Glaciers form where more snow falls in the long winter than melts in the short summer and, over a period of years, compacts into ice, becoming massive enough to begin to move. That is, a snow patch becomes a glacier when the deepest layers begin to deform due to the weight of the overlying snow and ice.

There are so many glaciers in Prince William Sound because moisture from storms sweeping in off the Pacific Ocean in the winter are trapped by the high mountains and drop that moisture as snow. In fact, in the higher elevations of the Chugach Mountains it is not uncommon for snow to fall 12 months of the year. The thick, accumulating snow layers compress over years into ice that gradually flows down to the sea like massive rivers of ice. In the high reaches of

the mountains, the glacier accumulates the snow that turns into ice. This area is called the accumulation zone. It can take the glacial ice about 100 years to move from this point to its terminus point at sea. The ice moves down from the mountains and begins melting and calving into the water. The area where more ice is lost to calving, melting, and evaporating is called the ablation, or melting, zone. The glacier’s thickness is about one-half of the surface width of the glacier. Although few glaciers have been measured, the measured thickness ranges from a few hundred feet for small glaciers to about 5,000 feet for the largest glaciers in Alaska. At the end of the last great Ice Age, the glaciers covering Prince William Sound may have reached two miles in thickness in places.

Most of the glacier ice in Alaska is only a few tenths of a degree below the melting temperature, except for a surface layer several feet thick that is cooled during winter. Because of this, most glaciers in Alaska are not frozen to their beds. These glaciers are referred to as "temperate" glaciers. Glaciologists refer to a glacier as a "cold" glacier if it is more than a few degrees below the freezing temperature throughout most of its thickness. Cold glaciers are frozen to the bedrock and do not move and are found in Greenland and Antarctia.

Tidewater glaciers show a cyclical behavior. Glaciers push debris in front of them as they slowly move down a mountainside. The moraine, which is formed of rocks, boulders, and debris, protects the face of the glacier from the melting effects of saltwater. Eventually too much of the glacier is in the melting zone compared to the amount in the accumulation zone and the glacier retreats off its protective moraine. When the glacier retreats, it is still flowing downhill due to gravity and its own mass. Now its ice-face is exposed to the relatively warm salt water and drastic retreat begins with the glacier calving millions of tons of ice daily. When the glacier retreats onto land or into shallow water, the melt rate decreases, the glacier stabilizes, and it once again begins to build a new terminal moraine from rock debris carried down the mountain by the glacier. Over time, the new moraine will protect the face of the glacier again slowing the melting and the glacier will begin to advance, slowly bulldozing its moraine down the mountainside.

We see many varieties of glaciers in Prince William Sound, tidal or tidewater, valley, cirque, and hanging, and of these we see all stages represented as well, advancing, where more snow is being added than is melting or calving, stable, where the same amount of snow is being added as is being