Australian Rock Coasts: review and prospects

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Australian Rock Coasts: review and prospectsWayne J. Stephenson a & Lukar E. Thornton aa University of Melbourne , AustraliaPublished online: 14 Oct 2010.

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Australian Rock Coasts: review and prospects

WAYNE J. STEPHENSON & LUKAR E. THORNTON, University of

Melbourne, Australia

ABSTRACT Studies of Australian rock coasts (except carbonate reefs) are reviewed and

considered in view of recent process and morphological studies. The unique nature of the

Australian coast, its geographical distribution and relative stability mean that it is a

productive environment in which to research fundamental questions concerning rock coasts.

Future research directions are identified, specifically in the areas of processes, morphology

and modelling.

KEY WORDS Shore platform; waves; bioerosion; subaerial weathering; shore platform

elevation.

Introduction

The investigation of rock coasts has long been an important area of study for coastal

geomorphology both in Australia and worldwide. However, the focus on cliffs and

platforms in Australia from the 1940s through to the 1960s, by workers such as

J.T. Jutson, E.S. Hills, A.B. Edwards, and E.D. Gill, was lost as coastal studies grew

and later workers turned their attention to other shore types. The main contribution

of these workers was to crystallise the debate between the ‘wave versus weathering’

schools of thought on the origin of platforms and highlight the complex relationships

between morphology, sea level and processes.

Internationally there has always been an interest in cliff recession, but usually

from the perspective of engineered hazard management. During the 1980s and

1990s worldwide work on rock coast was conducted by small number of individuals

(see Trenhaile 1987; Sunamura 1992). Similarly, in Australia only a small number

of individuals were investigating rock coasts (Nott 1990, 1994), while Bryant and

Young (1996), Bryant et al. (1996), Young et al. (1996) and Young and Bryant

(1998) focused on the role of tsunami in shaping rock coasts. In recent years there

has been an increased interest in platforms and cliffs, for example, the European

Commission’s Marine Science and Technology Program (MAST-III) for the

‘European Shore Platform Erosion Dynamics’ project, involving five European

universities. Stephenson (2000) and Trenhaile (2002a) have reviewed more recent

advances with respect to platforms and cliffs, but Australian research on rock coasts

remains limited. Such a pattern is not surprising given the preoccupation with

beaches, coral reefs and other coastal landforms, but rock coasts, like all others, are

coming under increasing pressure from human occupation of the coastal zone and

Australian Geographer, Vol. 36, No. 1,

pp. 95–115, March 2005

ISSN 0004-9182 print/ISSN 1465-3311 online/05/010095–21 q 2005 Geographical Society of New South Wales Inc.

DOI: 10.1080/00049180500050946

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therefore deserve greater attention (e.g. Thom 2004). In this paper we review

geomorphological studies of Australian rock coasts (excluding carbonate reefs) in

the context of global studies over the last 20 years and present some preliminary

results of recent fieldwork. Our intention is to illustrate that the Australian coast

offers significant opportunities and excellent environments to expand our

understanding of rock coasts.

Rock coast morphology

Jutson (1939, 1949a, b, 1950, 1954) proposed that platforms could be grouped into

three categories based on elevation. High-level platforms were those with average

heights of 1 m but could be found higher; normal platforms were found at elevations

between mean low tide and mean high tide; and the ultimate platform was well

below low water. Bird (1968, 2000) and Davies (1977) also advocated a tripartite

scheme based on elevation. However, elevation has proved to be an unreliable

morphology for categorising platforms because of the uncertainties of relating

elevation to sea and tide levels.

Sunamura (1983, 1992) and Tsujimoto (1987) proposed three broad morphologies

of rock coasts (see Figure 1): shore platforms that slope gently into the sea, platforms

that are nearly horizontal and terminate abruptly with a cliff or ramp at the seaward

edge, and plunging cliffs, i.e. where shore platforms have not developed. The two

platform morphologies were designated Type A for the sloping platform and Type B for

the horizontal platforms. However, it was Edwards (1941) working in Australia who

FIGURE 1. Three major rocky coast morphologies: (1) Type A or sloping platforms; (2) Type B or

horizontal platform; (3) plunging cliff (after Sunamura 1992).

96 W. J. Stephenson & L. E. Thornton

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first showed that rock strength played a role in platform morphology. Compressive

strength for different rock types on the Victorian and Tasmanian coast were measured

and showed a relationship between rock hardness and platform width. The best

developed platforms occurred in rock with compressive strengths between 10 290 and

20 594 kN/m2. Platforms were also well developed in rock with compressive strengths

between 20 594 and 109 838 kN/m2. At compressive strengths of 186 038 kN/m2 they

were absent or incipient. In weak rock between 6450 and 10 290 kN/m2 they were

narrow, absent or incipient.

