GLACIER Glaciers and Glaciation in Glacier National Park Special Bulletin No. 2

During the Pleistocene Period or Ice Age when most of Canada and a large portion of the United States were covered by an extensive ice sheet or continental glacier, all the valleys of Glacier National Park were filled with valley glaciers. These originated in the higher parts of the Lewis and Livingstone Ranges. On the east side of the Lewis Range they moved out onto the plains. From the Livingstone Range and the west side of the Lewis Range they moved into the wide Flathead Valley. During the maximum extent of these glaciers all of the area of the park except the summits of the highest peaks were covered with ice.

The great Two Medicine Glacier, with its source in the head of the Two Medicine and tributary valleys, after reaching the plains spread out into a big lobe (piedmont glacier) eventually attaining a distance of about 40 miles from the eastern front of the mountains. The stream of ice emerging onto the plains from St. Mary Valley also extended many miles out from the mountain front. In the major valleys these glaciers attained thicknesses of 2,000 or more feet. Although man probably never viewed this magnificent spectacle, the park at that time must have been similar in aspect to some of the present day ice filled ranges along the Alaska-Yukon border.

PANORAMIC VIEW OF GRINNELL GLACIER AS IT APPEARED IN 1945. THE CREVASSES IN GLACIER MAY BE OVER 50 FEET DEEP. (BEATTY PHOTO) (click on image for a PDF version)

PANORAMIC VIEW OF SPERRY GLACIER AS IT APPEARED IN 1946. NOTE MELT-WATER LAKES TERMINATING AGAINST MORAINES AT EXTREME LEFT. (DYSON PHOTO) (click on image for a PDF version)

At least twice these glaciers advanced down the park valleys, and at least twice they melted away and disappeared.

Evidence of these two distinct glacial advances is yielded by the deposits left by the ice. On the east side of the park the lower courses of the major valleys and the adjoining ridges are covered with moraines. The material in them ranges in size from clay to large boulders, and was deposited by glaciers after being transported down the valleys. The debris deposited by the latest stage of glaciation, known as the Wisconsin, is fresh in appearance and contains fragments of all park rocks. Moraines of the earlier stage, because of much greater age, are more weathered. They contain many fragments of diorite, from the layer of rock which appears as a conspicuous black band on many of the mountains, and almost no fragments of limestone, so common in the newest moraine. The diorite is resistant to weathering; the limestone slowly dissolves, thus during the interval between the first and second stages of glaciation most of the limestone boulders disappeared and the resistant diorite remained. The only localities where this older moraine occurs are the crests of the ridges which run eastward from the mountains out onto the plains. On top of Two Medicine Ridge along and just above the highway, fragments of this material have been cemented together into a comparatively hard tillite. Lower down on the slopes the older moraine cannot be found as it is covered by that of the Wisconsin glacier which was less extensive and did not override the ridge crests as did the earlier glaciers. The older debris is also found on top of Milk River Ridge, as the glacier (Wisconsin Stage) from the Cut Bank Valley did not attain the crest, although it did spill northward through the gap in the ridge (the highway now goes through this low place) for a distance of three miles into the Milk River Valley. Swiftcurrent and Boulder Ridges, north and south of Sherburne Lake, respectively, are also capped with this old moraine.

Following the maximum advance of the Wisconsin glaciers they slowly shrank until about 8,000 years ago when all glacial ice probably disappeared from the mountains. After this there was a warm, dry period during which no glaciers were present. Then about 4,000 years ago the present small glaciers were born. During the period of their existence they have fluctuated in size, probably attaining maximum dimensions around the middle of the last century. Since then they have been getting smaller.