From freezing conditions near the mountain crests the glaciers slowly grind their way down-valley into warmer temperatures far below the upper limit of forest. It may seem a contradiction that such glaciers occur in a temperate climate, and that ice and forest are juxtaposed, but there are simple reasons. A glacier is simply a large body of ice, formed from the snow left over at the end of summer, which is moving slowly downhill.
Glaciers occur at high elevations, where winter snowfalls exceed the snow lost from summer melting.
Over several years, as water seeps in and air is expelled under the accumulating weight of snow, the granules merge to form bluish glacial ice. Under the pull of gravity the ice mass actually flows as though it were a stiff liquid, slowly oozing its way down-valley where it melts away in warmer temperatures. This region, the highest part of the Southern Alps, lies in the path of the westerly airstream which sweeps in continually from the Tasman Sea, As moisture laden winds and frequent storm fronts ascend the mountains, the air cools rapidly causing heavy rain and snowfalls. Huge volumes of snow are dumped on the upper slopes of the mountains giving rise to the 140 glaciers which blanket over 200 square kilometres of Westland National Park. The Franz Josef and Fox glaciers are by far the largest of these, containing two thirds of the volume of glacial ice in the park.