Storm wave currents, boulder movement and ... - landforms · cuestas with scarps formed by rock steps in sandstone and agglom-erateandstrike-aligned,shallowvales—namedherestrikecanals—in

Marine Geology xxx (2010) xxx–xxx

MARGO-04595; No of Pages 8

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Marine Geology

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Storm wave currents, boulder movement and shore platform development: A casestudy from East Lothian, Scotland

Adrian M. Hall ⁎School of Geography & Geosciences, University of St. Andrews, St. Andrews, KY16 9ST, UK

⁎ Tel.: +44 131 311 6969; fax: +44 131 332 3081.E-mail address: [email protected].

0025-3227/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.margeo.2010.10.024

Please cite this article as: Hall, A.M., StormLothian, Scotland, Mar. Geol. (2010), doi:1

a b s t r a c t

Article history:Received 2 March 2010Received in revised form 25 October 2010Accepted 26 October 2010Available online xxxx

Keywords:shore platformstorm wavesboulder transportEast LothianScotland

Understanding the distribution of wave energy on shore platforms is important for models of the long termdevelopment of both shore platforms and backing cliffs. Little is known, however, of the magnitude of thewave currents generated on shore platforms in storms. This study uses the distribution of boulders of differentsizes on a shore platform on the North Sea coast of Scotland to investigate patterns of storm wave currentvelocities.In storms over the last 40–240 yr, blocks as large as 9 m3 have been quarried from the rock step at LWMS andboulders of N0.5 m3 have been moved landward over extensive areas of the platform. In earlier storms,boulders N1 m3 have been moved widely. Wave current velocities in storms have likely reached 3–4 m/s onmany parts of the platform, especially in the slightly deeper water along smooth-floored strike canalsdeveloped in marl beds. Attenuation of velocities across the platform is limited.Storm wave action is a key process in erosion across large parts of this shore platform. Wave impacts quarrylarge blocks at or near LWMS and wave current velocities in storms are sufficient to transport landwards largeclasts and smaller debris. Weathering of soft marl beds also contributes directly and indirectly to developmentof the shore platform. Lowering of marl beds at long term rates of ~1 mm/yr in response to recent sea-level fallleads to the undercutting and eventual collapse of overlying, more competent rock beds, releasing blocks forwave transport. Deepening of strike canals, by both water-layer weathering and wave action, allows fasterwave currents to reach and erode cliffs at the rear of the platform. On this exposed, macro-tidal coast, shoreplatforms are eroded by both wave action and weathering processes operating over various spatial andtemporal scales.

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wave currents, boulder movement and sho0.1016/j.margeo.2010.10.024

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The importance of storm wave activity in shaping shore platformshas long been recognised (Edwards, 1951; Naylor and Stephenson,2010; Trenhaile, 1972). Due, however, to the low frequency of highmagnitude storms and the formidable practical difficulties of fixedinstrumentation (Trenhaile and Kanyaya, 2007), little is known of thehydraulic properties of storm waves on shore platforms. Recent workon reefs indicates however that boulder size can be used to estimatethewave current velocities generated in storms and tsunamis (Goto etal., this volume; Paris et al., 2009). The main aim of this paper is to usespatial variations in block size to explore the distribution of stormwave energy across a shore platform in East Lothian, Scotland (Fig. 1).

2. Study site

East Lothian was deglaciated around 16,000 yr BP and relative sealevel has fallen over the last 8000 yr (Smith et al., 2000). Dates fromthe Forth estuary (Smith et al., 2010), combined with regional isobasepatterns (Smith pers. comm. 2010), indicate development of raisedshorelines in the study area as follows: the Storegga Shoreline (circa8100 BP) at +5.6 m above Ordnance Datum; the Main PostglacialShoreline (circa 7500 BP) at +6.4 m OD; the BlairdrummondShoreline (circa 4700 BP) at +7 m OD; and a later shoreline (circa3000–2000 BP) about +4.4 m OD. This is a macro-tidal coast, with aSpring Tide range of 4.5 m. The coast is exposed to waves from theNorth Sea in the northeast quadrant. Wave energy is high, with arecently-installed wave buoy located in 65 m of water in the Firth ofForth (Fig. 1) recording an Hmax of 6.24 m for the recording period 19/08/2008-01/01/2010 (Cefas WaveNet data). During an easterly stormwith gale force winds on 30/03/2010, observed breaking wave heightsat the outer edge of the shore platform were 3–5 m at The Gegan,compared to a recorded Hmax of 5.8 m from the wave buoy (Fig. 2). Aswave heights did not exceed the two year maximum, the 30/03/2010storm can be regarded as only of moderate intensity.

