Controls on the formation of coastal ventifacts

Ventifacted boulders are present within the intertidal zone of a mixed sand and boulder beach in Gweebarra Bay, northwest Ireland. The boulders show features typical of wind abrasion by sand including polished surfaces, pits and grooves. Orientation of ventifact keels was measured and direction of prevailing winds responsible for ventifaction was inferred from a sample of 50 boulders in each of two adjoining locations on the beach (30 m apart). The keel orientations and inferred wind direction are both strongly clustered but results from each location differ by 90° from one another, and neither corresponds closely to the present-day regional wind regime. Since wind flow patterns were not significantly different during the Little Ice Age, when the ventifacts were likely formed, the orientation of ventifact keels cannot be used uncritically, as in many studies, as a proxy record of prevailing wind direction. It is likely that ventifact development in Gweebarra Bay was controlled by sediment availability rather than by wind direction.