Although originally proposed by Hills (1949), Gill (1972) argued that both Type

A and B platforms were the products of an evolutionary shoreline process which

continued until an ultimate state of equilibrium was attained which was termed the

‘profile of equilibrium’. Sloping platforms (Type A) were cut into soft rocks such as

clay and siltstone or weathered basalt and granite. In contrast, no platform was

present in fresh granites, and a plunging cliff resulted. Horizontal platforms (Type B)

were cut into a group of rocks of intermediate strength. Type B platforms or

storm wave platforms were said to be in various stages of development, as was

evident from the fact that they occurred at a variety of elevations with differing

degrees of planation. Gill (1972) argued that since horizontal platforms occur in

harder rock and sea level has been at its present elevation for only about 6000 years

there has not been enough time for a sloping profile of equilibrium to have been

achieved. He proposed that a second type of equilibrium existed in platforms cut

into aeolianite and calcarenite, which were horizontal (essentially a Type B

platform) but inter-tidal. Gill considered the soluble nature of these rocks to be

important, although he never stated why. Therefore, any Type B platform in soluble

rock was considered by Gill (1972) as a profile of equilibrium, but Type B platforms

in insoluble rock were an evolutionary stage, the end result of which would be a Type

A profile. Gill and Lang (1983) also proposed that Type A and B profiles are

different stages of development towards an ultimate profile of equilibrium and

concluded that Type A and B platforms were not two distinct morphologies but

rather two stages in one evolutionary process. However, the morphological

distinction made by Sunamura (1983, 1992) and Tsujimoto (1987) is based on rock

properties, particularly compression strength. Hence, according to this approach, it

is unlikely that the Type B platforms, which occur in harder rock, evolve into a

Type A platform when this sloping morphology occurs in softer rock, i.e. they are

mutually exclusive.

More recently, Dickson et al. (2004) investigated coastal rock morphologies

(cliffs and platforms) on Lord Howe Island. They determined rock resistance using

the Schmidt Hammer test and made an assessment of discontinuities in rocks. In a

similar fashion to Tsujimoto (1987), a demarcation was found between platforms

and cliffs based on rock strength and nearshore water depth. Nearshore water depth

may be interpreted as a surrogate for wave energy, as it determines the size of waves

that break against platforms; nevertheless, the demarcation was not as strong as

Tsujimoto’s (1987), as it was complicated by anomalous platforms at the critical

threshold between cliff and platform. The work of Dickson et al. (2004) illustrates

the complex interaction between rock control and processes responsible for shaping

rock coast morphology.

In addition to the general form of platforms and cliffs, there has been international

interest in determining the controls of platform gradient, width and elevation.

Regrettably there has been almost no recent work to establish the relationship

Australian Rock Coasts 97

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between platform morphology and the process environment in Australia, despite the

Australian coast offering a suitable environment. The stability resulting from a lack

of tectonic activity during the late Quaternary and Holocene assists greatly in such

endeavours. However, deciphering the relationships between active platform

elevation and sea level is complicated by the possibility that apparently modern

platforms may have inherited morphology from previous high sea levels during the

Quaternary (Trenhaile 1987).

Outside Australia, workers have found no clear relationship between elevation

and wave energy or rock type. In fact contradictory evidence exists. So (1965),

Trenhaile (1972, 1974) and Sunamura (1978) report platforms that are higher in

mean elevation on exposed headlands than in sheltered bays. This suggests a

possible relationship between exposure to wave energy and elevation. Trenhaile

(1971) concluded that high water rock ledges in the Vale of Glamorgan were the

result of lithological control and higher stands of past sea level. Duckmanton (1974)

did not find any correlation between platform elevation and lithology on the

Kaikoura Peninsula, New Zealand. In Australia, Hills (1971) noted a relationship

between elevation and lithology: lower platforms occurred in softer rocks than

higher platforms in harder rocks. Gill and Lang (1983) did not find changes in

elevation between headlands and embayments along the Otway coast of Victoria,

where platforms were cut in greywacke, but presented few data to support this

conclusion.

In contrast, however, Sunamura (1991) used laboratory experiments and field

tests to show that elevation was linked to rock hardness and wave energy, with higher

elevations in harder rock provided wave energy remained constant. The relationship

between platform elevation and sea level is important because marine terraces are

used to reconstruct palaeo sea level and tectonic histories. Little effort has been

made to validate Sunamura’s (1991) work, but recent work on platform elevation

along the Otway coast (see Figure 2) has revealed interesting results in support of

Sunamura’s work (see Appendix A). This compared the elevation of 14 platforms

(relative to Australian Height Datum) with rock hardness using the Schmidt

Hammer test. A significant correlation between rock hardness and elevation existed

(r ¼ 0:661; p , 0:05).

Rock coast processes

Process studies on rock coasts involving wave erosion, weathering rates and

bioerosion are rare in Australia. Obviously the nature of rock coasts creates hazards

for equipment and researchers. In addition, the general lack of research activity on

rock coasts occurs at a time when process studies of beaches have exploded.

Nevertheless, in the case of shore platforms, a primary question remains over the

relative roles of marine, subaerial and biological processes in developing and shaping

platforms. Related to this is the same question applied to coastal cliffs. The diverse

range of environments that the Australian coastline encompasses offers excellent

opportunities to address this fundamental question.

Waves and rock coasts

Jutson (1939, p. 248) proposed that the origin of shore platforms was a result of the

‘sea up to the effective spray line’. The sea cut these surfaces through the process of


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‘marine abrasion’ and through ‘spray erosion’. A detailed description of how spray

erodes was not given, but it was credited with removing the products of erosion at

higher levels. In the upper portion of this zone, ‘atmospheric and marine erosion

combine to break down the rock, while the sea is the chief agent in their removal’

(Jutson 1939, p. 248). Above the spray line, platforms are formed by subaerial

decay, with the products of decay being carried away by wind, rain and gravity.

