Rock coasts, with particular emphasis on shore platforms

Geomorphology 48 (2002) 7 – 22 www.elsevier.com/locate/geomorph

Rock coasts, with particular emphasis on shore platforms Alan S. Trenhaile * Department of Earth Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4 Received 23 November 1999; received in revised form 15 March 2000; accepted 24 January 2002

Abstract Rock coasts are one of the most common elements of the world’s littoral zone, and they are often important sources of sediment for estuaries and beaches. Economic development, growing populations, and the potential effects of rising sea level provide a practical need to understand the dynamics of these coasts. Although there have been significant advances in our understanding of rocky coastal systems, further progress has been hindered by a lack of researchers and by the often imperceptible changes that generally occur within human time scales. This paper reviews rock coast processes and landforms in a variety of morphogenic environments. One of our most fundamental challenges is to determine the degree to which rock coasts are contemporary rather than inherited features from the Quaternary, when changes in sea level and climate were responsible for marked variations in the nature, intensity, and elevational distribution of the marine and subaerial processes that sculpture rock coasts. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rock coast; Shore platform; Morphogenic environment

1. Introduction The term ‘‘rock coast’’ is used in this review to refer to coasts that have rocky substrates, often, but not necessarily, in the form of cliffs and shore platforms. Cohesive clay coasts are not considered in this review, although they have many of the characteristics of weak rock coasts (Trenhaile, 1997). Although rock coasts generally lack large quantities of sand and other sediment, there are often coarse clastic or sandy beaches in the embayments and rock cliffs and platforms on the headlands of indented coastlines. Platforms covered by beach sediment are not considered in this paper if there is sufficient material to prevent *

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wave erosion of the underlying bedrock and the cliff behind. This paper is primarily concerned with the processes operating on rock coasts and with their modes of development, with an emphasis on shore platforms and whether these coasts are essentially contemporary or inherited features. For more detailed discussion on the wide range of processes and landforms that characterize rock coasts the reader is referred to the texts by Trenhaile (1987) and Sunamura (1992). The emphasis of most modern research is on beaches, salt marshes, and other economically important coastal features that change fairly rapidly and are potentially vulnerable to rising sea level. The lack of research on slowly changing rock coasts, however, belies their significance as one of the most common elements of the world’s littoral zone and as an

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X ( 0 2 ) 0 0 1 7 3 - 3

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important source of sediment for estuaries and beaches. Housing, transportation, recreation, and other economic activities are increasingly providing a practical need to understand rock coast dynamics and evolution, but although there have been some significant advances, progress has been hindered by the very low number of active researchers in this area. The lack of rapid progress in elucidating the evolution of rock coasts can also be attributed to the inherent complexity of rocky coastal systems. Whereas there are only limited variations in the substrates of sandy beaches, tidal marshes, and sand dunes, for example, the physical resistance of rocks varies enormously according to such factors as their chemical composition, angle of dip, strike, bed thickness, joint pattern and density, degree of weathering, and a myriad of other factors. Sandy beaches are largely controlled by waves and tidal variations in water level. Tidal marshes are controlled by tidal currents. Dunes depend on aeolian processes. Rock coasts, however, experience the effects of wave action, tidal variations, bioerosion, frost, chemical weathering, salt weathering, wetting and drying, mass movements, and other mechanisms. Furthermore, the importance of these mechanisms varies with climate and with the characteristics of the substrate.

2. Rock coast processes and evolution Modern analytical techniques, geochronometric dating, mathematical modelling and the careful measurement of processes and erosion rates have improved our ability to identify and quantify rock coast processes, but we are still largely ignorant of their precise nature or relative importance. It is difficult to acquire quantitative data on rock coast processes because of the imperceptible changes that generally occur within human lifetimes and the importance of high-intensity, low-frequency events. Rock coasts are also exposed and often dangerous environments for wave measurement and subaqueous exploration, and there is frequently a lack of access to precipitous or heavily vegetated cliffs. The nature and intensity of marine and subaerial processes have changed through time, with variations in relative sea level and climate, and rock coasts often retain vestiges of environmental conditions that were quite different from today. There

has therefore been only very slow progress in evaluating the relative and absolute contributions of the erosive mechanisms that operate on rock coasts, and their relationship to prevailing conditions. 2.1. Mechanical wave erosion vs. weathering Much of the century-old debate on the development of shore platforms has been concerned with whether they are primarily the product of marine or subaerial processes (Trenhaile, 1980, 1987). Weathering reduces rock resistance in the vigorous storm wave environments of the middle latitudes, particularly in sheltered areas. On the exposed coast of southern Wales, for example, the compressive strength of unweathered intact limestone is 123 MN m
2, compared with only 1.36 MN m
2 when it is weathered and jointed (Williams et al., 1993). It is generally accepted, however, that mechanical wave erosion is most important in the development of the gently sloping platforms (gradients of up to a few degrees) in the North Atlantic and other vigorous wave environments (Everard et al., 1964; Trenhaile, 1972, 1978; Sunamura, 1973, 1977). In Australasia and other warm temperate and tropical environments, where quasi-horizontal platforms terminate abruptly seawards in low-tide cliffs, mechanical wave erosion has often been accorded a secondary role to subaerial weathering (Bird and Dent, 1966; Stephenson and Kirk, 2000a,b). Nevertheless, there is no consensus on the way in which horizontal platforms are produced by weathering, and it should be noted that most weathering theories accord a major role to mechanical wave erosion in the removal of weathered rock (Trenhaile, 1987). Weathering theories for the development of quasihorizontal shore platforms are dependent on the occurrence of a saturation level within the intertidal zone, below which the rocks are permanently saturated with seawater. There is assumed to be an abrupt transition from the oxidation zone above the saturation level, where the rock is weathered and weak, into the saturation zone below the saturation level, where the rock is largely unweathered and resistant. Horizontal platforms are then considered to develop at the saturation level as the weathered rock is either washed away by weak waves in sheltered areas (Bartrum’s, 1916 ‘‘Old Hat’’ platform) or differentially eroded by

