Page 75 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
70 Geotechnical Engineering

4.4.4 Beaches and Strandlines
The extent of depression of the earth’s crust is indicated by the inclination of
uplifted beaches that originally were level. Such beaches are called strandlines.
Strandlines bordering Hudson Bay, the Great Lakes, and the Baltic Sea now are
tilted in directions consistent with a hypothesis of crustal rebound. The directions
of bedrock striations indicate that the Hudson Bay area was at the center of the
Laurentide ice sheet, and measurements and dating of strandlines show that the
Gulf of Bothnia in the northern part of the Baltic Sea is rising at a rate of about
1 cm (0.5 in.) per year.

Tilted strandlines also indicate rebound around Great Salt Lake as the lake level
lowered as a result of desiccation. The ancestral lake, known as Lake Bonneville,
was about 330 m (1000 ft) higher during the Pleistocene, and the land has risen
about 70 m (230 ft).

4.4.5 Mechanics of Glacial Sliding
Rock debris concentrated in basal ice is a kind of ‘‘glacial sandpaper.’’ The
drag marks, deep longitudinal gouges left in bedrock, are called striations.
Nevertheless the overall frictional resistance to sliding had to be low to carry
the ice for such long distances with a low slope angle, and must have been aided
by pressure-melting of ice at the bottom of the glacier. This would transfer
the weight to liquid water trapped between the ice and the soil, thereby creating
a positive pore-water pressure sufficient to support the ice. A similar decrease
in friction from positive pore-water pressures also plays an important role in
landslides.

4.4.6 Life in the Pleistocene
The time of glaciation is referred to as the Pleistocene epoch of the Cenozoic
(recent life) era, and extended from about 2.5 million to 10,000 years ago.
Subarctic cold and a generous food supply led to many experimental models of
cats, cave bears, sloths, mammoths, and mastodons. A few mammoths survived in
miniature on islands off the northern coast of Siberia almost into historical times,
5000 years ago. Mammoths found in permanently frozen ground, or permafrost,
yield frozen tissues that are being studied for their DNA.

The early Pleistocene saw the emergence of man, who now probably would be
referred to as intellectually challenged. The stocky and large-brained Neanderthal
man appeared about 100,000 years ago, and the taller and equally large-brained
Cro-Magnon man first appeared about 35,000 ybp (years before present) and
threw his weight around. Modern man is Cro-Magnon with shoes and a haircut.

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