Sabari, Sileru, Bhaskel, Indravati etc.) and the east-flowing rivers (Lingulia, Vamsadhara, Nagavalli etc).
Between the Krishna and Madras (Chennai) they continue as the Kondavidu hills mainly composed of quartzites and slates. The Nallamalai (900- 1100 m height) and Palkonda hills are composed of Cuddapah and Kurnool formations. These form an
extremely rugged and broken hilly country with jagged peaks and steep slopes. Their continuation is to be seen in the Javadi, Shevaroy and Biligiri Rangan hills of Salem and Coimbatore districts which finally join the Nilgiris.
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The last stretch beyond Chennai is formed primarily of charnokites together with gneiss, crystalline limestone, quartzites and mica-schits. Fermor believes their upliftment during the pre-Cambrian times which is supported by the evidence of high degree of metamorphism.
The Nilgiris (Blue Mountains) provide the converging site for three mountain ranges: the Sahyadri joining opposite of the Makurti peak; the southern Ghats across the Palghat in the south and the Eastern Ghats at the north-eastern corner.
They abruptly rise from the surrounding area. The Nilgiris enclose between them the Karnataka Plateau. The highest peaks are Dodabetta (2637 m) and Makurti (2254 m). Oootacamund (Udagamandalam) lies at the foot of Dodabetta.