cky Mountains rose. At some
stage of this process the range cracked along its crest from what is now
Marias Pass to a point just over the Canadian border, and, a couple of
hundred miles farther north, from the neighborhood of Banff to the
northern end of the Canadian Rockies.

Then the great overthrust followed. Side-pressures of inconceivable
power forced upward the western edge of this crack, including the entire
crust from the Algonkian strata up, and thrust it over the eastern edge.
During the overthrusting, which may have taken a million years, and
during the millions of years since, the frosts have chiselled open and
the rains have washed away all the overthrust strata, the accumulations
of the geological ages from Algonkian times down, except only that one
bottom layer. This alone remained for the three ice invasions of the
Glacial Age to carve into the extraordinary area which is called to-day
the Glacier National Park.

The Lewis Overthrust, so called because it happened to the Lewis Range,
is ten to fifteen miles wide. The eastern boundary of the park roughly
defines its limit of progress. Its signs are plain to the eye taught to
perceive them. The yellow mountains on the eastern edge near the gateway
to Lake McDermott lie on top of the Blackfeet Indian Reservation, whose
surface is many millions of years younger and quite different in
coloring. Similarly, Chief Mountain, at the entrance of the Belly River
Valley, owes much of its remarkable distinction to the incompatibility
of its form and color with the prairie upon which it lies but out of
which it seems to burst. The bottom of McDermott Falls at Many Glacier
Hotel is plainly a younger rock than the colored Algonkian limestones
which form its brink.

Perhaps thousands of years after the overthrust was accomplished another
tremendous faulting still further modified the landscape of to-day. The
overthrust edge cracked lengthwise, this time west of the continental
divide all the way from the Canadian line southward nearly to Marias
Pass. The edge of the strata west of this crack sank perhaps many
thousands of feet, leaving great precipices on the west side of the
divide similar to those on the east side. There was this great
difference, however, in what followed: the elongated gulf or ditch thus
formed became filled with the deposits of later geologic periods.

This whole process, which also was very slow in movement, is important
in explaining the confor