Glaciers and Glaciation - The Changing Earth: Exploring Geology and Evolution - page 368

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Figure 14.12 Erosional Landforms Produced by Valley Glaciers

A mountain area before glaciation.

a

b The same area during the maximum extent of valley
glaciers.

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Figure 14.11 Sediment Transport by Valley Glaciers Debris on
the surface of the Mendenhall Glacier in Alaska. The largest boulder
is about 2 m across. Notice the icefall in the background. The person
left of center provides scale.

Arete ˆ Cirque Truncated

spurs

Horn

Hanging
valley

During the Pleistocene, when glaciers were more exten-
sive, sea level was as much as 130 m lower than at present,
so glaciers flowing into the sea eroded their valleys below
present sea level. When the glaciers melted at the end of the
Pleistocene, sea level rose and the ocean fi lled the lower ends
of the glacial troughs so that now they are long, steep-walled
embayments called fi ords .
Fiords are restricted to high latitudes where glaciers
exist at low elevations, such as Alaska, western Canada,
Scandinavia, Greenland, southern New Zealand, and southern
Chile. Lower sea level during the Pleistocene was not entirely
responsible for the formation of all fi ords. Unlike running
water, glaciers can erode a considerable distance below sea
level. In fact, a glacier 500 m thick can stay in contact with
the seafl oor and effectively erode it to a depth of about 450
m before the buoyant effects of water cause the glacial ice to
fl oat! The depth of some fi ords is impressive; some in Norway
and southern Chile are about 1300 m deep.

U-shaped
glacial trough

c After glaciation.

of the hanging valley is perched far above the main valley's
fl oor (Figure 14.12c). Accordingly, streams fl owing through
hanging valleys plunge over vertical or steep precipices.
Although not all hanging valleys form by glacial erosion,
many do. As Figure 14.12 shows, the large glacier in the main
valley vigorously erodes, whereas the smaller glaciers in tribu-
tary valleys are less capable of erosion. When the glaciers dis-
appear, the smaller tributary valleys remain as hanging valleys.

Hanging Valleys Waterfalls form in several ways, but some
of the world's highest and most spectacular are found in
recently glaciated areas. Bridalveil Falls in Yosemite National
Park, California, plunge from a hanging valley , which is
a tributary valley whose fl oor is at a higher level than that
of the main valley. Where the two valleys meet, the mouth

Cirques, Arêtes, and Horns Perhaps the most spectacular
erosional landforms in areas of valley glaciation are at the upper
ends of glacial troughs and along the divides that separate adja-
cent glacial troughs (see Geo-inSight on pages 370 and 371. Val-
ley glaciers form and move out from steep-walled, bowl-shaped
depressions called cirques at the upper end of their troughs