Journal of Coastal Research, Special Issue No. 70, 2014 A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland 121 A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland Jasper Knight†, Helene Burningham‡ † School of Geography, Archaeology & Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa jasper.knight@wits.ac.za ‡ Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK h.burningham@ucl.ac.uk INTRODUCTION A range of coastal gravel structures including barriers, bars, banks and spits has been well documented worldwide. Most gravel structures are found where coastline orientation changes abruptly. They are usually elongate structures with a steep shoreface, attain a height of several metres above sea level, and are often characterised by systematic changes in clast size along the length of the structure (Carter, 1983; Orford et al., 2002). Most of the documented examples of gravel structures are formed by onshore transport of clasts during storm events and associated with strong storm wave setup and in response to long-term changes in sea level (Forbes et al., 1991; Carter and Orford, 1993; Orford and Anthony, 2011). As such, most gravel structures are assumed to form episodically under contemporary conditions, and to attain a maximum threshold height with respect to incoming storm waves and/or sea-level position (Orford et al., 1995). However, coarse clastic (gravel) structures that are documented from glaciated coasts are more ambiguous and probably polygenic in origin (Carter and Orford, 1988). This is because glacier retreat often leaves gravel-rich moraines and drumlins in coastal lowland settings where eustatic and glacioisostatic sea-level rise can rework these sediments, leaving a winnowed lag of glacigenic clasts from which the finer matrix has been washed away (Carter et al., 1990; Forbes and Syvitski, 1994; Greenwood and Orford, 2007; Hayes et al., 2010; Hoffmann et al., 2010). It is notable that many coastal dune fields across northwest Europe are anchored on gravel ridges (e.g. Orford et al., 2003) which are likely to have a similar history. Such gravel landforms are commonly overstepped by postglacial sea-level rise, forming a drowned or welded barrier, and winnowing may result in formation of sandy beaches, saltmarsh or lagoonal sediments adjacent to a residual moraine core (Carter and Orford, 1988). The term paraglacial has been applied to such coastal landforms (Forbes and Syvitski, 1994) because their morphology and post-depositional evolution have been strongly influenced by glaciation (cf. Church and Ryder, 1972). Although examples of coarse clastic paraglacial structures, in particular barriers, have been described from eastern Canada and USA (Forbes et al., 1995a; FitzGerald and van Heteren, 1999), few have been described from other glaciated coastal lowlands or drowned continental shelves (Ruz, 1989; Johnston, 2001). Here, we describe one element of a paraglacial coastal landscape in northwest Ireland – a cobble-mantled bank – where a combination of high glacigenic sediment supply and postglacial sea-level rise has resulted in a complex history of sediment reworking, morphological evolution, and polygenic physical attributes of these elements in today’s landscape. ABSTRACT Knight, J. and Burningham, H., 2014. A paraglacial coastal gravel structure: Connell’s Bank, NW Ireland. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13 th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 121-126, ISSN 0749-0208. Coastal gravel structures have been well documented worldwide and are formed dominantly by onshore wave transport of gravel, mainly during storm events. They are commonly observed along paraglacial coastlines where their origins are more ambiguous because of the effects of antecedent patterns of sediment supply, glacioisostatic sea-level change, and contemporary coastal processes. This paper describes the properties and polygenic origin of Connell’s Bank, a small paraglacial gravel structure on the Atlantic-facing coast of NW Ireland. This feature has been shown on historical maps, air photos and satellite imagery since c. 1850 but its outline has varied depending on seasonal migration, expansion and contraction of a sand veneer. It has also acted as a major control on tidal channel position within the estuary, and thus on sensitivity of the estuary system to ocean forcing. In detail, the bank surface is composed of cobbles sourced from outside of the immediate catchment. These surface cobbles are strongly winnowed, forming a lag deposit, whereas below the surface, cobbles exist within a granule and shell matrix. A significant proportion of surface cobbles shows evidence for recent ventifaction by blown sand at low tide. The paraglacial evolution of Connell’s Bank since the last glaciation comprises the following stages: (1) deposition of coarse glacigenic sediment as a moraine or proximal outwash fan during the late Pleistocene lowstand; (2) reworking of sediments onshore during early Holocene sea-level rise; (3) surface winnowing during mid to late Holocene tides and storms; and (4) surface cobble modification by contemporary wind abrasion. ADDITIONAL INDEX WORDS: Paraglacial, glacial sediment, ventifacts, erosional lag, gravel barrier, landscape palimpsest. www.JCRonline.org www.cerf-jcr.org ____________________ DOI: 10.2112/SI65-021.1 received 2 December 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2013