Geoscience Reference
In-Depth Information
Fig. 3.11
Scattered boulders
transported by tsunami and
deposited across the reef at
Agari-Hen'na Cape on the
eastern side of Miyako Island,
Ryukyu Island, Japan.
Photograph courtesy of Prof.
Toshio Kawana, Laboratory of
Geography, College of
Education, University of the
Ryukyus, Nishihara, Okinawa
cliff tops 15 m above sea level. Contentious is the docu-
mented appearance of a 200-tonne boulder measuring
6.1 m 9 4.9 m 9 3.0 m on an intertidal rock platform dur-
ing a storm in 1912 at Bondi Beach, Sydney (Sussmilch
1912
). However, Cass and Cass (
2003
) show that the boulder
appears in an 1881 photograph. It has not moved since, even
during the great storms of 1876, 1889, 1912 and 1974—the
latter judged the worst in a 100 years. While much higher
storm events can be, and have been, invoked for the move-
ment of the boulders now piled prodigiously along this coast,
tsunami offer a simpler mechanism for the entrainment of
joint-controlled blocks, the sweeping of these blocks across
platforms, and their deposition into imbricated piles.
In exceptional cases, in New South Wales, boulders have
been deposited as a single-grained swash line at the upper
limit of tsunami run-up (Young and Bryant
1992
; Young
et al.
1996
). Some of these tsunami swash lines lie up to
20 m above sea level and involved tractive forces greater
than 100 kg m
-2
(Bryant et al.
1997
). Just as unusual are
angular boulders jammed into crevices at the back of plat-
forms. For instance, at Haycock Point, north of the Victo-
rian border, angular blocks up to 2 m in length and 0.5 m in
width have been jammed tightly into a crevice, often in an
interlocking series three or four blocks deep (Young and
Bryant
1992
). What is more unusual about the deposit is the
presence along the adjacent cliff face of isolated blocks
0.4-0.5 m in length perched in crevices 4-5 m up the cliff
face. Boulders are also found in completely sheltered
locations along the coast. At Bass Point, which extends
2 km seaward from the coast, a boulder beach faces the
mainland coast rather than the open sea. Similarly at Hay-
cock Point, rounded boulders, some with volumes of 30 m
3
and weighting 75 tonnes, have been piled into a jumbled
mass at the base of a ramp that begins 7 m above a vertical
rock face on the sheltered side of the headland (Fig.
3.12
).
Chatter marks on the ramp surface indicate that many of the
boulders have been bounced down the inclined surface.
Perhaps the most dramatic deposits are those containing
piles of imbricated boulders (Young et al.
1996a
). These
piles take many forms, but include boulders up to 106 m
3
in
volume and weighing as much as 286 tonnes. The boulders
lie en echelon one against the other like fallen dominoes,
often in parallel lines. At Jervis Bay, New South Wales,
blocks weighing almost 100 tonnes have clearly been
moved in suspension and deposited in this fashion above the
limits of storm waves on top of cliffs 33 m above present
sea level (Fig.
3.13
). The longest train of imbricated boul-
ders exists at Tuross Head where 2-3 m diameter boulders
stand as sentinels one against the other, over a distance of
200 m at an angle to the coast.
The velocity of water necessary to move these large
boulders can be related to their widths as follows:
v
¼
5
:
2 b
0
:
487
ð
3
:
1
Þ
I
v
min
¼
2
:
06 b
0
:
5
ð
3
:
2
Þ
where
= mean flow velocity (m s
-1
)
v
= minimum flow velocity (m s
-1
)
v
min
b
= the intermediate axis or width of a boulder (m)
b
I
= intermediate diameter of largest boulders (m)
Equations
3.1
and
3.2
, derived from Costa (
1983
) and
Williams (
1983
) respectively, are based upon unidirectional,
