Following their early erosional history, there came a period of much colder climate during which time heavy snows fell and large ice fields were formed throughout the mountain region. At the same time huge continental ice sheets formed in Canada and also in northern Europe. This period, during which glaciers, sometimes over a mile thick, covered many parts of the world including all of Canada and New England and much of North Central United States, is known as the "Ice Age." Such a tremendous covering of ice had an enduring and pronounced effect upon the relief of the country.
In Glacier National Park some of the ice still remains in the higher portions of the valleys and a study of these ice fields helps in interpreting the history of the park during the Ice Age. It is evident that ice did not cover the entire range, but that the higher peaks stood out above the ice, which probably never reached a thickness of over 3,000 feet in this region. The V-shaped valleys which had been produced by stream erosion were filled with glaciers which moved slowly down the valleys. The ice froze onto all loose rock material and carried it forward, using it as abrasive to gouge out the rock, the valley bottoms, and sides. Gradually the valleys were molded until they had acquired a smooth U-shaped character (fig. 3). There are excellent examples of this work of ice in the park, among which are Two Medicine, Cut Bank, St. Mary, Swiftcurrent, and Belly River Valleys.