MECHANISMS OF WAVE TRANSPORT OF MEGACLASTS ON ELEVATED CLIFF-TOP PLATFORMS: EXAMPLES FROM WESTERN IRELAND RELEVANT TO THE STORM-WAVE VERSUS TSUNAMI CONTROVERSY

Abstract

The emplacement of very large clasts (megaclasts) onto cliff-tops many metres above sea-level has often been ascribed to the action of tsunami. In western Ireland, megaclasts are found on cliff-top platforms at up to 50 metres above sea-level having been emplaced by extreme storm waves without any tsunami action, making these some of the highest such modern deposits in the world. To test the role of storm-waves in this environment, an analysis of wave impact on vertical obstacles is presented partly based on previous work on artificial sea-walls and partly on examples from the cliffs of the Aran Islands. The impact of large waves onto cliffs can lead to overtopping of either green or white water. It is suggested that such overtopping results in the downward collapse of the overtopping water mass. This may result in the formation of a landward moving high-velocity bore. It is this bore which transports megaclasts, sometimes weighing tens of tonnes, across the cliff-top platforms. The impacting orbital wave is thus converted into a unidirectional bore and analysis is presented to ascertain the properties of such bores necessary to move megaclasts in the context of the Aran Islands.