The South Taranaki Bight is the large coastal indentation lying to the north and west of the western entrance of Cook Strait. The water shoals gradually inshore from about 70 fathoms in the west into the coastal shallows. There is reason to believe that the bight inshore of the 60 or 70 fathom line was dry land during the last glaciation of the Pleistocene Ice Age, which ended no more than 15,000 years ago. It is probable that at that time all the drainage of the area from Wanganui to the east flowed into a single large river draining into an arm of the sea in the area now occupied by Cook Strait proper. The rise of the sea to its maximum level was completed about 4,500 years ago with a then shore line marked by a line joining Bulls, Shannon, Lake Horowhenua, Otaki, Waikanae, Paraparaumu, and Paekakariki. The broad coastal plain now seawards of this line has been built forward since then, largely from debris brought down by the Wanganui and Rangitikei Rivers draining the volcanic plateau in the centre of the North Island. This area lying in the path of the Roaring Forties is subject to high winds; consequently, dry sand from above high-tide mark on the beaches has been blown inland to form a continuous area of sand dunes. Shortly after the great Taupo volcanic eruptions of 1,800 years ago, the highest of these dunes was formed of debris washed down the rivers and deposited along the south-eastern shore of the South Taranaki Bight. The beach sands of the area to the west of Wanganui are rich in titanomagnetite and have been suggested as the source of iron ore for an iron industry. Detailed estimates, however, show that the amount available is less than that to the north in the North Taranaki Bight and in the south-west of the Auckland Province.

During early Pleistocene times the sea extended much further inland than it does now, crossing the Manawatu Gorge and reaching well inland up the present valley of the Rangitikei River. In western Taranaki the shore line lay at the front of the central Taranaki highlands to the east of Stratford. It was only in the middle of Pleistocene times, when volcanic activity on the Egmont chain of volcanoes began, that the South Taranaki Bight gradually assumed its present form.

Four large rivers, Patea, Wanganui, Rangitikei, and Manawatu, draining almost a quarter of the North Island, flow into the South Taranaki Bight. There are no good natural harbours on this coast, although river ports once existed at Patea, Wanganui, and Foxton. Only the Wanganui port is still used and it faces considerable difficulties from a bar which continuously forms at the mouth of the river. Opunake, which lies to the north of the bight proper, has plans for harbour improvements, but for a number of reasons it is unlikely that this harbour will handle ships of any significance.

The main towns on or near the shores of the bight are Opunake, Hawera, Patea, Wanganui, Foxton, and Otaki.

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

• See also Geology, Sea Floor.