Arches are natural openings eroded out of the cliffs by marine processes. Some arches appear to have developed from surge channels, which are created by wave refraction causing the focussing of wave fronts on the side of a headland. More generally, arches develop where waves attack a plane of weakness which cross-cuts a promontory. Caves produced on either side of a promontory may become joined over time to become a tunnel and, finally, an arch.

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Gently-dipping Devonian sandstone and arch, Cusbay, Eday. Image by Stuart Rendall

Arches are fragile, ephemeral forms. The cave exploits a line of weakness provided by joints and other fractures. This line often extends vertically to give closely jointed blocks at the key stone of the arch which render it liable to collapse. On Orkney, the orthogonal fracture patterns in the Devonian sandstones provided by intersecting vertical joints and horizontal bedding planes give rectilinear shapes to arches and stacks.

  • 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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