Geomorphology 114 (2010) 42–54
Contents lists available at ScienceDirect
Geomorphology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Competing mechanisms for boulder deposition on the southeast Australian coast
Adam D. Switzer a,⁎, Joanna M. Burston b
a
b
Department of Earth Sciences, The University of Hong Kong, Hong Kong SAR, China
School of Geosciences, Madsen F09, University of Sydney, Sydney, Australia
a r t i c l e
i n f o
Article history:
Received 25 November 2007
Received in revised form 24 February 2008
Accepted 5 February 2009
Available online 15 February 2009
Keywords:
Boulder transport
Tsunami
Storm surge
Rocky shorelines
Imbrication
a b s t r a c t
This study investigates the role of late Holocene sea-level change, large storms and possible pre-historic
tsunami in the deposition of boulder features on an exposed headland and sheltered rock ramp. Large
accumulations of boulders are found on coastal rock platforms, cliff tops and ramps in the Jervis Bay region of
southeastern Australia. These deposits are elevated above sea-level and in places exhibit obvious signs of
imbrication as a response to flow in a landward direction. The event history of these features is controversial
and Holocene sea-level change, storms and tsunami can be considered as possible, non-mutually exclusive
mechanisms of deposition. It is apparent that even if the detachment site, transport distance, elevation and
final orientation of a boulder can be identified, deciphering the event history remains a difficult task.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Despite recent advances in the study of the sedimentology and
geomorphology of cliffed and rocky coasts (Scoffin, 1993; Nott, 1997;
Hansom, 2001; Felton and Crook, 2003; Sommerville et al., 2003;
Noormets et al., 2004; Williams and Hall, 2004; Stephenson and
Thornton, 2005; Felton et al., 2006; Hall et al., 2006; Kennedy et al.,
2007; Hall et al., 2008; Hansom and Hall, 2009), the action of coastal
processes on rocky and cliffed coastlines remains understudied and
complicated (Stephenson, 2000; Felton 2002; Felton and Crook, 2003;
Stephenson and Thornton, 2005; Dominey-Howes et al., 2006). One
aspect that has received recent attention is the detachment, transport
and deposition of boulders and large clasts (Nott, 1997; Scheffers,
2002; Nott, 2003; Scheffers and Kelletat, 2003; Noormets et al., 2004;
Morton et al., 2006; Goto et al., 2007; Imamura et al., 2008). Boulder
deposition on the southeast Australian coast has been subject to
considerable research dating back to the early 1900s (Sussmilch, 1912;
Young and Bryant, 1992; Bryant et al., 1996; Young et al., 1996; Bryant,
2001; Felton and Crook, 2003; Switzer, 2005; Saintilan and Rogers,
2005; Dominey-Howes et al., 2006; Dominey-Howes, 2007).
The southeast Australian coast (Fig. 1A) is a microtidal (b2 m),
temperate continental margin that experiences a dominance of highenergy ocean waves often exceeding 2 m in amplitude offshore.
Superimposed on this wave-dominated environment are periodic
storm events that cause swells in excess of 5 m high with offshore
waves often in excess of 10 m (Roy and Boyd, 1996; Switzer and Jones,
2008a). The coast is best described as wave-dominated due to a
⁎ Corresponding author.
E-mail address: [email protected] (A.D. Switzer).
0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2009.02.009
persistent background swell (Short and Trenaman, 1992). Dominant
low-pressure systems from the south produce frequent high-energy
pulses of swell to the coast. Periodic cyclones and locally produced
east-coast lows can also generate swell from the north to east but are
generally much less frequent (Short, 1993). In particular low pressure
systems in the Tasman Sea have the demonstrated ability to produce
waves capable of transporting boulders on rocky shorelines (Saintilan
and Rogers, 2005). One storm in 1912 is reported to have quarried and
moved a boulder in excess of 200 tonnes a considerable distance
(possibly up to 50 m (see discussion of Felton and Crook, 2003))
across coastal rock platforms just above the high water mark
(Sussmilch, 1912).
1.1. Tsunami deposited boulders?
Initial evidence for tsunami activity on the southeast coast of
Australia came from two papers. Young and Bryant (1992) focused on
boulder deposits from rock ramps on the south coast of New South
Wales at Tura Point, where the authors attributed several large rock
accumulations on coastal ramps to a tsunami, possibly the Pleistocene
tsunami of 105 ka identified by Moore and Moore (1984) in Lanai,
Hawaii. This hypothesis was challenged by Jones (1992) who showed
that such an event in the Hawaiian Islands would reach the Australian
east coast with little possibility of catastrophic effects. Bryant (2001)
re-examined the age of the event, using radiocarbon dating of
encrusting shells and suggesting the deposits are much younger. A
second paper, by Bryant et al. (1992), furthered the tsunami
hypothesis with more boulder evidence from a number of sites on
the southeast coast of Australia including Batemans Bay, Gum Getters
inlet on Beecroft Head and Bass Point (Fig. 1).