re platform development: A case study from East

Fig. 1. The Seacliff–Scoughall shore platform. (A) Geology. (B) Geomorphology. (C) Distribution of basalt boulders. (D) Location.

2 A.M. Hall / Marine Geology xxx (2010) xxx–xxx

The shore platform between Scoughall and Seacliff, East Lothian,extends for ~2 km, reaches a maximum width of 400 m and slopesseaward at 0.6–3.0° (Fig. 1). The platform is backed locally by rockcliffs, up to 25 m high in places, but more generally just a few metreshigh at the eroding edges of raised shore platforms. Much of the cliffline is inactive, however, and fronted by postglacial raised beachesand modern storm beaches (Figs. 1, 3).

The shore platform is developed across a range of Carboniferousrock types, including vent agglomerate, sandstone and marl, intrudedby a series of volcanic vents (Davies et al., 1986)(Fig. 1). Here, aselsewhere (e.g. Cruslock et al., 2010; Kennedy and Dickson, 2006;Naylor and Stephenson, 2010), lithology and structure are majorcontrols on the morphology of the shore platform. Isolated segmentsof the platform developed in the most resistant sandstone and ag-glomerate beds occur up to 3 m above Low Water Mark Spring Tides

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

(LWMST) (Fig. 1B), and remain exposed for much of the tidal cycle.These segments probably represent remnants of postglacial or earlierraised shore platforms that are found a few metres above sea level onthis coast (Hall, 1989). On the main platform, differential weatheringand erosion of gently-dipping rock beds has produced a series of lowcuestas with scarps formed by rock steps in sandstone and agglom-erate and strike-aligned, shallow vales— named here strike canals— inthe weaker marl beds (Figs. 1, 3). Parts of the strike canals retain ashallow water layer at low tide and flaking and broken rock surfaceson the canal floors demonstrate significant water-layer weathering.This, together with the presence of basalt and sandstone bouldersresting on marl pedestals, indicates lowering of the floors of the strikecanals. Boulder and gravel beaches occur locally on lower parts of theshore platform (Fig. 1B) and boulder clusters and isolated bouldersare widespread.

vement and shore platform development: A case study from East

Fig. 2.Wave behaviour at The Gegan during the storm of 30/03/2010. (A)Waves duringthe vernal Spring Tide at 1530. Waves are breaking on the rock step at the outer edge ofthe agglomerate shore platform, causing broken wave water to surge across the plat-form. Maximum water depths on the platform estimated from photographs are 3–4 m.Wave energy is dissipated throughwave refraction towards the back of the shore. Videofootage of the storm waves is available at http://www.landforms.eu/Lothian/Shore platformprocesses.htm. (B) The same segment of the shore platform at lowtide on the following day. Boulders moved during the storm are circled. The bouldercluster shown in Fig. 4B is marked with an X.

3A.M. Hall / Marine Geology xxx (2010) xxx–xxx

3. Methods

The geomorphology of the shore platformwasmapped at a scale of1:10,000 on the ground (Fig. 1B) and from oblique aerial photos takenfrom a light aircraft on 23/08/2008 (Fig. 3A).

The shore platform carries an irregular scatter of large clastsderived from three sources:

1. Basalt boulders are widespread on the shore platform (Figs. 1C, 3C).Only a few, narrow and closely-jointed basalt dykes occur on theplatform (Fig. 1A), which do not generate large boulders. The largebasalt boulders were originally sourced from volcanic outcrops afew km to the west and were carried as glacial erratics (Davieset al., 1986) and then deposited within till or as perched boulders.During the postglacial fall in relative sea level, marine erosion hasstripped away any till cover and raised beach deposits from theunderlying bedrock surface and released the basalt boulders on tothe platform surface.

