Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 198

FIGURE 4.39 Idealized cross section of a valley glacier showing the relationship between lateral and
medial moraines along with their subsurface extension into the glacier. Note that the moraines
from the small tributary glacier on the right maintain themselves at the depth at which they join
the main glacier. The photo in Figure 38 shows actual field example of this moraine-glacier rela-
tionship. (T. M. Oberlander, University of California.)
Glaciers as Landscape-Forming Tools
MECHANISMS OF GLACIAL EROSION
When a glacier moves over an area, the ice undergoes plastic deformation to fill every
nook and cranny. Movement at the ice-rock interface results in modification of the un-
derlying surface through glacial erosion and transport. The primary processes are ab-
rasion, crushing, and plucking or quarrying. Abrasion is the scratching, gouging, and
grooving of the surface as the ice, carrying rock particles as tools, moves across it. Ob-
viously, this is most effective when the rocks in the ice are harder than the surface over
which they are passing. Pure ice or ice containing softer rocks is relatively ineffective
at abrasion, although it may produce smoothed and shiny surfaces ( glacial polish ). A
bedload of fine material will result in tiny scratches and smoothing of surfaces, while
large embedded rocks can produce scratches several centimeters deep. Such striations
are found in greatest abundance on gently inclined terrain where the ice was forced to
ascend, since this places greater pressure on the glacial base. Striations provide good
evidence for the direction of glacial movement; some caution should be used in their in-
terpretation, however, since they can be caused by other processes, such as avalanches
and mass movement, as well as lake- and sea-ice pushed ridges. Crushing is the pul-
verization of rock because of the glacial mass above. Plucking or quarrying is generally
considered to be the most potent erosional tool of glaciers. Plucking involves regelation
and the lifting and incorporation of ground surface rubble and bedrock segments into
the moving ice. Plucking is aided in its work by crushing and frost weathering, which
operates in front of the glacier, producing frost-shattered rock with many cracks and
crevices. As the ice moves, it easily incorporates the loose material, and the ice under-
goes plastic deformation around the larger rocks until they too are swept along with
the mass. This debris becomes part of the glacier's bedload and serves as a tool for ab-