LEGEND
1. TRACE OF LEWIS OVERTHRUST
2. ALLUVIAL FANS
3. RANGE CRESTS
4. HIGHWAYS
Movement along this fault was slow--so slow that had people been present
at the time they probably would not have been aware that anything of an
unusual nature was occurring. Occasionally along many large faults,
however, there is sudden movement of small magnitude, usually not more
than a few inches, but strong enough to vibrate the crust. These
vibrations are earthquakes, and their frequent occurrence in California
and elsewhere along the Pacific coast indicates the presence of numerous
active faults. Their occurrence also in the northern Rockies, as at
Helena, Montana in 1935 and 1936, attests to the fact that some of the
faults here are still active.
The Lewis overthrust comes to the surface at the base of the Altyn
formation along the entire precipitous east front of the Lewis Range and
can be traced nearly 100 miles northward into Canada and for almost an
equal distance south of the park. The section lying within the park is
tilted very gently toward the southwest, the angle of dip seldom
exceeding ten degrees. In some places it is practically horizontal. For
this reason the lower courses of all the largest, and some of the small,
valleys on the east side of the Lewis Range have been cut entirely
through the upper block (overthrust) of Belt rocks down into the weak
Cretaceous shales underneath. This causes the trace of the overthrust to
be very sinuous and also accounts for the deep indentations in the
mountain front formed by Swiftcurrent, St. Mary, Two Medicine, and other
valleys. The floors in the lower courses of these valleys, because they
lie below the level of the thrust surface, are composed of Cretaceous
shales. In most places these rocks are covered by glacial moraine, but
they are exposed along the highway from Babb into the Swiftcurrent
Valley, especially along the shore of Sherburne Reservoir and near the
entrance station. Because these shales readily disintegrate when exposed
to the atmosphere they give rise to slumps and landslides which,
although of small proportions, cause a great deal of damage to the
highway, sections of which must be rebuilt annually. At most damaged
spots along the route the shales appear as a dark mud or clay in the
roadcuts. The bumpy topography of the whole slope lying north of the
road has been formed by innumerable such small landslides.
A deep well located near Cameron Falls in Waterton Townsite (Waterton
Lakes National Park) about one mile west of the edge of the mountains
passes through 1,500 feet of Belt rocks and then penetrates the Lewis
overthrust and the Cretaceous shales beneath.
In the southern part of Glacier National Park just north of Marias Pass,
Debris Creek has cut a hole or "window" (known as a fenster by
geologists) through the overthrust block (Figure 2). Thus a small area
of Cretaceous rock completely surrounded by the Belt series lies within
the mountains. This is the only such Cretaceous outcrop in the park, but
like the well at Waterton, it serves as a reminder that the rocks of
this period are everywhere present under the mountains, and their
surface constitutes the "sliding board" over which the upper, more
massive block of Belt rocks was pushed. And so we see that the mountains
of Glacier National Park, unlike many of the world's great ranges, have
no roots, for they rest on a base of greatly different and much less
resistant material, the Cretaceous shales. Presumably the Lewis
overthrust and Cretaceous rocks beneath it would be penetrated by a well
drilled anywhere within the mountains, although in the Livingstone Range
the depth of such a well would be very great (Figure 3D).