Edwards (1941) discussed ‘storm wave platforms’ along the coasts of Victoria and

Tasmania in Australia. He considered these shore platforms to be identical in nature

and mode of origin to those discussed by Bartrum (1924, 1926) in New Zealand.

Like those described by Bartrum, they occurred a ‘few feet above high tide’,

although the Australian examples appear to have greater lateral extent, with widths

up to ‘300 feet’. Edwards (1941) considered that the continued survival of these

shore platforms depended on the relative rates of retreat of the cliff backing the shore

platform which he called the ‘high tide cliff ’ and the seaward edge of the platform, or

the ‘low tide nip’ as he termed it (also known as the low tide cliff or seaward cliff).

For a shore platform to widen, the high tide cliff must retreat faster than the low tide

nip. Conversely if the low tide nip retreats faster than the high tide cliff, then

eventually the high tide cliff is overtaken and the platform ceases to exist. The

erosion of the high tide cliff was thought to occur at high tide while the low tide nip

was eroded at low tide. Edwards (1941) proposed that at high tide, larger, more

powerful waves were able to reach the high tide cliff because of an increase in water

FIGURE 2. The Otway coast from Lorne to Cape Otway. Numbered arrows indicate the locations of

platform profiles surveyed in the investigation of platform elevation.


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depth in front of the platform. These waves were also ‘armed’ with abrasive material

supplied by the eroding cliff. Based on these two factors, Edwards (1941) concluded

that the high tide cliff would inevitably erode faster than the low tide nip. Edwards

proposed that erosion of the high tide cliff was caused by waves during storms, hence

the term storm wave platforms.

Edwards (1951) added to his theory by arguing that there was ‘maximum erosion

above a defined level’; such a level existed because tides and storm waves elevated

the water level. This ‘defined level’ was that of the platform. A defined level helped

to explain the higher elevation of what are now called Type B surfaces compared

with Type A, because of the occurrence of lower Type A platforms in sheltered

embayments away from storm waves of the open coast. According to Edwards, the

role of storm waves in platform development could not be doubted on the open

coast. Edwards (1951) considered that the amount of energy delivered by waves was

an important control in platform initiation and development. He proposed that on

the open coast there was an excess of energy that elevated the level at which waves

erode. This excess energy was delivered by storm waves. He did make a concession

to the role of weathering, by suggesting that as platforms widen with age the role of

water layering increases in importance.

Waves have long been identified as causes of shore platform and cliff development.

However, such identification has always been based on interpretation of visual

observations. There are few field data of wave characteristics on rock coasts or how

waves are transformed as they impinge on rock coasts. Previously, Johnson (1919)

and Trenhaile (1980, 1983, 2000, 2002a) had proposed that wave forces are

diminished due to increasing dissipation as platforms widen over time. This is

intuitive and consistent with standard wave theories but has remained theoretical

due to the lack of field measurements. However, Stephenson and Kirk (2000)

showed that during storms significant amounts of wave energy were dissipated by

waves breaking before arriving on shore platforms. This is because larger waves

break further from shore in deeper water. Furthermore, the energy reaching the cliff

platform junction (where platform extension occurs) was reduced by five orders of

magnitude relative to that in deep water, so much so that waves were incapable of

eroding intact rock. Stephenson and Kirk (2000) also showed significant wave decay

across platforms, with only 5–7 per cent of energy at the seaward edge of a platform

reaching the cliff platform junction.

Given the lack of wave data on rock coasts, an attempt has been made to measure

waves on shore platforms under a variety of energy conditions (see Appendix B).

Two wave recorders were deployed across a shore platform at Marengo (see Figures 2

and 3) in Victoria, and the results are shown in Figure 4. The most interesting aspect

of these data was the similar wave heights at both recorder positions. The inner

recorder was placed at the foot of the cliff platform junction and showed similar and

in some cases larger wave heights than the recorder on the seaward edge of the

platform. Our results also showed that significant dissipation did occur off shore of

the platform, but very little wave decay occurred on the platform at Marengo, with

as much as 67–90 per cent of the energy arriving at the back of the platform (see

Table 1). The most likely explanation for this is the relatively narrower (35 m) and

near horizontal nature of the platform compared with those monitored by

Stephenson and Kirk (2000). The platforms studied by Stephenson and Kirk

(2000) on Kaikoura Peninsula were either 80 or 100 m wide and gently sloping

(1.58). In addition, it is likely that at Marengo wave reflection from the cliff


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contributed to the higher wave heights recorded at the cliff foot. These could readily

be observed during the deployments interacting with incoming waves to produce

momentarily higher waves. This may well explain why on the 26 June 2002 there was

more wave energy at the rear of the platform than the front.

Our recordings have interesting implications for consideration of the longer term

evolution of platforms. Our data, when compared with those of Stephenson and

Kirk (2000), offer, for the first time, field evidence to support this proposition, since

little or no dissipation occurred at Marengo and significantly more dissipation

occurred at Kaikoura on wider sloping platforms. Furthermore, it would seem that

near horizontal platforms are able to receive significantly more wave energy at the

cliff platform junction than sloping platforms where widths are comparable.

However, this energy is reflected from the cliff face and is unavailable for work at the

cliff platform junction.