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stronger waves in more exposed areas (Edwards, 1958; Gill, 1967; Bradley and Griggs, 1976; Sunamura, 1978a). The presence of an intertidal saturation level has been a basic tenet of the influential Australasian literature on rock coasts since almost the beginning of the last century (Bartrum, 1916). However, laboratory and field measurements suggest that the water content of intertidal rocks varies according to the time of exposure and inundation during tidal cycles and gradually increases from the high to the low tidal level (Trenhaile and Mercan, 1984). This suggests that rocks can only be permanently saturated below the low tidal level, where they are constantly submerged, and that there is a gradual transition, rather than an abrupt change, in the degree of weathering within the intertidal zone. Another of the arguments used in support of the weathering origin of quasi-horizontal Australasian platforms can now be refuted. It has been proposed that the lack of horizontal platforms in the storm wave environment of the North Atlantic demonstrates that waves operate, according to wind and tidal conditions, over too great a range of elevations to cut quasihorizontal shore platforms (Hills, 1971). Whereas sloping platforms are generally found in the macrotidal regions of the North Atlantic, however, there are quasi-horizontal platforms in micro- to mesotidal environments in eastern Canada. The gross morphology of these Canadian platforms is indistinguishable from those in Australasia, where there is a similar tidal range. The occurrence of platforms with a seaward gradient of up to 4.5j in the Bay of Fundy in eastern Canada, where the spring tidal range is more than 14.5 m, provides further support for the role of tidal range in determining platform morphology (Trenhaile, 1978, 1987, 1999). 2.2. The role of tidal range Most of the traditional debate on the origin of shore platforms emphasized the role of climate and wave regime, and it was largely conducted in almost complete ignorance of the fundamental role of tidal range. It has been found that there is a moderately strong positive correlation in many areas between mean regional platform gradient and the spring tidal range (Trenhaile, 1987, 1997, 1999) (Fig. 1). Mechanical wave erosion takes place through water hammer, the

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Fig. 1. The relationship between shore platform gradient and tidal range. Each point represents the local mean of a large number of surveyed profiles.

generation of high shock pressures by breaking waves, and, probably most importantly in most rock types, by air compression in joints and other crevices. These processes occur in the zone where air and water alternate and they therefore operate only in a narrow zone above and below the fluctuating waterline. Although abrasion by rock fragments, pebbles, or sand is not as closely associated with the water level, its efficacy rapidly decreases below the waterline (Robinson, 1977a; Trenhaile, 1987, 1997). Most mathematical models suggest that waves also exert the greatest pressures at, or slightly above, the mean water surface (Trenhaile, 1987). As tides control the elevation of the mean water surface and the degree to which wave erosional processes are concentrated within the vertical plane, they must therefore also determine where, and to what degree, wave erosion takes place. The mean water surface most frequently occupies elevations that are at, or close to, the mean high and low water neap tidal levels. Frequencies are about one-third lower at the mid-tidal level and they decrease very rapidly from the neap to the spring high and low tidal levels (Carr and Graff, 1982). The occurrence of the mean water surface is increasingly concentrated between the mean high and low water neap tidal levels as the tidal range (Tr) decreases. Although tidal duration distributions

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suggest that waves most frequently operate between the mean high and low water neap tidal levels, however, the strength of the waves varies according to weather conditions, bottom gradient, and the depth of the water. 2.3. Models of platform development The first attempts to model the evolution of slowly changing rock coasts were qualitative and structured within a cycle of erosion (Davis, 1896; Johnson, 1919; Challinor, 1949). The moderate correlation that exists between platform gradient and tidal range and the feedback effect of changing platform width and gradient on wave energy and erosion rates suggest that platforms attain a state of equilibrium through a balance in the rates of erosion at the high and low tidal levels (Edwards, 1941; Bird, 1968; Trenhaile, 1972). Mathematical models have provided further support for the equilibrium concept (Sunamura, 1978b; Trenhaile, 1983, 2000, 2001a, 2001b; Trenhaile and Layzell, 1981), and it has been suggested that submerged platforms on the consolidated clay coasts of the lower Great Lakes also maintain their shape as they shift landwards (Bishop et al., 1992). Although mechanical wave erosion is largely accomplished by processes that are closely associated with the water level (Sanders, 1968; Robinson, 1977a; Trenhaile, 1987), most models have been concerned with submarine erosional processes in tideless seas (Flemming, 1965; Horikawa and Sunamura, 1967; Scheidegger, 1970; Sunamura, 1976, 1977, 1978b). Several models have considered the effect of mechanical wave erosion at the high and low tidal levels (Trenhaile, 1983, 1989; Trenhaile and Byrne, 1986). Other models have incorporated the tidal duration concept into models that examine intertidal erosion and the evolution of shore platforms under constant and changing sea level conditions (Trenhaile, 1983, 2000, 2001b, 2001c; Trenhaile and Layzell, 1981). These models provide further support for the contention that mechanical wave erosion can produce quasihorizontal shore platforms in low tidal range environments, and for the presence of a direct relationship between the regional mean gradient of intertidal shore platforms and the spring tidal range. Modelling has also suggested that weathering plays a significant role in influencing the gradient, width,

