Fox Glacier is the largest West Coast glacier at 13 km's in length. However, once it extended almost to the sea, as evidenced by the remnant moraine walls. Since the 1950's the glacier has ultimately been retreating, but has experienced a number of small advances also.
The glacier is fed by the large snowfields that drain the Southern Alps at a height of around 2000 metres. Snowfall occurs as the moisture-heavy winds that travel across the Tasman Ocean are forced upwards and over the mountains they meet along their path. As the winds rise, the air cools and the moisture converts to rainfall.
Because of the Alps gradient and their closeness to the coast, this cooling is rapid and the resulting rainfall prolific. For instance, rainfall of 5840mm has been measured near Haast in recent years. At higher altitudes, this rain becomes snow and during heavy snowfall years, this snow compiles & compacts down in the glacial basins; forcing the river of ice to continue its path along the river and down through the Fox Glacier valley.
As the front of the ice pushes out and down the river, it bulldozers boulders and debris down valley. When the melt rate of the Glacier exceeds the accumulation of the snowpack, the Glacier appears to retreat, leaving the wall of boulders and debris marking the further most point of progress. This wall is called the terminal moraine and is evidence of past advance.
Glaciers ebb and flow with changing global weather patterns. The melt rate of the Fox Glacier currently exceeds the accumulation in the upper snow fields; so today, the Fox Glacier is in a state of retreat - but that can quickly change as New Zealand experiences some of the best winter snowfalls in many years.