Page 73 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
68 Geotechnical Engineering

Figure 4.3
Boulders that
have been faceted
and striated are
evidence for
dragging by
moving ice. The
grinding action of
continental
glaciers was the
ultimate source for
most agricultural
soils, whether
deposited by ice
or by wind or
water.

glaciers, may have made some people question his sobriety. Aggasiz’s spe-
cialty at the time was the study of fossil fish. However, he was a competent and
critical observer, and he saw similarities between deposits in North America and
glacial deposits in the Alps. In particular he saw linear drag marks with a roughly
north-south orientation scored into bedrock, which reminded him of home.

Boulders within glacial deposits also show scrape marks and often are flat-
tened on one or more sides, Fig. 4.3. Agassiz became a professor at Harvard
and revolutionized the teaching of natural sciences by emphasizing field study,
an emphasis that also has a home in geotechnical engineering.

4.4.2 Extent
Prior to continental glaciation the Missouri River flowed north into what now
is Hudson Bay, which occupies a basin that was pushed down below sea level
by the weight of the glacial ice.

Approximately 30 percent of the continental land mass has been covered at
one time or another by continental glaciers. Nearly 10 percent still is covered,
including Greenland, Antarctica, and the northern islands of Canada. The top of
the Greenland icecap is at an elevation of over 3000 m (10,000 ft) and the bottom
is below present sea level, so the maximum thickness of continental glaciers would
be measured in kilometers or miles. Further glacial melting therefore is a major
concern because of the rise in sea level.

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