be mixed down and results in a brackish layer typically thick. The interface, called a pycnocline, between the brackish near-surface water and the denser water below supports internal wave motion. Waves on the pycnocline, called internal waves, are ubiquitous in Pelorus Sound. They propagate at and involve vertical displacements of the pycnocline through . They are often generated by the tide, in which case they are called internal tides. They play an important part in driving flow and horizontal exchange in bays off the main channel. For example, current meter records at the entrance to Beatrix Bay show a significant semi-diurnal component with velocities up to . This is not simple, uniform tidal flow in and out of the bay because its amplitude does not follow a spring-neap cycle and there is a phase difference of several hours between the surface and the bottom. Timeline Mists of time: The founding father of the Ngāti Kuia arrives in the South Island in his canoe Te Hoiere 1642: Abel Tasman sails past D'Urville Island. Late 17th century: French and Russian explorers arrive. 1770s: Captain James Cook arrives. 1827: Jules Dumont d'Urville has a disconcerting ride through French Pass in the French Navy corvette Astrolabe. 1838: HMS Pelorus, carries out the first survey. The sound is named in honour of this survey. 1864: Gold is discovered in the Wakamarina Valley, and briefly Havelock becomes a boom town. 1865: Milling of native timber begins 1870: A flax mill starts operating on the Ruapeka Stream 1888: First appearance of the dolphin Pelorus Jack. c. 1900: The timber camps are giving way to dairy and sheep farms. 1912: Pelorus Jack dies. 1939: The last native timber mill closes. Only two small areas of native timber remain 1942: Coastal fortifications are constructed on Maud Island to protect Cook Strait, and the entrance to the sound. 1953: NZS Amokura, New Zealand's first naval ship, is towed into St Omer Bay in Kenepuru Sound, an arm of Pelorus Sound, and retired on the beach, where her remains lie to this day. 1960s: There is a general decline in wet fish and scallop harvests, as some wild fisheries collapse due to overfishing. Late 1960s: Mussel farming begins. 1981: The first specially designed mussel processing factory is established at Havelock. See also Fairy Bay, a small settlement and camping area within the Sound Tawero Point Notes Further references Rutland, Joshua (1900)