Geology of the Christchurch urban area Christchurch is situated on the middle part of the east coast of the South Island of New Zealand. The city is located on Holocene deposits at the Pegasus Bay coast of the Canterbury Plains, and on the northern slopes of the adjacent Port Hills of Banks Peninsula. Originally the site of Christchurch was mainly swamp lying behind beach dune sand; estuaries and lagoons; and gravel, sand, and silt of river channel and overbank flood deposits of the coastal Waimakariri River floodplain. The Avon and Heathcote rivers originate from springs in western Christchurch and meander through the city to form the main drainage system. These rivers feed an estuary at the coast where dune sand backs the Pegasus Bay beaches. The remnants of older beach dune sand occur up to 6km inland from the coast. Loess mantles the Port Hills on the northern rim of the Lyttelton Volcano (late Miocene) which has been eroded and drowned to form Lyttelton Harbour. Lava flows, dikes, airfall tuff, and laharic deposits of the Lyttelton Volcanic Group are present in the southern part of the map area. Potential geological hazards of concern to Christchurch include flooding, variable foundation conditions, slope instability on the Port Hills, and coastal erosion. The Waimakariri River regularly flooded Christchurch prior to stopbank construction and river realignment which began shortly after the city was established. The high groundwater table and lateral changes from river floodplain, swamp, and estuarine-lagoonal environments have created variable foundation conditions which impose constraints on building design and construction. Christchurch is adjacent to the more tectonically active central region of New Zealand. Consequently, the geology and tectonic setting of the area indicate that future large earthquakes may have a major impact on the city. Earthquakes would be expected to produce liquefaction, landsliding, ground cracking, and tsunamis. Planning and design to mitigate the consequences of these phenomena are an essential prerequisite of preparedness. $19.90 There is a map only available.