Formation of Bedrock‐Cut Ventifacts and Late Holocene Coastal Zone Evolution, County Donegal, Ireland

Abstract

Wind‐etched rocks (ventifacts) at Gweebarra Bay, County Donegal, northwestern Ireland, are located <+2 m above mean high‐water springs (MHWS) on a glacially eroded bedrock shore platform (<10 m wide, 200 m long at MHWS). Prevailing southwesterly winds transport sand across the platform from the adjacent beach. Ventifacts, found on the windward‐most 20 m of the platform, decrease in frequency and degree of wind alteration with distance. Near the beach, ventifacts have two to three wind‐etched faces (facets), both linear and serrated keels, and are pitted, grooved, and polished. Farther from the beach, ventifacts are less frequent; have fewer facets, pits, and grooves; and show curved keels. Facets nearest the beach lie perpendicular to prevailing winds but are wind parallel farther away. This reflects structural control on the shape and orientation of bedrock blocks protruding into the wind stream. Nine possible ventifact morphologies are identified on the basis of wind‐approach angle and dip angle of the windward face. Three morphological types dominate at Cashelgolan (moderately inclined, perpendicular facets; moderately inclined, oblique facets; steep, oblique facets). Calculated sand transport is likely only for limited (25%–28%) periods of time under the contemporary wind regime. This suggests ventifacts are not developing actively at this time but may reflect long‐term (more than hundreds of years) wind activity and/or stronger winds. Historical evidence for high‐magnitude winds and storms suggests ventifacts formed within the last 2000 yr, possibly during the Little Ice Age (LIA). Regional evidence also shows that coastal regimes intensified during the LIA, with increased onshore winds, waves, sediment fluxes, and periods of sand dune building.