Swona. Photo courtesy of Alan Moar.

Definition: a blow hole, where a chimney has developed behind the cliff face, often above a cave, and spray is blasted out during high seas. From Old Norse glup, a throat. The Irish would call these ‘puffer holes’ from the way in which spray erupts from them in high seas.

Gloups are a reminder that cliffs may become riddled with tunnels which remain largely unseen from the cliff top. The excavation of blowholes is a hidden process but the opening of the shaft must involve the removal of material along intersecting lines of weakness provided by crossing joints and fractures. Gloups often lead down into large caverns and the opening of the gloup can be seen as the first step in the block by block collapse of the cave roof. The hydraulic forces as waves enter caves and compress water and air in fractures must be very considerable and the withdrawal of the water may also induce instantaneous vacuums and may promote cavitation. In the absence of detailed observations inside caves and gloups, we remain, much like these cavities, mostly in the dark about the processes involved.

The sequential development of caves and gloups is well seen at the Kilns of Brin Novan

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