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
43
Fig. 1. A) Location of the study in the southeast of Australia. B) Sites mentioned in publications by Switzer, 2005 and Switzer et al. (2005, 2006), the monograph of Bryant (2001) or in
this text. The figure also shows the area of Jervis bay which is presented in detail in Fig. 2.
Three subsequent publications by Bryant and Young (1996), Bryant
et al. (1996) and Young et al. (1996) described a broad spectrum of
evidence which they attributed to late Quaternary tsunami activity
along the southeast coast of Australia. Much of this published
evidence was recently reviewed in a paper by Bryant and Nott
(2001) and a monograph by Bryant (2001). Recent work on large scale
overwash into estuaries including Dunmore embayment and Killalea
Lagoon (Fig. 1) has focussed on sandsheet deposits and dune
modification, and has furthered the tsunami hypothesis suggesting a
tsunami struck the coast between 800–200 yrs BP (Switzer et al.,
2005, 2006; Switzer and Jones, 2008b).
2. Regional setting: tsunami evidence from Jervis Bay
The area around Jervis Bay in southeast Australia (Fig. 2) received
particular attention from Bryant et al. (1997), who attributed numerous
boulder accumulations on Beecroft Head, Gum Getters Inlet and at
Greenfields Beach to tsunami deposition in a range of environments
from cliff tops to gently sloping sheltered rock shelves. This paper
revisits two of these sites and shows that although detachment sites,
landward transport routes, boulder orientation and final deposition can
be identified the definitive chronology, event history and depositional
mechanism of the boulder accumulations remains difficult to determine.
2.1. Study site
Large accumulations of boulders are found at several sites on
coastal rock platforms and ramps in the Jervis Bay region of
southeastern Australia. These deposits are elevated above sea level
and in many places show signs of imbrication. Two sites are presented
here: the first at Little Beecroft Peninsula occurs on the more exposed
parts of the coast and the second at Greenfields Beach within the
relative shelter of Jervis Bay (Fig. 2). The Greenfields Beach site is a
small shallow-dipping rock ramp with elevated boulders some 7 m
above contemporary sea level. Twenty boulders are found in two
imbricated piles of fourteen and six respectively at the northern end of
the platform. The imbricated group start at approximately 4.5 m above
sea level and rises to 5.8 m. This deposit contrasts directly with
boulders on top of Little Beecroft Peninsula that exist up to 30 m above
sea level. The high-energy shoreline of the Little Beecroft Head is
characterised by steep cliffs and rock falls that topple periodically to
the rock platforms or sea below (Fig. 3A). In both cases the boulders
are erosional remnants of the local pebbly sandstone stratigraphy.
Some boulders at both sites exhibit obvious signs of inversion or
imbrication as a response to flow in a landward direction whilst other
larger clasts appear characteristic of fallen blocks with no hydraulic
reworking.
44
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
Fig. 2. A) Location of Little Beecroft Head and Greenfields Beach in the Jervis Bay region. Little Beecroft Head (B) is a dramatic cliffed coastline that is open to consistent high-energy
waves and contrasts directly with the Greenfields Beach site that has the morphology of a shallow dipping shore platform (C) and lies within the shelter of Jervis Bay. (Photos by
Charlie Bristow).
2.2. Modern erosion at Little Beecroft Head and Greenfields Beach
Modern erosion at both Little Beecroft Head and Greenfields Beach
is best described as blocky erosion along two sets of near
perpendicular joints in the shallow dipping strata (Fig. 3). The blocky
nature of the erosion results in blocky boulders that are found along
the shoreline around Little Beecroft Head and Greenfields Beach.
Erosion is ongoing and modern rock falls are common. Observations
from local residents indicate that boulders that fall into the ocean are
often reworked and transported either offshore or landward during
storms. It should also be noted that in October 1999 at Whale Point
approximately 1.5 km to the north of Little Beecroft Head the onshore
transport and breaking of a boulder estimated at 15.7 tonnes was
recorded on a rock platform. The boulder split into two smaller
boulders and the largest clast had moved more than 40 m laterally and
2 m vertically during a large storm (Saintilan and Rogers, 2005).
At Greenfields Beach, boulder elevation and transport distance were
also recorded. The boulder transport equations for subaerial and joint
bounded boulders proposed by Nott (2003) were applied to eight
measured boulders from four sites at Greenfields Beach. The two largest
transported boulders at each of four sites were selected for analysis. Of
the eight boulders, seven were of the fine-grained cross-bedded
lithology of the lower platform and the remaining one was of the
pebbly lithology of the upper platform. This analysis was not conducted
on the boulders at Little Beecroft Head as this environment is elevated at
30+ m above sea level and the equations were deemed not applicable.
4. Results
The two contrasting sites allowed direct comparison of the boulder
accumulations on an exposed cliffed coastline and a sheltered embayment. At both sites, transported boulder accumulations were identified
by their lithology, orientation and sedimentary features (Tables 1 and 2).
3. Methods
4.1. Little Beecroft Head
Boulders at both sites were measured for dimension, orientation
and lithology and compared to the local stratigraphy. Supplementary
notes were also taken on grain size, imbrication, dip direction,
jointing, cross-bedding, bioturbation and relative size and abundance
of lichens. These features were used as indicators of boulder source
and for signs of transport and direction.
