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.
No one knows exactly how many times glaciers moved down the Park valleys
during the million or more years of the Pleistocene period, but
geologists have found evidence for at least eight distinct advances. It
is difficult to determine just when the first advance took place but it
may have been very early in the period. Most of the advances, however,
occurred during the past 70,000 years or so in what is known as the
Wisconsin stage of the Ice Age. Large glaciers flowed down the Park
valleys probably as late as 7,000 years ago. Between each of the major
times of ice advance, the glaciers, responding to warmer or drier
climate, shrank to small size and in some instances disappeared. These
warmer intervals varied in length from 2,000 to tens of thousands of
years.
Evidence of the several distinct glacial advances is yielded by the
moraines, deposits of rock debris left by the ice. On the east side of
the Park the lower courses of the major valleys and the adjoining ridges
in the Park and on the adjacent plains 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 ice advance is fresh in appearance and
contains fragments of all Park rocks. Moraines of the earlier stages,
because of much greater age, are more weathered. They contain many
fragments of much weathered diorite, from the layer of rock that appears
as a conspicuous black band on many of the mountains, and almost no
fragments of limestone, so common in the newest moraines. The diorite is
more resistant to weathering than the limestone which slowly dissolves
in ground-water. The only localities where the oldest moraine occurs are
the crests of the ridges which run eastward from the mountains out onto
the plains. This material is especially abundant on St. Mary Ridge. 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 later glacial advances which were 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 and
Boulder Ridges.
Following the last maximum advance of the Wisconsin glaciers they slowly
shrank until about 6,000 years ago when all glacial ice probably
disappeared from the mountains. After this there was a warm, dry period
during which it is probable that 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.
[Illustration: PANORAMIC VIEW OF GRINNELL GLACIER AS IT APPEARED IN
1945. THE CREVASSES IN GLACIER MAY BE OVER 50 FEET DEEP (BEATTY
PHOTO)]
[Illustration: PANORAMIC VIEW OF SPERRY GLACIER AS IT APPEARED IN
1946. NOTE MELT-WATER LAKES TERMINATING AGAINST MORAINES AT EXTREME
LEFT (DYSON PHOTO)]