Chapter 1 Introduction to the rock coasts of the worldChapter 1 Introduction to the rock coasts of the world DAVID M. KENNEDY1*, WAYNE J. STEPHENSON2 & LARISSA A. NAYLOR3 1Department

Chapter 1

Introduction to the rock coasts of the world

DAVID M. KENNEDY1*, WAYNE J. STEPHENSON2 & LARISSA A. NAYLOR3

1Department of Resource Management and Geography, The University of Melbourne, Parkville, Victoria 3010, Australia2Department of Geography, University of Otago, PO Box 56, Dunedin, New Zealand

3School of Geographical and Earth Sciences, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK

*Corresponding author (e-mail: [email protected])

Rock coasts are erosional environments which form as a result ofthe landward retreat of bedrock at the shoreline. Vertical facesplunging into deep water form imposing cliffs on many shorelines.Other cliff forms may be steeply sloping, but no strict definitiondelineates a slope from a cliff. In many instances the retreat ofthe cliff leads to the formation of a rock ledge, or shore platform,at or close to sea-level. The surface of these platforms may beeither subhorizontal or slope gently in a seaward direction.

Rock coasts have been considered a neglected coastal landform(Trenhaile 1980, 2002; Stephenson 2000). The two seminal bookson the topic (Trenhaile 1987; Sunamura 1992) were written overtwo decades ago and they still form a core academic sourcewhich people refer to when seeking to understand the morphologyand dynamics of rocky coasts. In the past decade there has beengrowing interest in rock coasts; however, the subdiscipline isstill in its infancy when compared with other landform systemssuch as sandy beaches, rivers or glaciers. Much of the recentresearch is focused on a case-study approach, with small teamsof researchers using their local field sites to infer wider morpho-logical models of platform or cliff systems.

A major driver for the growing interest in the rocky coast hasbeen the advent of new and emerging technology that enables usto address problems not previously accessible and at scales notpreviously achievable. In the late nineteenth and early twentiethcenturies scientific investigation was observational and lackedquantitative data (Dana 1849; Bartrum 1926). Such qualitativedescriptions of the rocky coast continued into the mid twentiethcentury, when quantitative data from field surveying became thenorm. The use of techniques for measuring rock hardness, suchas the Schmidt hammer, developed soon after (Goudie 2006),but it is in the last decade that the greatest advances in technologyhave been made. The development of laser surveying, which canbe operated from aerial and terrestrial platforms, has greatlyincreased our understanding of these landform systems (Kennedy2013). Light detection and ranging (LiDAR) techniques now areable to survey landforms to centimetre scale from the air (Limet al. 2005; Palamara et al. 2007) and can also penetrate thewater column, allowing seamless surveying of the subaerial andsubmarine portions of the coast (Kennedy et al. in press). Terres-trial laser scanners can produce digital elevation models of cliffand platform systems to millimetre scale and have been used formonitoring both long- and short-term cliff retreat in the UK(Lim et al. 2005; Chapter 3 by Lim 2014). The sheer volume ofdata that can now be collected is unprecedented. These data,which give a very precise indication of morphology at the timeof survey, can now be combined with other techniques such asmicro-erosion metres to provide data on rates of erosion at themillimetre scale from hourly to decadal timescales (Stephensonet al. 2004; Trenhaile 2006).

The process-side of morphodynamics has been particu-larly neglected in the discipline. Wave-tank experiments were

undertaken in the mid-late twentieth century, mainly inJapan (Chapter 12 by Sunamura et al. 2014), but also in Australia(Chapter 14 by Kennedy 2014); however, it is only in the pastdecade that field experimentation has started to precisely quantifyenergy transfers on the shore and resulting sediment movement.This more quantitative approach has been driven by a new gener-ation of researchers taking advantage of new technologies, particu-larly the miniaturization of sensors, greater computing memorycapacity and increased battery life. As a result, the transformationof wave energy from gravity to infragravity frequencies isnow being quantified through field deployments of pressuresensors (e.g. Ogawa et al. 2011; Marshall & Stephenson 2011;Beetham & Kench 2011) and individual pebbles traced in the lit-toral zone using radio-frequency identification tags (e.g.Benelli et al. 2012). The result is that researchers are now startingto identify and quantify erosion processes at different spatial andtemporal scales.