and other aspects of platform morphology, and in determining rates of platform development (Trenhaile, 2001a). In general, however, even severe weathering, which reduced the strength of the rocks at the high tidal level by 75% in the model, only played a secondary role in platform development. In 53% of the model runs, for example, the introduction of severe weathering conditions increased the equilibrium width of unweathered platforms by less than 20%. The model therefore suggests that the morphology of platforms that are cut by waves that are powerful enough to erode severely weathered rocks is largely determined by tidal control of the distribution of wave energy within the intertidal zone. This is because any portion of a shore platform consisting of weaker rock than other portions of the platform is eroded more rapidly, and it becomes wider and gentler sloping. This, in turn, reduces the rate of erosion. Therefore, although weathering reduces rock resistance and facilitates wave erosion, its influence is limited by the negative feedback relationship between platform gradient and rates of wave and surf attenuation. Static equilibrium was attained in model runs when, because of gentle bottom gradients, the wavegenerated forces become too weak to continue eroding the weathered rock. Platforms could continue to evolve, however, if fine-grained, weathered material was carried away in suspension. Lowering of the upper portions of the platform surface by water layer levelling and other weathering processes would then periodically allow renewed wave erosion at the cliff base, until a gently sloping or horizontal platform was produced at the low tidal level, below which the rocks are permanently saturated with seawater. Although weathering assists wave erosion in more exposed areas, the model indicated that it was of more fundamental importance in situations with resistant rocks in weaker wave environments. Equilibrium platform width was more than 40% greater with severe weathering conditions than with no weathering in about 30% of these runs (Trenhaile, 2001a). Weathering may therefore be a dominant influence on the development of narrow shore platforms in resistant rocks in sheltered environments. It is conceivable that platforms can only develop in rocks that are severely weathered in environments that are less conducive to effective wave action than were considered in the present study. In such cases, the role of the weak

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waves would be essentially limited to removing the weathered debris. This conclusion is consistent with some elements of Bartrum’s (1916) Old Hat theory, although there is no evidence that this type of platform is associated with a permanent level of saturation within the intertidal zone. In such cases, where waves wash away fine weathered debris, the morphology of the resulting platform could be independent of tidal and wave conditions. Although the relationship between platform gradient and tidal range, and modelling incorporating tidal duration distributions, suggest that the gross morphology of shore platforms is the product of wave erosion, this cannot be corroborated with field data. Wave recorders need to be set up along rocky coasts to record simultaneous differences in the wave energy in bays and on headlands. Wave data are also needed on shore platforms along profiles normal to the cliff, to determine rates of attenuation across smooth and uneven platform surfaces in relation to rock strike and dip, platform gradient, width, and other aspects of platform morphology, Wave data would also show variations in wave conditions with tidal stage. Sunamura’s (1992) evolutionary model attributed the development of horizontal or gently sloping shore platforms to the type of breaking wave and to the relationship between wave height and the compressive strength of the rock. The ‘‘relative strength of the rock’’ is only one of the many factors that must be considered, however, to account for global and regional variations in platform morphology. Trenhaile (2000) used a mechanical wave erosional model derived from basic wave equations to explore the interaction among wave dynamics, tides, coastal morphology, and the evolution of wave-cut shore platforms under constant mean sea level conditions. There were complex, multivariate relationships among platform morphology and the geological, topographical, and morphogenic variables considered in the model. It was found, for example, that simulated equilibrium platform width increased with tidal range and decreased with rock resistance, roughness of the platform surface, amount and persistence of the cliff-foot debris, and wave period. Because higher waves break in deeper water than smaller waves, and therefore lose much of their energy before they reach the shoreline, there was no consistent relationship between simulated platform width and wave height. The model

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indicated that platform evolution reflects the interaction of a multitude of interrelated and independent variables, which cannot be represented by a few, fairly simple mathematical expressions. 2.4. Recent developments and continuing problems Despite slow progress in understanding the evolution of rock coasts, there have been some interesting recent developments. For example, the principles of rock mechanics are being applied and incorporated into simulation models to determine failure mechanisms and to account for erosion rates and variations in the form of coastal cliffs. These can also be used to account for the formation and shape of marine caves (Davies and Williams, 1986; Allison and Kimber, 1998; Belov et al., 1999). In southern Wales, for example, it has been shown that translation failures commonly result from the formation of tension cracks near the cliff face and toppling where there are tension cracks and basal undercutting. Translation failure is dominant where mudstones are prevalent at the cliff base, but toppling is more common where limestones provide a fulcrum at the foot of the cliff. The safety factor is inversely related to the ratio of the depth of undercutting at the cliff base to the distance of the tension crack from the cliff face (Williams et al., 1993). Numerical modelling, based on a range of geotechnical parameters, has also been used to predict the most likely modes of failure in the cliffs of southwestern Wales (Davies et al., 1998). There have also been improvements in our ability to collect field data. Several workers have designed instruments to measure and record the roughness of rock surfaces (McCarroll, 1992, 1997; Whalley and Rea, 1994; McCarroll and Nesje, 1996). Surface roughness can provide an indication of the degree of weathering, and possibly, in some circumstances, a measure of relative age. Although roughness-measuring instruments have not yet been used on shore platforms, they have potential applications in the measurement of variations in weathering intensity with elevation relative to the tides. These could also possibly be used to compare the relative age of different parts of a platform surface to determine whether, or to what degree, the platform is contemporary or inherited. The Schmidt rock test hammer is another useful field instrument that provides a simple