Although many boulder piles and individual boulders exist on top
of Little Beecroft Head (Fig. 4), only three main sites are presented
here. Examination of the stratigraphy interpreted by Tye (1995)
suggests that many of the boulders are likely to be remnants of a
formerly overlying gravelly sand lithology as no such unit exists in the
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
45
Fig. 3. A) Modern erosion at Little Beecroft Head. Erosion occurs along prominent joints (Fig. 4A). The blocky nature of modern erosion on the steep cliffed environments at Little
Beecroft Head. Large blocks are periodically dislodged due to collapse after erosion of the weaker underlying unit. Such erosion can create very large boulders (Fig. 4B). It is important
to note the dipping nature of the eroded blocks (IV) as it is likely that subsequent erosion and the sliding of blocks marked B1, B2 and B3 would give a imbricated nature to the deposit
(see discussion). B) Boulder accumulations on the rock platform south of Greenfields Beach. The blocky nature of modern erosion is clearly visible along joint lines in the upper unit of
pebbly sandstone. Some boulders, including the one the author is standing next to, appear anomalous.
underlying stratigraphy (Fig. 4C). Two lithologies of boulder were
found on top of Little Beecroft Head. One of these lithologies is found
in both the immediately underlying unit and overlying stratigraphy,
the other lithology is quartz rich sand with larger gravel clasts is not
found in the units below. Reconnaissance of the larger headlands that
are more than 50 m above present mean sea level (PMSL) to the south
of Little Beecroft Head show many randomly orientated boulders exist
as eroded remnants of the formerly overlying stratigraphy.
The first boulder site presented here lies at an elevation of
approximately 30 m above PMSL on the southeastern end of the
headland. Here are two small detached boulders that exhibit clear
signs of that they are not in situ because they rest on other small
cobble and boulders (Fig. 5A). One boulder weighing just over 1 tonne
rests on a smaller boulder and the detachment site is interpreted as
being approximately 3 m to the southeast.
The second site on Little Beecroft Head consists of one large
boulder that rests across a large prominent joint in the platform
(Fig. 5B). This boulder is the largest on the platform and weighs more
than 22 tonnes. The heavily bioturbated sandstone block appears to
have been transported because to rest across a prominent joint and
analysis of jointing through the block shows that it is currently rotated
approximately 55° to the jointing in the surface below.
The third site occurs approximately 15 m to the northwest of the
large boulder, and consists of five boulders of different lithology and
up to 3 tonnes, in an imbricated stack stretching over approximately
8 m (Fig. 6). These boulders are found at an elevation ∼ 33 m above
PMSL and are orientated facing southeast suggesting flow from the
southeast across the platform.
4.2. Greenfields Beach
Within the shelter of Jervis Bay, another site on a rock platform
south of Greenfields Beach contrasts with the high energy shoreline
of Little Beecroft Head. (Figs. 2B, 3B, and 7A). The Greenfields Beach
46
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
Table 1
The dimensions and mass of the boulders identified at five sites along the top of Little Beecroft headland.
Site
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
LBH
1
1
1
1
1
1
2
3
3
3
3
3
3
5
5
5
5
5
5
Boulder #
Rock unit
a-axis
b-axis
c-axis
Vol.
Mass (tonne)
Inv.
Imb.
Moved (horiz)
Moved (vert)
1.1
1.2
1.3
1.4
1.5
1.6
3.1
4.1
4.2
4.3
4.4
4.5
4.6
5.1
5.2
5.3
5.4
5.5
5.6
1
1 (J)
1
1
1
1
1
1
2
1
2
1
1 (J)
1
2
2
1
2
1
2.30
2.50
1.30
1.20
1.05
1.15
3.50
2.40
2.10
3.00
2.00
3.60
1.80
2.2
0.7
1.5
1
1.9
1.9
2.20
2.30
1.10
1.00
0.90
0.60
2.10
2.40
1.90
3.50
2.00
1.90
1.65
1.75
0.8
1.6
0.8
1.8
0.8
0.35
0.35
0.30
0.30
0.30
0.30
1.30
0.25
0.30
0.55
0.40
0.35
0.30
1.4
0.3
0.3
0.4
0.4
0.4
1.77
2.01
0.43
0.36
0.28
0.21
9.56
1.44
1.20
5.78
1.60
2.39
0.89
5.20
0.17
0.72
0.32
1.37
0.53
4.25
4.83
1.03
0.86
0.68
0.50
22.93
3.46
2.87
13.86
3.84
5.75
2.14
12.47
0.40
1.73
0.77
3.28
1.28
N
N
N
?
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Y
Y
Y
Y
N
Y
Y
Y
Y
N
N
N
N
Y
Y
Y
Y
N
N
2
2
8
8
?
Y
Y
Y
Y
?
?
Y
?
Y
Y
Y
?
