The Earth periodically undergoes glaciation. This is a time during in which global temperatures drop and environments change. Glaciers, or large masses of ice, are gigantic erosional forces. There are many different types of glaciers and most contain a large portion of the earth's fresh water resources. During the Ice Age, ten thousand years ago, glaciers covered large areas of land, wore away the surface, and disrupted the drainage system.
Alpine glaciers create large U-shaped glacial valleys and cut through large mountains.
Large boulders have been moved and deposited in the most unusual locations and the only explanation is the glaciers. A glacial landscape, as seen in northern Wisconsin and Minnesota and large parts of Canada, is one in which the natural drainage system has been disrupted and many lakes were formed. The earth had a cooling period again before the ice which had formed years before had a chance to melt. This is viewed as a cyclical event. We are presently in an Interglacial Period.