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. [Illustration: Figure 1.--_Sketch showing structure of Chief Mountain. The ancient limestone above is not appreciably altered, but the lower part is broken up by many oblique thrust faults. The entire mountain is composed of ancient rocks and rests on shale of a very much younger age. After Bailey Willis._] [Illustration: Figure 2.--_The Lewis overthrust. Diagram illustrating how pressure from the northwest affected the rocks of the Glacier Park region._] [Illustration: Figure 3.--_A, An irregular V-shaped valley produced by stream erosion; B, the same valley after it has been occupied by a glacier. Note the smooth topography and U-shaped form._] 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. In addition to smoothing the valley down which they moved, the glaciers produced many rock basins called cirques. These are the result of ice plucking in the regions where the glaciers formed. Alternate freezing and thawing cause the rock to break and the resulting fragments are carried away by the moving ice mass. In the majority of cases the cirques have lakes on their floors. The park is dotted with these beautiful little lakes scattered throughout the high mountain country. The valley lakes are usually larger than the cirque lakes and have a different origin. As the glaciers melted they deposited huge loads of sand, mud, and boulders in the valley bottoms called moraines. Debris of this nature has helped to hold in the waters of St. Mary, Lower Two Medicine, McDonald, Bowman, and numerous other lakes in the park. FLORA AND FAUNA