N
N
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
Notes are also presented on whether the boulder appears inverted (Inv.), imbricated (Imb.) or has moved horizontally (horiz) or vertically (vert) are also presented.
Boulder lithologies.
1 — medium quartz sandstone with pebbly storm lags and mild bioturbation (J = jointed).
2 — medium-coarse quartz sandstone with pebbly storm lags, larger clasts and mild bioturbation.
lower rock ramp appears cleared of boulders although large boulder
accumulations appear on the upper ramp and at the northern end
but not at the southern end, possibly indicating wave approach from
the south.
site is a small shallow dipping rock ramp with boulders elevated
some 7 m above contemporary sea-level, much lower than the Little
Beecroft Head site. The boulders in this area are also considerably
more block-shaped than those found on Little Beecroft Head. The
Table 2
The dimensions and mass of the boulders identified at four sites on the rock platform south of Greenfields beach.
Site
Boulder #
Rock unit
a-axis
b-axis
c-axis
Vol.
Mass (tonne)
Inv.
Imb.
Moved (horiz)
Moved (vert)
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
1.1
1.2
1.3
1.4
1.5
2.1
2.2
2.3
2.4
2.5
3.1
3.2
3.3
3.4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.14
4.15
4.16
4.17
4.18
4.19
4.20
3
3
3
3
3
3
3
3
3
3
3
3
3
3
1
1
1
1
1
1
1
3
1
1
1
1
1
1
1
3
1
1
1
1
1.80
1.10
2.50
2.20
2.30
1.70
1.40
1.20
0.90
1.50
2.80
2.50
2.90
1.70
2.10
2.00
1.70
1.60
1.90
1.50
1.40
2.00
3.00
2.00
1.60
1.00
1.50
0.70
1.40
1.80
1.40
0.80
1.20
2.10
2.00
1.70
1.2
2.30
1.30
1.30
0.90
1.80
1.00
0.90
1.60
2.60
2.10
1.40
1.50
1.90
1.40
0.90
1.40
1.00
0.90
1.90
2.10
1.30
1.60
0.80
1.00
0.70
1.40
1.50
1.10
0.70
1.50
1.20
0.50
0.55
0.35
0.65
0.50
0.60
0.50
0.35
0.20
0.45
0.70
1.00
0.90
0.90
0.70
0.55
0.55
0.25
0.60
0.40
0.20
0.40
0.50
0.30
0.35
0.25
0.30
0.20
0.60
0.50
0.60
0.25
0.80
0.40
1.80
1.03
1.05
3.29
1.50
1.33
0.63
0.76
0.18
0.61
3.14
6.50
5.48
2.14
2.21
2.09
1.31
0.36
1.60
0.60
0.25
1.52
3.15
0.78
0.90
0.20
0.45
0.10
1.18
1.35
0.92
0.14
1.44
1.01
4.32
2.47
2.52
7.89
3.59
3.18
1.51
1.81
0.43
1.46
7.53
15.60
13.15
5.14
5.29
5.02
3.14
0.86
3.83
1.44
0.60
3.65
7.56
1.87
2.15
0.48
1.08
0.24
2.82
3.24
2.22
0.34
3.46
2.42
N
N
Y
N
Y
N
?
Y
N
Y
N
N
N
Y
?
?
?
?
?
?
?
Y
?
?
?
?
?
?
?
Y
?
?
?
?
N
N
N
N
N
N
Y
Y
Y
N
Y
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
12
8
8
3
4
21
25
26
28
26
20
30
28
35
?
?
?
?
?
?
?
4
?
?
?
?
?
?
?
4
?
?
?
?
2
2
2
4
3
5
5
5
6
5
2
2
2
3
?
?
?
?
?
?
?
15
?
?
?
?
?
?
?
16
?
?
?
?
1
1
1
1
1
2
2
2
2
2
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
Notes are also presented on whether the boulder appears inverted (Inv.), imbricated (Imb.) or has moved horizontally (horiz) or vertically (vert) are also presented.
Boulder lithologies.
1 — medium quartz sandstone with pebbly storm lags and mild bioturbation (J = jointed).
2 — medium-coarse quartz sandstone with pebbly storm lags, larger clasts and mild bioturbation.
3 — fine- to medium-grained cross-bedded sandstone.
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
47
Fig. 4. A) Aerial photograph of study sites at Little Beecroft Head B) Topographic map with key geomorphic features. The structural control of prominent jointing is apparent as are
several fallen large blocks. C) Stratigraphy of the Little Beecroft Head sequence (redrawn from Tye, 1995). The boulders on top of Little Beecroft Head can only come from the gravely
sequence at the immediate top of the sequence or from eroded remnants of the formerly overlying stratigraphy.
The upper ramp is composed of bioturbated pebbly sandstone
which erodes to form joint bounded blocks (Fig. 3C). This lithology
provides little structural and lithological information making it
difficult to decipher whether a rock has been transported or not. In
contrast a finer cross-bedded sandstone unit is found in two large
wave-cut notches on the lower part of the ramp which exposes the
stratigraphy of the underlying sequence. The overlying stratigraphy is
composed of pebbly, bioturbated medium-grained sandstone indicating that any boulders that contain cross-bedding must have originated
from this lower unit (Fig. 7B).
