During the peak of the Kaikoura Orogeny, rain, rivers, wind, and waves were joined in their attack upon the rising land by another powerful erosive agent — glacial ice. The Pleistocene period, which saw the climax of the New Zealand mountain-building movements, was the time of the world Ice Age. In the northern hemisphere there were four main glacial episodes, when ice sheets a mile or more thick advanced over northern Europe and North America. These glacial episodes were separated by “inter-glacials”, tens or hundreds of thousands of years in duration, when the glaciers melted away and the climate was warm again. Enormous loads of boulders and finer debris, carried as moraine within and upon the ice, were deposited as thick carpets of “boulder clay” or “till”; for example, in northern and central Europe and in Canada and the northern United States. Long ridges of moraine mark temporary halts in the recession of the latest ice sheets. A study of the deposits of these glaciers suggests at least four main Pleistocene glacial episodes, which are termed in Europe the Günz, Mindel, Riss, and Würm (youngest), and in North America the Nebraskan, Kansan, Illinoian, and Wisconsin. Radiocarbon dating of charcoal fragments has shown that this latest ice sheet (Würm, Wisconsin) began its advance about 50,000 years ago, reached its maximum extent about 25,000 years ago, and had retreated from Europe and most of North America by 10,000 years ago. Although the Pleistocene period is often stated to have begun about 1 million years ago, the age of the earlier glaciations is uncertain.
New Zealand was of low enough latitude for its high mountains to be heavily glaciated during the Ice Age. Although the landscape of the South Island shows strikingly the imprint of the latest Pleistocene glaciation, and some relics of the one before, almost all traces of the earliest Pleistocene glaciations have been destroyed by erosion. Strong indirect evidence of the first onset of the chill climate of the Ice Age is recorded, however, by fossils in the younger marine sedimentary rocks of New Zealand; the Pliocene-Pleistocene boundary is recognised by the disappearance of warmth-loving shells characteristic of the Pliocene sediments and their replacement by shells and foraminifera that now live only in sub-Antarctic waters. An alteration of warm and cool faunas was repeated also in younger Pleistocene strata. Wanganui and the southern Wairarapa are two areas where indications of changes of sea temperature have been studied most closely. An examination of fossil pollen preserved in Pleistocene peats and in old lake sediments has also shown how climate fluctuated during the Ice Age. It has been estimated that at the maximum phase of the last glaciation permanent snow level was at least 3,500 ft lower than at present and huge areas of once-forested land were then clothed only by tussock: the changes in the vegetation are reflected by the changing sequence of pollen grains. At the height of the Pleistocene glaciations, glaciers extended from Fiordland to west Nelson, and probably existed in the Tararua Range in the North Island.
The oldest known New Zealand glacial deposit is at Ross, in Westland: it comprises a small accumulation of boulders and laminated silts dating, perhaps, from the earliest glaciation of the Pleistocene. The glaciation that produced it has been named the Ross Glaciation: it probably coincides in time with the first onset of cooling recorded by marine fossils, but very little is known about the extent and position of the glaciers of that time.
A time interval representing probably hundreds of thousands of years separates these old Ross deposits from New Zealand's late Pleistocene glacial deposits, formed during two glacial stages, the Waimaungan and the Otiran. The Otiran glaciation is probably equivalent in age approximately to the last glaciation of Europe and America (the Würm and the Wisconsin); the Waimaungan glaciation may be equivalent to the Riss and Illinoian.
During the great Waimaungan glaciation more than 100,000 years ago, glaciers were thicker and more extensive in New Zealand than at any time since. Great rivers of ice flowing from the Southern Alps merged on lower ground to form local ice sheets that deposited thick beds of moraine; in south-west Nelson, for example, ice tongues joined to form a huge glacier that advanced along the depression between Inangahua and Greymouth, where it probably entered the sea. On the eastern side of the Southern Alps erosion has removed most of the Waimaungan glacial deposits, but a thick accumulation of outwash gravels, till, and varve silts survives at Avoca in the Waimakariri Valley. It is unlikely that any of the landforms carved by the ice in the mountains during this early glaciation remain.
At the time of the latest Pleistocene glaciations (Otiran) the Southern Alps had probably attained more or less their present elevation and their present general pattern of major ridges and valleys: the late Pleistocene glaciers followed these valleys, deepening and steepening them. All the ice-sculptured landforms that add so much to the scenic beauty of the South Island are the product of the Otiran glaciations, particularly the latest which “ended” only about 10,000 years ago. Manapouri, Te Anau, Wakatipu, Pukaki, Rotoroa, and almost all the other lakes of the South Island occupy hollows scooped out by the Otiran glaciers; Milford Sound and other fiords are deep glacial valleys drowned by the sea; great troughs, such as the Cleddau, Hollyford, and Arthur, owe their formation to glaciers; and Cook, Sefton, Aspiring, and other splendid peaks of the high Alps have been sharpened and steepened by ice. Each successive advance of the Otiran glaciers modified the glacial features carved in earlier advances and partly or wholly buried the earlier moraines. Evidence of four major ice advances during the Otiran glaciation has resulted from the study of the sequence of moraines both east and west of the Southern Alps. The closest studies of the Otiran glaciations have been made in the Kumara area of Westland, in the Waimakariri Valley, and Lake Pukaki areas of Canterbury, and at lakes Wanaka and Hawea.
The enormous volumes of broken rock carried by the glaciers and worn by streams from deforested mountains were deposited on the lowlands to form broad sheets of boulders, gravels, and sand: the Canterbury Plains were built in this way.