The period of mountain-building which gave final form to New Zealand was not confined to this country. It coincided with a time of world-wide mountain-building which created the present mountain chains of the earth. The great changes of relief thus brought about were accompanied by great changes in climate. In both hemispheres polar ice-caps spread into the temperate zones. In the Northern Hemisphere, where the pole is surrounded by large areas of land, the polar ice sheet advanced far into North America and Europe; but in the South, where the Antarctic Continent is surrounded by ocean, conditions were somewhat different. Here, as a result of the intense cold, immense glaciers were formed in the newly raised mountains of the South Island and in Patagonia.

From the great snowfields on the high moun­tains of the South Island glaciers descended to the lower country. Their former extent, far beyond the limits of the present-day glaciers, may be de­tected by the tell-tale land-forms which the moving ice sculptured, and by the terminal moraines dropped at their farthest points of advance.

Probably one-third of the South Island was covered by ice during the Pleistocene Period. From north-west Nelson to Hokitika glaciers spilled down the valleys, sometimes uniting to form extensive ice-sheets on the lower country. Behind them they left great piles of ice-borne rubble, some of which still exist to dam back lakes such as Brunner and Kanieri. Farther south the moving ice reached the sea, there to break off as icebergs.

On the Canterbury side of the Alps the ancient glaciers made their way down all the main valleys, actually reaching, in the Rakaia and the Rangitata,’ as far as the Canterbury Plains. South of Mount Cook a great glacier system extended far into the Mackenzie Plain. Lakes Ohau, Pukaki, and Tekapo are each bounded by huge moraines left by glaciers of this period. Western Otago was heavily glaciated, but the east coast was probably free from ice.

The retreat of the glaciers was followed by interesting climatic changes, and as the land was freed from ice, plants advanced from the non-glaciated regions. Recent study suggests that the first plants to re-people the newly exposed land were grasses and sedges. These were followed by invasion of beech-forest, and finally by a mixed, warm rain-forest in which such trees as the kahikatea, the miro, and the totara were dominant.

As the glaciers melted away, they left behind a deposit of the finest sediment produced by the grinding of the ice over its bed. This ‘ rock-flour’ was spread by streams over the wide valleys, picked up later by the prevailing north-west winds, and so carried over much of Canterbury and parts of Otago. This is the ‘ loess,’ a wind-borne silt of glacial origin, which forms an excellent cover to much of the shingle of the Canterbury Plain.