Numerous cross-bedded boulders are found at Sites 1 and 2 on the
southern and central part of the platform (Fig. 6A andB). Many of
these boulders lie at least 8 m away from the notch and weigh
between 2 and 8 tonnes (Fig. 8A). The boulders are often found mixed
with boulders of the pebbly sandstone lithology and many are found
imbricated against other boulders. At Site 3 to the north, several large
cross-bedded boulders are also identified with the largest weighing
more than 15 tonnes. This boulder (Boulder 3.2) must have come from
the seaward wave-cut notch (Fig. 6B and C) and moved at least 30 m
horizontally and more than 2 m vertically (Table 2). This suggests that
an event of significant magnitude detached and transported the
boulders up the ramp and deposited them up to 30 m from their
original position. In addition to transporting the boulders, such an
event also plucked the boulders from the notch and lifted them over
the lip of the notch.
Of these boulders, 16 were of the fine-grained cross-bedded
lithology of the lower platform and the remaining 14 were of the
pebbly lithology of the upper platform. Of the twenty boulders found
in two imbricated piles at the northern end of the platform (Fig. 9)
two boulders were deemed to be transported because one was of the
fine grained crossed bedded lithology (Boulder GF-4.4) and the other
(Boulder GF-4.9) was clearly imbricated and dipping seaward at a high
angle (Fig. 9B).
4.3. Application of the boulder transport models and equations of
Nott (1997, 2003)
Recently a considerable number of studies have relied heavily on
equations published by Nott (1997, 2003) when attempting to
determine if tsunami or storm waves are the most likely mechanism
for boulder detachment and transport in a variety of locations.
Application of the boulder transport equations (see Appendix A) for
subaerial and joint bounded boulders proposed by Nott (2003) were
applied to the two largest measured boulders from four sites at
Greenfields Beach (Table 3). The Nott equations suggest that the
largest wave heights are required for boulder 4.4 which would require
a tsunami wave height of 5.70 m or a storm wave height of 22.80 m if
joint bounded, or a tsunami wave height of 2.19 m or a storm wave
height of 8.78 m if subaerially exposed and wave heights only slightly
higher if submerged (refer to Table 3). Note that the heaviest boulder
at Greenfields Beach (Boulder 2.3) did not require the greatest wave
height to move it as the shape of the boulder is the primary determinant in the Nott equations.
To provide a modern analogue and test the validity of the method,
the boulder called ‘Mermaid Rock’ at Ben Buckler Head north of Bondi
Beach in Sydney (Fig. 1B) was also analysed using the equations of
Nott (2003). This sandstone block measures 6.1 × 4.9 × 3.0 m and
weighs approximately 235 tonnes (Sussmilch, 1912). The block
appears inverted relative to its original orientation in the bed and
was deposited by a large storm on or in the days preceding July 15,
1912 (Sussmilch, 1912; Felton and Crook, 2003). Assuming that the
boulder at Ben Buckler Head was joint bounded, then Nott's equations
48
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
Fig. 5. A) Small scale sketch map of the boulders at Site 1 on Little Beecroft Head showing the present positions of the boulders and the likely detachment site. Photographs shown in
B and C show the cliff top setting and boulders 1.1–1.4. Boulders 1.3 and 1.4 appear to have been transported in a landward direction and rotated. Photograph C shows boulder 1.4
resting on a smaller boulder of the same lithology. D) A sketch map of site 2 is provided, a large boulder weighing almost 23 tonnes (boulder 3.1) rest across a prominent joint. The
boulder also contains joints and appears rotated ∼55° to the joint orientation of the underlying bedrock. The boulder is of a pebbly lithology similar to the material directly below in
the stratigraphy. Photograph (E) shows boulder 3.1 and the head land at Little Beecroft Head. Site 1 is at the end of the headland. Larger cliffs (50 m+) can be observed in the top right
of the photograph.
indicate that detachment and transport of this boulder would require
a tsunami wave in excess of 11.5 m in height or a storm wave in excess
of 46 m in height (Table 3). Records of the 1912 storm indicate storm
wave heights of less than 10 m (Sussmilch, 1912), indicating that
Nott's equations overestimate the required wave height. Although the
pioneering work of Nott is admirable, the applicability of these
equations appears limited and suggests the equations need to be
tested against other clasts from known events and reworked and
extended to include other variables (see discussion of Kelletat, 2008).
A recent study by Morton et al. (2006) also highlighted problems
with the Nott equations, stating that the simple mathematical
assumptions lead to the conclusion that storm waves must be
approximately four times higher than tsunami waves to transport
the same size particle. The mode of transport assumed by Nott (1997,
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
49
Fig. 6. Imbricated boulders at an elevation of more than 30 m at Little Beecroft Head. This stack of 5 imbricated boulders contains 2 different lithologies and indicates flow from the
left of the photo to the right.