While broader generalisations are difficult when based on just two small data sets,

it would appear that the two types of platforms (Types A and B) mitigate wave

energy in two different ways. This can be understood by considering wave energy in

the same way as the beach morphodynamic framework. Narrow horizontal

platforms can be considered reflective, and cliffs may well be the most reflective rock

coast morphology, while wide sloping platforms may be viewed as dissipative. While

clearly individual platforms do not change state in the way beaches do, there may be

a range of morphologies akin to intermediate beach states where platforms have a

combination of reflective and dissipative mechanisms for mitigating wave energy.

This is clearly the case at Marengo where significant dissipation occurs at wave

breaking prior to the wave arriving on the platform. Any wave energy that does

transit across the platform is largely preserved but then reflected by the cliff face.

This method of dealing with wave energy is similar to intermediate beaches that have

dissipative surf zones and reflective beach faces. Future process studies of platforms

may benefit by utilising the morphodynamic approach that revolutionised our

understanding of beaches.

Consideration of wave processes on rock coasts has also been extended to extreme

events, in the form of tsunami, and challenges the gradualist view of Australian

coastal evolution. Young and Bryant (1993, 1998), Bryant et al. (1996, 1997),

FIGURE 3. Cross-section of the Marengo platform showing locations of wave recorders.


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FIGURE 4. Significant wave heights measured on the Marengo platform. Note the preservation of wave

height, and hence energy, across the platform.

TABLE 1. Percentage of wave energy transmitted across the Marengo shore platform

Date Percentage of energy at seaward edge measured at rear of platform

25 June 2002 91

26 June 2002 104

27 June 2002 81

28 June 2002 88

24 October 2002 67

7 December 2002 91

8 December 2002 81

9 December 2002 71


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Bryant and Young (1996), Young et al., (1996), Bryant and Nott (2001) and Bryant

(2001) have all argued that rock coasts have been shaped primarily by tsunami. The

evidence for the occurrence of tsunami, such as imbricated and elevated boulders,

may have some merit, but Nott (2003a, b) and Noormets et al. (2004) have noted

the difficulty in differentiating between storm waves and tsunami deposition. Felton

and Crook (2003) have been critical of Bryant’s book on tsunami hazard and Rubin

et al. (2000) and Keating and Helsley (2002) have cast doubt on the giant Hawaii

tsunami hypothesis identified by Bryant (2001) as a significant event shaping the

NSW coast.

Bedrock sculptured features identified on platforms are much less convincing

morphological evidence for tsunami than boulder fields. Mii (1962), Kirk (1977)

and Stephenson (2000) noted that processes shaping shore platforms are usually

inferred from morphology. The inferred ‘process’ is then used to explain further

morphologies. The problem with this approach is that morphology is a notoriously

ambiguous indicator of process. More specifically, it remains to be shown

convincingly that bedrock sculptured features result from tsunami-generated flow.

Such features can also be accounted for by the interaction of weathering processes

and lithological structure. Thus research is required in Australia to determine when

and where large tsunami have occurred, and whether or not such events are capable

of the erosive work attributed to them. With respect to the first research

requirement, evidence should be sought in depositional environments such as

estuaries adjoining rock coasts by following methods outlined by Scheffers and

Kelletat (2003) and used by Lario et al. (2001), Goff et al. (2001), Williams and

Hutchinson (2000) and Dawson and Smith (2000). The erosional effectiveness of

tsunami waves may be determined through modelling or by direct observation in the

event that a large tsunami arrives on the Australian coast.

Subaerial processes

There have been relatively few dedicated studies of subaerial processes on rock

coasts, especially in Australia. However, the variety and age of the Australian

continent ensures that weathering has played an important role in shaping rock

coasts. Nott (1990) found that flow collapse and slumping were the dominated

processes causing cliff retreat at Long Beach in south-eastern Australia.

Furthermore, although waves removed basal debris during storms, material was

also washed away by drainage water from the cliff, so that cliff retreat continued in

the absence of wave action.

Nott (1994) investigated the role of subaerial processes in the development of

rock coasts in the seasonally dry tropics of northern Australia. In this setting rock

coast configuration and smaller scale morphologies (platforms and reefs) are

controlled by deep weathering processes. This involves the development of a lateritic

profile more than 30 m deep. Undulations in the weathering front correspond to the

position of cliff tops and beaches. Cliff tops coincide with bioturbated or

phosphorite beds, and beaches occur where these descend below sea level. Thus

these beds control the position of the weathering front and the configuration of the

coast for distances of some 30 km.

At a smaller scale individual platforms result from etch planation of the

weathering front where this front occurs within the marine zone. Nott (1994)

proposed that platforms developed in a two-stage process, where the first stage


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is the development of the weathering front and the second is the removal of the

saprolite. Significantly, the elevation of these platforms is not related to sea level

but rather to the level of weathering. A second type of shore platform was

identified by Nott (1994), namely an iron-encrusted platform. Of this type of

platform, two subtypes were identified: ferricrete and saprolite platforms. Each

results from erosion down to different parts of the weathering profile. Ferricrete

platforms occur where an iron indurated layer coincides with sea level, and

saprolite platforms occur where the iron indurated layer occurs well above sea

level and the underlying saprolite is eroded by marine processes. According to

Woodroffe et al. (1992) and Nott (1994), such platforms are common along the

Northern Territory coast.

