Page 76 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 71

4.4.7 River Valleys in the Pleistocene
An indirect consequence of continental glaciation was a lowering of sea level, as
much water was locked up in cold storage in the glaciers. The most telling
evidence that this occurred is thick deposits of sediments in modern river valleys,
particularly close to their outlets into the sea. These sediments only could have
been deposited if sea level were lower because of a basic requirement for water to
run downhill.

This influence of continental glaciation was world-wide because as sea level
lowered, rivers were free to cut downward and create deep, entrenched river
valleys; then as the glaciers melted and sea level rose, river valleys close to the
sea became drowned as estuaries or, if scoured out by glacial ice, fiords.

4.4.8 Sedimentation of River Valleys
Rivers draining glaciated areas carried large amounts of sediment, a process that
still can be observed in Alaska. During Pleistocene periods of glacial retreat the
availability of this glacio-fluvial sediment and the simultaneous rise in sea level
caused valleys to be filled with sediment, creating the base for broad floodplains
that now extend hundreds of miles upstream from the sea. The broad valley of the
Lower Mississippi River south of Cairo, Illinois, is an example. The thick sedi-
ment fill in river valleys directly influences foundation designs for bridges, etc.,
because of the large depth to bedrock.

Question: What volume of glacial ice would be required to lower sea level 100 m?
2
The total surface area of the Earth is approximately 509,600,000 km , of which
oceans cover about 71 percent.
3
Answer: 36,200,000 km ¼ 8,700,000 cubic miles. This does not include displace-
ment as the weight of the ice pushed down the Earth’s crust.

4.4.9 Glacial Drift, the Deposit

Deposits from glaciers are collectively called glacial drift. The rock and mineral
composition of drift reflects the source area from which it came, which in the
case of continental glaciation may be only a few hundred kilometers to the north.
Where glacial erosion attacked mainly granite, the resulting glacial drift will
contain large quantities of sand, gravel, and boulders. Where the glacial ice
gouged into shale, the resulting drift has a high percentage of clay. Incorpora-
tion of ground-up limestone by glaciers causes much glacial drift to be calcareous,
meaning that it contains calcium carbonate. Copper and traces of silver and gold

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