Geography Reference
In-Depth Information
FIGURE 4.43 Sequence of events showing headward erosion by cirque glaciers to create steep saw-
tooth ridges (arêtes) and glacial horns. (From Davis 1911; Lobeck 1939; Cotton 1942)
Although cirque erosion is the dominant glacial process operating on upper slopes
and depressions, larger glaciers may overflow the cirque basins to form valley glaciers.
The ice commonly adapts to and modifies a preexisting drainage system, and the former
stream channels are eventually transformed into glacial troughs (Figs. 4.41a, b, c). Most
stream-cut valleys in mountain regions have roughly V-shaped cross-profiles, whereas a
glaciated valley is typically U-shaped (see Fig. 4.42 for an example). Streams are lim-
ited to channel cutting along their beds, while other processes (especially mass wast-
ing ) erode the valley slopes and transport material to the stream. A glacier, on the other
hand, occupies the entire valley, and its much greater mass and erosive capacity soon
widen and deepen the valley into a semicircular or elliptical cross section with steep
rock walls (Hooke 2005). The valley floor may be bare rock, or it may be back-filled
with glacial meltwater deposits, resulting in flat valley bottoms, as seen in Yosemite Val-
ley, California. In longitudinal profile, glacial valleys have a more irregular surface than
stream valleys and often display a series of steps and risers (or reigels ). Various ori-
gins have been postulated for the stepped nature of glacial valleys, including differen-
tial rates of erosion controlled by valley width, different rock types, more intensely frac-
tured zones within the same rock type, greater erosion occurring at the base of deep
crevasses, and association with places where tributary glaciers join the main stream
(Thornbury 1969; Flint 1971; Embleton and King 1975; Sugden and John 1976; Ander-
son et al. 2006). Massive erosion and excavation of material by the ice deepens, widens,
and straightens the former stream valley along its axis so that the lower reaches of trib-
utary streams and their interfluves are cut off, leaving them truncated at some height
above the main valley. After the glacier melts, the water of these streams cascades down
as waterfalls over the trough sidewall. Such tributary valleys with floors higher than
the floor of the trunk valley, known as hanging valleys, are a scenic feature of glaciated
mountains (Fig. 4.41c).
Many glaciated mountain regions are situated close to oceans and experience pre-
dominantly onshore winds responsible for their characteristic heavy snowfall. Examples
include the west coasts of Norway, Alaska and British Columbia, New Zealand's South
Island, southern Chile, and parts of Greenland. These areas display one of the most
spectacular of glacial landforms, the fjord. Fjords, above all, exemplify the glacial
erosional phenomenon of overdeepening, that is, deep valley bottom erosion below base
level. For river systems, ultimate base level is represented by sea level, and fluvial
erosion can occur only slightly below this level. Fast-moving glaciers have the ability to
erode far below sea level—more than 600 m below in Sognefjord, Norway. The counter-
parts of coastal fjords in glaciated mountains far from sea coasts are the great trough
valleys such as the Lauterbrunnen Valley in the Swiss Bernese Oberland and Yosemite
Valley in California's Sierra Nevada.
Because of the rough and irregular terrain left behind by glaciers, lakes are common
in such landscapes (see Fig. 4.42 for examples). Tarns are lakes commonly found in
cirques (also known as cirque lakes ) . Tarns are characteristically clear and blue, since
the glacier has removed most of the loose debris, leaving a smoothed bedrock depres-
sion. Some tarns and other glacial water bodies may appear “milky” or opaque if they
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