Page 81 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
76 Geotechnical Engineering

As will be pointed out, braided streams are not limited to glacial rivers, but
also occur in arid and semiarid climates where sediment is abundant and
water limited.

4.5.3 Special Alluvial Features of Valley or Alpine Glaciers

Although continental glaciers did not invade high mountains, valley glaciers in
these areas advanced during the Pleistocene and left soil deposits in lower parts of
their valleys. Glacially sculpted valleys have a characteristic U-shape, compared
with a V from stream downcutting, and uneven gouging often leaves a series
of small lakes arranged like beads down a valley.

Another feature of glacial valleys that differentiates them from stream valleys is
that tributary glaciers have elevations that are even at the top of the ice, whereas
tributary streams cut down to elevations dictated by the bottom. Therefore after
the valley glaciers have melted, tributary valleys are ‘‘hanging valleys’’ marked
by waterfalls.

4.5.4 Glacial Damming

An interesting and significant byproduct of glaciation is temporary damming of
large river valleys by ice. The scale of damming created by continental glaciation
can be huge. The largest example in North America is Lake Agassiz, which was
created by damming of a Missouri River that originally flowed northward
through what is now the Red River valley into Hudson Bay. The Lake Agassiz
plain extends from Canada southward into eastern North Dakota and western
Minnesota. It is known for its flatness, its fertility for agriculture, and its deposits
of expansive clay. Glacial Lake Agassiz was larger than the combined areas of
all of the present Great Lakes.

4.5.5 Varved Clays

The rate of filling of glacial marginal lakes was seasonal, with more rapid melting
during summers contributing layers of relatively light-colored silt, followed by
a more gradual sedimentation of darker-colored and finer clay during winter
months. The seasonal cycling results in varved clays, and the number of years
during which sedimentation was active can be determined by simply counting
the layers or varves.

4.5.6 Causes of Continental Glaciation
Continental glaciation was cyclical, resulting in a series of glacial advances
separated by mild interglacial periods A mathematical theory developed by
a Serbian engineer, M. Milankovich, and published in 1920, related cold
periods to periodic changes in eccentricity of the Earth’s orbit around the Sun.
In the 1950s radiocarbon dates of glacial sediments challenged the theory, but

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