Page 82 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 77

detailed studies of cores of deep-sea sediments in the 1970s reveal better
agreement with the predicted dates. This is the ‘‘Milankovich’’ or ‘‘astronomic’’
hypothesis.

Many other explanations also have been suggested, including climatic oscilla-
tions caused by surges of glaciers in Antarctica, or changes in reflectance of the
Sun’s energy as snow cover accumulated during a series of harsh winters. The
Ewing-Donn hypothesis, first proposed in the 1950s and modified in the 1960s,
suggested that melting of the Arctic ice pack created a source for snow that then
accumulated sufficiently to depress global temperatures, which led to freezing
over of the source for snow, thereby setting up a cycle.

The discovery of lithified glacial till called ‘‘tillite’’ indicates that continental
glaciation also occurred during earlier geological eras. The causal factors affect-
ing such dramatic climatic changes are relevant to the interpretation of global
warming.

4.6 ALLUVIAL DEPOSITS

4.6.1 Down the River

Rain falling on a slope initially runs off as sheet wash, then concentrates into
parallel channels or ‘‘rills’’ that can be observed on bare roadcuts. Rills randomly
intersect and combine downslope into streams that in turn connect with one
another into rivers. The random branching resembles limbs in a tree, and is called
a dendritic drainage pattern.

Streams that are aggressively downcutting are considered ‘‘youthful.’’ Youthful
streams have relatively steep downhill gradients that enable them to flow rapidly,
and therefore erode and move large particles. The flow may only be intermittent
after periods of rain. During waning stages of each cycle, sand and gravel
accumulate in the stream bottom. Prospectors for gold or other heavy minerals
therefore pan their way upstream until traces of gold run out, and then up the
adjacent hillsides.

Alluvial deposits from youthful streams are confined to narrow valleys and
generally are thin and temporary, being washed away by the next major runoff
event. They often co-mingle with colluvial soil brought down by gravity and sheet
wash from adjacent slopes. The combined deposit can arbitrarily be called ‘‘local
alluvium.’’
Headward erosion by youthful streams slows down when the collection area
for rainfall diminishes, and essentially stops at the previously mentioned
‘‘critical distance.’’ This distance defines the width of intervening hilltops, or
‘‘interfluves.’’

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.