Glacial motion
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Glacial motion--that is, the motion of the rivers of ice called glaciers--has played an important role in sculpting many landscapes. Typically, glacial motion occurs on a scale of hundreds of years, but much more rapid movements can occur.
The most familiar kind of glacial motion is glacial advance. This occurs when a glacier's terminus is pushed forward by the glacier's downhill flow faster than its rate of ablation by melting or vaporization.
Glacial retreat occurs when a glacier ablates more material from its terminus than it transports into that region.
Glacial movement has recently become politicized in discussions of global warming. Opponents of global warming like to point out that although some glaciers are retreating, others are advancing and therefore must be growing.It might also be mentioned that while some glaciers are retreating, all glaciers are advancing as they never stop moving downward. Glacierologists consider that trends in mass balance for glaciers are more important than the advance or retreat of individual glaciers. In the years since 1960, there has been a striking decline in the overall volume of glaciers worldwide.
During the Pleistocene Epoch (more commonly called the Ice Age) huge sheets of ice called continental glaciers advanced over much of the earth. The movement of these continental glaciers created many now-familiar glacial landforms.
As the glaciers were expanded, due to their accumulating weight of snow and ice , they crushed and redistributed surface rocks, creating erosional landforms such as striations, cirques, and hanging valleys.
Later, when the glaciers retreated leaving behind their freight of crushed rock and sand, depositional landforms were created, such as moraines, eskers, drumlins, and kames. The stone walls of New England contain many glacial erratics, rocks that were dragged by a glacier many miles from their bedrock origin.
Lakes and ponds can also be caused by glacial movement. Kettle lakes form when a retreating glacier leaves behind an underground chunk of ice. Moraine-dammed lakes occur when a stream (or snow runoff) is dammed by glacial till.
Studying glacial motion and the landforms that result requires tools from many different disciplines: physical geography, climatology, and geology are among the areas sometime grouped together and called earth science.