@ Alchemy - You described the movement of glaciers across the bedrock, but glacial movement can also be examined as a natural force much like the ebb and flow of the tides. In this context glacial movement is referred to in terms of accumulation and ablation.
Accumulation is the addition of snow that turns to ice on a glacier; resulting in the glacier flowing down slope. The distance the glacier expands every year is the accumulation. Ablation is just the opposite. When glaciers actually melt, and retract this is referred to as ablation.
This natural cycle of accumulation and ablation has been thrown completely out of sync in the past century due to climate change. Some of the climate change is natural, but some is also the result of human influences on the environment. In many glaciers ablation is occurring at a faster rate than accumulation; resulting in the overall decrease in ice cover.