These linear clefts in the cliff line reflect marine erosion along the line of weakness. Where joints and other planes weakness are close together, these long, narrow slots develop in response to selective marine erosion between the neighbouring planes or weakness. They can also form through roof collapse of narrow sea caves. Many geos have a boulder beach at their head, generally with large amounts of flotsam. Too many Orkney geo heads continue to be used as rubbish chutes perhaps in the mistaken view that the dumped debris will be taken out to sea in storms.
The restricted width of many geos is partly a reflection of the narrowness of the line of geological weakness that is being exploited. On Orkney most geos align to trap dykes and faults. These zones of weakness are often surprisingly narrow. Furthermore, not all faults are associated with geos. The location and spacing of these spectacular cliff forms awaits further study.