In situ exhumation from bedrock of large rounded boulders at the Giantʹs 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 of the boulders has been used to suggest that contemporary marine processes, acting within their normal spatial and energy range, are unlikely to have moved them. Explanations for the presence of such boulders include transport by infrequent very high-energy marine processes (storms or tsunamis), mass movement from backing cliffs, transport by ice, or exhumation from glacial deposits. This paper advances an alternative explanation which does not involve transport by any of the marine or glacial processes, or gravity. It is proposed that, in a very specific geological and topographic setting, large boulders are exhumed in situ by storm waves acting on heavily weathered jointed basalts. Eventually wave action liberates residual blocks from the deeply weathered matrix. These liberated boulders will be mobile only if they lie within wave competence, and the larger ones will remain as stationary residuals. The same in situ weathering processes, followed by removal of the friable matrix material debris by wave action, also progressively round the boulders. Consequently, despite their appearance of being transport-rounded, the larger boulders have not transported at all. In specific locations, the assumption that the presence, and rounding, of such large clasts in the shore zone can be attributed to marine transport can lead to erroneous interpretations of very high-energy storm wave (or tsunami) activity.