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

2. Quarried blocks are found near the low water mark. Wave quarry isfrom angular sockets (Hall et al., 2008; Knight et al., 2009) anddetachment sites (Switzer and Burston, 2010) in the sandstone andagglomerate rock steps found along the seaward edge of theplatform (Figs. 1B, 3A).

3. Collapsed blocks occur widely. Large tabular blocks of sandstoneand agglomerate cap rocks are found on strike canal floors adjacentto undercut and retreating rock steps.

Wave transport of blocks away from quarry and collapse siteshas left boulder trains on the shore platform (Fig. 3A). Such bouldersoften can be matched to nearby sources on the basis of bed thickness,lithology and weathering features.

Boulders were located using a hand-held GPS unit. Boulders weremeasured along a, b and c axes using a metre rule. For sandstone andagglomerate clasts with tabular shapes, these measurements give agood approximation of volume, but basalt boulders tend to moreellipsoid shapes, and the volumes given here are over-estimates.Density of basalt, agglomerate and sandstone was estimated in thefield using a spring balance and by weighing in air and water(Table 1). A total of 600 basalt boulders were measured (Fig. 1C),including all boulders N1 m3 on the shore platform. Three areas withabundant large boulders, The Gegan, Seacliff and Scoughall (Fig. 1),were chosen for detailed mapping at a scale of 1:250, with locationand measurement of all large clasts of any rock type, along withrecording of the orientation of imbricate boulder clusters (Figs. 4–6).

Any large clast on the shore platform may have been moved bywave currents in the past. Evidence of wave transport includes orga-nization of boulders into imbricate clusters facing to seaward, thepresence of chock stones or gravel deposits beneath boulders anddisplacement of sandstone and agglomerate boulders away frombedrock sources and towards the rear of the platform. It is likelyhowever that some clasts with highly weathered and pitted surfaceshave not been moved recently. A measure of how long ago movementlast occurred can be provided by clasts that stand on marl pedestals(Fig. 3C). Raised shore platforms belonging to the youngest raisedshoreline developed between 3000 and 2000 BP (Smith et al., 2010)developed in marl and covered by raised beach deposits stand 2.0–2.5 m above the present strike canal floors at the rear of the modernshore platform, implying rates of vertical erosion of N0.7–1.3 mm/yr.These rates are comparable to those estimated from short-term fieldand laboratory measurements on comparable soft rock lithologies(Porter et al., 2009; Stephenson et al., 2010). Pedestals are 40–240 mm in height (median 100 mm, n=76) and, assuming averagerates of erosion of 1 mm/yr, require that the boulders have been staticfor ~40–240 (median 100) yr. On this basis, boulders on pedestals orwith highly weathered surfaces have not been moved by waves in atleast the last few decades and these are described as recently static. Incontrast, boulders without pedestals and with fresh surfaces aredescribed as recently mobile. The largest boulders for which there is noevidence of wave transport are described as static.

Various equations have been used to estimate the wave currentvelocities needed to quarry or entrain blocks on coastal platforms(Goto et al., 2009, 2010 and this volume; Hansom et al., 2008;Noormets et al., 2004; Nott, 2003; Switzer and Burston, 2010). TheNott (2003) equations underestimate the capacity of storm waves tomove blocks by sliding (Hall et al., 2010). Observations during the 30/03/2010 storm (Fig. 2) indicate that wave currents on the shoreplatform mainly derive from broken wave water. The Barltrop equa-tions developed to model the movement of bore water from brokenwaves (Hansom et al., 2008) are applied here to quarrying of tabularblocks and to transport by sliding across the shore platform. Theresults for wave current velocity are reported as maximum values asthe drag forces needed to remove blocks from pedestals or fromsloping or uneven surfaces would be less than those necessary tomove for blocks in full contact with the platform surface. For recently

