The Sea Floor

Cook Strait is the stretch of water separating the North and South Islands of New Zealand. It connects the South Pacific Ocean and the Tasman Sea through the centre of the New Zealand land mass, and is 14 miles at its narrowest, between Cape Terawhiti on the south-east coast of Wellington, and Wellington Head near Tory Channel. Much of the shore of Cook Strait on both sides is composed of steep cliffs. The beaches of Cloudy Bay, Clifford Bay, and Palliser Bay are composed of boulders with steep and high storm beaches. Although these bays shoal gently down to 70 fathoms, where there is a more or less extensive submarine plateau, the bottom topography of the strait is complex. The major features of this bottom topography are, however, an eastern Cook Strait Canyon with steep, and in some places, precipitous walls descending eastwards into the bathyal depths of the Hikurangi Trench which lies off the east coast of the North Island. The upper part of the canyon divides into three arms: the South Arm, lying near the centre of the strait with its head south-west of Wellington; the Middle Arm with its head south of Tongue Point on the South Wellington Coast; and the Wairarapa Arm with its head at the 20-fathom contour near the western side of Palliser Bay. Centre Bank at a depth of 70 fathoms divides the South and Middle Arms. To the north-west of the Cook Strait Canyon, in the Cook Strait Narrows, lies the Narrows Basin, where depths of water between 150 and 200 fathoms predominate. Fisherman's Rock in the north end of the Narrows Basin near the centre rises to within a few feet of low tide, and is marked by wave break in rough weather. Leading into the Narrows Basin from the north-west is the North West Trough, a rather shallow submarine “valley” lying across the northern end of the Marlborough Sounds. Its head lies near the centre line of Tasman Bay. Near shore on both coasts from the Narrows both to north and west, the bottom topography is most irregular, particularly around the coast of the South Island where the presence of offshore islands, submerged rocks, and the entrances to the sounds, create violent eddy conditions. Cases in point are Koamaru Hole, 100 fathoms off the entrance to Queen Charlotte Sound; Jacksons Hole, 150 fathoms off Cape Jackson; Chetwode Hole, 80 fathoms off Chetwode Island; and Sentinel Rock and Stephens Hole, 150 fathoms off Stephens Island.

Formation

Cook Strait appears to have been formed by a flooding by the sea of a continuous land ridge in Opoitian times, late in the Tertiary Period. Although the sea withdrew several times during the Pleistocene Glacia-tions, due to world-wide lowering of sea level with the accumulation of vast quantities of ice on the land, it always returned, unlike the Manawatu Gorge further north, when the level rose again. It is believed that, during the last glaciation, sea level dropped about 300 ft, thus joining the North and South Islands and forming a great sound in place of Cook Strait. The sound appears to have had its northern end somewhere west of Kapiti Island. In the calmer water undisturbed by the present violent tidal currents, Cook Strait filled up to form a flat sea floor about 70 fathoms below present sea level, giving about 20 fathoms of water in the Pleistocene sound. Concentrations at this depth off Cape Campbell have been radiocarbon dated, and shells from them, N.Z. 150, have dated as 19,500± 1,000 demonstrating a last glaciation age for the concretions and the Sound. At a depth of 50 fathoms off Kapiti Island the leg bone of a small moa, probably Anomalopteryx oweni (Haast) was recovered. The bone was probably deposited in the alluvium of a valley floor and is unlikely to have travelled a great distance. It appears clear that depths of water greater than 70 fathoms in Cook Strait have been eroded since the rise of the sea following the close of the last Glaciation, and in all likelihood most of this erosion in a very short space of time following the reopening of Cook Strait.

Major Faults

Cook Strait is crossed by four major faults with a dominating horizontal displacement. One is the Wairau, which lies on the western side of the Wairau Valley, passing into Cook Strait to the east of the Marlborough Sounds and probably continuing along the western side of Kapiti Island. This fault is considered to have been responsible for the extension of the Marlborough Sounds across a one-time linear strait, by bodily moving the western portion of the South Island relatively northwards. The Awatere Fault in the Awatere Valley, South Island, continues in the North Island as the Wellington Fault, again with the western side moving northwards relatively to the eastern. The Clarence Fault of the South Island appears in the North Island as the West Wairarapa Fault, movement on which caused the 1855 earthquake which seriously damaged the infant town of Wellington. The Hope Fault passes across the strait to continue north-eastwards in the East Wairarapa.

Tides and Winds

Since the early years of European settlement, Cook Strait has been notorious for its treacherous currents and high winds, often of gale force. The tidal currents now generated in the strait reach a velocity of about 8 knots westwards and 4 knots eastwards, but are very erratic depending on conditions of wind and weather. The strait lies between 41 degrees and 41 degrees 40 minutes South Latitude in the westerly wind belt known as the Roaring Forties and wind velocities of up to 150 m.p.h. have been measured in the vicinity of Wellington. As Cook Strait is the only large gap in the chain of mountains extending north-eastwards for 900 miles from Puysegur Point to East Cape, it is thus a natural channel through which air streams approaching central New Zealand are diverted and accelerated to pass between the North and South Islands. This local acceleration of the surface winds is most pronounced when the air approaches from directions between west and north-west or between south and south-east. The map below illustrates the pattern of flow through the strait on an occasion when the general air-flow aloft was from the west.

