Page 91 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
86 Geotechnical Engineering

In desert areas where there is no outlet, fans can build up until they submerge
bases of mountains. Some formidable examples in the U.S. are the Basin and
Range Province of Nevada, Utah, and southern California, and Death Valley in
California. Alluvial fan deposits reflect their localized sources but also exhibit
some sorting action because as a fan builds outward, coarser particles are depos-
ited first and fines are carried farther out. In closed desert basins the fine particles
that are not deposited in fans are carried into an intermittent lake or playa,
to build up a clay deposit that may be alkaline and highly expansive.

Braided streams and alluvial fans may be seen in miniature in roadside ditches
after rain.

4.7 SEA AND LAKE DEPOSITS

4.7.1 Deltas

Deltas were named by Herodotus in the fifth century B.C.E., from the shape of the
Nile delta, like a Greek with the apex pointing upstream. However, most deltas
extend outward so a delta shape is not necessarily an identifying feature. A delta
is deposited as a river or stream flowing into a lake or ocean loses velocity and
deposits its sediment.

As in the case of alluvial fans, coarser particles carried into a delta are deposited
first and finer materials are carried farther out to constitute ‘‘bottomset beds.’’
As the delta builds outward, the bottomset beds are covered with ‘‘foreset beds’’
that are deposited on a steeper slope, as illustrated in Fig. 4.12. The last materials
to be deposited are the ‘‘topset beds.’’

4.7.2 Freshwater vs. Saltwater Deltas

Freshwater deltas differ markedly from those deposited in salt water because
of the flocculating effect of salt on suspended clay. As clay enters salt water the

Figure 4.12
Schematic
cross-section of a
delta.

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