Dr P. A. Sabine enquired about the relationship of the spectacular long narrow inlets of the Shetland cliffs, the geos. Many appeared to be excavated along joints, but did they also occur along the faults; why was there no comparable topographic effect with the main faults, and did this indicate the conditions under which the faulting occurred?
In reply Dr Flinn remarked that geos are narrow inlets in the cliffs formed by preferential marine erosion along faults or joints where these form narrow zones of shattering. The preferential erosion within a geo probably arises as much from the way its narrow form intensifies the erosive power of the sea as from the presence of a zone of weakness in the rocks. The main faults are associated with broad zones of shattering so that the zone of preferential erosion is much too wide to give rise to geos. Their absence indicates nothing about the conditions under which faulting occurred but much about the conditions under which geo formation takes place.