Page 74 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 69

Figure 4.4
Glaciated areas in
the U.S. Vertical
shading indicates
younger
(Wisconsin age)
glacial advances
that are dominated
by glacial features.
Cross-hatching
shows earlier
glacial deposits
that are incised by
streams and
covered with
wind-blown silt or
loess.

Continental glaciers once covered broad northern areas of the continents.
In Europe, most of the British Isles, Scandinavia, and northern parts of Germany,
Poland, and Russia were glaciated. Glaciation in North America pushed the
Missouri and Ohio Rivers to their present locations and left broad areas of
glacially derived sediments shown in Fig. 4.4.

Glaciation was a temporary inconvenience for mankind, but after the development
of agriculture it has been a huge plus because it created a mantle of fertile soil.
Several soil deposits are associated with continental glaciation—soil deposited
from the ice itself, soil deposited by water from melting ice, and soil picked up by
winds crossing exposed river bars that deposited dust across broad areas of
uplands.

4.4.3 Glacial Erosion
Alpine glaciers are confined along the edges by mountains, and scoop out
characteristic U-shaped valleys. In contrast, continental glaciers are free to
spread, stripping soil from bedrock, leveling hills, plucking out lake basins, and
carrying the glacial sediment southward to be deposited wherever the ice runs out
and melts. It is no coincidence that the Great Lakes of the north-central U.S.
are elongated in the directions of former glacial movement. Farther north, large
seas such as Hudson Bay and the Baltic Sea owe their existence to depression
of the Earth’s crust under the weight of glacial ice, and even now, thousands of
years after the ice has melted, the crust still is rebounding upward.

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