marinas of Westhaven and the suburbs of Freemans Bay and the Viaduct Basin. Further east from these, and close to the harbour's entrance, lies the Port of Auckland. There are other wharves and ports within the harbour, notable among them the Devonport Naval Base, and the accompanying Kauri Point Armament Depot at Birkenhead, and the Chelsea Sugar Refinery wharf, all capable of taking ships over . Smaller wharves at Birkenhead, Beach Haven, Northcote, Devonport and West Harbour offer commuter ferry services to the Auckland CBD. Geology The harbour is a drowned valley system that was carved through Miocene marine sediments of the Waitemata Group. Recent volcanism in the Auckland volcanic field has also shaped the coast, most obviously at Devonport and the Meola Reef (a lava flow which almost spans the harbour), but also in the explosion craters of Orakei Basin and in western Shoal Bay. Over the last two million years, the harbour has cycled between periods of being a forested river valley and a flooded harbour. In periods of low sea level, a tributary ran from Milford into the Shoal Bay stream. This valley provided the harbour with a second entrance when sea levels rose, until the Lake Pupuke volcano plugged this gap. Approximately 17,000 years ago during the Last Glacial Period when sea levels were significantly lower, the river flowed north-east along the Rangitoto Channel, meeting the Mahurangi River to the east of Kawau Island. The resulting river flowed further north-east between modern day Little Barrier Island and Great Barrier Island, eventually emptying into the Pacific Ocean north of Great Barrier Island. The current shore is strongly influenced by tidal rivers, particularly in the west and north of the harbour. Mudflats covered by mangroves flourish in these conditions, and salt marshes are also typical. History Prior to European settlement, the harbour was the site of many Tāmaki Māori pā and kāinga, including Kauri Point in Chatswood, Okā at Point Erin, Te Tō at Freemans Bay, Te Ngahuwera, Te Rerenga-oraiti at Point Britomart, and Ōrākei. Herald Island and Watchman Island were both settled by the Waiohua confederation. The Waitematā Harbour was traditionally used as a fishery used by Tāmaki Māori for sharks and snapper. In the late 18th century and early 19th century, the waters were fished together by Ngāti Whātua-o-Ōrākei and Ngāti Pāoa. In traditional legend, the Waitematā Harbour is protected by a taniwha named Ureia, who