|ॲक्सेसदिनांक=2007-09-12}}</ref> <!-- The city is classified into three regions based on geology, sandy areas, clayey areas and hard-rock areas. --> Sandy areas are found along the river banks and coasts, such as [[Thiruvanmiyur]], [[Adyar (Tamil Nadu)|Adyar]], [[Kottivakkam]], [[Santhome]], [[George Town, Chennai|George Town]], [[Tondiarpet]] and the rest of coastal Chennai. Here rainwater [[Surface runoff|runoff]] [[percolation|percolates]] quickly through the soil. Clay underlies most of the city including [[T. Nagar]], [[West Mambalam]], [[Anna Nagar]], [[Villivakkam]], [[Perambur]] and [[Virugambakkam]]. Areas of hard [[rock (geology)|rock]] include [[Guindy]], [[Perungudi]],[[Velachery]], [[Adambakkam]] and a part of [[Saidapet]].<ref name=rain_water_harvesting_TN_gov>{{स्रोत बातमी