Making New Zealand vol 01 no 01: The Beginning
After the Ice Age
After the Ice Age
A glaciated 'U'-shaped valley in Fiordland, from the air. Glacial valleys are broad-floored and steep-sided; they have few curves and ape not interrupted by overlapping spurs. Former spurs have been cut back or truncated by the ice. The floor of the valley is often uneven, with sudden steps and lake-filled irregularities. Tributary streams, which flow in hanging-valleys often high above the main valley, cascade in as waterfalls, whereas tributaries of a normal stream meet it at stream level. M. C. Lysons
This scraper-board diagram shows the origin of a mountain such as Mount Cook. Cirques (armchair-like hollows) have been, enlarged until they met in converging ridges, which are worn away until a sharp peak is left. Geologists foretell that some day Mount Cook will be reduced to the fragment shown at the bottom of the diagram.
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.
In Westland, in Fiordland, in Canterbury, and in the Lake District of Otago, it is now possible to 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 from those which result from the operation of rain and running 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.' The more 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.
Land-forms of yet another type are frequently found in the lower parts of glacial valleys where moraines, which consist of debris transported by the glacier, have dammed back a stream to form a 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 into delta-plains. The Southland Plains and the Heretaunga Plain of Hawke's Bay are of this origin.
Shingle slopes on the Whitcombe Pass—the 'No Man's Land' between Canterbury and Westland.
This photograph from Mount Oakden, near Lake Coleridge, shows the Rakaia, Mathias, and Wilberforce Valleys. The floors of the main valleys form valley-plains of shingle and sand, over which the streams wander in the 'braided' channels so characteristic of many South Island rivers. J. D. Pascoe