Definition: steep to vertical or overhanging rock slope at the coast, free of soil and regolith

Geo extension along a 1 m wide line of breakage parallel to the cliff. Near Scaun, Westray.

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.

Lang Geo, north Mainland- Birsay

In some parts of the coast, the presence of till in geo heads indicates that the sea is only now completing the task of cleaning out pre-existing canyons. Elsewhere major rock falls leave no doubt that geo extension continues today.

The processes of landward extension are a little better understood. Where the geo ends in a rock wall then cavitation and hydraulic action must be dominant. Geos with bay head beaches have the possibility of abrasion but lichen-covered boulders must only be mobilised during major storms. Where fracturing or weathering has reduced the rock into small blocks or even grit then the sluicing effects of spray will remove this debris and may undermine larger blocks. Although no measurements exist, it is likely that incoming storm waves can generate high water and air pressures in fractures at the heads of geos, leading to extension of fractures landwards. The suction of the withdrawing wave water may also allow plucking of loosened blocks.

  • The outer coast of Orkney is an outstanding location to study the erosion of hard rock coasts. Evidence of major erosion is not hard to find. The great bowl of Enegars corrie is losing its edge to the sea. The egg-shaped headland of Marwick Head is a dome half lost.

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  • Amongst the many beauties of Orkney are the beaches which fringe the bays and ayres of the inner coast. These are dynamic forms, changing shape with the seasons and gradually retreating landward as sea level rises.

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  • Orkney is a dissected landmass drowned by postglacial sea level rise. A drop in sea level of just 35 m would unite the archipelago into a single island. The individual islands may represent hills that formed the watershed areas of preglacial drainage basins but it is linear glacial erosion which has separated one island from another and severed Orkney from the rest of Scotland.

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  • World coasts have seen sea-level variation of approximately 100 metres within the past 11,500 years through melting of the ice caps. When the ice caps melt the sea level rises globally (eustatically). The relative sea-level at any location is measured proportionate to the nearby land, which is itself subject to tectonic movement both up and down.

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