MEASURING THE EFFECTS OF WINTER STORMS (2013-2014) ON BOULDER AND MEGAGRAVEL ACCUMULATIONS IN WESTERN IRELAND
Cobbles and boulders were moved in abundance, but some of the megagravel was impressively large. Examples of masses include an isolated block of ~430 t transported ~5 m laterally on a supratidal platform; slabs >20 t ripped from bedrock and incorporated into a ridge at 11 m AHW and 90 m inland; and an 18.5 t block moved from ridge base to crest (translated 12 m inland and 4.5 m vertically) in a ridge located 130 x m inland and 17 m AHW.
Our data document reorganisation of existing boulder accumulations as well as creation of new clasts from ripped-up bedrock. Within the boulder ridges, clasts shifted upward and inland. Boulders that had been low down on the seaward face of a ridge in last year’s photos were found further up the face, or in some cases at the ridge crest. Other clasts moved from the ridge face over the crest and into the back-ridge area. Within the back ridge, clasts travelled further landward. These observations indicate that the ridges migrate progressively landward. Mass was also added to the deposits. Uplift and dismemberment of bedrock slabs subjacent to the ridge base was the most common source of new material, but in some cases bedrock blocks were quarried and transported several 10s of m to their final resting places
The changes documented in this study prove the role of storms in growth and migration of supratidal boulder ridges, and provide insight into the sediment dynamics of these enigmatic deposits.