Nott (1994) proposed that the etch planation he observed had not been

recognised previously. He termed this ‘land surface refraction’, in which subsurface

structural and lithological controls influence surface topography and determine the

development of coastal landforms. The significance of this work is that it is the only

study to highlight the importance of pre-weathering of landscapes prior to rock coast

initiation and the unique nature of rock coastal evolution in a tropical setting. Pre-

weathering is possible because of the age and stability of the Australian landscape in

this tropical setting. A later study by Young and Bryant (1998) also showed the

importance of modern weathering processes on the lateritic coast around Darwin.

Solution weathering of the laterite was deemed the most important contemporary

process causing platform development. The role of tsunami was also identified,

based on the deposits of large boulders on platforms. Because of their large size,

Young and Bryant (1998) rejected the hypothesis that these boulders were cyclone

deposits.

Early debates about platform genesis were concerned with the roles of marine and

subaerial processes. According to Hills (1949), storm wave platforms, as named by

Bartrum (1924, 1926) and Edwards (1941), were not a result of waves. He proposed

that these platforms, with a horizontal profile, were modified sloping platforms. This

was attributed to water layer weathering since this process produced level surfaces.

Elsewhere, Gill (1981) investigated honeycomb weathering along the temperate

Otway coast. Using seawalls constructed in 1943 and 1949, Gill (1981) showed

significant honeycomb development over the intervening period. Unfortunately the

rate at which this process occurred was not well defined. Stephenson et al. (2004)

investigated the dynamic behaviour of inter-tidal and supra-tidal coastal rock in New

Zealand and at Apollo Bay in Victoria. Using a traversing micro-erosion meter

(TMEM) they made daily observations of a bolt site and recorded both surface

lowering and rising in the order of 1.681 mm (surface rise) to 21.697 mm

(lowering). They interpreted this dynamic rock behaviour as evidence for active

weathering processes on coastal rock. There are ample opportunities to use seawalls,

gravestones and monuments to determine the effectiveness and rates of coastal

weathering around Australia using established methodologies (Mottershead 1997,

1998, 2000; Inkpen & Jackson 2000).

Bioerosion and protection

Bioerosion is little studied as a geomorphic process on Australian platforms and

cliffs despite the importance of its being recognised by Hills (1949) and more

recently by Naylor et al. (2002). Exceptions occur but such studies are usually


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ecological (e.g. Braley et al. 1991) in their focus. Besides erosion, biology on rock

coasts also serves to provide protection from other erosive processes. Hills (1949)

recognised this protective role on platforms in Victoria but no effort has since been

made to quantify the relative roles of protection and erosion either in Australia or

elsewhere. Hills (1949) also noted the relationship between algae distribution on

platforms and morphology. Since biota take advantage of platform morphology

(Schreider et al. 2003), particularly elevation, as platforms develop and lower,

biology may well play some role in determining the ultimate elevation that platforms

attain. The remarkably horizontal aeolinite platforms in Victoria, for example, are

covered extensively by Hormosira banksii, and marine and subaerial processes are

unlikely to be effective on these surfaces. Of course this algal cover does not prevent

the extension of the platform but may well hinder further lowering. Field studies are

needed to establish what role organisms play in platform evolution.

Erosion rates

Establishing the rates at which shore platforms and cliffs erode improves our

understanding of their long-term development. In addition, erosion patterns and

rates provide insight into the processes operating on platforms (Stephenson & Kirk

1998; Inkpen et al. 2004). Rates of platform development are also needed to

develop models of landform evolution. Furthermore, erosion rates can be used to

untangle the question of inheritance, that is, the possibility that some platforms

may have been developed on two or more high sea levels during the Quaternary.

Given that the Australian coast has been relatively stable through the last 0.5 ma

(Murray-Wallace 2002), inherited morphologies are likely. However, the rates at

which cliffs and platforms retreat and lower are poorly documented in Australia.

While a number of international studies have established rates of surface lowering

on shore platforms (Stephenson 2000), only one study has occurred in Australia.

Gill and Lang (1983) used a micro-erosion meter (MEM) to investigate erosion of

shore platforms on the Otway coast of Victoria in south-east Australia. These

platforms are cut in greywacke and siltstone, and are exposed to a swell wave

environment. They installed 62 measurement sites on nine profiles along a 50 km

stretch of the Otway coast. By the end of the study period readings were being

taken from only 50 bolt sites as some could not be reached because of swell waves,

sand covering some sites, or the growth of biota. The mean annual erosion rate for

the Otway shore platforms was 0.37 mm/year. The highest mean annual rate for a

profile was 0.9 mm/year and the minimum was 0.2 mm/year. Mean annual rates

between individual bolt sites ranged from 0.02 mm/year on greywacke to

1.8 mm/year on a siltstone platform.

Recent work (see Appendix C) at Apollo Bay on the Otway coast includes

measurement of erosion rates using a MEM. Ten bolts sites were established across a

platform to assess in more detail a small area of a platform than the broader scale

approach used by Gill and Lang (1983). Table 2 shows the total amount of surface

lowering and annual rates of lowering for each bolt site. Over 3.4 years an erosion

rate of 0.302 mm/year (range 0.062–0.986 mm/year) was measured. This rate

compares favourably with those from other studies (see Stephenson 2000),

including Gill and Lang (1983).

While the MEM has been applied successfully to Victorian shore platforms, it

would be useful to expand such studies to other Australian locations. However, the


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MEM does not account for larger scale block erosion common on many platforms.

