Even more significant is the lowering of the glacier’s surface, from which volume shrinkage may be obtained. In 1938 Sperry Glacier had a thickness of 108 feet at the site of the 1946 ice margin. At this same place in 1913 the thickness was nearly 500 feet, and the average thickness of the glacier over the area from which it has since disappeared was at least 300 feet.
The average thickness of Grinnell Glacier in 1937 at the site of the 1946 ice front was 73 feet. The surface of the entire glacier was lowered 56 feet during that nine-year period. This means that each year the glacier was reduced in volume by an amount of ice equivalent to a cube 450 feet high.
GRINNELL GLACIER AS IT LOOKED PRIOR TO 1926 WHEN THE LOWER AND UPPER SEGMENTS WERE STILL CONNECTED.
At the northern terminus of Grinnell Glacier, which is bordered by a small marginal lake, a large section of the ice front fell into the water on or about August 14, 1946, completely filling it with icebergs. This event, although witnessed by no one, must have been comparable to many of the icefalls which occur at the fronts of the large glaciers along the southeast coast of Alaska.
The volume of Grinnell Glacier was reduced by about one-third from September 1937 to September 1946. Several other glaciers have exhibited a more phenomenal shrinkage than Sperry or Grinnell. The topographic map of Glacier National Park, prepared in 1900-1902, shows several comparatively large glaciers such as Agassiz, Blackfoot and Harrison. Their shrinkage has been so pronounced that today Agassiz has virtually disappeared and the other two are pitifully small remnants, probably less than one-fifth the size they had been when originally mapped.
Since 1945, because of above-normal snowfall and subnormal temperatures, glacier shrinkage has slowed down appreciably, coming virtually to a standstill in 1950; and in 1951, for the first time since glacier changes have been recorded in the Park, Grinnell Glacier increased slightly in volume. This was also reflected by a readvance of the front. Although no measurements were made in 1951 on other Park glaciers some of them certainly made similar readvances. Thus the climatic conditions which caused glaciers to shrink for fifty or more years seem to have been replaced by conditions more favorable to the glaciers. Time alone will tell whether the new conditions are temporary or mark the beginning of a long cycle of wetter and cooler climate.
Former Extent of Park Glaciation
During the Pleistocene Period or Ice Age when most of Canada and a large portion of the United States were covered several times 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 and ridges 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. Several of these long valley glaciers extended far enough out onto the plains to meet the edge of the vast continental ice sheet moving westward from a center in the vicinity of Hudson Bay. In the major Park valleys these glaciers attained thicknesses of 3,000 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.