The unprecedented level of data quality and quantity that cannow be collected produces its own challenges for coastal research-ers. Specifically, how does the data scale from, and between, thelocal to regional level? How applicable are measurements ofmicron-scale change of a rock surface to understanding theregional evolution of a rocky coast? In part, such questions arerelated to the dominant processes driving landscape evolution.For example, in areas where wave plucking of the bedrock is thedominant erosive process, the millimetre-scale granular disinte-gration and water layer weathering may be less important. Howquickly the joints themselves are eroded is however criticallyimportant as this can determine how quickly wave quarrying cantake place (Paris et al. 2011). On the other hand, if the bedrockis homogenous and grain disintegration is the primary mecha-nism of platform erosion (e.g. Kaikoura, New Zealand) then themicro-scale surface downwearing is critically important (Ste-phenson & Kirk 1996). The challenge for researchers is there-fore both to understand the scale of their particular study and tocontextualize it within the boundary conditions of the system(Naylor & Stephenson 2010). Such a task can be very complex,particularly when investigating a new area where the boundaryconditions are unknown.

Boundary conditions

The evolution of rocky shore landforms is driven by the action ofsubaerial, biological and marine processes. Subaerial weatheringbreaks down the bedrock by either directly removing material ormaking it more susceptible to erosion by marine processes,namely waves and tides. Biological activity is complicated byvirtue of being erosive, protective and constructive, or a varietyof combinations of each type. Biological activity is also closelyinterlinked with other weathering and erosive agents (Chapter

From: Kennedy, D. M., Stephenson, W. J. & Naylor, L. A. (eds) 2014. Rock Coast Geomorphology: A Global Synthesis.

Geological Society, London, Memoirs, 40, 1–5. http://dx.doi.org/10.1144/M40.1

# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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5 by Coombes 2014). The present-day form of a shoreline is there-fore dependent on the balance between the assailing forces oferosion and the resisting forces of the bedrock. On shorelineswhere the bedrock is highly resistant to forcing agents, such asthose composed of basalt, plunging cliffs may occur (e.g.Chapter 13 by Dickson & Stephenson 2014). In instances wherethe bedrock is highly erodible, such as in mudstone, sloping plat-forms are more common.

Predicting the form and evolutionary direction of a rock coastbased on their formative processes is, however, difficult. In factit is becoming apparent that rock coasts evolve through a rangeof differing, and contrasting, processes (Naylor et al. 2010). Asdescribed above, the suite of processes that dominate landformevolution in a given location are modulated by the external bound-ary conditions (e.g. sea-level history, climate and tidal range).The rate of variation within the boundary conditions is significantover large spatial scales, such as between countries or hemi-spheres, but on a regional level or local scale the variation inboundary conditions is often much less. For example, frost andice comprise a significant geomorphic agent on cliffs and shoreplatforms developed in high latitudes (Chapter 16 by Hansomet al. 2014), but it are irrelevant at low latitudes. Tides areanother major boundary condition. For example, Japan (Chapter12 by Sunamura et al. 2014), New Zealand (Chapter 13 byDickson & Stephenson 2014) and southern Australia (Chapter14 by Kennedy 2014) are microtidal, which leads to the develop-ment of semi-horizontal rock platforms, while eastern Canada(Chapter 8 by Trenhaile 2014a) is predominantly macrotidaland intertidal rocky surfaces which slope at greater than 58 arefound. The distribution of wave and weathering processes acrossthese varied sloped surfaces is very different which means thatdirect comparisons of landform evolutionary models are notalways appropriate; however, models of landform change devel-oped in these regions can have applicability for those areas withsimilar tidal range. In addition, rocky landforms may evolveover multiple eustatic cycles, which means the magnitude ofthe erosive process may change significantly through time (Tren-haile 2001).