Larger scale erosion may also be related to low frequency but high magnitude events.

Very large storms, cyclones or even tsunami presumably erode at the scale of blocks

and boulders. It may well be that micro-erosion represents the day-to-day

development of platforms and cliff, and block erosion occurs during high-

magnitude events. Thus there is a need to establish the relative roles of these

different scales of erosion. It follows that long-term erosion data sets are required

that utilise a variety of measurements techniques. Not surprisingly, cliff retreat is not

well documented and rates are not regularly measured, except where erosion

presents a management problem (e.g. Bird & Rosengren 1987). While aerial

photographs still offer the best coverage of the Australian coast (Fryer 1992), the

application of LiDAR (Light Detection and Ranging) to cliff retreat offers a

significant advantage in that it captures the spatial and temporal variability of

erosion (Sallenger et al. 2002; Stockdon et al. 2002).

Inheritance

Trenhaile (2002a) has argued that the most important question regarding rock

coasts is whether or not morphologies are inherited from previous high sea levels

during the Quaternary, but also noted that there is no widely accepted or clear

definition of inheritance. Despite the lack of definition, it is undoubtedly an

important issue with regard to the Australian coast, given its relative stability during

that later part of the Quaternary. A number of studies have shown that coastal rock

morphologies in Australia are indeed inherited from at least the last two Quaternary

high sea levels (Bryant et al. 1990; Woodroffe et al. 1992; Brooke et al. 1994;

Dickson et al. 2004), and in some cases even the Tertiary (Young & Bryant 1993).

Bryant et al. (1990) utilised uranium/thorium dating of Fe/Mn deposits on coastal

rocks. This approach allowed them to determine the age of rock platforms and ledges

along the Illawarra coast of NSW. Three surfaces at 2.0–3.0 m, 4.0–5.5 m and

6.0–7.0 m were dated. The 2.0–3.0 m surface is frequently washed over by waves

during storms and, as previously noted by Bird and Dent (1966), is a Pleistocene

surface and most likely formed during the last inter-glacial. The 4.0–5.5 m and

TABLE 2. Summary erosion rate data from Apollo Bay shore platform

Total (mm) mm/year

AP1A 0.541 0.158

AP2A 1.674 0.490

AP3A 3.371 0.986

AP4Aa 1.083 0.354

AP1B 0.741 0.217

AP2B 0.552 0.161

AP3B 0.354 0.103

AP1C 0.901 0.264

AP2C 0.764 0.224

AP3C 0.214 0.063

Mean 0.302

Note:a Covered in sand for mostof the study period and measured only three times, givinga record length of 3 years.


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6.0–7.0 m surfaces pre-dated the penultimate inter-glacial. Young and Bryant (1993)

also used uranium/thorium dating of Fe/Mn deposits on coastal rocks along the

southern NSW coast and similarly found that surfaces from 2 m to 30 m were

inherited features from the Quaternary, and in the case of the 30 m surface, the

Tertiary. Woodroffe et al. (1992) investigated the Quaternary history of the coastline

of Cobourg Peninsula in the Northern Territory, using uranium/thorium to date

ferricrete encrusting platforms. These were shown to have formed during previous

inter-glacials. These surfaces occurred presently in the inter-tidal zone so that earlier

sea levels must have been similar to today. The dating of these platforms has been

successful because of the encrusted surfaces providing datable material, but many

platforms are not covered with suitably datable material. However, cosmogenic

exposure dating holds much promise for further insight into the evolution of rock

coasts and particularly the question of inheritance (Cockburn & Summerfield 2004).

Dickson et al. (2004) proposed that higher platforms on Lord Howe Island were

inherited based on weaker rock surfaces than inter-tidal surfaces, when tested with a

Schmidt Hammer. However, testing surfaces with a Schmidt Hammer is problematic

since on supra-tidal surfaces weathered material is not removed continuously by

marine processes as is the case on inter-tidal platforms. Removal of weathered debris

on inter-tidal surfaces exposes unweathered rock, thus yielding higher rebound values

from the Schmidt Hammer test.

Another difficulty is that the current sea level does not coincide precisely with any

previous inter-glacial sea level, so that the degree of inheritance is difficult to discern.

It is for this reason that modern process studies are important. If we hope to untangle

the degree to which rock coasts are inherited features, we must understand how and at

what rates they develop. We must be able to determine the relationship between

platform elevation, width and gradient with sea level under formative conditions.

Of course, it is also necessary to have an accurate picture of Quaternary sea level

changes. In addition to questions of inheritance, understanding the long-term

evolution of shore platforms and cliffs will undoubtedly be advanced by numerical or

simulation modelling. To date, no attempt has been made to model the evolution

of Australia rock coasts. Such attempts have generally been limited to the work of

Trenhaile (2000, 2001a, b, c, 2002b). However, the fundamental problem

of modelling is that it is constrained by the lack of understanding of how rock

coasts develop.

Prospects

Future research directions in relation to Australian rock coasts can be categorised

broadly under three headings, within which a number of associated questions follow:

(1) Process studies: a morphodynamic approach to processes operating on rock coasts

has been advocated. If platforms are viewed as dissipative or reflective of wave

energy, or some combination, what are their associated morphologies? What is the

role of extreme events in rock coast development? How important are subaerial

and biological process of erosion relative to marine processes? What is the relative

importance of bio-protection? How effective are waves at eroding cliffs?

