Results: Organization and Architecture of Boulder Clusters and Boulder Ridges

Abstract

The organization patterns of large coastal boulder deposits on the Aran Islands and within Galway Bay exhibit many different patterns: single platy boulders are leaning in low or steep inclination at structural steps on the upper cliff platform, but also show imbrication from different sides at an obstacle, or form imbrication trains even if the single clasts are many tens of tons in mass. Another setting is balancing on the crests of ridges, even if their mass is more than 10 tons and the crest is positioned at +15 m or more above MHW. The source for single boulders often is the cliff top front, or low structural steps on it, and only in rare cases in the tidal and supra-tidal zones. Most of freshly activated boulders, however, derive from older ridge fronts. The most significant form of deposition is a very long and 20–50 m wide ridge with a relative height of many metres. Cross sections of these ridges are asymmetrical with steep seaward faces. Lower ridges may occur to landward, separated from the main one, and at some promontories more than three ridges or boulder piles exist, all older than the main one. A special aspect are two different types of ripples: small ones parallel to the ridge crests at the landward slope, and wider diagonal and curved ones forming the main ridge in a longer row.