Significant variations in coastal morphology can occur as aresult of variations in another major boundary condition – thegeology. Both rock mass and rock material properties exert majorcontrols on rock coastal processes (Naylor et al. 2012). The effectsof geology are most obvious when crossing from one geologicalunit to another in the same region. For example, along the GreatOcean Road of Victoria, Australia, low cliffs and shore plat-forms are common on the Cretaceous sandstones, but adjacent tothis unit, on Cenozoic limestones, high cliffs fronted by narrowbeaches, arches and stacks are common (Fig. 1.1a, b). On thesmaller scale of a single outcrop alternating beds of lithologiesof different hardness can dominate the micro–mesoscale reliefof a shore platform forming a stepped (Fig. 1.1c) or washboardmorphology (Fig. 1.1d). Geological control on rock coast erosionand evolution is often complex, especially when other parame-ters can exert a stronger control than lithology. For example,rock mass properties (i.e. discontinuities) were found to exert amuch stronger control on erosion processes than the rock typeitself when comparing limestones and dolerite (Cruslock et al.2010). Here the rocks with more similar structural properties (butdifferent lithology) produced similar erosion products despitehaving different boundary conditions (i.e. waves and ice).

The variations in boundary conditions pose difficulties for thecreation of holistic models of landform development as the con-ditions in which one shore platform or cliff form will be verydifferent from place to place, region to region and through time.This has led to many of the significant debates within the sub-discipline. It also means that problems arise when relationships,such as between wave energy and platform width, or the dynam-ics of platform lowering measured in one area are applied inanother location. The original inferences are often correct, but

their application in areas with a different set of boundary con-ditions can yield contradictory results.

The opportunity to understand and develop holistic modelsof rocky coast evolution therefore becomes greatest when shore-lines with similar boundary conditions are compared. This oftenoccurs at a regional level as climate and tidal range tend to havegreater similarity at this spatial scale, although as advocated byStephenson (2000), collaboration should still also occur acrossregional boundaries for a holistic understanding of the globalrocky shore.

Sea-level

Rock coast landforms are by definition found at the shoreline;sea-level is therefore a fundamental boundary condition (Chapter2 by Trenhaile 2014b). However, the position of the sea is notconstant over the millennial timescales of rocky shore evolu-tion. Rocks of high erosional resistance tend to erode over mul-tiple eustatic cycles with multistoreyed cliffs often being theresultant landscape form (Fleming 1965). For platforms, signifi-cant inheritance can occur between sea-level cycles with manyshores bearing the imprint of higher sea-level during the Last Inter-glacial period and in some cases the penultimate interglacial suchas seen on the Australian coast (Chapter 14 by Kennedy 2014).There are also suggestions that many of the subhorizontal plat-forms that characterize microtidal shorelines have been primarilycut during the mid Holocene highstand (Trenhaile 2010). On aglobal scale local Holocene sea-level history is strongly affectedby vertical land movement related to glacial rebound, tectonicmovements and hydro-isostacy. The north of the northern hemi-sphere is dominated by glacial rebound from the last GlacialMaximum, while the central Pacific is considered a truly eustaticsignature being characterized by a c. þ1 m highstand around 5–6 ka (Pirazzoli 1991).

Geology

The geology in which cliffs and shore platforms are formed isfundamental to determining both how fast they erode but alsothe form that they will take. The role of geology includes manyfactors, such as the rock type, mineralogy, jointing and bedding.In general harder rocks form higher cliffs or platforms at greaterelevation. Rock hardness does not, however, imply a certain ero-sional resistance. This is because fractures and jointing willprovide lines of weakness in the rock, which increases its suscep-tibility to erosion. The degree of jointing and fracturing not onlyaffects the amount of erosion (Kennedy & Dickson 2006), but italso influences the products of erosion (Stephenson & Naylor2011). This is especially the case for coastal settings dominatedby storms where large accumulations of boulders may occur.The size of individual boulders and the volumes of these depositscan often be directly related to the jointing and fracturing of thebedrock in the area where they are found (Paris et al. 2011; Ste-phenson & Naylor 2011).

It is also often the case that on a particular stretch of coastlinemany different lithological units will outcrop, each with theirown unique geotechnical properties. The orientation of the bed-ding planes, both vertically and horizontally, will have a stronginfluence on different landforms that emerge as softer materialsare eroded preferentially to harder ones, as seen on the rock coastsof the USA and Mexico (Chapter 9 by Hapke et al. 2014) as wellas South America (Chapter 10 by Blanco-Chao et al. 2014).Gulches and sea caves are often formed in this manner throughthe preferential erosion of weaker units (Trenhaile 1987).