(2) Morphology: understanding controls of morphology (gradient, elevation and

width), particularly interactions of rock properties and processes, is vital to enable

palaeo-reconstruction of sea level and to model platform development.


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(3) Modelling rock coast evolution: this is required to understand the long-term

evolution of such landforms and the role of inheritance. In Australia this is

simplified by the absence of rapid tectonic activity but complicated by the

inheritance from earlier inter-glacial sea levels. Such models will be successful only

if there is a better understanding of process-form relationships.

Acknowledgements

Special thanks to Gerald and Catherine Nally, Andrew Sharpe, Ben Davidson,

Danielle Inwood and Karen Stephenson who assisted with fieldwork at Apollo Bay.

Thanks to Joanne Greenwood and family who make their house at Marengo available.

Bruce Thom and Andy Short offered helpful comments on the original draft of this

paper. The comments of two anonymous reviewers also improved the manuscript.

Gerd Masselink first used the term dissipative to describe shore platforms at Kaikoura,

and it is from this comment that some of the ideas in this paper were developed.

Financial support was received from the University of Melbourne Careers

Establishment Grant Scheme and continued support has been received from the

Melbourne Research Development Grant Scheme (Grant 424925).

Correspondence: Dr Wayne Stephenson, School of Anthropology, Geography and

Environmental Studies, University of Melbourne, VIC 3010, Australia. E-mail:

[email protected]

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Appendix A: rock hardness and platform elevation

Introduction

Platform elevation in relation to mean sea level (M.S.L) is determined by a number of

factors, namely wave energy, weathering, tidal range, and lithology including: rock

strength, joints, faults and bedding planes (Sunamura 1991). Edwards (1941), Gill

(1967), Hills (1971) and Gill and Lang (1983) have noted a relationship between rock

hardness and platform morphology which, with the exception of Edwards (1941), was

reported qualitatively. Trenhaile (1978) concluded that the elevation of the platforms at

Gaspe was determined by processes operating on the platform and not rock hardness.

Sunamura (1991) undertook a laboratory investigation into the influence of wave

height and rock strength on the elevation of a horizontal plaster model, Type B, shore


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platform (see Figure 1). It was found that elevation increases with increased rock

strength if all other factors are constant. This conclusion is problematic in that it is

based on only six data points so that field testing is required to confirm this relationship.

In an attempt to model whether shore platforms are inherited or contemporary

features, Trenhaile et al. (1999) tested the morphological difference of nine platforms

found in three separate bays in north-western Spain. ThirtySchmidt Hammer readings

were taken at ‘quasi-regular’, intervals along surveyed profiles of shore platforms. It was

concluded that ‘variations in platform morphology cannot be attributed to differences

in rock hardness, as determined by the Schmidt Rock Test Hammer’, (Trenhaile et al.

1999, p. 613). However, no statistical test was applied to determine whether a

relationship was indeed absent.

Although many factors may have some control on elevation, the true effect of rock

hardness remains unconfirmed. What determines the elevation of a shore platform is

also important since marine terraces are used to reconstruct palaeo sea level and

tectonics. The quantification of a relationship between rock hardness and elevation

must be tested extensively in the field either to verify the relationship presented by

Sunamura (1991) or the conclusion of Trenhaile et al. (1999).

Study site

The Otway coast (see Figure 2) is composed of non-marine Lower Cretaceous

sandstones and mudstones (Gill 1977). Of particular interest to this study is the

section between Cape Otway and Eastern View which trends north-east for 60 km

(see Figure 2). This stretch of coast is notable for extensive shore platforms of

various elevations (Gill & Lang 1983). The morphology of the coast is controlled by

geological structures, such as the dips of bedding, faults and jointing. Shore

platforms occur extensively, although sandy bays and bay heads interrupt their

continuity. The Otway coast has a Mediterranean climate and it is characterised by a

microtidal environment; with a mean tidal range of 1.05 m at Apollo Bay. High-

energy storm waves and a strong south-east oceanic swell occur because of exposure

to the Southern Ocean (Gill & Lang 1983).

Methods

Shore platforms between Apollo Bay and the Lorne area were selected for this study

because wave energy, climate and tidal range were essentially the same and based on

the availability of benchmarks relative to the Australian Height Datum (A.H.D.).

Sites were selected on the basis that they were accessible and could be surveyed

relative to a benchmark. A total of 14 profiles across platforms were surveyed using a

total station. The Schmidt Hammer methodology has been described by Day and

Goudie (1977). Schmidt Hammer readings were recorded and later averaged for

each survey point along the profile using a Type L hammer. A total of 4180 reading

were recorded, yielding 209 averages. The mean rebound value for each survey point

was then used to calculate the mean for the whole platform.

Results

Pearson’s correlation with a 2-tailed significance test was used to determine whether a

significant correlation existed between elevation and rock hardness. A significant


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correlation was found to exist between all elevation readings and all rebound numbers

(r ¼ 0:47; p , 0:01). Higher Schmidt Hammer values related to higher elevations.

An even higher correlation was found between mean platform elevation and mean

Schmidt Hammer values (r ¼ 0:66; p , 0:05) when aggregated for each profile.

