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Firnification
A glacier is an area of layered ice that has undergone a process known as firnification. According to the Tongass National Forest website, layers upon layers of snow that build up eventually compact. The new layers of snow put pressure on the old layers and change the snow to firn, a denser type of snow. The firn eventually becomes ice and these ice sheets are what form glaciers.
Conditions
Specific conditions must occur for a glacier to form. Glaciers only form in areas where there is more snow than melt. This means that as the years pass, instead of the snow melting during each warm season it stays intact. Over years and years more snow piles up on top of old snow in a glacier's accumulation area. This is what creates perfect conditions for firnification and glacier formation.
Anatomy of Glaciers
The area of accumulation in a glacier is in the higher elevations. This refers to where snow is building and compacting and growing the glacier. As it compacts, the glacier actually expands and retreats down the mountain. Like a river, glaciers flow from precipitation that accumulates in higher elevations in the accumulation zone. They flow down the mountain and are constantly melting in the ablation zone. The ablation zone of a glacier is where it enters a lower elevation and melts away.
Glacial Movement
When the accumulation zone of a glacier reaches a critical thickness, the pressure becomes too much and the glacier begins to move. It is twisted and formed to the shape of the land as it advances down the mountain. During this process the glacier scrapes out valleys, forms peaks and collects sediment. The sediment is held within the glacial ice until it is deposited in the ablation zone. Because of this natural process, glacial deposits are good areas to search for precious stones.