The extent of the Southern Alps has never been officially defined. This account deals with that part of the axial range of the South Island extending from Haast Pass to Arthur's Pass. The geology, vegetation, and economic sections apply also to the northern continuation, including the Spenser Range, as far as Tophouse Saddle on the Wairau River, Marlborough. Between the Spenser Range and Arthur's Pass, the ranges are low, 5,000–6,000 ft, with no notable peaks, and contain four passes: Harper Pass (3,152 ft); Hope Pass (3,383 ft); Amuri Pass (3,301 ft); and Lewis Pass (2,837 ft) the latter providing an important road route between Canterbury and the West Coast.
The Alps and the major flanking ranges are largely the upturned edges of tilted blocks considerably modified by water and ice erosion by rivers and glaciers. The main divide culminates in the Mt. Cook area with 17 peaks above 10,000 ft, from Mt. Sefton (10,359 ft) in the south to Mt. Elie de Beaumont (10,200 ft) in the north. Mt. Cook (12,349 ft) is on a south-trending spur from Mt. Dampier (11,287 ft) near Mt. Tasman (11,475 ft) on the main divide. South from Mt. Sefton, the Southern Alps maintain a height of about 7,500 ft to Haast Pass (1,847 ft), the lowest alpine pass.
North of Mt. Elie de Beaumont the main divide decreases in height, with a few peaks above 8,000 ft (e.g., Mt. Tyndall, 8,282 ft; Mt. Whitcombe, 8,656 ft), descending to Mt. Rolleston (7,453 ft) just south of Arthur's Pass. Some subsidiary spurs and flanking ranges contain peaks higher than those on the adjacent main divide (e.g., D'Archiac, 9,279 ft, and the Arrowsmith Range (Mt. Arrowsmith, 9,171 ft)). Whitcombe Pass (4,025 ft) and Browning Pass (4,752 ft) are low passes free from snow in summer, but only Arthur's Pass (3,020 ft) in the north and Haast Pass in the south are used for transalpine roads.
Peaks and ranges above 5,000 ft show the effects of glaciation with cirques and glacial valleys dominating the landscape, and the major valleys show glacial features – moraines and ice-sheared walls – for many miles down stream from existing glaciers.
On the western side of the divide, temperate rain forest reaches from the sea to a height of 4,000 ft, and in some places glaciers extend well below the bush line, e.g., Fox and Franz Josef Glaciers. The forest is composed of beech (Nothofagus menziesii and N. fusca) with rata (Metrosideros lucida), and rimu (Dacrydium cupressinum) at lower elevation. This is succeeded by a belt of subalpine scrub and finally by snow grass up to the vegetation limit which is permanent snowline (about 6,000 ft).
On the east the vegetation is largely subalpine tussock with small areas of beech and totara forest in gullies and patches which have escaped destruction by fire, possibly set alight by early Maoris. Evidence for the former extent of the eastern forest is given by the widespread occurrence of charred logs of totara and matai, and charcoal horizons in the soil. Changing climate was possibly the reason for the failure of the forest to regenerate.
The most notable of New Zealand's flowering plants, the subalpine and alpine plants, are found in the Southern Alps. Generally the forest flowers are inconspicuous.
The Southern Alps throughout their length are composed of Upper Paleozoic to Mid-Mesozoic sediments, greywackes, and argillites, and their transformed (metamorphosed) underlying equivalents, the schists of the western side of the main divide. They have been deformed by two periods of diastrophism which gave the rocks their complicated fold-structures and generally steep attitudes. The second deformation, which is probably still in progress, was a block-faulting and tilting which reached an extreme in the Alps, and produced the basis of the present topography. The maximum tilting was to the west along the Alpine Fault where schist underlying the eastern greywacke is now exposed, implying a displacement of tens of thousands of feet.
No economic mineral deposits are known in the Southern Alps, but the low-grade schists west of the main divide contain quartz veins from which the gold deposits of Westland are thought to be derived. The quartz veins themselves are poor in gold but the concentrating action of rivers and sea has produced rich leads now very largely worked out. Greywacke gravels derived from the alps and foothills are the main source of road metal and concrete aggregate in the South Island.
The main economic product of the Alps is the waterpower produced by the heavy precipitation throughout the high country. This is at present exploited only by power stations on the Waitaki River and at Lake Coleridge. (Others on the Waitaki River and its major tributaries are in course of construction.) Several other rivers, though difficult to exploit, are potential hydro-electrical producers.
by Alan Copland Beck, M.SC., New Zealand Geological Survey, Christchurch.