 The two major rivers draining Tiruchirappalli are the Kaveri and its tributary the Kollidam, but the city is also drained by the Uyyakondan Channel, Koraiyar and Kudamuritti river channels. The land immediately surrounding the Kaveri River — which crosses Tiruchirappalli from west to east — consists of deposits of fertile alluvial soil on which crops such as finger millet and maize are cultivated. Further south, the surface is covered by poor-quality black soil. A belt of Cretaceous rock known as the Trichinopoly Group runs to the north-east of the city, and to the south-east there are layers of archaean rocks, granite and gneiss covered by a thin bed of conglomeratic laterite. The region falls under Seismic Zone III, which is moderately vulnerable to earthquakes.