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measure of rock strength or durability, which can be correlated with compressive strength, in the field. It can be used to assess rock strength for comparative purposes between sites, and to measure changes in mechanical strength as a result of weathering (Trenhaile et al., 1998, 1999). Micro-erosion meters (MEMs) have been used to measure rates of platform erosion in several regions (Kirk, 1977; Robinson, 1977a,b; Gill and Lang, 1983; Mottershead, 1989; Stephenson and Kirk, 1998). Stephenson (1997) fitted a traversing MEM, capable of making numerous measurements at each site, with a digital dial gauge connected to a laptop computer to allow the data to be logged directly into a spreadsheet. This important development allows far more data to be collected and processed than was previously possible using manual instruments. Nevertheless, although MEMs are invaluable for measuring contemporary rates of platform lowering, one must clearly understand the nature of their limitations. Although MEMs can determine rates of platform lowering, the responsible processes must be inferred from the spatial and temporal characteristics of the erosional and morphological data. MEMs monitor platform lowering through abrasion and weathering, but they cannot measure the effect of wave quarrying or frost riving of large rock fragments and joint blocks. Modelling has demonstrated that wave-and-surf zone energy is increasingly dissipated in crossing shore platforms as they become more gently inclined and trend towards an equilibrium state (Trenhaile, 2000, 2001a, 2001b). Weathering must therefore become more important as platforms become wider and more gently sloping. Consequently, assessments of the relative importance of the formative processes cannot be based on contemporary measurements of erosion rates within very small portions of the platform surface. Because of these limitations with the MEM, it is impossible to accept Stephenson and Kirk’s (1998) contention, based on MEM data, that the shore platforms of the Kaikoura Peninsula in New Zealand are the product only of weathering processes, especially as there are fresh rock scars and large, angular blocks of debris on these platforms. This is not, of course, to deny that weathering is possibly the dominant process in Kaikoura or on similar platforms today, or that weathering-induced reduction in rock strength was

possibly an essential precursor to platform development in this area. Although there has been significant progress in some areas, because of the complex interaction between the processes, which vary according to the nature of the substrata and climatic conditions, we may always have to rely, at least in part, on the interpretation of sometimes ambiguous field evidence. For example, the importance of wave quarrying is usually assessed according to the occurrence of fresh rock scars and coarse, angular debris consisting of joint blocks and other rock fragments. Although there are other possible interpretations (Gill, 1972a), the occurrence of residual ramparts or ridges at the seaward edge of platforms has been considered by some workers to be indicative of platform lowering by weathering processes that operate around the margins of rock pools. These processes are collectively referred to as ‘‘water layer levelling’’ (Bartrum and Turner, 1928; Sanders, 1968; Bird and Dent, 1966; Hills, 1971; Takahashi, 1977). A more recent example of our reliance on ambiguous field evidence concerns the possible effect of tsunamis on coastal regions near their source and in tectonically stable regions further away. Bryant et al. (1992) found evidence of catastrophic wave erosion on rock ramps up to at least 15 m above present sea level in southeastern Australia, and boulders have been tossed onto shore platforms and jammed into crevices. The occurrence on shore platforms of cemented cobble sheets and V-shaped grooves, broad potholes, flutes, transverse troughs, sinuous grooves, sichelwannen- and cavetto-type forms, and other bedrock erosional features that are similar to glacial p-forms, have been considered to be indicative of high velocity flows generated by tsunamis (Bryant and Young, 1996; Young and Bryant, 1998; Aalto et al., 1999). Although these features have been identified in areas such as California and Australia that are known to have experienced repetitive tsunami events, many of the erosional features do not appear to be markedly different from those found on platforms that do not experience tsunamis. Detailed measurements will be needed to determine whether there is a significant difference in the erosional bedrock features on tsunami and nontsunami coasts, and whether these features can be attributed to tsunami events, or if they are simply a reflection of structurally or chemically controlled wave erosion and weathering.

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2.5. Quantification Japanese scientists have attempted to quantify some relationships between rock coasts and a limited number of geological and morphogenic factors. Horikawa and Sunamura (1967) and Sunamura (1973) related rates of cliff erosion (dx/dt) near Tokyo to the compressive strength of the rock at the cliff base (Sc). According to Sunamura (1977): dx=dt a½1nðqgH=Sc Þ þ C

ð1Þ

where q is the density of seawater, g is the acceleration due to gravity, H is the wave height at the cliff base, and C is a dimensionless constant. Although this attempt to relate the erosive ability of the waves to their height and the resistance of the rocks to their compressive strength represents a commendable effort to quantify some aspects of coastal erosion using variables that can be measured in the field, many other factors need to be considered before we can derive a equation that will reliably predict rates of wave erosion in different areas. Although compressive strength is fairly easy to measure, the ability of waves to erode rocks depends less on the strength of the rock than in the exploitation of joints, bedding planes, and other structural weaknesses. Representations of a rock’s resistance to wave erosion by compressive strength can therefore provide results that are contrary to the field evidence. For example, variations in the compressive strength of the rocks along the coast of southern England is countered by fracture patterns that result in faster cliff erosion in rocks that have the greatest compressive strength (Allison, 1989). Sunamura (1982, 1992) also derived an equation for the minimum height of a wave that is able to erode the base of a cliff: Hcrit ¼ ðSc =qgÞe
c ;

ð2Þ

where c, a nondimensional constant, is equal to 1n(A/ B). The value of A may reflect the occurrence of beach sediments acting as an abrasive and B may reflect the discontinuities in the cliff material. Many of the other factors that need to be considered, including the effect of rock structure, the presence, mobility, and quantity of the cliff-foot deposits, and whether they function as protective or erosive agents, may prove to be essentially unquantifiable.