When the general air-flow aloft is roughly parallel to the mountain chain (and south-westerly flows are relatively frequent), a slight shift in direction causes a complete reversal of the surface winds in Cook Strait. The southerly change which is usually very sudden and on occasion violent, often arrives as a “line squall”. Winds from the north-westerly quarter predominate, however, being about twice as frequent as those from the southerly quarter. Strong gales (60 m.p.h. or more) occur about Wellington and in the strait about 30 times per annum, on the average, with maximum frequency in the late spring.

Communications

Regular passenger services across Cook Strait are maintained by a rail-road ferry (Aramoana, 4,160 tons) shortly to be duplicated, running between Wellington and Picton in Queen Charlotte Sound. The inter-Island Steamer Express between Wellington and Lyttelton is maintained by Maori, 8,300 tons, and Hinemoa, 6,911 tons, with Rangatira, 6,152 tons, in reserve. A new vessel, Wahine, will enter the service in early 1966. Until 1953 a passenger ferry service ran between Wellington and Nelson, using Arahura, 1,596 tons, and Matangi, 1,366 tons. Regular passenger flights by DC3 and Fokker Friendship are maintained both to Nelson and to Blenheim. Freight shipping services are extended across the strait on a routine basis; rail-air freight between Wellington, and Nelson and Blenheim, using Bristol Freighter Aircraft, operates continuously.

Telegraphic and telephone communications across the strait have been maintained by cables. The first across the mouth of the Narrows from Oteranga Bay to Port Underwood was continuously disrupted by breaks, usually during storms, and was supplemented by two other cables. These cables had a mean service between breakages of three months. Later, two other cables were laid from Titahi Bay to Queen Charlotte Sound and, although several breaks occurred, they gave better service. The latest pair of modern coaxial cables were laid between Island Bay and Blind Bay off the coast of the Awatere Valley. A break in one of these cables appears to have been caused by a turbulent mass of mud and water which washed the cable into the head of Cook Strait Canyon. Planning for a Cook Strait power cable has been completed and the work was finished by May 1965. The route taken was that followed by the early telegraph cables. It is anticipated that the much greater weight of the power cable will prevent movement along the bottom that was the cause of most of the failures of the other cables.

Shipwrecks

Because of its unreliable weather, very strong and variable currents and often violent seas, Cook Strait has always been a hazardous place for ships, especially in the days of sail when the presence of lee shores in most directions and restricted sea room caused many shipwrecks. Among the worst on record were the barque Maria, 23 July 1851, with the loss of 26 lives; steamer City of Dunedin, May 1865, with the loss of 40 lives; ship Lastingham, September 1884, with the loss of 18 lives; ship Zuleika, 16 April 1897, with the loss of 12 lives; steamer Penguin, 12 February 1909, with the loss of 75 lives; and steamer Ripple, 7 August 1924, with the loss of 16 lives.

What could have been a terrible disaster was narrowly averted on Sunday, 6.10 a.m., 2 February 1936, when the inter-Island steamer express Rangatira in a howling southerly gale struck a submerged object off the Wellington Heads and was badly holed. She had 600 passengers on board. A quarter of an hour later she backed off and, although badly down by the bow, managed to enter the harbour. When she finally berthed at Clyde Quay Wharf, she was aground forward. Under the weather conditions at the time, the survival of the Rangatira was due to a combination of good fortune and good seamanship.

Swimming of the Strait

It is quite possible that, before the coming of the European, the strait was swum by a Maori. Within European times it is recorded that a South Island Maori named Whakarua-tapu, of the Kai Tahu tribe, swam the strait in order to save his life. Apparently after Te Rauparaha had returned from his onslaught on Kaiapohia (Kaiapoi) in 1831, he attacked the people of Wairau, and among the captives whom he carried back to the North Island in his canoe was Whakarua-tapu. When near the coast of the North Island, Te Rauparaha ordered Whakaruatapu to kill his own daughter, a child about nine years old; instead, he flung his daughter into the sea. Te Rauparaha then tried to kill Whakarua-tapu with his hatchet but the latter dived overboard and set off for the shore. Fearing death from Te Rauparaha's people, he changed course and made for the South Island which, by swimming and floating, he eventually reached. It is alleged that he landed somewhere in the vicinity of Queen Charlotte Sound.

The first European to swim the strait was Barrie Devenport who on 20 November 1963 made the crossing in the time of 11 hours 13 minutes. This feat was repeated by Keith Hancox on 7 February 1964 in the time of 9 hours 34 minutes.

Canoeists

The first European canoeists to cross the strait were the brothers G. and J. Park, who in 1890 made a journey from Mana Island to Dunedin in 14 ft canoes. The first canoeist to cross solo was H. Sherman, a 16-year-old Wellington boy, who in 1896 went from Mana Island to Cape Koamaru in a tiny craft 12 ft long.

Discovery

Abel Tasman was in the Cook Strait area from 18 until 26 December 1642. Although he showed the strait as a bight on his chart, he noted the tide was running from the south-east and therefore concluded that there might be a passage. He named it Zeehaens Bocht, after one of his ships. The true nature of the strait was discovered by Cook on 22 January 1770. While the Endeavour was undergoing repairs in Ship Cove, Cook, Banks, and Solander took the ship's pinnace to explore the head of what seemed to be a large inlet. After rowing 4 or 5 leagues without reaching their objective, the party landed on the south-east shore. Cook then climbed a hill and sighted the eastern sea, thus proving the strait's existence.

Name

The Maoris knew the strait as Raukawa. The origin and the meaning of the name are obscure.

The European name is, of course, after Captain Cook and it appears on the chart of 1770. It seems likely that the name for the strait was agreed upon by Cook only because he was pressed to do so by members of the Endeavour party, especially Joseph Banks.

by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.