The hydrogeology of a particular section of coast can also driveits evolution. Groundwater can be concentrated within specificlithological units based on their permeability and porosity and in

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turn this influences the erosional resistance of the bedrock and itsgeotechnical strength. In some areas, such as the soft-rock cliffs ofFrance (Chapter 6 by Gomez-Pujo et al. 2014) and the UK(Chapter 3 by Lim 2014), groundwater flow can determine thenature and timing of cliff collapse and hence shoreline retreat(Duperret et al. 2005; Castedo et al. 2012).

Climate

Climate, particularly temperature and humidity, strongly influ-ences the mechanisms of physical, biological and chemical weath-ering. In the tropics, terrestrial weathering profiles are often deep,which in turn lowers the erosional resistance of subaerial rocks(Nott 1994). Higher latitudes, on the other hand, tend to havelower rates of chemical erosion but high rates of physicalerosion. Ice, in the form of frost as well as sea ice, can dominaterock coast erosion in the polar regions (Chapter 16 by Hansomet al. 2014); however these processes do not occur close to theequator in our current climate (Chapters 10 by Blanco-Chaoet al. 2014 and 15 by Woodroffe 2014). The effect of these cli-matic gradients is expressed most obviously by the landforms

created on carbonate lithologies. In the classic study of Guilcher(1953), he noted that cliffs in the tropics were characterized bydeep notches and overhanging visors, while in the warm Mediter-ranean (Chapter 7 by Furlani et al. 2014) the notches were less dis-tinct. In temperate Britain (Chapters 3 by Lim 2014 and 4 byMoses 2014), profiles tend to lack notches and slope seawardrather than being vertical. Weathering is not, however, the solereason for this change in form; other boundary conditions suchas waves and tides are also important.

Tides and wind wave

Rock coasts occur in a range of tidal environments. In macrotidalenvironments (such as Canada, Chapter 8 by Trenhaile 2014a),platforms tend to slope seaward while microtidal areas (Chapters11–14 by Choi & Seong 2014; Sunamura et al. 2014; Dickson& Stephenson 2014; Kennedy 2014 respectively) are character-ized by subhorizontal forms (Trenhaile 1987). Wind–wave pro-cesses on the other hand greatly influence the rates of erosionthat can occur through physical destruction of the rock. This ismost often represented in the presence of boulder debris, which

Fig. 1.1. (a) Platforms and low cliffs formed in Cretaceous sandstones near Lorne, Australia. (b) High vertical cliffs fronted by narrow beaches characterize the

rock coast of the 12 Apostles region of Victoria, Australia where Cenozoic limestones dominate the shore. (c) A stepped shore platform morphology on Lord Howe Island,

Australia where more resistant basaltic dykes intrude between softer breccia units. (d) A classic washboard morphology of a shore platform in the Wairarapa region of

New Zealand. Harder sandstone units interbedded with softer mudstones project above the platform surface.

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is more common in the high-latitude storm belts (see Chapters 3 byLim 2014 and 7 by Furlani et al. 2014).

Aim of this volume

The purpose of this Memoir is to bring together the global researchof the past couple of decades alongside the established theoriesand debates within the discipline. Each chapter within the Memoiris structured around the boundary conditions and forcing factors ofthe region in which the chapter is based. The focus of the chaptersis also framed within the key methodologies of the researchers thathave focused their attention in those regions. For example, fieldsurveying is core to Australian studies (Chapter 14 by Kennedy2014) while hydrodynamic modelling and cosmogenic dating hascharacterized Japanese (Chapter 12 by Sunamura et al. 2014) andKorean (Chapter 11 by Choi & Seong 2014) work, respectively.For the British Isles the breadth and depth of information hasmeant that separate chapters are included to cover cliffs (Chapter3 by Lim 2014), shore platforms (Chapter 4 by Moses 2014) andweathering and biologic processes (Chapter 5 by Coombes 2014).The conclusion of the volume (Chapter 17 by Naylor et al. 2014)seeks to identify and synthesize the key messages emerging fromthe volume and to identify future research needs.

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