Geoscience Reference
In-Depth Information
Wave-cut bluff
Notch
Beach
Wave-cut platform
(a)
Figure 19.18 Marine terraces on San Clemente Island,
California.
Each of these terraces formed during a high sea lev-
el associated with an interglacial period, with the highest surface
representing the oldest high-level sea stand. Sea level subse-
quently dropped during glacial periods when enormous volumes
of water were stored as ice. At the same time, the old wave-cut
bluff was slowly uplifted due to tectonic activity. With each suc-
cessive interglacial, sea level rose again, creating new wave-cut
bluffs that were progressively lower than the prior ones.
Old cliff
Marine terrace
(old platform)
New wave-cut bluff
New notch
New beach
common along the active margin of the west coast of the United
States (Figure 19.18).
In addition to wave-cut bluffs and terraces, additional
evidence for coastal retrogradation comes in a variety of
forms, which evolve in a continuum as
the shoreline recedes.
Sea caves
are
semicircular notches cut into the
base of rock bluffs. If the rock is
entirely cut through so that water
passes through the notch, a
sea
arch
results (Figure 19.19). With
New wave-cut platform
(b)
Figure 19.17 Evolution of a marine terrace.
(a) Waves cut a
bluff and platform. (b) Tectonic uplift or sea-level fall raises the
previous wave-cut features relative to the position of current ero-
sion, resulting in a marine terrace.
4° W
0°
58° N
8° W
SCOTLAND
called a
wave-cut platform
into the rocks in front of it. During
higher water stages or strong storms, waves run up and cut a
notch into the lower rocks of the bluff outcrop and further plane
the nearshore platform (Figure 19.16c). If the notch is cut suffi-
ciently deep, the rock overhang falls under the influence of grav-
ity and the bluff retreats farther inland, leaving a longer wave-cut
platform in front of it (Figure 19.16d). As the coastline retreats
due to erosion, it is said to be undergoing
retrogradation
.
As described earlier, one way that water moves along
coastlines is through eustatic changes in sea level. In addi-
tion to the submerged river channels that occur on the conti-
nental shelf, one of the best lines of evidence for prehistoric
eustatic sea-level changes is the presence of abandoned bluffs
and marine terraces. Similar to a fluvial terrace, a marine ter-
race represents the former position of the active shoreline on
the landscape. Figure 19.17 shows how this kind of landscape
evolves. In Figure 19.17a, waves create a wave-cut bluff and
platform similar to that shown in Figure 19.16. Imagine what
would happen if the landscape were subsequently uplifted or if
sea level fell. If such a eustatic change occurred, a new bluff and
platform would form below the abandoned one (Figure 19.17b),
leaving a stair-stepped coastal landscape. Such landscapes are
NORTHERN
IRELAND
North
Sea
54° N
REPUBLIC OF
IRELAND
ENGLAND
WALES
London
Atlantic
Ocean
50° N
Figure 19.19 Erosional landforms along the coast of Wales.
Sea arches develop when sea caves are entirely cut through, al-
lowing free water passage beneath the arch. If the top of the arch
collapses, an isolated sea stack remains, such as the one to the
left. Note the tilted rock strata.
Retrogradation
The process through which a shoreline
retreats through erosion.
