Glaciers and glaciation

Press, 21 July 1988, Page 13

Glaciers and glaciation

Glaciation in the South Island is a mere remnant today of what it was during the Pleistocene Period, popularly known as the Ice Age, which spanned the time of 1.8 million years to 10,000 years ago.

During the Pleistocene massive ice sheets spread over much of North America and Eurasia several times, and mountain glaciers expanded considerably in other parts of the world in response to a cyclic pattern of cooling and warming climate. These great Pleistocene glaciers left unmistakable evidence in the landscape of their former widespread extent. For a glacier to form, the neve or snow catchment area has to be sufficiently high and large for snow accumulation to exceed melting over many seasons. When a thick snow pile accumulates, the lower layers will gradually become compacted and air forced out, causing the snow to slowly transform into granular ice, which is able to flow downhill under gravity.

Qlaciers in the Southern Alps today are small and localised. East of the main divide they hug the flanks of the higher peaks or occupy the uppermost reaches of the major valleys. Larger glaciers are supported in the Mt Cook area because of the higher mountains and larger neves, but they have, like other New Zealand glaciers, been affected by marked recession since at least the mid-nineteenth century. Rapid downwasting has occurred leaving the lower reaches of glaciers such as the Tasman

Glacier, our longest, covered with a thick layer of morainic debris that gives it a “dead” appearance. Incidentally, the morainic cover acts as an insulating layer, slowing the melting process, and it is known that isolated blocks of ice can exist for many years buried in gravel. The glaciers of south Westland are the jewels of the New Zealand landscape, and virtually unique in the world because they are temperate latitude glaciers which descend to low altitude rain forest. It is the combination

Part I of a two-part series

of blue ice, bush and steep mountain front, which has made Franz Josef and Fox Glacier such important scenic areas. The south Westland glaciers owe their vigour to the steep slopes of the mountain front along the Alpine Fault and the extreme precipitation on the high slopes resulting from the prevailing westerly winds. Although the Franz Josef Glacier is at present advancing rapidly due to higher than average snowfall in the neve area several years ago, its general decline is indicated dramatically by park board signs showing the terminal position at various times back into the nineteenth century. Several kilometres down valley is the 1750 A.D. terminal moraine, and out beyond Franz Josef township is a distinctive

forested moraine loop marking the glacier terminus about 11,500 years ago.

The landscape of Canterbury has been profoundly affected by past glaciations. The sculpturing effects of ice are evident over much of the Southern Alps, and the depositional effects extend way beyond the limits of the great Pleistocene glaciers in the form of low-angle fans of gravel deposited by meltwater issuing from the ice front. Large parts of the Canterbury Plains originated in this way. What is it about moving ice that enables it to remove parts of mountains? When ice moves down valley under gravity it picks up any loose debris in its path; at the same time frostshattered debris and rock slips are continually falling down on to the glacier from above. Armed with rocks the glacier acts like a giant rasp, exerting tremendous abrasive power on the valley sides and bottom. Underlying rock surfaces and boulders become scratched and polished, weaknesses in the rock are picked out and ice is able to pluck rock from the valley floor or wall. Sitting on the floor in the Haast Hall of Geology at Canterbury Museum is a large greywacke boulder, about one metre across. This boulder from the Arthur’s Pass area shows polish and striations caused by glacial transport. So great is glacial erosive power that it tends to straighten and overdeepen former river valleys, changing them to U-

By

HOWARD KEENE

shaped troughs, trimming the spurs on the valley sides and leaving tributary valleys hanging

high above the main valley. These hanging valleys often produce spectacular waterfalls in alpine areas, of which the Devils Punchbowl in Arthur’s Pass is a good example. Rock debris carried by a glacier varies in size from fine silt and clay — the “rock flour” which is formed by the grinding process and gives glacial streams a milky appearance — up to gigantic slabs of rock many metres across which have fallen on to the ice. This material is eventually deposited directly underneath, alongside and in front of the ice as moraine, or it is carried beyond the limits of the ice by meltwater streams issuing from the glacier and deposited as a sheet of alluvial gravel. Terminal moraines mark the maximum position of a glacial advance. These heaps of rubble have had a marked effect on the landscape of the South Island because they have acted as barriers, impounding lakes behind them in glacially overdeepened valleys. The terminal moraines of the last major Pleistocene ice advances are responsible for impounding many of our largest lakes. These moraines, probably about 13,000 to 14,000 years old, hold lakes Sumner, Coleridge, Heron, Clearwater, plus the great lakes of the Mackenzie country, Pukaki, Tekapo and Ohau, and the scenic southern lakes, Wakatipu, Hawea and Wanaka.