2003) involves overturning and the wave height required is
determined by the square of the velocity, which is proportional to
the height of storm waves and to one-quarter the height of tsunami
waves. The assumption of particle overturning may not be correct
because there is growing evidence from high wave energy coasts that
movement of blocks that are not fully submerged is dominated by
sliding rather than overturning or suspension (Noormets et al., 2004;
Williams and Hall, 2004; Morton et al., 2006; Goto et al., 2007) or
rolling or saltation (Imamura et al., 2008).
5. Event history: storm waves vs tsunami
It is often not possible to work out the prehistoric event history for
a boulder, given that it may have been transported by several storm
and/or tsunami events. Even if it is assumed that a single event
transported the boulder, and the boulder dimensions, detachment
position and final position are known, it is still difficult to determine
the type and size of the wave that emplaced it. The problem is an
inverse one, and given the chaotic nature of turbulence and the
associated non-linearity of the hydrodynamics at the shoreline, there
may be non-unique solutions. With a numerical model of wave
dynamics and boulder transport, it may be possible to find the
solution space (i.e., the range of possible waves) by using reverse
Monte-Carlo simulations. However, the possibility of non-unique
solutions means that such an approach may provide little constructive
meaning.
The dynamics of quarrying and transport were also studied by
Noormets et al. (2004) who suggested that both tsunami and storm
waves are capable of quarrying mega clasts from cliff edges. They
concluded that a higher quarrying capacity is expected from short
period storm waves when combined with super elevation of sea level
associated with storm surge. Such events may flood the rock platform,
thus moving the potential zone of wave impact and possibly promoting
block transport. Noormets et al. (2004) also noted that tsunami waves
are of considerably longer period, and may have higher transporting
capacities because each wave has the potential to transport blocks for a
longer time than a storm wave. Recently the work of Goto et al. (2007)
has examined the transport of boulders in Thailand by the 2005 Indian
Ocean tsunami. Here they examined the boulder fields on a reef flat
and using the equations of Noji et al. (1993) and assuming the boulder
is a sliding block (not overturned or rolling) found that the inferred
velocity to move the largest clasts (2–3 m s− 1) is far below that likely
to be experienced during tsunami inundation.
Recent work by Imamura et al. (2008) may also show promise in
assisting the hindcasting of wave heights and possible mechanisms for
palaeoevents. In hydraulic experiments conducted in an open channel,
blocks of cubic and rectangular shape were observed to be mainly
transported by rolling or saltation rather than by sliding. This
observation differs significantly from previous models of boulders
on rocky coasts that have assumed sliding as a mode of transport for
the boulder (Goto et al., 2007). Imamura et al. (2008) concluded that
the sliding assumption in tsunami transport models may underestimate the distance a boulder moved when it was transported.
Furthermore they developed a practical model for the transport of a
boulder by tsunami, introducing an empirical variable coefficient of
friction and assuming that the coefficient decreases with decrease in
ground contact time when the block was transported by rolling or
saltation. Thus this model can explain various modes of transport, i.e.,
sliding, rolling, and saltation. Importantly, Imamura et al. (2008)
tested their model against a tsunami deposited boulder transported
by the 1771 Meiwa tsunami on Ishigaki Island, Japan, and found
transport distances consistent with the descriptions in historical
documents.
Morton et al. (2006) also suggested that applying the equations of
Lorang (2000) may aid the prediction of boulder size transport
considering wave height, wave period, runup elevations, and swash
velocities. The recent studies summarised above suggest that the wave
mechanics and hydrodynamics of both storm waves and tsunami
require more detailed investigation and modelling. Modern examples
have shown that single extreme-wave event (storm or tsunami) have
the ability to move and transport large boulders on rocky coasts.
However, despite significant advances in numerical modelling over
the last decade the current numerical and empirical models available
for the analysis of boulders do not allow the differentiation of storm or
tsunami transport based solely on the size boulders and their position
in the landscape.
5.1. Boulders accumulations at Jervis Bay: can they be attributed to
an event type?
The boulders on top of Little Beecroft Head and at Greenfields
Beach provide evidence of landward transport. Although the evidence
for landward movement is fairly compelling there are a number of
unresolved issues that require attention before attributing these
deposits to either toppling of eroded remnants of overlying stratigraphy, deposition during higher sea level, storm, tsunami or a
combination of factors. The first problem lies with the age of the
deposits and the history of the events. Radiocarbon dating of
encrusting shells on transported boulders by Young and Bryant
(1992) suggested that many of the boulders in the Jervis Bay region
are late-Holocene in age. If this is a true indication of the age then sea
level change cannot reasonably invoked as a mechanism for deposition. Even if the deposits are much older (eg. OIS5e) there is no
evidence that sea level on this tectonically stable coast has exceeded
2 m above present levels for the last 200 ka (Murray Wallace and
Belperio, 1991). If the deposits are mid-Holocene in age then higher
sea level (up to 2 m) may be a possibility (Flood and Frankel, 1989).