vement and shore platform development: A case study from East

Fig. 3. The Seacliff–Scoughall shore platform. A. Shore Platform near Seacliff. (A) Cliff line of Main Postglacial Raised Beach. (B) Raised beach of the Blairdrummond Raised Shoreline.(C) Gravel and boulder beaches incorporating recently static sandstone boulders earlier carried along the rear of the shore platform from the cliff. (D) Strike canal with pond.(E) Sandstone rock steps near LWMS from which blocks have been quarried to form boulder trains. B. Platform between Scoughall and Seacliff. Backing cliffs fronted by stormbeaches, with an extensive area of elevated platform developed on resistant sandstone with few boulders. In the foreground, a strike canal developed in red marl and thin limestoneand sandstone beds. C. Recently static perched basalt boulders with marl pedestals. Metre rule for scale. D. Recently moved sandstone boulder, with exposed pedestal.

4 A.M. Hall / Marine Geology xxx (2010) xxx–xxx

static blocks, the estimated maximum wave current velocity for en-trainment has not been exceeded in at least the last 40 yr.

4. Results

In exposed locations along the seaward edge of the platform,boulders are confined to traps between bedrock highs, including thefloors of large pot holes. Elevated areas of the shore platform alsooften lack boulders of any rock type (Fig. 3B). Other parts of theshore platform carry scatters of boulders of various frequencies andsizes, with the densest concentrations found towards the inner edgeof the platform, especially to the south. In addition, spreads ofclosely packed boulders and gravel are found near LWMST (Fig. 1).

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

Sharp lateral variations in the sizes of recently static boulders existin relation to the elevation of the shore platform surface. Strikecanal floors tend to have few boulders whereas adjacent strikeridges 1–2 m above the canal floors have numerous perched andweathered blocks.

Quarry sites near LWMS are evident from angular sockets anddetachment scars. Boulder trains can be traced back to sources in bothquarry and collapse sites (Fig. 1B). Sandstone sockets and bouldersshow variable degrees of surface pitting and edge rounding (Fig. 3C),indicating that quarrying and transport have been episodic. Furtherevidence of episodic boulder movement over short distances isprovided by boulders that have been recently removed from pedestals(Fig. 3D).

vement and shore platform development: A case study from East

Table 1Summary data for boulder movement at three study sites. Sliding velocities for tabular clasts calculated using the Barltrop formula in Hansom et al. (2008).NGR: National Grid Reference.

NGR (all NT) Rock type Size(m)

Volume(m3)

Density(g/cm3)

Mass(t)

Sliding velocity(m/s)

Easting Northing a b c

The GeganSeaward edge of platform Largest recently static 60,383 84,776 Agglomerate 3.75 3.66 1.02 14.0 2.36 33.04 8.6

Largest recently mobile 60,368 84,759 Agglomerate 3.50 3.30 0.78 9.0 2.36 21.26 7.9Mid-platform Largest recently static 60,328 84,778 Agglomerate 2.15 1.50 0.73 2.4 2.36 5.56 5.0

Largest recently mobile 60,323 84,742 Agglomerate 1.70 1.25 0.30 0.6 2.36 1.50 4.4Largest moved on 30/03/2010 60,322 84,750 Agglomerate 0.88 0.58 0.22 0.1 2.36 0.26 3.0

Landward edge of platform Largest recently static 60,258 84,771 Agglomerate 1.80 1.60 0.50 1.4 2.36 3.40 5.5Largest recently mobile 60,271 84,785 Agglomerate 1.25 0.84 0.33 0.3 2.36 0.82 3.7Largest moved on 30/03/2010 60,327 84,777 Agglomerate 0.27 0.23 0.14 0.0 2.36 0.02 2.1

SeacliffSeaward edge of platform Largest recently static 61,074 84,595 Sandstone 2.60 1.86 0.76 3.7 2.55 9.37 6.0

Largest recently mobile 61,122 84,534 Basalt 1.18 1.05 0.57 0.7 3.04 2.15 5.6Mid-platform Largest static 61,178 84,456 Basalt 3.00 1.87 1.90 10.7 3.04 32.40 6.8

