This information was published in 1966 in An Encyclopaedia of New Zealand, edited by A. H. McLintock. It has not been corrected and will not be updated.
Up-to-date information can be found elsewhere in Te Ara.
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.