Journal of Coastal Research, Special Issue 56, 2009 Journal of Coastal Research SI 56 1646 - 1650 ICS2009 (Proceedings) Portugal ISSN 0749-0258 The geomorphology and controls on development of a boulder-strewn rock platform, NW Ireland J. Knight†, H. Burningham‡ and C. Barrett-Mold‡ † Department of Geography, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK j.knight@exeter.ac.uk ‡ Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK h.burningham@ucl.ac.uk, c.barrett-mold@ucl.ac.uk ABSTRACT KNIGHT, J., BURNINGHAM, H. and BARRETT-MOLD, C., 2009. The geomorphology and controls on development of a boulder-strewn rock platform, NW Ireland. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 1646 – 1650. Lisbon, Portugal, ISSN 0749-0258. A boulder-strewn rock platform on an Atlantic-facing coastline in Gweebarra Bay, NW Ireland, is examined using an integrated geomorphological, ecological and geotechnical methodology. Here, a steep granite shore platform is cut by intersecting subvertical and subhorizontal fractures and shows a clear biological and geomorphological zonation associated with level within the tidal frame. Minimum surface strength values of the bedrock platform (as derived from Schmidt hammer tests) corresponds to the supralittoral zone, which is dominated by Ramalina siliquosa. Bedrock surface strength increases into the littoral zone, which is characterized by a succession through Verrucaria maura, Semibalanus balanoides and Mytilus edulis communities. Granite boulders in the upper intertidal zone are clustered and stacked into ridges that have a consistent spacing and northeast-southwest alignment. Boulders within the ridges are imbricated, stacked, and have west-facing dips. Boulder surface strength is higher on northwest-facing ridge sides than on southeast- facing sides, and is higher at the landward than seaward ridge end. Variations in surface weathering across the platform show where blocks have been recently removed by storms. Pulverized bedrock and boulder surfaces show where boulders have knocked into each other and been dragged across the platform by backwash. It is likely that such boulder features are formed and destroyed on decadal or shorter time scales. ADDITIONAL INDEX WORDS: Boulder ridges, Storms, North Atlantic, Schmidt hammer, Coastal ecology INTRODUCTION AND AIMS Intertidal bedrock platforms, and the boulder-built structures that are developed upon them, are common features of exposed, high-energy coasts (TRENHAILE, 2002). These geomorphological features reflect a combination of strong onshore winds, open- ocean swell waves, and onshore storms that are generated by low- pressure atmospheric cells over the ocean (TRENHAILE, 2005; HALL et al., 2008). Such conditions characterize open-ocean coasts such as those in the British Isles, Iberia, Japan, and Canada (TRENHAILE, 2002). Bedrock platforms and boulder-built structures are particularly common in meso- and macrotidal settings in these paraglacial, mid-latitude areas (BLANCO-CHAO et al., 2007), where platform surfaces are swept clean by breaking waves that are able to shape the platform surface and erode, transport and organize any loose boulders located upon it (TRENHAILE, 2005). Studies in these areas have focused on the formation and controls on rock platform morphology (including width, steepness and microtopography) and rock cliff morphology and failure (e.g. DICKSON, 2006). Apart from boulder and gravel beaches, the boulder-built structures found in association with rock platforms, such as narrow ridges or scattered, isolated boulders, have been less commonly investigated. This may be due in part to the perception that low-density and isolated boulders are moved rapidly over the platform surface until they are lodged against other boulders or are lost from the platform by backwash transport seaward. Boulder-built structures within the intertidal zone of a rock platform are therefore perceived as transient features that have low preservation potential. Distinctive isolated and outsized boulders, however, have been extensively described from many locations, in particular those interpreted as tsunami deposits (e.g. SCICCHITANO et al., 2007). More widely, the role of high-magnitude storms has been argued to be a primary control on boulder-structure morphology and dynamics (e.g. HALL et al., 2006; HANSOM et al., 2008), because storms are accompanied by large waves which have a disproportionately large capacity to move coastal boulders. This highlights the potential of rocky coasts in exposed Atlantic-facing settings to be very responsive to storm and other events over short time scales. The aims of this present paper are to examine interrelated shore platform characteristics from the high-energy rocky coast of County Donegal, northwest Ireland (Fig. 1). In detail, this paper (1) describes the morphology, ecology and geotechnical properties of the rock platform; (2) describes the boulder-built structures developed in the upper intertidal zone of the platform; and (3) considers the controls on morphology of the platform and associated boulder structures. LOCATION AND METHODS The Atlantic-facing coastline of County Donegal, northwest Ireland, is high-energy, being exposed to high-magnitude onshore 1646