Largest recently static 60,980 84,525 Sandstone 2.10 1.32 1.07 3.0 2.55 7.56 5.7Largest recently mobile 60,975 84,515 Basalt 1.15 0.88 0.80 0.8 3.04 2.46 4.9Largest moved on 30/03/2010 61,084 84,635 Sandstone 0.45 0.42 0.14 0.0 2.55 0.07 3.5

Landward edge of platform Largest recently static 61,003 84,425 Basalt 1.59 1.15 0.59 1.1 3.04 3.28 5.5Largest recently mobile 61,093 84,372 Basalt 1.02 0.91 0.43 0.4 3.04 1.21 5.2Largest moved on 30/03/2010 61,112 84,358 Sandstone 0.73 0.48 0.22 0.1 2.55 0.20 3.4

ScoughallSeaward edge of platform Largest static 61,987 83,589 Basalt 2.51 1.67 0.74 3.1 2.55 7.91 6.4

Largest recently static 61,845 83,766 Basalt 3.05 1.93 1.23 7.2 3.04 22.01 6.9Largest recently mobile 61,715 83,766 Basalt 1.89 1.39 0.40 1.1 3.04 3.19 5.8

Mid-platform Largest recently static 61,697 83,665 Basalt 3.56 1.58 0.90 5.1 3.04 15.38 5.8Largest recently mobile 61,625 83,752 Basalt 1.40 1.22 0.87 1.5 3.04 4.53 6.0Largest moved on 30/03/2010 61,584 83,577 Sandstone 0.48 0.27 0.16 0.0 2.55 0.05 2.2

Landward edge of platform Largest static 61,799 83,317 Basalt 2.40 1.62 1.26 4.9 3.04 14.89 5.6Largest recently static 61,555 83,589 Sandstone 2.10 1.57 0.85 2.8 2.55 7.15 5.6Largest recently mobile 61,594 83,580 Basalt 1.90 0.84 0.61 1.0 3.04 2.95 4.3

5A.M. Hall / Marine Geology xxx (2010) xxx–xxx

4.1. Distribution of basalt boulders across the shore platform

Basalt boulders are irregularly distributed (Fig. 1C). A few, isolated,very large (4–10 m3) static boulders with strongly weathered surfacesare found in mid-platform locations and there is no evidence thatthese have been moved by waves for many centuries, if ever. Alongthe outer edge of the shore platform from north to south, there is adrop in the volume of the smallest recently static basalt boulders from1.0–2.0 m3 to 0.50–0.76 m3 with a corresponding decrease from1.07 m3 to 0.32 m3 along the inner edge for the largest recentlymobileboulders. Moving across the southern part of the shore platform,recently static basalt boulders decrease in minimum size from 0.50–0.76 m3 at LWMST to 0.21–0.43 m3 in boulder spreads near HWMST.

4.2. Distribution of boulders within three study areas

4.2.1. The GeganThis area shows the largest wave-transported blocks on the entire

shore platform (Fig. 4A). A cluster of agglomerate blocks includesclasts of 5.0–9.0 m3 (Fig. 4B) that have been quarried by waves andlifted from sockets on the low tide cliff andmoved 20–30 m landward.The uppermost blocks have sharp edges and little weathered surfacesand so probably have been recently mobile. Photo comparisonsindicate that this boulder cluster existed in 1985 but not in 1952(Fig. 4C, D). It is likely that these boulders were emplaced during theNorth Sea storm of 31/01/1953, probably the greatest storm to affectthis coast in the last 500 years (Hickey, 2001).

Clast imbrication and boulder train orientation indicate that thedominant storm waves arrive at the leading edge of the platform atThe Gegan from the north-east. The rapid reduction in clast sizetowards HWMST is consistent with dispersal of wave energy acrossthe platform by wave diffraction, as observed during the 30/03/2010storm (Fig. 2). This storm was associated with movement of boulders

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

of agglomerate and masonry of up to 0.1 m3 across the mid and rearparts of the platform (Table 1).

4.2.2. SeacliffThe shore platform near Seacliff is backed at HWMST by boulder

and coarse gravel storm beaches where the largest recently mobileclasts generally are b0.4 m3 (Figs. 3A, 5). The front edge of stormbeaches and rear of the shore platform have recently static blocksof 0.2–0.4 m3. Near LWMST a few basalt boulders up to 0.7 m3 arerecently mobile. Recently static sandstone boulders as small as0.15 m3 are found in places close to LWMST but 0.4 m3 is a moretypical minimum size on the seaward edge of the platform (Table 1).

