[Illustration: Lake McDonald, 16 kilometers long, 2 kilometers wide, and 134 meters deep, is the largest lake in the park. Its basin is the classic U-shaped glacial valley. Forested lateral moraines on either shore gently rise 600 meters above lake level. Going-to-the-Sun Road snakes along the eastern shore, and Logan Pass lies near the center of the photograph, behind the peaks of the Lewis Range.] [Illustration: Subjected to the drying and shaping effects of wind both winter and summer, this Douglas-fir, growing in the prairie community near St. Mary, will attain neither the symmetrical shape nor the great size of the Douglas-firs growing in moister, more sheltered sites on the western slopes of the Continental Divide.] [Illustration: Freeze-and-thaw cycles continually fracture and loosen rocks along joints, making them subject to removal by the actions of water, gravity, and avalanche. The resulting fans of rock debris (talus cones) indicate the extent of erosion since the withdrawal of the Pleistocene glaciers.] [Illustration: Although moving water is an agent of erosion—the primary destructive force of mountain masses—it also permits life. Even small watercourses, such as this freshet, abound with plant and invertebrate life.] [Illustration: Going-to-the-Sun Mountain, towering above the St. Mary Valley, from the trail to Siyeh Pass. The coniferous forest at its base and the alpine tundra plants at its summit are closely juxtaposed in space; but if these two communities grew at the same elevation they would be separated by thousands of kilometers. Hiking from St. Mary Lake up to Siyeh Pass is going, in effect, from Montana to the Arctic Circle; but here the life zones are compressed and sharply divided rather than extended and overlapping.] [Illustration: Setting moon and snow shelf near the summit of Heavens Peak. Note stratification of Precambrian sediments.] [Illustration: Western redcedars line the shores of Lake McDonald. Because of prevailing air currents from the Pacific coast, winters in the protected western valleys are moist and comparatively mild, and this deep body of water freezes over an average of only one winter in four.] [Illustration: Moose often follow the spring snowmelt upwards to the headwaters of drainages. This bull will remain at Thunderbird Pond, at the base of Brown Pass, until autumn, when it will return to its Waterton Valley wintering ground.] [Illustration: Because of the high reproductive capacity of insects and small mammals, if all their offspring survived the earth’s plant life would be consumed within one year. This is prevented by natural controls such as predation and parasitism. The American kestrel (“sparrow hawk”) feeds primarily on large insects and on small rodents such as the meadow vole here.]