DISCUSSION OF MEGACLAST TRANSPORT AND EROSION OF ROCKY SHORELINES: A COMMENT ON WILLIAMS (2010)

Abstract

DISCUSSION OF MEGACLAST TRANSPORT AND EROSION OF ROCKY SHORELINES: A COMMENT ON WILLIAMS (2010) JASPER KNIGHT A recent paper by Williams (2010) described very glacial sediment and/or preglacial weathered debris large boulders (megaclasts) from the Aran Islands, (Moles and Moles 2002). This variable sediment Galway Bay, and argued that wave overtopping of cover strongly influenced patterns of postglacial high cliffs caused clast detachment and landward weathering. Although the extent of glaciation on transport by wave bores. Here I highlight limita- the Aran Islands is unclear (McCarron 2007), tions of Williams's arguments, and show that clast glacially-influenced preconditioning of rock surfaces detachment and transport involves complex pro- through weathering is extremely significant but was cesses and conditions not considered in his paper. not considered by Williams (2010). Pre-detachment conditioning of rocky shorelines Processes of clast detachment It is important to consider clast properties (size, Williams (2010) was concerned with identifying shape, strength, lithology, degree of weathering) the wave height required for clast transport, but prior to considering processes of clast detachment did not first consider processes by which clasts and transport. This is because pre-detachment are detached from rockhead. Limestone weather history strongly influences clast susceptibility to ing decreases rock strength, widens joints and detachment (Naylor and Stephenson 2010), the force makes clasts more easily detachable. Recent studies required for detachment (Nott 2003), and processes show a close relationship between weathering, rock of clast transport (Goto et al. 2009). Williams strength, and likelihood of clast detachment (e.g. does not consider pre-detachment history, but it Hall et al. 2008; Naylor and Stephenson 2010). is significant for the Aran Islands because of its Williams (2010) assumes that all areas of the underlying Lower Carboniferous limestone. Weath- upper cliff face are potential sites for clast detach ering and karstification has led to joint widening ment. However, recent studies show that rocky and block déstabilisation, evidenced by surface shoreline erosion is highly selective, strongly litho grikes, karren and dolines and subsurface caves, logically controlled, and that clasts are detached The Quaternary history of the region is also sig- preferentially from the intertidal zone (Naylor and nificant. Frozen ground during glaciations allowed Stephenson 2010; Knight and Burningham 2011). meltwater erosion to cut channels and oversteepen Williams (2010) applied Nott's (2003) equation for slopes. Permafrost and meltwater erosion were joint-bounded blocks in calculating the significant both active in western Ireland during the late Mid- wave height required for clast detachment. However, landian glaciation (Orme 1967; Lewis 1985). Differ- Nott (2003) was concerned with sloping shore ential glacial erosion also led to exposure of some platforms rather than cliffs, and his equations have bedrock surfaces whilst others were protected by now been superseded (Nandasena et al. 2011). Irish Journal of Earth Sciences 29 (2011), 23-24 ■© 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.23 23 24 Irish Journal of Earth Sciences (2011) Wave conditions required for clast movement Williams (2010) argued that a landward-moving bore formed as a result of cliff overtopping by white- (broken) or green-water (unbroken) waves. Bores can only form where waves are topographi cally funnelled across a shore platform or cliff-top (Hall et al. 2008). No topography was presented by Williams, hence it is unclear if bores are likely or not. Williams' paper's title explicitly stated it would illustrate the storm versus tsunami wave debate, but failed to do that. Scheffers et al. (2010) presented radiocarbon ages from organics below megaclasts on the Aran Islands and County Clare. They showed clasts were emplaced episodically throughout the last 3,000 years and, although there may be correspondence with periods of high stormi ness (Hansom and Hall 2009), a tsunamigenic origin cannot be rejected. Knight and Burningham (2011) mapped the distribution of upper shoreface boulders in County Donegal and showed that, although clasts can be detached under contempor ary wave conditions, there is a mismatch between the size of contemporary clasts and those that form ridges on the shore platform. This suggests that a spectrum of past and present wave sizes and there fore wave-generating mechanisms are responsible for clast detachment and transport. JASPER KNIGHT School of Geography...