Recently static boulders of sandstone and basalt resting on lesscompetent beds occur widely. Blocks quarried from rock steps nearLWMST include clasts up to 3.7 m3. Boulder trains include individualboulders of 0.6–1.0 m3, and show only a limited decrease in size tolandward. At the cliff in the south-east corner of the study area(Fig. 3A), boulders up to 1.1 m3 released by rockfall have been trans-ported to both north-west and south-east by wave currents thatbifurcated at the head of a strike canal. Boulders up to 0.1 m3 weremoved here in the 30/03/2010 storm.

4.2.3. ScoughallThe elevated northern part of the shore platform near Scoughall is

free of boulders (Fig. 3B), but those parts of the rest of the platform atmid-elevation carry numerous large boulders that have not beenmoved recently (Fig. 6). These include isolated 5–7 m3 basalt boulderswhich are regarded as static. The largest recently mobile basaltboulders reach 0.97–1.49 m3 on the floor of the strike canal. Perched,recently static basalt boulders 1–2 m higher on adjacent cuesta creststend to be 0.2–0.4 m3 larger. Recently static sandstone boulders reach1.4–2.8 m3. Storm beaches at HWMST include several recently mobilebasalt boulders up to 0.33 m3. Recently mobile clasts towards LWMST

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6 A.M. Hall / Marine Geology xxx (2010) xxx–xxx

are larger, with maximum sizes of 0.62 m3 for sandstone and 1.05 m3

for basalt, but the largest clast moved in the 30/03/2010 storm wasonly 0.02 m3 (Table 1).

Fig. 4. (A) Distribution and size of agglomerate boulders at The Gegan. Blue arrowsindicate directions of wave current flowdirections based onmatching boulder lithology toquarry sites. (B) View to the NE at The Gegan, showing wave-transported agglomerateblocks in relation to the wave-quarry zone. The white flashes on the broken face of theblock next to the metre stick are fresh calcite crystals that can be matched to those at thelow tide cliff. Metre rule for scale. (C) Vertical 1:10,000 air photograph from 1952.(D) Detail of Fig. 4D. The present location of the boulder cluster shown in Figs. 2B, 4A and Bis within box X but no large boulders are visible. In contrast, boulders emplaced before1952 are visible in box Y. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

Fig. 5. Distribution of recently mobile and recently static basalt and sandstone bouldersnear Seacliff. Black arrows indicate recent wave current flow directions as indicated byimbricate boulders.

Fig. 6. Distribution of recently mobile and recently static basalt and sandstone bouldersnear Scoughall.

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

5. Discussion

The sizes of the largest blocks on shore platforms, reefs and cliff topshave been increasingly used recently as a proxy for estimating themaximum tractive forces generated by waves in storms and tsunamis

vement and shore platform development: A case study from East

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(Etienne and Paris, 2010; Goto et al., this volume; Hansom et al., 2008;Nott, 2003; Switzer and Burston, 2010). Only in exceptional instances(Goto et al., 2010; Noormets et al., 2004), however, do sufficient largeblocks occur to allow identification of spatial variations in wave energyalong and across a reef or platform. This East Lothian shore platformrepresents the first such shore platform to be examined in these termson a high energy, meso- to macro-tidal coast.

All basalt boulders that remain on the Seacliff–Scoughall shoreplatform have not been removed to the storm beaches to landwardand so represent a lag deposit. Sandstone and agglomerate bouldersmay also include clasts with a long residence time on the shoreplatform but significant numbers of fresh clasts are generated byquarrying and collapse. It is clear, however, from the organization ofthe boulders into clusters, and from movement away from sourcesthat all, but perhaps the very largest static boulders have been movedlandward by waves, either recently or during the past few thousandyears when sea level has been within a fewmetres of its present level.With no known record of tsunami impacts on this coast since 8000 yrBP (Smith et al., 2004), the transport of large boulders must relateexclusively to wave currents set up on the platform surface duringNorth Sea storms. Recent movement off pedestals and away fromcollapse sites and observations after the 20/03/2010 storm indicatethat at least some block movement during storms proceeds by asequence of short landward steps.

