Paper No. 8
Presentation Time: 10:00 AM


COX, Rónadh, Geosciences, Williams College, Williams College, Williamstown, MA 01267,

The Aran Islands, off Ireland’s west coast, are exposed to strong Atlantic storms but have not been subject to tsunami activity in recent centuries. Their coastal boulder deposits therefore preserve an excellent record of the effects of large storm waves on decadal to millennial timescales. But although regional studies show large-scale patterns of sedimentation, it is difficult to pinpoint specific transportation events, and in particular to document the motions of very large boulders (masses several 10s of tonnes). Archival imagery can help bridge that gap.

Between 1931 and 1933 Robert Flaherty shot the classic film “Man of Aran” on Inishmore, the largest of the three Aran Islands, depicting island life and showcasing the rocky Atlantic coast with its battering storm waves. Eighty years later, his footage provides a documentary pinning point for analysing large-boulder movements. Fifteen minutes into the film, Flaherty shows his characters working their way homeward across a coastal limestone platform among the boulder piles. The area is identifiable as being near the Grí Óir at the western end of Inishmore. In 2012 it was possible to find the exact location where Flaherty positioned his camera and re-take the same shots. Comparison of photographs taken in June 2012 with frames from the movie reveals substantial rearrangement of the coastal boulder clusters and addition of new blocks.

Boulders up to 61 tonnes weight have been added to the coastal boulder deposit since 1933, at 6 m above highest high tide (11.2 m o.d.), and 45 m inland from the high water mark. The minimum force required to transport rocks of this mass (based on equations of Hansom et al. 2008, Marine Geology v. 253, p. 36-50) would be provided by a bore 7.5 m thick travelling at 8.6 m/s .

This result underscores the usefulness of historical imagery in chronicling the effects of storms on decadal to centennial time scales. It dovetails with previous work showing that Aran Islands boulder ridges have migrated since the mid-19th C , but is the first photo-documentation of specific coarse-boulder movements in Ireland in the 20th century.