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Old Man of Hoy, north-west coast of Hoy. Orkney. A 137 m. sea stack composed of red sandstone of Upper Old Red Sandstone age. The stack rests on a pediment of Upper Old Red Sandstone basalt lava which unconformably sits on flagstones of Middle Old Red Sandstone age. The stack in former times had an arched base. Sea stacks are produced by the combined action of marine and subaerial erosion, their bases may be completely submerged or just above sea level.