The basin is located between Ponnaiyar and Cauvery basins. The Vellar river rises at an elevation of 900 m near the village of Tumba in the Chittori hills, of the Eastern Ghats in the Salem district of Tamil Nadu. It flows generally in an Easterly direction for a total length of 210 Km in the Salem and Cuddalore districts in Tamil Nadu and finally out falls in to the Bay of Bengal near Porto Nova in Cuddalore district. The Gomukinadhi and Manimukthanadhi are the important left tributaries and Swetanadhi and Chinnar are the right tributaries of the Vellar. The Vellar drains an area of 8,922 Sq.Km.

Vellar river basin comprises of crystalline rocks of Archaean age on the western part of the basin and sedimentary formation of Cretaceous, Tertiary and Quaternary ages on the eastern part. The oldest rock members, are pyroxenites, amphibolites, gneisses, granulites, quartzites, migmatite. Later, charnockites, granites and younger intrusives of pegmatite occur within the older rocks. The general trend of foliations are NE-SW with variation to East-west and dipping South-east North-west, showing minor flextures in the folding in several locations . 82 % of the basin area comprises of hard rock terrain. 18% of the area is underlain by Cretaceous argillaceous sandstone and shell limestone,Tertiary Cuddalore sandstones and Quaternary alluvium.

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