After the Ice Age

THE improvement of climate which gradually succeeded the intense cold of the Ice Age in New Zealand was accompanied by the gradual retreat, still in progress, of the valley glaciers in the mountains of the South Island. Westland, in Fiordland, in Canterbury, and the Lake District of Otago, it is now possible examine country that was formerly glaciated, and so gain some appreciation of the powerful erosive action of moving ice. It is found that glacial erosion, slow but relentless in its action, produces characteristic land-forms very different hose which result from the operation of rain inning water. Such land-forms are developed on a magnificent scale in New Zealand.

The valley head in glaciated country is usually as wide as the rest of the valley, or may even expand into an armchair-like hollow, or ‘cirque.’ tore or less perpendicular walls of adjacent cirques, each eating backwards into a mountain peak, may meet in a steep-sided, jagged ridge, of which Mitre Peak in Milford Sound is a superb example. A number of ridges, each separated from its neighbour by a cirque, may converge to form an isolated, pyramidal mountain. Many of the finest peaks in the glorious pile which dominates South Westland — Elie de Beaumont, Cook, Tasman, La Perouse, and Sefton — owe their origin and their beauty to this process; and on them all it is still in progress.

Valley-head cirques sometimes intersect to produce a gap or ‘ col’ which may serve man as a mountain-pass. Such is the- origin of the Lake Harris Saddle between the Routeburn Valley and the Hollyford; and of McKinnon’s Pass, between Te Anau and Milford Sound, which separates the Clinton Canyon, a magnificent glacial trough, from the Arthur Valley.

Of the same type and origin as the Clinton Canyon, but differing in that they are drowned or invaded by the sea, are the fiords of south­western New Zealand. These scenic masterpieces have all the characteristics of heavily glaciated country—vertical walls, great depths, broad floors, cirques at their heads or in side valleys, and hanging-valleys from which plunge spectacular water-falls such as Stirling Falls or Bowen Falls,.

Land-forms of yet another type are frequently found in the lower parts of glacial valleys where raines, which consist of debris transported by glacier, have dammed back a stream to form lake. Such is the origin of Lakes Wakatipu, Wanaka, and others in Otago; of Tekapo, Pukaki, and Ohau, the triple gems of Canterbury; of lovely Lake Matheson at Weheka; of Mapourika and Ianthe near Waiho; of Brunner, Kanieri, Rotoiti, and Rotoroa.

The bulk of the waste derived from the land by the agents of erosion is deposited finally on the sea-floor. Part of it, however, may build land-forms of some permanence. In the glacially-formed valleys of Canterbury, for example, waste descends to valley level as screes or ‘shingle-slips,’ Where a tributary enters the main valley, this type of deposit may develop into an alluvial fan or cone. Land-forms of this origin are developed to perfection in the Rakaia and Waimakariri Valleys.

When a number of streams emerge from a mountain tract on to a lowland they deposit much of their load to form broad, gently sloping fans. As these grow forward and enlarge they merge and ultimately form a continuous apron of waste. The Canterbury Plains are a fine example of such a land-form. If the waste carried to the river mouth is greater than can be   removed by off-shore currents, deltas will form, and these may grow to delta-plains. The Southland Plains and the Heretaunga Plain of Hawke’s Bay are of this origin.