Laboratory Manual for Introductory Geology 4e

15.4 Landscapes Produced
by Mountain Glaciation
Alpine, Himalayan, Rocky Mountain, and Sierra Nevadan vistas, with their jagged
peaks, steep-walled valleys separated by knife-sharp divides, and spectacular waterfalls,
are the result of mountain glacier erosion. Small glaciers form in depressions
high up in the mountains and flow downhill, generally following existing stream
valleys. The depressions are deepened and expanded headward by glacial plucking,
which eats into the mountainside, and the ice flows downhill, filling the former
stream valleys and modifying them by abrasion (FIG. 15.11). When the ice melts,
the expanded depressions become large, bowl-shaped amphitheaters (cirques); formerly
rounded stream divides are replaced by knife-sharp ridges (arêtes); and the
headward convergence of several cirques leaves a sharp, pyramidal peak (a horn
peak, or horn, named after the Matterhorn in the Alps).
Mountain glaciers transform pre-existing stream valleys in unmistakable ways,
including their cross-sectional and longitudinal profiles and the way that their streams
flow into one another. Figure 15.11c shows the characteristic U-shaped cross-sectional
profile of a valley carved by a mountain glacier. Why are glaciated valleys U-shaped
whereas stream valleys are V-shaped? Let’s see if your ideas in Exercise 15.2 were right.
The answer lies in how the two agents of erosion operate (FIG. 15.12). A stream
actively erodes only a small part of its valley at any given time, cutting its channel
downward or laterally (on the left in Fig. 15.12). Mass wasting gentles the valley
walls, creating the V shape. In contrast, a mountain glacier fills the entire valley,
abrading the walls not only at the bottom, but everywhere it is in contact with the
bedrock (center). When the ice melts, it leaves a U-shaped valley (right).
FIGURE 15.11 Erosional Tributary features valleyformed by mountain glaciers.
V-shaped valley
Tributary valley
Main valley
V-shaped valley
Main valley
Main valley
Main valley
Tributary valley
Tributary valley
(a) Before mountain glaciation, the area has a normal fluvial topography.
(b) The extent of the glaciers during mountain glaciation.
U-shaped valley
U-shaped valley
Cirque
Cirque
Arête
Arête
Horn
Horn
Hanging valley
Hanging Truncated valley spur
Truncated spur
(c) The area’s erosional features after mountain glaciation.
15.4 LANDSCAPES PRODUCED BY MOUNTAIN GLACIATION
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