Waves breaking on the platform edge are capable of generatingforces sufficient to release large joint-bounded blocks from socketsand to move these and any pre-existing blocks off the front edge ofthe platform (Table 1). At The Gegan, the largest blocks reach 5–9 m3

(Fig. 4B), implying generation of wave current velocities forquarrying of up to 10.0–14.9 m/s and for transport of 5.5–7.9 m/s(Table 1). Near Seacliff, a large sandstone block, probably not movedrecently, indicates a transport velocity of 5.6 m/s. South of theScoughall study area, the largest transported agglomerate boulderrequires a transport velocity of 4.3 m/s. These largest recentlymoved boulders weigh over 20 t (Table 1). Quarrying and transportof much larger blocks of up to 100 t during storms have beenreported from a few shore platforms and reefs in other parts of theworld (Bishop and Hughes, 1989; Etienne and Paris, 2010; Gotoet al., 2009 and this volume; Saintilan and Rogers, 2005) and recentobservations suggest that 1–10 m3 wave-transported blocks occur atnumerous sites on exposed platforms close to sea level around thecoast of the British Isles (Hall et al, 2010; Knight et al., 2009).Hydrodynamic forces calculated from waves measured on shoreplatforms in a high energy wave environment at Kaikoura, NewZealand, indicate, however, that, in this location, these forces do notexceed the compressive strength of the platform rocks (Stephensonand Kirk, 2000). This contrast may be a result of different waterdepths offshore. Shallow water offshore causes waves to breakbefore reaching the platform edge at Kaikoura (Stephenson and Kirk,2000) but observations during the moderate intensity storm of 30/03/2010 showed 3–5 m high waves breaking on the seaward edge ofthe Seacliff–Scoughall platform (Fig. 2).

Maximum wave current velocities in storms within the last ~40–240 yr are constrained at any location on the shore platform by thesizes of the smallest recently static boulders and of the largest recentlymobile boulders. Maximum wave current velocities in earlier stormsare constrained by the sizes of large, weathered sandstone bouldersthat form boulder trains across the shore platform. There is no clearevidence that the largest (4–10 m3) basalt boulders found in mid-platform locations have ever been moved, implying that wave currentvelocities at these sites have never exceeded 6.3–7.6 m/s (Table 1).Nonetheless, the largest recently mobile boulders imply wave currentvelocities of 3–4 m/s have been achieved in storms in the last ~40–240 years. Moreover, the large, weathered sandstone blocks inboulder trains show that earlier storms were associated with wavecurrent velocities of up to 5 m/s across parts of the shore platform.

Please cite this article as: Hall, A.M., Storm wave currents, boulder moLothian, Scotland, Mar. Geol. (2010), doi:10.1016/j.margeo.2010.10.024

These estimates leave little doubt that fast-moving wave currentscross parts of the shore platform during storms.

Sharp lateral variations in boulder sizes, together with thepersistence of boulder and gravel spreads in outer and mid-platformlocations, indicate, however, that significant spatial variability existsin storm wave current velocities across the platform. Shore platformtopography is an important local control onwave current velocity. Notonly do larger blocks occur along the rims of strike canals where stormwater depths are 1–2 m less, the only locations identified whereblocks of ~1 m3 have been moved by storm waves reaching the cliffline is at the rear of strike canals. At Scoughall, the smooth floor anddeeper water within the strike canals allow relatively rapid stormwave currents to cross the platform and erode the base of the backingcliff.

The absence of boulders from the most elevated parts of the shoreplatform is anomalous under these circumstances, as storm waterdepths would be relatively shallow in these locations. Away from themost exposed sites on the seaward edge of the platform, it issuggested that all boulders were removed from these bedrock highsduring the postglacial sea level maximum phase before 3400–1800 yr(Smith et al., 2000).

