The existence of a glacier is a balancing act between competing forces: snowfall at the névé, high in the mountains, battles with rapid melting at the terminal lower down the valley, the victor determining whether the glacier will advance or retreat. Snowfall, metres thick, gradually compacts to form clear blue ice, accumulating until it starts to flow downhill under its own weight. Friction against the valley walls slows the sides while ice in the centre slips down the valley, giving the characteristic scalloped effect on the surface, which is especially pronounced on such vigorous glaciers as Franz Josef and Fox. Where a riverbed steepens, the river forms a rapid: under similar conditions, glaciers break up into an icefall, full of towering blocks of ice known as seracs, separated by crevasses.
Visitors familiar with grubby glaciers in the European Alps or American Rockies will expect the surface to be mottled with rock debris which has fallen off the valley walls onto the surface; however, the glaciers here descend so steeply that the cover doesn’t have time to build up and they remain pristine. Rock still gets carried down with the glacier though, and when the glacier retreats, this is deposited as terminal moraine. Occasionally retreating glaciers leave behind huge chunks of ice which, on melting, form kettle lakes.
The most telling evidence of past glacial movements is the location of the trim line on the valley wall, caused by the glacier stripping away all vegetation. At Fox and Franz Josef, the advance associated with the Little Ice Age around 1750 left a very visible trim line high up the valley wall, separating mature rata from scrub.