Geological controls on boulder production in a rock coast setting: Insights from South Wales, UK

Abstract

We investigate some of the geological controls on boulder size, using a heterogeneous outcrop of Blue Lias limestone. Discontinuity and predicted boulder size data were collected on different layers of Blue Lias limestone on a shore platform in Wales, United Kingdom. Significant differences in joint spacing were found between the four layers examined and significant differences in the predicted boulder size were found between many of the ten stratigraphic layers examined. Predicted boulder size ranged from 0.18 to 4.00 m measured on the longest axis. These data demonstrate that the shape and size of boulders liberated from the platform is dependent on which layer of limestone is at the surface. Since the size of boulders varies between layers depending on the thickness of beds and joint spacing, the magnitude of wave energies required to detach, entrain and transport boulders must vary. Boulder erosion is therefore geologically contingent and highly variable through time and space. This variability requires us to develop methods to predict where boulder-scale erosion is likely to occur and what wave conditions are required to add these materials to the sediment supply. We therefore considered the reliability of predictions of boulder size. For two layers examined (layers 19 and 24), predicted boulder sizes were compared with boulders recently detached by storm waves (i.e. within 2.5 months of detachment) from the same layer. Results show that boulder volume was statistically similar between predicted and actual boulders, and either the long or intermediate axis measurements were statistically similar. Variations between predicted and actual boulder axis lengths are attributed to the sampling design for measuring predicted boulders, variable detachment processes and the rapid breakdown of rock masses post-detachment. Our results demonstrate that in locations where repeated entrainment, transport and boulder breakdown are observed, loose boulder size may not be a reliable means of reconstructing wave energies required to detach, entrain and transport boulders.