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In Tsujimoto’s (1987) model, sloping platforms develop where downward erosion eliminates the low-tide cliff—where the ability of the waves to degrade the platform surface is greater than the ability of the rocks to resist. The compressive strength of the rocks was used to represent the resisting force and wave pressure to represent the assailing force in order to derive an expression for the transition from horizontal to gently sloping platforms in Japan. This occurs when: so > 0:005Ss

ð3Þ

where so is the wave-induced shear stress on the bed and Ss is the shear strength, including the influence of discontinuities in the rock. Although Tsujimoto’s rock strength equation may help to account for the occurrence of horizontal and sloping platforms within regions with similar morphogenic conditions (tides, waves, climate, etc.), it cannot explain the general occurrence of sloping platforms in macrotidal, storm wave environments. Nor can it explain the quasihorizontal platforms in micro- and mesotidal, warm temperate environments.

3. Inheritance One of our most important challenges is to determine the degree to which rock coasts are contemporary rather than relict features. Because of very slow rates of change, it is difficult to measure and observe coastal evolution, and rocky coasts may consist of a variable combination of essentially contemporary and inherited elements. The problem of determining whether or to what degree rock coasts are a legacy of glacial or interglacial stages when sea level and climate were different or similar to today’s is further compounded by the fact that these erosionally dominated features usually lack datable sediments. 3.1. Quaternary sea level and climate Quaternary changes in sea level and climate were responsible for marked variations in the nature, intensity, and elevational distribution of the marine and subaerial processes that sculpture rock coasts. During the Quaternary, cold, dry, and windy glacial maxima alternated with warm, wet interglacials, and there

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were associated changes in fauna and flora. Temperatures in the glacial maxima were about 2– 3 jC lower than today on a worldwide basis. Because of an apparent shift in the position of the Gulf Stream, however, temperatures were from 12 to 18 jC lower in the North Atlantic, and even greater cooling was experienced in coastal areas near ice sheets (Goudie, 1983). Although sea surface temperatures were lower than today in most areas, they were similar or slightly warmer in some equatorial regions during the last glacial maximum. In the North Atlantic, cold polar water flowed much further south than it does today, and cold water also flowed westwards across the equatorial Pacific and Atlantic Oceans. Sea ice was therefore more extensive in the glacial stages than today, and in winter, pack ice extended as far south as Spain and New York. Isotopic deep-sea records suggest that sea level was lower than today in interglacial stages 7, 13, 15, 17, and 19, and similar to today in stages 5e, 9, and 11 (Shackleton, 1987). It is therefore unlikely that eustatic sea level has been more than a few meters higher than at present during the last 2.5 million years. Quaternary changes in sea level and climate, and in some places glaciation, have had a profound effect on the development of rock coasts. The sea was from about 80 – 200 m lower than today during glacial stages (Trenhaile, 1987), and the main effect of glacial stage climates on inherited interglacial platforms and cliffs was therefore subaerial rather than marine. The isotopic record from deep-sea core V28-238 (Shackleton and Opdyke, 1973) suggests that, in the last 600,000 years, mean sea level was within modern

macrotidal intertidal zones for only about 60,000 years and within microtidal intertidal zones for about 30,000 years (Trenhaile, in press). Therefore, the modern shore was exposed to subaerial conditions during glacial stages for much longer than it was attacked by waves during interglacial stages; there were corresponding changes in climate with sea level variations in mid to high latitudes, but changes in temperature were less significant in tropical regions. Although the effect of lower temperatures during glacial stages in extra-tropical regions may have been partly offset by the occurrence of drier conditions, abandoned interglacial platforms and cliffs must have experienced severe frost shattering and periglacial mass movement. Wave erosion would also have been very effective during the early stages of interglacials and interstadials, when sea level was still rising, the inherited platforms were severely frost shattered and the foot of former cliffs were likely covered in scree and other loose debris. 3.2. What is inheritance? The term ‘‘inheritance’’ has not been precisely defined in the geomorphological context used here, and it can mean different things to different people. Obviously, inheritance did not occur if the rock – low tide interface, at the time that the sea first reached its present level, was located landward of the position of the cliff base during the last interglacial (Fig. 2) (Trenhaile, 1989). The term could be applied to all other situations, where the rock –low tide interface was initially seawards of the interglacial cliff base.

Fig. 2. Simulated platform development during glacial and interglacial cycles (Trenhaile, 1989). Note that, according to this model, the contemporary platform was not inherited from the platform formed in the last interglacial stage.

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Some workers might consider that inheritance had occurred if a platform developed from an interglacial surface that was several meters higher, whereas others might consider it to be a contemporary platform cut into a low cliff (Fig. 3). On the other hand, if the degree of downcutting and backwearing were much greater, most workers would consider the platform to be contemporary. The question arises as to the amount of downcutting, backwearing, and other modification of old surfaces that can take place before platforms are no longer considered to be inherited. The somewhat arbitrary definition used in this paper assumes that an inherited shore platform has experienced downcutting of no more than a few centimeters since the sea reached its present level and cliff retreat of no more than a few meters. The rock – low tide interface of partially inherited platforms, when the sea first reached its present level, was seaward of the interglacial cliff base. A partially inherited platform is considered to be one that has experienced downcutting of up to 20 cm during the Holocene. If platform width and gradient remain constant, simple geometry suggests that the corresponding amount of cliff retreat in the Holocene would have been no more than about 20 m. 3.3. The evidence for inheritance Elevated rock ledges and wave ramps extending to well above the present level of the waves testify to the

Fig. 3. Possible relationship between the contemporary shore platform, and the platform formed in the last interglacial. At what point should we consider the distances between ab and de and between cb and fe to be too great to consider the modern platform to be inherited?

