Geoscience Reference
In-Depth Information
Fig. 3.21
Small dissected
potholes at the top of a 15 m high
headland at Atcheson Rock,
60 km south of Sydney,
Australia. The potholes lie on the
ocean side of the canyon
structure shown in Fig.
3.23
At the crests of headlands, flow can separate from the
bedrock surface, forming a transverse roller vortex capable
of eroding very smooth-sided, low, transverse troughs over
50 m in length and 10 m in width (Bryant and Young
1996
). Under optimum conditions, the bedrock surface is
carved smooth and undular. In some cases, the troughs are
difficult to discern because they have formed where flow
was still highly turbulent after overwashing the crest of a
headland. In these cases, the troughs are embedded in
chaotic, hummocky topography. This is especially common
on very low-angled slopes. Transverse troughs can also
form on up flow slopes where the bed locally flattens or
slopes downwards. Under these circumstances, troughs are
usually short, rarely exceeding 5 m. The smoothest and
largest features develop on the crests of broad undulating
headlands.
controlled rock slabs from the underlying bedrock. Where
standing waves have formed, then bedrock plucking can
remove two or three layers of bedrock from a restricted
area, leaving a shallow, closed depression on the ramp
surface devoid of rubble and unconnected to the open ocean
(Fig.
3.22
). Ramps are obviously controlled structurally and
have an unusual juxtaposition beginning in cliffs up to 30 m
above sea level and, sloping downflow often into a cliff. If
these high velocities are channelised, erosion can produce
linear canyon features 2-7 m deep and pool-and-cascade
features incised into resistant bedrock on the lee side of
steep headlands (Bryant and Young
1996
). These features
are most prevalent on platforms raised 7-8 m above modern
sea level. All features bear a resemblance to the larger
canyon-and-cascade forms carved in the channeled Scab-
lands of the western United States (Baker
1978
,
1981
).
Wave breaking may also leave a raised butte-like structure
at the seaward edge of a headland, separated from the
shoreline by an eroded depression. This feature looks sim-
ilar to an inverted toothbrush (Fig.
3.23
). Large-scale flut-
ing of a headland can also occur, and on smaller rock
promontories, where the baseline for erosion terminates
near mean sea level, the resulting form looks like the
inverted keel of a sailboat. Often this form has a cockscomb
peak produced by the rapid, random erosion of multiple
vortices (Fig.
3.24
). While technically a sculpturing feature,
the cockscomb looks as if it has been hydraulically ham-
mered. On narrow promontories, vortices can create arches.
While arches have been treated in the literature as products
of chemical weathering or long-term wave attack along
structural weaknesses, close investigation shows that many
are
3.3.2
Large-Scale Features
Large-scale features can usually be found sculptured or
eroded on rock promontories, which protrude seaward onto
the continental shelf (Young and Bryant
1992
). Such fea-
tures require extreme run-up velocities that can only be
produced by the higher or longer waves (mega-tsunami)
generated by large submarine landslides or comet/asteroid
impacts in the ocean. One of the most common features of
high-velocity overwashing is the stripping of joint blocks
from the front of cliffs or platforms forming inclined sur-
faces or ramps (Fig.
3.12
). In many cases, this stripping is
aided by the detachment of flow from surfaces, a process
that generates enormous lift forces that can pluck joint-
formed
by
vortices.
It
is
also
possible
for
these
