Environmental Engineering Reference
In-Depth Information
the land submerging. Exceptions are found in areas with high tectonic activity such as the
west coast of South America, where emergence is the dominant factor.
Evidence of sea level transgression is given by Pleistocene terrace formations, which
along the Serra do Mar coast of Brazil are 60 ft above sea level and along the east coast of
the United States as high as 250 ft above sea level (the Brandywine terrace). The eroded
surface of the Cretaceous clays in Chesapeake Bay (see
Figure 7.37)
indicates that sea level
was once 200 ft lower in that area, during the immediate post-glacial period.
Primary Shoreline
Landforms resulting from some terrestrial agency of erosion, deposition, or tectonic activ-
ity are classified as primary shorelines (Shepard, 1968). The subaerial erosional coast, sub-
aerial depositional coast, and structural or diastrophic coast are illustrated in
Figure 7.43.
Volcanic coastlines are not shown.
Secondary Coastlines
Secondary coastlines result from marine processes and include wave-erosion coasts,
marine depositional coasts, and organically built coastlines, as illustrated in
Figure 7.44
and classified by Shepard (1968).
Significant Classes
From the point of view of soil formations, the most significant classes are the marine depo-
sitional coast and the emerged depositional coast.
Marine Depositional Coastline Deposits
Barrier Beach or Offshore Bar
As a result of abundant stream deposition from the land and shallow offshore conditions,
waves and currents pick up sand particles and return them landward. Eventually, a
(a)
(b)
(c)
FIGURE 7.43
Primary shorelines and coasts. (a) Subaerial erosion coast. The landform along the shoreline was developed by
erosion during a period of emergence above sea level; then sea level rose or the land subsided leaving
drowned valleys and offshore islands. The coast of Maine is a classic example. (b) Subaerial depositional coast.
Features resulting from deposition of sediments from rivers, glaciers, wind, or landslides. The most common
and significant are deltas (see
Section 7.4.2)
;
(c) structural or diastrophic coast. The most common are formed
by faulting along the coastline. Wave attack usually results in very straight cliffs with steep slopes and deep
offshore water, such as common to California coasts (see
Figure 7.49)
.
(d) Volcanic coastlines (not shown).
Results from volcanic activity and include volcanoes or lava flows. (From Hamblin, W. K. and Haward, J. L.,
Physical Geology Laboratory Mamal
, Burgess Publishing Co., Minneapolis, 1972. With permission.)