Whilst quarrying of large blocks by breaking waves occurs alongthe leading edge of the platform, the Barltrop equations indicate thatstorm wave current velocities of 3–4 m/s on the rest of the plat-form are capable of the extraction of only small (b0.4×0.3×0.2 m)fracture-bounded blocks from sockets (c.f. Trenhaile and Kanyaya,2007). On much of the platform therefore the release of larger blocksis probably largely reliant on weathering and erosion of weak marlbeds and the resultant undercutting and collapse of competent units.The estimated wave current velocities have the potential, however, toextract small blocks from the thinly-bedded marls on the strike canalfloors and to transport all small fragments to landward.

Early theories of shore platform formation considered that waveenergy became progressively more attenuated as shore platformswere widened until wave erosion was no longer effective at the baseof the cliff (Johnson, 1919). Attenuation in maximum wave currentvelocities across the Seacliff–Scoughall platform is not strong,however, except at The Gegan, where storm wave energy is refractedand so dissipated across the rear of the platform (Fig. 2A). For recentlymoved basalt boulders, the size range across the southern part of theplatforms indicates that maximum wave current velocities decreaseonly from 5 to 4 m/s from LWMST to HWMST. Similarly, the recentlystatic sandstone boulders in trains at Seacliff indicate a landwarddecrease of maximum velocity from 4.6 to 3.3 m/s over a distance of140 m. On the floor of a strike canal at Scoughall, clast size reductionover 90 m indicates a drop in velocity of only 5.8 to 4.8 m/s. Theseresults support mathematical models (Trenhaile, 2000) that indicateonly limited attenuation of wave current velocities away from theseaward edge of shore platforms.

The range of evidence for effective wave erosion on this shoreplatform should not obscure the important role of weathering in itsdevelopment. Falling sea level has induced lowering at rates of~1 mm/yr of those parts of the platform developed in soft or thinly-bedded Carboniferous sedimentary rocks. Erosion of the floors ofstrike canals has led directly to undercutting and retreat of smallcuestas. The consequent progressive increase in water depths hasallowed faster wave currents to access parts of the cliffline. In contrast,the erosion of the resistant beds that form the highest, and likelyinherited parts of the shore platform (Hall, 1989), has been muchmore restricted, as indicated by the limited amount of collapse debrisaround fringing rock steps, except where breaking waves can quarryblocks from rock steps at LWMST. As in many other parts of the world(Trenhaile, 2008; Trenhaile and Porter, 2007), the erosion of thisshore platform in East Lothian can only be understood in terms of thecombined and spatially-variable action of wave action andweatheringon diverse rock types operating over timescales of 101–103 yr.

vement and shore platform development: A case study from East

8 A.M. Hall / Marine Geology xxx (2010) xxx–xxx

6. Conclusion

The distribution of wave-transported boulders provides evidenceof storm wave dynamics across a shore platform in East Lothian. Instorms over the last 40–240 yr, blocks as large as 9 m3 have beenquarried at the low tide cliff and boulders of N0.5 m3 have carriedlandward over extensive areas of the platform, leaving the seawardedge of the shore platform largely free of boulders. In earlier storms,boulders N1 m3 have been moved across large areas of the shoreplatform. Wave current velocities have likely reached 3–4 m/s butattenuation of storm wave current velocities across the platformsurface is weak. The distribution of boulders of different sizes indi-cates sharp lateral variations in storm wave current velocities. Thetopography of the shore platform is an important control on wavecurrents, with the fastest flows developed along strike canals wherewater depths are greatest.

Wave action is a key process in shore platform erosion through thequarrying and removal of coarse clastic debris. The role of water-layerweathering is also important, as lowering of soft marl beds at rates of~1 mm/yr leads to the undercutting and eventual collapse ofoverlying, more competent rock beds. Falling sea level over the lastfew thousand years has led to rapid erosion of weak rock beds on theplatform, but the platform retains inherited bedrock highs developedin resistant sandstone and agglomerate beds. Even on an exposed,macro-tidal coast, shore platform evolution involves spatially andtemporally variable interactions between wave erosion and weath-ering processes.

Acknowledgement

Field work was supported by The Carnegie Trust for theUniversities of Scotland.

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