Signatures of Tsunami in the Coastal Landscape - Tsunami - page 50

Fig. 3.18 A star-shaped impact mark on the face of the raised
platform at Bass Point, New South Wales, Australia. Note as well the
drill holes. The sheltered juxtaposition of these forms suggests that
cavitation rather than the impact of rocks thrown against the rock face
eroded them

at right angles to it. Such marks also appear on the inner
walls of large whirlpools. While it would be easy to attri-
bute these features to marine borers, they often occur pro-
fusely above the limit of high tide.
The most common type of drill mark appears at the end
of a linear or sinuous groove and extends downwards at a
slight angle for several centimeters into very resistant
bedrock (Bryant and Young 1996 ). In some cases, grooves
also narrow with depth to form knife-like slashes a few
centimeters deep. Sinuous drill marks are useful indicators
of the direction of tsunami flow across bedrock surfaces.
Sinuous grooves tend to extend no more than 2 m in length
and have a width of 5-8 cm at most. Depth of cutting can
vary from a few millimeters to several centimeters. In some
cases, the sinuous grooves become highly fragmented lon-
gitudinally and form comma marks similar to those found in
sub-glacial environments. Often they form en echelon in a
chain-like fashion (Fig. 3.19 ). They are not a product of
storm waves or backwash because they show internal
drainage and do not join downslope. Sinuous grooves have
been described for the southeast coast of New South Wales
(Young and Bryant 1992 ; Bryant and Young 1996 ) and for
platforms near Crescent City, California (Aalto et al. 1999 ),
where tsunami appear to be a major process in coastal
landscape evolution. It is tempting to credit their formation
to chemical erosion along joints, micro-fractures, or igneous
inclusions. Four facts suggest otherwise. First, while they
may parallel joints, sinuous grooves diverge from such
structures by up to 10. Second, joints in bedrock are linear
over the distances, which grooves develop. The grooves
described here are sinuous. Third, sinuous grooves often
appear as sets within the spacing of individual joint blocks.
Finally,

Fig. 3.19 Sinuous grooves on a ramp at Tura Point, New South
Wales, Australia

rounded surfaces characteristically produced by tsunami,
and

not on the highly weathered, untouched surfaces

nearby.
S-forms also develop on surfaces that are smoothed and
polished. This polishing appears to be the product of sedi-
ment abrasion. However, high water pressures impinging on
bedrock surfaces can also polish rock surfaces. Flow vor-
tices sculpture S-forms that can be categorized by the three-
dimensional orientation of these eddies (Kor et al. 1991 ).
The initial forms develop under small roller-like vortices
parallel to upslope surfaces. In this case muschelbrüche,
sichelwanne, and V-shaped grooves are created (Fig. 3.17 ).
Muschelbrüche (literally mussel-shaped) are cavities scal-
loped out of bedrock, often as a myriad of overlapping
features suggestive of continual or repetitive formation.
While the features appear flat-bottomed, they have a
slightly raised pedestal in the center formed by uncon-
strained vortex impingement upslope onto the bedrock
surface towards the apex of the scallop. They vary in
amplitude from barely discernible forms to features having
a relief greater than 15 cm. Their dimensions rarely exceed
1.0-1.5 m horizontally. Coastal muschelbrüche inevitably
develop first on steeper slopes and appear to grade upslope
into sichelwanne, V-shaped grooves, and flutes, as the
vortices become more elongated and erosive (Bryant and
Young 1996 ). Sichelwanne have a more pronounced ped-
estal

in the middle of the depression, while V-shaped

grooves have a pointed rather than concave form

sinuous grooves occur only on polished and