The mountains of Glacier National Park are made up of many layers of
limestone and other rocks formed from sediments deposited under water.
The rocks show ripple marks which were made by waves when the rock
material was soft sand and mud. Raindrop impressions and sun cracks
show that the mud from time to time was exposed to rains and the drying
action of the air. These facts indicate that the area now known as
"Glacier National Park" was once covered by a shallow sea. At intervals
muds were laid down which later became consolidated into rocks known as
"shales" and "argillites." Limy or calcareous muds were changed into
limestone. The geologist estimates that these depositions were made
several hundred million years ago.
In the plains area east of the mountains are other lime and mud
formations. These are younger and softer than the rocks which make
up the mountains but were undoubtedly formed under much the same
conditions. These contain much higher forms of life, such as fish
and shells.
When originally laid down all these layers must have been nearly
horizontal, just as they are deposited today in bodies of standing
water all over the world. Then came a time when the sea slowly but
permanently withdrew from the area by an uplift of the land, which
since that time has been continuously above sea level. This uplift, one
of the greatest in the history of the region, marks the beginning of
a long period of erosion which has carved the mountains of Glacier
National Park.
The geologist observes that the rock layers are no longer in the
horizontal position in which they were laid down. There are folds
in the rocks and many breaks or faults cutting across the layers.
Furthermore, the oldest rocks in the region are found to be resting on
the younger rocks of the adjacent plains. One of the best examples of
this is to be seen at Chief Mountain where the ancient limestone rests
directly on the young shale below (fig. 1). The same relationship is
visible in Cutbank, St. Mary, and Swiftcurrent Valleys. In these areas,
however, the exact contact is not always so easy to locate principally
because of the debris of weathered rocks that have buried them. What
has happened? How did this peculiar relationship come about? The
answers to these questions unravel one of the grandest stories in earth
history. Forces deep in the earth slowly gathered energy until finally
the stress became so great that the rocky crust began to move.
The probable results of the movement in the crust of the earth are
shown in the diagram (fig. 2). Section A represents a cross section
of the Glacier Park region, as it most likely appeared, immediately
following the long period of sedimentation. The rock strata are
horizontal. Section B shows the same region after the rock layers have
been slightly wrinkled due to the forces from the southwest, which,
although slightly relieved by the bending, still persisted and the
folds were greatly enlarged as shown in section C. At this stage the
folds reached their breaking limit, and the strata broke in a number of
places as indicated by dotted lines in the diagram. As a result of this
fracturing, the rocks on the west side of the folds were pushed upward
and over the rocks on the east, as shown in section D. The mountain
rocks (represented by patterns of cross lines) were shoved over the
rocks of the plains (represented in white), producing what is known as
an "overthrust fault." It has been estimated that the rocks have moved
a distance of at least 15 miles.
As the rocks were thrust northeastward and upward they made a greatly
elevated region, but did not, however, at any time project into the
air, as indicated in section D, because as the rocky mass was being
uplifted, streams were wearing it away and cutting deep canyons in its
upland portion. The rocks of the mountains, owing to their resistant
character, are not worn away as rapidly as the plains formations with
the result that great thicknesses of limestone and argillite tower
above the plains. Where the older, more massive strata overlie the
soft rocks the mountains are terminated by precipitous walls as
shown in section E. This explains the absence of foothills that is
so conspicuous a feature of this mountain front and one in which it
differs from most other ranges.
While the region now known as "Glacier National Park" was being
uplifted and faulted, the streams were continually at work. The sand
and other abrasive material being swept along on the beds of the
streams slowly wore away much of the rock. The uplifting gave the
streams life and they consequently cut deep valleys into the mountain
area. They cut farther and farther back into the mountain mass until
they dissected it, leaving instead of an upland plateau a region of
ridges and sharp peaks. This erosional process which has carved the
mountains of Glacier Park has produced most of the mountains of the
world.
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.