Page 363 - Laboratory Manual in Physical Geology

Cirque Lateral moraine
glacier
Horn
Bergschrund Snowline Medial Arête Cirque
moraines
glaciers
Snowfield

Transverse
crevasses
ZONE OF
Valley
ACCUMULATION
glacier

Longitudinal
Snowline ZONE OF
crevasses
ABLATION

Ground moraine

FIGURE 13.3 Active mountain glaciation, in a hypothetical region. Note the cutaway view of glacial ice, showing flow lines and

direction (blue lines and arrows).

Glacial Processes and Landforms FIGURE 13.3 shows the results of mountain

glaciation after the glaciers have completely melted.
Glaciated lands are affected by either local to regional Notice the characteristic landforms, water bodies, and
“mountain glaciation” or more continent-wide “ continental sedimentary deposits. For your convenience, distinctive
glaciation.”
features of glacial lands are summarized in three figures:
erosional features in FIGURE 13.4 , depositional features in

Mountain Glaciation FIGURE 13.5 , and water bodies in FIGURE 13.6 . Note that
some features are identical in mountain glaciation and
Mountain glaciation is characterized by cirque continental glaciation, but others are unique to one or
glaciers, valley glaciers, piedmont glaciers, and ice caps. the other. Study the descriptions in these three figures
Poorly developed mountain glaciation involves only and compare them with the visuals in FIGURES 13.2
cirques, but the best-developed mountain glaciation and 13.3 .
involves all three types. In some cases, valley and
piedmont glaciers are so well developed that only Continental Glaciation
the highest peaks and ridges extend above the ice.
Ice caps cover even the peaks and ridges. FIGURE 13.2 During the Pleistocene Epoch, or “Ice Age,” that
shows a region with mountain glaciation. Note the ended 11,700 years ago, thick ice sheets covered most
extensive snowfield in the zone of accumulation. of Canada, large parts of Alaska, and the northern
Snowline is the elevation above which there is contiguous United States. These continental glaciers
permanent snow cover. produced a variety of characteristic landforms

Also note that there are many cracks or fissures in the ( FIGURE 13.7 , FIGURE 13.8 ).
glacial ice of FIGURE 13.2 . At the upper end of the glacier Recognizing and interpreting these landforms

is the large bergschrund (German, “mountain crack”) that is important in conducting work such as regional
separates the flowing ice from the relatively immobile soil analyses, studies of surface drainage and
portion of the snowfield. The other cracks are called water supply, and exploration for sources of sand,
crevasses —open fissures that form when the velocity of gravel, and minerals. The thousands of lakes in the
ice flow is variable (such as at bends in valleys). Transverse Precambrian Shield area of Canada also are a legacy
crevasses are perpendicular to the flow direction, and of this continental glaciation, as are the fertile soils
longitudinal crevasses are aligned parallel with the of the north-central United States and south-central
direction of flow. Canada.

Glaciers and the Dynamic Cryosphere ■ 333