STORM WAVES MOVE LARGE BOULDER RIDGES ON THE ARAN ISLANDS, IRELAND

Boulder ridges on the Atlantic coasts of the Aran Islands are linear or arcuate deposits of cobbles, boulders, and megagravel. The ridge deposits occur at elevations 1-40 m above Higher High Water (HHW), and at horizontal distances that range from a few m to 250 m inland from HHW. Some are perched on top of sheer cliffs, and others are at the back of wide, gently sloping platforms. The boulders come from seaward, eroded from the cliff top or platform surface and transported landward. Clast size is variable: average boulder size decreases at higher elevation, but lower elevation ridges (≈12 m above HHW) incorporate boulders weighing up to 78 tonnes.

Debate continues as to whether these features—and specifically the megagravel that they include—can be transported by storm waves. Work by Williams and Hall (2004) (http://tinyurl.com/3mxfqcn ) indicates that storms are the most likely agent, but these results have been challenged by Scheffers et al. (2010) (http://tinyurl.com/432vf4a ) who argue that tsunami must be responsible because storm waves are insufficiently powerful to move such huge blocks.

We present evidence that the ridges are formed and moved by storm waves. First, shells of Hiatella arctica (subtidal rock-boring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, GIS comparison of 19^th C Ordnance Survey maps with 21^st C orthophotos shows that in several areas the boulder ridges have advanced 10s of metres inland since the mid-19^th C, overrunning old field walls in the process. The advancing ridges include the segment with boulders up to 78 tonnes at 12 m above sea level, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Finally, recent motion is attested to by eye-witness accounts that pin movement of several individual 40-80 tonne blocks to a specific 1991 storm, and by repeat photography over the last few field seasons (2006-2011) that captures movement of boulders (masses up to 12 tonnes) even in years without exceptionally large storms. Thus there is abundant evidence for ridge activity since the 1839 mapping. As there have been no tsunami in the northeastern Atlantic in that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.