Definition: pillar-like eroded remnant of a coastal cliff, with margins often defined by major fractures. Stac Gaelic – a rock pillar

Stacks can be formed by the loss of the keystone of an arch in response to marine erosion. Stacks can also develop directly from erosion of the cliff face or headland where the overlying rock is not strong enough to form a supporting roof. Orcadian sea stacks offer challenging and spectacular climbing.

Stacks may form relatively rapidly through selective erosion along weaknesses in otherwise resistant rock. Once isolated from a host cliff the stack may become a relatively persistent feature, although we know next to nothing about the survival time of stacks. The stacks may have few joints and erode more slowly in comparison with the retreating cliff. Secondary arches may develop within the stack where strong local joint patterns are exploited by marine processes. Eventually the stack will collapse leaving a stack remnant or stump.

The old man of Hoy- Image courtesy of Tristan Cameron Harper

  • 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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