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.