STORM-WAVE EMPLACEMENT OF CLIFF-TOP BOULDER RIDGES, ARAN ISLANDS, IRELAND

Boulder ridges decorate the steep western ocean-facing edges of Ireland’s Aran Islands. The largest ridges (up to 5 m high, 75 m wide, with clasts up to 70 tonnes at their crests) occur at lower elevations (1-10 m); but ridges up to 20 m wide and 2 m high, with half-tonne clasts, occur up to 28 m a.s.l. on Inishmore. The scale of the ridges has fuelled debate as to whether they require tsunami for emplacement or whether storm waves are capable of moving the very large clasts into position.

The ridges are deposited on bedding-plane platforms, at the tops of rock terraces and cliffs. Constituent clasts are tabular limestone blocks, broken along bedding planes and joints. Lithologic matching indicates that the clasts come from the subjacent platforms: they are pried up from the top 3 m of the ledges or cliffs, shoved inland, and piled into ridges. Gouged tracks on platform surfaces demonstrate recent movement of specific large blocks: e.g. a 3.5 tonne clast moved 12 m at 5 m a.s.l. on Inis Mór (measured July 2008), and a 20 tonne clast left a 10 m track at 5 m a.s.l. on Inis Oírr (measured June 2009). White powdery scrapings in the boulder tracks attest to their freshness.

Hydrodynamic calculations indicate that storm waves can transport the blocks in the Aran Islands boulder ridges. Irish Marine Institute buoy data show that the local wave climate can be extreme: waves 14-22 m were recorded on 9 days in 2001-2008. The islands rise out of deep water, with 10 m depth <50 m from shore in many places. Large waves therefore reach their breaking depth less than 1 wavelength from the coast. Coast steepness promotes reflection, and standing waves up to 2x original height can form under these conditions. Thus, cliffs 40 m high could be overtopped by waves of dimensions recorded in recent years, with resultant bores up to several m thick. A 1 tonne clast requires only a 1 m thick bore for storm-wave transport, and such bores can be generated at the top of 30 m cliffs by 20 m waves amplified by reflection. Moving a 70 tonne clast requires a bore about 5 m thick, which is no problem with a 20 m wave coming ashore at 10 m a.s.l. Although the same clasts could be moved by tsunami bores of lesser thickness, there is no evidence for tsunami on the timescales in question; and the local wave climate is demonstrably capable of generating bores of the appropriate magnitude.