Toward the end of Cretaceous time tremendous crustal forces, principally
from the west, were directed against the geosyncline with the result
that its rocks were compressed and uplifted, converting the site of the
former sea into a mountainous region. Similar activity took place
throughout the length and breadth of the entire geosyncline, which
resulted in the formation of the Rocky Mountain system stretching
between Mexico and Alaska. A number of mountains were formed on other
continents during this period. So widespread and tremendous was the
deformation, especially in the present day Rocky Mountain region, that
it is known as the Rocky Mountain, or Laramide (after the Laramie Range
in Wyoming), revolution. Mountain-building forces continued for several
million years in the Glacier Park area, finally squeezing the rocks into
a great fold (anticline). Continued pressure from the west overturned
the fold and put additional strain on the rock layers, eventually
causing them to break along a great low-angle fault. The western limb of
the fold, now a great slice of the crust, was driven upward and eastward
over the eastern limb ultimately reversing the order of rock layers by
placing older on top of younger ones (Figure 3). These younger layers
are Cretaceous shales and sandstones underlying the plains immediately
east of the mountains. The mountains themselves have been carved by
streams and glaciers from the Belt formations comprising the upper block
of older rock, that slice of the crust which has been moved more than 15
miles toward the east. The surface over which it was pushed is the Lewis
overthrust. At the time this great break occurred the part of it now
exposed in Glacier National Park was deeply buried. It was long after
that when removal by erosion of overlying Belt rocks, possibly several
thousand feet of them, finally exposed the fault.