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occurrence of sea levels that were higher than at present, but it is more difficult to find evidence of sea levels that were similar to today’s. The main evidence for coastal inheritance was usually provided by the occurrence of wide platforms in resistant, slowly eroding rocks, the apparent exhumation of platforms from beneath ancient glacial, periglacial, or other sediments, and other ambiguous morphological and sedimentary evidence (Guilcher, 1969; Everard et al., 1964; Orme, 1962; Whittow, 1965). Modern dating techniques are now confirming that inheritance has played an important role in the development of some rock coasts. It is sometimes possible to estimate the age of caves, shore platforms, and other rock surfaces using a variety of exposure-dating techniques (Brooke et al., 1994; Stone et al., 1996). Amino acid analysis and uranium series dating have shown that sea caves in the Channel Islands, southern Wales, and southern Australia are at least last interglacial in age (Goede et al., 1979; Davies, 1983). In New South Wales, U/Th dating of ferruginous and calcareous crusts, and thermoluminescent dating of associated sediments show that a subhorizontal platform, which is awash at high tide, developed in the last interglacial when sea level was higher than today. The platform was then buried under soil in the last glacial stage, when sea level was much lower, and exhumed and partly modified by waves in the Holocene. Sloping rock ramps, rising to more than 10 m above present sea level in places, are probably polygenic, having developed under rising and falling sea level during the Cenozoic (Bryant et al., 1990; Young and Bryant, 1993). Palaeosea-level evidence is preserved selectively, according to the hardness of the rock. In resistant rocks, features develop very slowly and they may never form during shorter stillstands, although, once formed, they have a high survival potential. On the other hand, features that form fairly quickly in weaker rocks have a low survival potential. Although there is considerable evidence of inheritance on resistant rock coasts, it is less clear in areas of weaker rock. Most workers, in the absence of compelling evidence to the contrary, believe that shore platforms formed in fairly weak rocks are postglacial features (Hills, 1971; Gill, 1972b; Sunamura, 1973; Takahashi, 1977; Kirk, 1977). This is difficult to substantiate on fairly rapidly eroding coasts, where, even if there had been evidence

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of inheritance, such as till covers, raised beaches, and structural remnants, it would usually have been removed by wave erosion at the present level of the sea. Nevertheless, as wide erosional terraces were able to develop on tectonically active coasts during fairly short interglacial stages, one cannot assume that platforms in fairly weak rocks exposed to vigorous wave action are necessarily inherited (Trenhaile, 1980, 1987). Many shore platforms are being cut into interglacial platforms that are only a few meters above present sea level. Ledges at, or somewhat above, the high tidal level are often the result of structural variations or thin seams of easily eroded rock. It is difficult to determine whether they are inherited features cut during periods of higher sea level, or contemporary features formed by storm waves, spray, and splash (Trenhaile, 1971). Relationships between platform gradient and tidal range and between other aspects of platform morphology and the morphogenic environment are usually quite strong in fairly weak rocks, but they are often very weak in resistant rocks that have not had enough time to adjust to today’s sea level. Strong relationships do not prove that platforms are entirely contemporary, but they do show that even if platforms are partly inherited they have adjusted to the present morphogenic environment. This appears to have occurred in the St. Lawrence Estuary, where well-adjusted shore platforms may have been partly inherited from an older, Micmac surface (Brodeur and Allard, 1983). There is abundant evidence for the inheritance of shore platforms and other rock coast features in fairly weak rocks in Galicia in northwestern Spain. Tropical conditions during the Tertiary, which became progressively less humid during the latter stages of this period, resulted in deep weathering of the rocks and sediments along this coast, and glacial and periglacial processes were important during the Pleistocene. There is an inherited shore platform near the village of Caaman˜o on the western coast of Galicia. The wide, gently sloping intertidal platform is backed in places by a supratidal rock ledge, and in other places by a steeper and narrower supratidal ramp. The gradient of the intertidal platform is consistent with the relationship between platform gradient and tidal range, but the slope of the ramp is much too high. The abandoned and degraded sea cliff is grass-covered along most of this coast, and the ledges and the ramp,

which extend up to several meters above the highest tides, are covered by lichen and, in places, by salttolerant plants. Radiocarbon-dated sediments in the cliff, which range up to 36,000 years in age, lie on top of an ancient beach deposit. The former beach, remnants of which are found in situ on the ramp and rock ledges, as well as two caves that are filled with the dated sediments, are probably last interglacial in age. The morphological and sedimentary evidence suggests that the supratidal ramp and ledges were formed during the last interglacial stage, whereas the wider intertidal platform is probably the product of several older interglacials, when sea level was generally similar to today’s (Trenhaile et al., 1999). There is evidence of coastal inheritance in many other places around the coast of Galicia, including dated periglacial sediments at the back of the intertidal zone and in caves, coves, and bays, and remnants of ancient beaches and dated peat deposits on shore platforms. Some inherited features were formed during the last interglacial stage, when climate and sea level were similar to today’s, whereas others developed in the cool to cold conditions of the last glacial stage, when the present coastline had been abandoned by the sea. Rock coasts may therefore consist of two distinct components: elements from the last interglacial stage that were already close to equilibrium under contemporary conditions; and elements from the last glacial stage that were in strong disequilibrium with modern conditions. Glacial stage elements may change fairly rapidly as they adjust to contemporary conditions; this could include, for example, the removal of scree, other frost shattered material, and periglacial slope deposits, and wave undercutting and destruction of composite cliff profiles. Ancient shore platforms and other interglacial features were much closer to equilibrium when they were inherited by rising postglacial sea level, and changes are therefore much slower. Measured and estimated backwearing rates on cliffs and shore platforms range from virtually nothing up to 50– 70 m year
1, and platform downcutting rates range from 0.1 to 35 mm year
1 (Sunamura, 1973, 1992; Kirk, 1977). Unfortunately, few measurements are reliable, and the most accurate techniques have usually been used over only very short periods. Erosion rates may be related to the degree to which coastal elements are in disequilibrium with contemporary conditions. Glacial – interglacial differences