However, the reliability of the use of fixed biological indicators to
50
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
Fig. 7. A) Aerial photograph of study sites at Greenfields Beach B) Topographic map with key geomorphic features. The two large wave cut notches can be observed in the lower
platform. This is the site of wave quarrying before transport for the cross-bedded boulders. C) The wave cut notch at the southern end of the platform. The lower rock ramp is
composed of cross-bedded sandstone and is overlain by pebbly bioturbated sandstone. Site GF1 lies to the northwest of the notch and contains boulders of both lithologies.
provide accurate spatial and temporal controls on prior sea level has
recently been challenged by Sloss et al. (2006, 2007). Higher sea levels
may be invoked to be partially responsible for at least the Greenfields
boulders but the affect of 2 m sea level rise on the boulders at Little
Beecroft Head would be minimal.
The second and major problem lies in defining the event history. It
is impossible to know if the boulders were emplaced by single or
multiple events. One fundamental and unanswered question limits
analysis of these types of deposit: Is it possible that multiple or single
tsunami, a series of high-energy storms or combination of both
processes can deposit a boulder accumulation that exhibits
imbrication?
Young et al. (1996) and Bryant (2001) suggested that the largest
boulder (Boulder LBH2-3.1) found on top of Little Beecroft Head was
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
51
Fig. 8. A) Many boulders at site GF1 are composed of cross-bedded sandstone and could only have come from the seaward wave cut notch. B) The boulder in the main photograph
(Boulder 1.3) must have moved at least 8 m horizontally and at least 2 m vertically to get to this position on the upper platform. C) Boulder 3.2 at Greenfields beach (site GF3). This
cross bedded sandstone boulder has dimensions of 2.6 × 2.5 × 1.0 and weighs approximately 15.6 tonnes has been transported at least 30 m laterally and 2 m vertically. It rests against
a pile of boulders with mixed lithologies of cross bedded and pebbly bioturbated sandstones.
emplaced by a Holocene tsunami where they cited its different
lithology, elevation and irregular contact as evidence for transport
from much lower in the cliff sequence. Analysis of the stratigraphy of
Tye (1995) shows that the pebbly lithology of the boulder suggests
that it is more likely to be an erosional remnant of the coarser
overlying stratigraphy (now eroded) than the slightly gravelly
sequences immediately below (Fig. 4C). Although the boulder has
obviously been transported it is impossible to say whether its present
position is the result of washover of the headland and plucking from
the underlying unit or deposition as an eroded remnant produced by
rock fall during earlier erosion. It is possible that both scenarios have
occurred.
The identification of imbricated, reworked and transported
boulders from coastal rock platforms and cliff tops shows vivid
evidence of large washover events (Noormets et al., 2002; Sommerville et al., 2003; Noormets et al., 2004; Williams and Hall, 2004; Hall
et al., 2006, 2008; Hansom and Hall, 2009). Some boulders at both
sites exhibit obvious signs of imbrication as a response to flow in a
52
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
Fig. 9. At site 4 on the northern end of the rock platform at Greenfields Beach 20 imbricated boulders are found in 2 piles of 14 and 6 at the northern end of the rock platform. All
boulders in this pile appear imbricated in response to landward flow and exist from 6–7 m above present sea level.
landward direction whilst other larger clasts appear characteristic of
blocks with no hydraulic reworking. Interestingly, the imbrication of
these deposits appears to only occur in boulders up to a threshold size,
thus providing a possible constraint on wave power for the
depositional event or events.
Both of the sites in this study were studied by Bryant et al. (1992,
1996, 1997) and Young et al. (1996) who attributed the boulder
deposition to tsunami. Although striking, these deposits present
significant analytical problems in dating and depositional history
some of which have been discussed controversially (Felton and Crook,
2003; Dominey-Howes et al., 2006). It is also important to note that
recent research on Northern Atlantic coasts by Hansom (2001),
Sommerville et al. (2003), Williams and Hall (2004), Hall et al. (2006,
2008), Hansom and Hall (2009) have convincingly related the
frequent removal and movement of very large boulders into organised
imbricated ridges to wave activity during large storms over the last
century or so. Much of this activity occurs at heights of around 20 m
above sea level, but more chaotic piles and smaller individual boulders
also occur at up to 50 m above sea level. Hansom and Hall (2009)
demonstrate that these processes have been ongoing for at least
1400 years. Other recent work by Morton et al. (2006) suggests that
reinterpretation of boulders and other deposits attributed to tsunami
in the Caribbean are also now overdue.
6. Conclusions
The imbrication, mixed lithology and sedimentary characteristics
of boulder deposits at Little Beecroft Head and Greenfields Beach
provide compelling evidence for large-scale movement attributed to
washover by single or multiple events. However, it is impossible to
attribute these deposits to a unique event. It cannot definitively stated
that the boulders are purely the result of tsunami washover. If the
deposits are late-Holocene in age then a hypothesis of higher
Holocene sea level can be discarded and it is likely that storms and
tsunami may have both played a role in the development of the high
elevation boulder deposits. However, for the sites reported here, it
remains that the mechanism of emplacement, and the number of
steps in boulder movement, is not yet definable, and may never be.
