In situ exhumation from bedrock of large rounded boulders at the Giant’s Causeway, Northern Ireland: an alternative genesis for large shore boulders (mega-clasts).

Very large boulders (mega-clasts) are found on some coasts. The size and position ofthe boulders has been used to suggest that contemporary marine processes, actingwithin their normal spatial and energy range, are unlikely to have moved them.Explanations for the presence of such boulders include transport by infrequent veryhigh energy marine processes (storms or tsunamis), mass movement from backingcliffs, transport by ice, or exhumation from glacial deposits. This paper advances analternative explanation which does not involve transport by any of the marine orglacial processes, or gravity. It is proposed that, in a very specific geological andtopographic setting, large boulders are exhumed in situ by storm-waves acting onheavily weathered jointed basalts. Eventually wave action liberates residual blocksfrom the deeply weathered matrix. These liberated boulders will be mobile only ifthey lie within wave competence, and the larger ones will remain as stationaryresiduals. The same in situ weathering processes, followed by removal of the friablematrix material debris by wave action, also progressively round the boulders.Consequently, despite their appearance of being transport-rounded, the larger bouldershave not transported at all. In specific locations, the assumption that the presence, androunding, of such large clasts in the shore zone can be attributed to marine transportcan lead to erroneous interpretations of very high-energy storm-wave (or tsunami)activity.