3 A large fold has been created, forcing the rock strata to double
over and overturning some layers. A break, or _fault_, is forming at
the plane of greatest stress.
4 The break has been completed and the strata west of the fault have
slid eastward, up and over the rocks east of the fault.
5 The Glacier landscape today. Throughout the millions of years
during which the folding, faulting, and overthrusting have been
taking place, the process of erosion has continued; a thousand
meters of stratified rocks have been worn away, so that only a
remnant of the overthrust layers can be seen today. Because
Glacier’s eastern slope represents the eroded face of the overthrust
block, the mountain range rises precipitously from the prairie, with
no foothills breaking the abrupt transition from open prairie to
mountain valley.]
[Illustration: The peaks in this photograph (a view to the northwest
from Marias Pass) are remnants of the overthrust block, which moved
eastward. The dividing line between the light-colored rocks and the
gray talus slopes beneath them is the Lewis Overthrust Fault.]
[Illustration: Glaciation
1 This is how the landscape in this region might have appeared
before the onset of the Pleistocene, millions of years ago. Note the
stream-eroded, V-shaped valleys. The climate at that time was dry.
2 Glaciers began to form high on the peaks, crept downward, and
joined to form larger glaciers.
3 After many centuries of glaciation, tributary glaciers have cut
back into the peaks, forming basins called _cirques_. Thick
glaciers, moving rapidly and carrying rock fragments, have abraded
the main valleys’ floors and sides, widening and deepening the
valleys into characteristic U-shapes.]
V-shaped Valley
Tributary Glacier
Unglaciated Peak
Headwall
Meltwater Stream
Nose of Glacier
Crevasse
[Illustration: 4 In the present landscape, free of all but remnant
glaciers, small lakes called _tarns_ occupy many of the cirque
basins; and waterfalls plunge into the main valleys from higher,
shallower, tributary valleys, called _hanging valleys_. _Alluvial
cones_—recent accumulations of rock debris—have begun to build along
the valley walls. In the main valley, a _moraine_ (a deposit of rock
materials left by the retreating glacier) has formed a dam that
holds back a large lake.
During all this time, all parts of the terrain not buried under ice
and snow have been weathered and eroded by nonglacial forces. Thus
the contours of the jagged peaks and sheer cliffs have been
softened.]
Unglaciated V-shaped Valley
U-shaped Valley
Hanging Valley
Cirque
Tarn
Alluvial Cone
Moraine
Morainal Lake
[Illustration: Glacial landforms can be identified in this view of
the Mokowanis Valley.]
[Illustration: A Divided Climate
Because of an eastward flow of cool, moist Pacific air masses, the
climate of northwestern Montana, including the western portion of
Glacier National Park, differs from that of other portions of
Montana. As a result of increased precipitation, Glacier’s western
valleys support a rich flora, more typical of the Pacific Northwest.