Acknowledgments
This project was supported by Dunmore Sand and Soil Pty Ltd.
Kerry Steggles, Managing Director of Dunmore Sand and Soil, is
thanked for his enthusiasm for this research. Funding for the project
came from ARC SPIRT grant C00107062 (now ARC Linkage grants). The
Landscape Research Centre (now GeoQuEST) at the University of
Wollongong also supported this study. Field visits and discussions
A.D. Switzer, J.M. Burston / Geomorphology 114 (2010) 42–54
53
Table 3
Analysis of the two largest boulders from each of the four sites at Greenfields Beach compared to the storm deposited boulder at Bondi using the equations of Nott, 2003.
Parameters
Explanation of term
Minimum height of wave (m)⁎
T (sm)
Tsunami submerged boulders
S (sm)
Storm wave submerged boulders
T (sa)
Tsunami subaerial boulders
S (sa)
Storm wave subaerial boulders
T (lift)
Tsunami joint bounded boulders
S (lift)
Storm wave joint bounded boulders
Input
a
a-axis of boulder (m)
b
b-axis of boulder (m)
c
c-axis of boulder (m)
Vol
V of boulder (a ⁎ b ⁎ c) (m3)
p(s)
Density of boulder (g/cm3)
p(w)
Density of water (g/ml)
Cd
Coefficient of drag
Cl
Coefficient of lift
Cm
Coefficient of mass
u''
Instantaneous flow acceleration (m/s)
g
gravitational acceleration (m/s/s)
Results — site (boulder #)
1a (1.3)
1.96
7.82
1.82
7.28
4.37
17.48
1b (1.4)
1.21
4.86
1.06
4.24
4.75
19.00
2a (2.1)
0.83
3.32
0.73
2.94
3.23
12.92
2b (2.3)
1.20
4.79
1.09
4.35
3.25
13.00
3a (3.2)
1.74
6.97
1.64
6.55
4.94
19.76
3a (3.3)
1.44
5.76
1.34
5.35
5.51
22.04
4a (4.4)
2.36
9.46
2.19
8.78
5.70
22.80
4b (4.4)
1.25
4.98
1.12
4.48
3.42
13.68
Mermaid rock
2.42
9.70
2.35
9.40
11.59
46.36
2.3
2.2
0.65
3.29
2.4
1.025
2
0.178
2
1
9.81
2.5
1.2
0.35
1.05
2.4
1.025
2
0.178
2
1
9.81
1.7
1.3
0.6
1.33
2.4
1.025
2
0.178
2
1
9.81
1.71
1.64
0.62
1.74
2.4
1.025
2
0.178
2
1
9.81
2.6
2.5
1
6.5
2.4
1.025
2
0.178
2
1
9.81
2.9
2.1
0.9
5.48
2.4
1.025
2
0.178
2
1
9.81
3
2.1
0.5
3.15
2.4
1.025
2
0.178
2
1
9.81
1.8
1.5
0.5
1.35
2.4
1.025
2
0.178
2
1
9.81
6.1
4.9
3
89.67
2.4
1.025
2
0.178
2
1
9.81
Note that the formerly joint bounded and now storm deposited boulder from Bondi would require a storm wave greater than 46 m according to the analysis of Nott, 2003.
T (sm) = submerged boulders; T (sa) = subaerial boulders; T (lift) = joint bounded boulders (requires lift).
with Ted Bryant, Simon Haslett, Anne Felton, Keith Crook, Charlie
Bristow, David Fink and Wayne Stephenson provided stimulus for
much of the material in this paper. An informal review from Craig
Sloss and the reviews of two anonymous reviewers greatly assisted
the drafting of this paper.
Appendix A. Boulder transport equations from Nott, 2003
For a submerged boulder
Tsunami: H ≥ 0.5a[(ρs − ρw) / ρw] / [(ac / b2)Cd + Cl]
Storm wave: H ≥ 2a[(ρs − ρw) / ρw] / [(ac / b2)Cd + Cl]
For a subaerial boulder
Tsunami: H ≥ 0.25[((ρs − ρw) / ρw)(2a − C m(a / b) × (ü − / g))] /
[(ac / b2)Cd + Cl]
Storm wave: H ≥ [((ρs − ρw) / ρw)(2a − 4Cm(a / b) × (ü / g))] / [(ac /
b2)Cd + Cl]
(Cd = 1.5, Cm = 2 and ü = 1 m/s2)
For a joint-bounded block
Tsunami: H ≥ 0.25a[(ρs − ρw) / ρw] / Cl
Storm: H ≥ a[(ρs − ρw) / ρw] / Cl
Where H is the wave height at breaking point, ρs = density of the
boulder (2.7 g/cm3), ρw = density of water (1.02 g/ml), a = A-axis of
boulder, b = B-axis of boulder, c = C-axis of boulder, Cd = coefficient
of drag (1.5) and Cl = coefficient of lift (0.178), Cm = coefficient of
mass, ü = instantaneous flow acceleration., g = acceleration due to
gravity (9.8 m/s2). Equations derived from Nott (2003).
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