Kolkata is located in the eastern part of India. It has spread linearly along the banks of Hooghly River. The Kolkata Municipal Corporation has an area of 205 square kilometres. The city is near sea level, with the average elevation being 17 feet. The whole area is in the Ganges Delta which starts within 100 km south to the city. Most of the city was originally marshy wetlands, remnants of which can still be found especially towards the eastern parts of the city. Geology Indo-Gangetic Plain, the soil and water are predominantly alluvial in origin. Kolkata is located over the "Bengal basin", a pericratonic tertiary basin. Bengal basin comprises three structural unit: shelf or platform in the west; central hinge or shelf/slope break; and deep basinal part in the east and southeast. Kolkata is located atop the western part of the hinge zone which is about wide at a depth of about below the surface. The shelf and hinge zones have many faults, among them some are active. Total thickness of sediment below the Kolkata is nearly above the crystalline basement; of these the top is quaternary, followed by of tertiary sediments, trap wash of cretaceous trap and permian-carboniferous Gondwana rocks. The quaternary sediments consist of clay, silt, and several grades of sand and gravel. These sediments are sandwiched between two clay beds: the lower one at a depth of ; the upper one in thickness. According to the Bureau of Indian Standards, on a scale ranging from in order of increasing susceptibility to earthquakes, the city lies inside seismic zone III. City geography Spread roughly north–south along the east bank of the Hooghly River, Kolkata sits within the lower Ganges Delta of eastern India; the city's elevation is . The city is fringed by extensive mangrove ecosystems and tidal flats. Urban structure The Kolkata metropolitan area is spread over and comprises 4 municipal corporations (Kolkata Municipal Corporation, Bidhannagar Municipal Corporation, Howrah Municipal Corporation and Chandernagore Municipal Corporation), 37 local municipalities (Bally Municipality is merged with Howrah Municipal Corporation and Rajarhat-Gopalpur Municipality is merged with Bidhannagar Municipal Corporation, so total number of municipalities becomes 37 from 39) and 24 panchayat samitis. The urban agglomeration encompassed 72 cities and 527 towns and villages, . Suburban areas in the Kolkata metropolitan area incorporate parts of the following districts: Kolkata, North 24 Parganas, South 24 Parganas, Howrah, Hooghly and Nadia. Kolkata, which is