A.S. Trenhaile / Geomorphology 48 (2002) 7–22

could therefore account, in part, for the large differences in erosion rates that have been reported for rock coasts, although geological factors and the lack of reliable measurement techniques are probably of greater importance. Although shore platforms may be entirely postglacial features in fairly weak rocks, the present rates of erosion often appear to be too low to account for the formation of wide platforms in unweathered igneous and other resistant rocks since the sea has been at its present level. Uncertainty over the morphology of the coast at the end of the last glacial stage is one of our greatest problems in determining, from contemporary erosion rates, whether wide shore platforms could have been cut since the sea reached today’s level. If it is assumed that interglacial platforms had approximately the same width and gradient as today, then last interglacial platforms would have had to be lowered by about 3 – 5 m to adjust to modern conditions. This would have required downcutting rates of 0.1 – 0.17 cm year
1 or backwearing rates of 7– 10 cm year
1 to produce 200– 300-m-wide platforms in the approximately 3000 years since the sea reached its present level. Although these backwearing rates are considerably higher than those in resistant rocks today, erosion would have been accelerated by rising Holocene sea level and by frost shattering of the inherited platform during the last glacial stage. Furthermore, as most previous interglacials were generally similar or lower than today, it is quite possible that little modification of inherited platforms would have been required beyond removal of the landward ramps or ledges that were cut during the brief period of higher sea level in the last interglacial. 3.4. Inheritance modelling Inheritance played no role in the formation of simulated shore platforms in fairly weak rocks over five glacial/interglacial cycles (Trenhaile, 1989). Nevertheless, there was a progressive decrease in the amount of cliff erosion accomplished in model runs during each interglacial stage. This implies that if more glacial/interglacial cycles had been represented in the model, as well as more resistant rocks and less vigorous wave environments, inheritance would have played a role in the development of the simulated profiles. More recently, a wave erosional model has

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demonstrated that high wave and surf attenuation can protect inherited, gently sloping shore platforms from erosion by high storm waves (Trenhaile, 2001c). Therefore, whereas relationships between elements of platform morphology and the morphogenic environment suggest that platforms are often well adjusted to modern conditions, this does not preclude the possibility that they were inherited, largely in their present form, from previous interglacial stages when sea level was similar to today’s. The model indicated that whether an ancient surface is inherited, modified, or completely replaced by a contemporary platform depends upon the multitude of factors that determine the erosive efficacy of the waves, including wave size and period, tidal range, the strength of the rock, cliff height and the size and mobility of the cliff-foot debris, and the morphology of the platform (Trenhaile, 2000, 2001b, 2001c). 3.5. Ancient composite cliffs Other elements of rock coasts may also have been inherited. Wide shore platforms in resistant rocks are often backed by high cliffs with composite profiles. This suggests that these are also inherited features that are being slowly modified by contemporary marine and subaerial processes. Composite cliffs have two or more major slope elements. Bevelled (hog’s back, slope over wall) cliffs have a steep wave-cut face below a convex or straight seaward-facing slope, whereas multistoried cliffs have two or more steep faces separated by more gentle slopes (Orme, 1962; Fleming, 1965). Although the two elements of bevelled profiles can develop contemporaneously in fairly weak rocks, high composite cliffs cut into resistant rocks are of great antiquity. Composite cliffs are widely distributed in resistant rocks in areas that were not under ice during the last glacial stage, including parts of western Britain and Ireland, Brittany, Galicia in Spain, and Cape Breton Island in eastern Canada (Griggs and Trenhaile, 1994). In southwestern Britain for example, raised beaches at the foot of composite cliffs contain middle and upper Pleistocene material, which suggests that the cliffs have been only slightly modified by contemporary wave action (Andrews et al., 1979; Davies, 1983). Ancient composite cliffs are the product of changing sea level and, in mid and high latitudes, climate.

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Falling sea level caused sea cliffs to be abandoned at the end of the last interglacial stage and exposed to frost action and other subaerial processes during the subsequent glacial stage. These steep cliffs were then gradually replaced by convex slopes developing beneath the accumulating scree. When the sea returned to its present level, the scree was removed. Depending on wave intensity and rock hardness, the

convex slopes were either trimmed back to form composite cliffs or completely removed to form steep, wave-cut cliffs. Modelling (Fig. 4) suggests that bevelled profiles may have developed where the scree extended up to the top of the cliff during the last glacial stage, and multistoried profiles where the debris rose only part of the way up the cliff face (Griggs and Trenhaile, 1994).

