Christchurch, largest city in the South Island, is situated on the coastal margins of the vast alluvial-built Canterbury Plains at the base of the Port Hills, which divide the city and the plains from Lyttelton Harbour and Banks Peninsula. The peninsula was once a group of volcanoes, which have since become extinct and have been intruded by the sea, forming many inlets, including Lyttelton Harbour and Akaroa Harbour. The great outpourings of gravel from the Southern Alps gradually linked up with this bastion to create a peninsula, on the neck of which Christchurch is built. Stretching away in a wide sweep to the north are the sandy Pacific beaches along Pegasus Bay, while to the south shingly beaches extend for more than 100 miles. A backdrop to the city's environs is provided by the majestic Southern Alps, rising some 30 miles inland, which exert a strong influence on the climate of the Canterbury region.