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Old Man of Hoy sea stack
The Old Man of Hoy, Orkney: a 135m-high sandstone sea stack
A pillar of rock that forms an islet at the end of a headland. A stack is the result of a series of events caused by erosion. Sea caves striking a headland first carve a cave by opening up joints in the rock. The cave gradually penetrates the headland until an arch is formed. Finally, the top of the arch collapses and leaves a stack separated from the rest of the headland. The process may be repeated several times, giving a series of stacks.

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