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

  • 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.

  • 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.

  • 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.

  • 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.