During the British Raj, Bangalore developed as a centre for colonial rule in South India. The establishment of the Bangalore Cantonment brought in large numbers of migrant Tamil Nadu and Andra Pradesh and North Indian workers for developing and maintaining the infrastructure of the cantonment. Bangalore has a handful of freshwater lakes and water tanks, the largest of which are Madivala tank, Hebbal lake, Ulsoor lake and Sankey Tank. Groundwater occurs in silty to sandy layers of the alluvial sediments. Bangalore receives adequate rainfall from the Northeast Monsoon as well as the Southwest Monsoon. The Peninsular Gneissic Complex (PGC) is the most dominant rock unit in the area and includes granites, gneisses and migmatites, while the soils of Bangalore consist of red laterite and red, fine loamy to clayey soils. Vegetation in the city is primarily in the form of large deciduous canopy and minority coconut trees. Bangalore has not been affected by major seismic activity due to it being located in a seismically stable zone (Zone II). Only mild tremors have been recorded in the city.