Geography Reference
In-Depth Information
began a long slow ride through the bowels of the glacier following the flow stream-
lines, only to reemerge in 2000 to be discovered by climbers ascending the nearby peak
(NOVA 2001).
MECHANISMS OF GLACIAL DEPOSITION
Using the transport mechanisms described above, glaciers may liberate their load (col-
lectively called
drift
) in one of three ways: (a) by deposition directly from the melting
ice (the deposit material is then referred to generically as glacial
till
); (b) by intermedi-
ate deposition from meltwater within the ice, then by melting of the ice (referred to as
glaciofluvial
deposits); or (c) by direct deposition from melt-water below the terminus of
the ice (referred to as
outwash
). The importance of knowing these depositional mechan-
isms is evident when trying to interpret various landforms found in previously glaciated
terrain. Only by understanding these processes can anomalous features such as huge
boulders of alien rock type littering a landscape (
glacial erratics
) or hundred-foot high
sinuous mounds of sorted stream deposits running miles across a landscape (
eskers
) be
explained (Benn and Evans 2010).
Glaciated Mountain Landscapes
The landscapes of glaciated mountains are among the most distinctive and striking on
Earth. The features and forms created by ice sculpting are very different from those
caused by running water, and glaciated mountains possess a ruggedness and grandeur
seldom achieved in unglaciated mountains. For most of us, the visual image of high
mountains is typified by glaciated landscapes, with their pyramidal peaks, jagged saw-
tooth ridges, amphitheater-like basins, and deep elongated valleys where occasional
jewel-blue lakes sparkle amid surrounding meadows. It is a landscape largely inherited
from the past, when the ice was much more extensive than now (see Fig. 4.30 for an
example). In the western United States alone, there were over 75 separate high-altitude
glacial areas (Fig. 4.40). Cirque or valley glaciers occupied most of these, but in some
areas there were mountain ice caps. The largest examples are in the Yellowstone-Grand
Teton-Wind River ranges, the Sierra Nevada, the Colorado Rockies, and the Cascades
(Flint 1971: 471-474). Mountains farther north (i.e., the Canadian Rockies, the Coast
Ranges, and the Alaska and Brooks Ranges) were almost totally inundated, while the
Yukon River Valley remained ice free.
The most characteristic and dominant feature of mountain glaciation is erosion. Gla-
cial erosion in mountains is facilitated by the channeling of ice into preexisting valleys
which accentuate its depth and velocity. For this reason, glaciers erode deeper in moun-
tain areas than the former ice sheets did in continental areas, with erosive depths of-
ten exceeding 600 m (2,000 ft) (Flint 1971: 114). There is a sharp contrast between
the appearance of glaciated uplands and valleys. The ice on upper surfaces is thin-
ner and prone to earlier melting than that in the valleys, where the ice is deeper and
more sheltered. The higher surfaces are thus exposed to prolonged weathering. Typical
features include sharp, angular ridges and peaks, and accumulations of frost-loosened
rock. By contrast, the valleys (
glacial troughs
) are so smoothed and shaped by the ice
that very few sharp or rugged features remain. An exception occurs where the entire
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