Fig. 4. Simulated cliff development during two glacial – interglacial cycles on mid to high latitude coasts (Griggs and Trenhaile, 1994; Trenhaile, 1997). The initial marine cliff profile was assumed to be vertical in the penultimate interglacial stage. Scree accumulated at the cliff foot during the period of low sea level in the penultimate and last glacial stages and was removed by wave erosion during the last interglacial and in the Holocene. The development of vertical, bevelled, or two-storied marine cliffs in the Holocene was determined by rates of wave erosion and whether the scree partly or completely covered the cliff face during glacial stages.

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3.6. Erosional terraces The hinterlands of many rock coasts have one or more ancient erosional terraces. Some terraces have caves and stacks and there may be shallow-water marine and terrestrial deposits. Tectonically stable plate-imbedded coasts generally have only a small number of very wide marine surfaces, whereas tectonically active collision coasts have flights of terraces ranging from a few meters up to several kilometers in width, and extending up to hundreds of meters above sea level (Orme, 1964; Bradley and Griggs, 1976; Pillans, 1983). The gradients of terraces and contemporary shore platforms are generally similar, but terrace widths are often much greater. Although it was once assumed that wide surfaces were cut by waves or currents at considerable depths below the surface of the sea (Johnson, 1919; Rode, 1930), the most effective erosional processes operate at or close to the water level. A mathematical model was used to study the effect of middle and late Pleistocene changes in sea level on marine planation surfaces (Trenhaile, 1989). The model was based on the assumption that cliff recession is the result of alternating periods of wave undercutting and debris removal at the high tidal level. The model, which was primarily concerned with mechanical wave erosion within the intertidal zone, suggested that, depending upon rock hardness and wave energy, erosion surfaces between 1 and 3 km in width could be produced at the end of one glacial –interglacial cycle: much wider surfaces were

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created at the end of five cycles. The width, gradient (0.5 – 2j), and concave shape of the simulated erosion surfaces were similar to those of marine terraces around the world. The model therefore suggests that very wide marine erosion surfaces were formed largely by mechanical wave erosion operating within the intertidal zone, especially during periods of rising sea level. Marine terraces in tectonically mobile areas developed during interglacial stages and were then uplifted to their present positions, whereas terraces in stable regions were formed in the Tertiary during slow transgressions or long periods of high, stable sea level. 3.7. Crenulated coasts It is probable that the plan shapes of crenulated rock coasts have developed over long periods. Bays and headlands usually develop as a result of longshore differences in the resistance of the rocks to erosion. Some headlands consist of rocks that are clearly more resistant than those in the adjacent bays, but others are expressions of more subtle differences in joint density, bedding thickness, and other structural influences. Because of wave refraction, the plan shape of crenulated coasts in fairly erodible rocks may eventually attain an equilibrium state: this occurs when the more resistant rocks on the exposed headlands are eroded at the same rate as the weaker rocks in the more sheltered bays (Fig. 5). The actual shape of an equilibrium coast would reflect a multitude of factors, including offshore and intertidal topography, the direction and wavelength of the incoming waves,

Fig. 5. Crenulated coastal equilibrium. The depth of an embayment at equilibrium is a function of the distance between the headlands and the difference in the hardness or erosive resistance of the rocks in the headlands and the bay.

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the distance between adjacent headlands, cliff height, and the difference in rock resistance between bays and headlands. Each of these affects or is affected by other variables. The rock resistance is determined by such factors as rock strength, chemical composition, bedding thickness, joint density, and variations in rock strike and dip relative to the direction of the incoming waves (Trenhaile, 1999). Cliff height affects the amount of debris that has to be removed. The distance between adjacent headlands is determined by the location of resistant and less resistant rocks. Although the general principles continue to apply, the development of an equilibrium profile is also strongly influenced by the thickness and the cross-shore and longshore mobility of any beach material accumulating in the bays. Although, given present rates of cliff erosion, it would require a considerable period to produce equilibrium, the plan shape of rock coasts has been sculpted over successive interglacial stages, when the sea was close to its present level. Only minor changes, including the removal of last interglacial rock ledges at the back of shore platforms, may therefore have been necessary to attain a quasi-equilibrium form at the present sea level. Unfortunately, the lack of reliable data on long-term cliff erosion rates, longshore variations in wave energy, and rock structure and strength make it very difficult to determine whether coastal plan shape does represent a quasi-equilibrium form.

4. Conclusions To understand the dynamics and evolution of rocky coasts and to assess the potential effects of rising sea level, we need reliable, quantitative data on processes and rates of erosion. We also need to know how rock coasts adjusted to rising sea level in the early Holocene and at similar times in the past. We are gradually improving our ability to evaluate the role of geological factors on rock coast development, through measurements of rock structure and hardness (compressive strength, Schmidt rock test hammer values, etc.) and the application of the principles of rock mechanics. MEMs can be used to accurately determine rates of platform lowering, but we are presently unable to measure the effects of large-scale

quarrying of joint blocks and other large rock fragments. Modelling provides an important means to investigate the long-term development of slowly eroding rock coasts, but model predictions must be substantiated through field measurement and experimentation. Models are also limited by the lack of reliable field data, which are needed to determine the value of the model coefficients and to provide a reliable time scale for model predictions. There were marked changes in the nature, intensity, and elevational distribution of the processes operating on rock coasts in response to changes in Quaternary sea level and climate, and they frequently retain vestiges of former environmental conditions. Therefore, one of our greatest challenges is to determine whether rock coasts are contemporary or ancient features inherited or partly inherited from interglacial stages when sea level was similar to today’s. Although there is often evidence of inheritance on resistant rock coasts, it is much more difficult to determine the age of coasts in fairly weak rocks.

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