Appendix B: wave energy on shore platforms

Introduction

A long-standing issue in platform studies has been whether platforms are wave cut or

weathered. Stephenson (2000) noted that the wave cut view has dominated thinking

in the last 20 years despite there being no clear demonstration that waves are

erosional on platforms. However, a more useful question is: what are the relative

roles of marine, subaerial and biological processes in developing and shaping

platforms? Such a question recognises that the development of platforms is

dependent on the interactions of three main processes and the degree of interaction

varies significantly from one environment to another. In order to address the above

question there is a need to quantify the erosional effectiveness of all those processes.

Very few attempts have been made to quantify the erosive capability of waves and

weathering rates. A number of studies exist that quantify bioerosion (e.g. Andrews &

Williams 2000) but the relative significance of this compared with wave action and

weathering remains unknown. Despite these difficulties, a number of numerical

models for shore platforms have been developed (Sunamura 1983; Trenhaile 2000,

2001a, b, 2002b). These models rely on deep-water wave data as input variables and

link platform erosion with that variable without accounting for shallow water

transformation across complex nearshore topography. Usually, transformations are

made using linear or Stokes wave theories over simple topography. Such models

could be improved by better understanding how waves are transformed as they

arrive on and shoal across platforms. In order to understand the erosive capabilities

of waves on shore platforms it is first necessary to describe and characterise wave

transformation from offshore onto, and then across, platforms. Here we present

results from an investigation of the wave transformation on shore platforms

measured under a variety of wave conditions along the swell storm wave coast of

Victoria, south-eastern Australia (see Figure 2).

Study site

Marengo is located 185 km south-west of Melbourne (see Figure 2). The study site

is located 100 metres west of Haley Point (Marengo) and is characterised by a 30 m

wide and flat (Type B) shore platform developed on grey arkosic sandstones (see

Figure 2). The seaward edge of the platform presents an abrupt low tide cliff,

dropping about 1.5 m; beyond, a lower platform extends another 30 m further

offshore. At the rear of the platform is a cliff 3.5 m in height.

Methods

Two wave recorders were deployed across shore platforms on eight separate

occasions (see Figure 3). Deployment times were determined by high tides so that


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waves were recorded as long as the recorder was submerged. However, high tides are

not always sufficiently high to submerge platforms. Based on observations and a

review of tide predictions, on occasions the Marengo platform is subaerial during

neap tides. Therefore, the opportunities for wave recording with the highest high

tide, and therefore deepest water over the platforms, are limited to the spring tides

2.0 m above chart datum, which occurred only four times in 2002 (see Table A1).

Lower spring tides (1.8 m and 1.9 m above chart datum) also cover the platforms

but obviously the water over the platforms is not as deep. Higher spring tides were

targeted for wave recording. In 2002 three deployments occurred in June for 4 days,

in October for 1 day and December for 3 days. In June, high spring tides coincided

with a significant storm event, while more moderate seas occurred in October and

December. Regrettably no deep-water wave data were available so observations of

offshore breaking wave height were made from the shore.

Results

Figure 4 shows measured significant wave heights for the eight deployments. The

figure also contains the visually estimated wave breaking height offshore of the

platform for each deployment. During the 4 day June deployment, breaker heights at

high tide were estimated to be 5 m for 25 June, decreasing to 3 m on 26 June, but

rising again on 27 and 28 June to 4 m and 5 m, respectively. Over these 4 days the

largest significant waves recorded on the seaward edge of the platform were 0.76 m

while at the rear of the platform Hs ¼ 0.80 m (see Figure 4). On 24 October 2002

offshore breaking heights were estimated to be 1.5 m, on the outer edge of the

platform Hs ¼ 0.35 m and at the back Hs ¼ 0.17 m. December 2002 breaking

waves were estimated to be 2—3 m and on the platform Hs ¼ 0.59 m on the outer

edge and 0.53 at the rear.

Appendix C: micro-erosion measurements on shore platforms

Introduction

There have been relatively few attempts to quantify either the processes or the rates

of morphological change on shore platforms. Most studies are restricted by the time

period they cover, typically 2 years, and the relatively small number of studies means

that there is restricted geographical coverage, therefore necessitating further

measurement of shore platform erosion rates.

TABLE A1. Spring tide elevations at Apollo Bay and number of times each occurred in 2002

Tide elevation above chart datum (m) Months in 2002 (number of tides)

2.0 January (2); October (2); November (4);

December (2)

1.9 February (3); April (3); May (3);

June (4); July (3); August (3);

September (2)

1.8 March (5)


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Methods

A micro-erosion meter (High & Hanna 1970) was used to investigate erosion rates

on shore platforms at Apollo Bay, Victoria. Ten bolt sites were established in 2000 in

a grid measuring 20 m £ 65 m (see Figure 2), the greater length being the width of

the platform. Across the platform, bolts sites were located at the seaward edge of the

platform, the middle and at the cliff platform junction. This pattern was intended to

assess in more detail a small area of a platform than the broader scale approach used

by Gill and Lang (1983). After installation, one bolt site at the back of the platform

was covered over by sand and only periodically exposed. The last time was 3 years

after it was installed. While detailed analysis of data from these bolts will be the

subject of a future paper, we are able to report erosion rates from 3.4 years of

monitoring.

Results

Mean annual erosion rates for each bolt site and the grand mean for all bolts are

presented in Table A1. The grand mean is 0.302 mm/year with a range from 0.062 to

0.986 mm/year.


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Documents

Australian Rock Coasts: review and prospects