Laboratory Manual for Introductory Geology 4e

FIGURE 17.13 Coastal-bluff
erosion caused this house in
Destin, Florida, to collapse.
The erosion was exacerbated by
high waves during Hurricane Dennis
in 2005.
FIGURE 17.14 Creation of a sea stack.
Headland
Sea arch
Sea stack
Waves carve
sea caves.
A sea arch
forms.
The arch
collapses.
(a) Formation of sea arches and stacks by erosion at a bedrock headland.
Time
glacial deposits, as is the case along the Atlantic coast
(FIG. 17.13). Over long periods, wave-cut benches
become wider as the coastal cliffs retreat landward. In
tectonically active areas, old wave-cut benches may have
been uplifted tens of feet above present sea level, where
a newer bench is being carved today. By dating the
uplifted benches, geologists can estimate the amount
and rate of tectonic uplift.
The second type comprises sea arches and sea stacks:
distinctive, often dramatic bedrock landforms found a
short distance from the shore. When a bedrock cliff
retreats, it does not do so at the same rate everywhere.
Waves may erode zones of weak rock quickly, isolating
stronger material, and then start to cut into that
(b) Morro Rock in California, a classic sea stack.
material as well. Eventually, a sea arch—named for its
distinctive shape—is formed. When further erosion removes the support for the
arch, it collapses, leaving an isolated remnant of the bedrock called a sea stack
(FIG. 17.14a). Morro Rock (FIG. 17.14b), off the coast of California, is a spectacularly
beautiful sea stack. Sea stacks along a coastline mark the former position of
the bedrock cliffs, letting us measure the amount of cliff retreat.
Exercise 17.6 explored the factors controlling coastal erosion. In Exercise 17.7,
we’ll examine the landforms resulting from that erosion.
448 CHAPTER 17 SHORELINE LANDSCAPES