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Gloup List

Duncansby Head, John o' Groats, Caithness.

The Gloup List is under 'Links and Resources' below. It has entries from all over Scotland and the islands.

A gloup is a hole in the roof of a sea-cave, caused by the collapse of part of that cave's roof. It is separated from the sea by an arch or a longer tunnel. In some cases the tunnel continues landward beyond the landward end of the gloup. If the arch or tunnel at the sea end collapses, it becomes a geo.(eg N Reeva, Fairisle). Pot is an occasional synonym on the Grampian and Angus east coast. The term gloup is usually applied to sea-caves, while sink-holes and shake-holes are applied to similar holes in limestone areas inland. However the mechanism of gloup, sink-hole and shake-hole formation involves the hollowing out of a void below ground level which reaches a point such that a smaller or larger collapse into that void occurs.

Some definitions overlap with 'blowhole'. It is suggested here that the term 'blowhole' be reserved for holes connected to a sea-cave out of which the sea does in fact get thrown or blown out. On this view a gloup can be a blowhole, but on the evidence so far available is more usually not a blowhole.

While voids in calciferous rock are mostly formed by acidic dissolution, sea-cave tunnels are formed by wave action and are observed to have been formed in all lithologies of hard rock cliffs. This usually occurs either by erosion along a joint or a fault, or by eroding a weaker layer, in which case the tunnel follows the strike of the rock layers. For a tunnel to form, this erosion has to work faster than the rate at which the face of the cliff is eroded. Also the rock above the tunnel has to be strong enough hold the roof up and not immediately collapse into the tunnel. It may be noted that sea-caves often occur in clusters, sometimes very close clusters presumably because the above conditions are met.

If we ask how a sea-cave tunnel roof might collapse to produce a gloup, some of the possibilities are:

1. Excessive widening.

Many sea-caves have chambers inside which are wider than the entrance, often towards the end of the tunnel. This may partly be caused by the wave energy being concentrated at the end, or the rock being more cracked or softer than it is near the entrance. Widening may also be enhanced in caves where tunnels intersect. This may be why the gloup of Brei Holm Cave was formed.

Widening within the tidal plus wave height range is assisted by the milling action of pebbles and larger worked on by waves.

If the chamber becomes too wide then at some point, the strength of the ceiling rock will be insufficient to hold it up.

One might expect to see a rather rounded gloup by this mechanism, with the gloup significantly wider than the arch/tunnel connecting it to the sea. Gaylet Pot, Arbroath may be an example. Note that in that case some low level widening at the landward end of the current tunnel may be observed.

2. Excessive thinning from below.

On this model, the ceiling of the tunnel goes on losing material from below until it collapses. The actual mechanism for this is more difficult to understand as many caves have ceilings well above the height that one would expect wave action to exert significant hydraulic action or milling effects (eg Cathedral Cave, Duncansby Head). Whatever the mechanism, the spalling of rock from a cave ceiling may be observed in some supratidal sea-caves in parts beyond the reach of any sea-water apart perhaps from foam.

However, if this selective thinning of the ceiling occurs, then one might expect to see a relatively narrow gloup with more or less vertical sides. A number of the examples in the list below fit that description. (eg The Gloup (Deerness)).

3. A near vertical joint or fault on which the sea level tunnel is formed, breaks through the cliff top surface and later widens. An arch-gloup-tunnel cave which suggested this is 'cave (S Newtonhill Bay)'. The inner cave is 3-4 times wider than the tunnel width under the gloup. This short (3m) narrow gloup might have started as a blowhole/shakehole. It seems rather unlikely that any of the larger gloups formed in this way, but it appears hard to prove what the intermediate stages were.

4. An expanded blowhole.

On this model, a blowhole forms and then, over time, expands to gradually form the gloup.

Such holes are known - such as the Hole of Troupa, Caithness. On the basis of only seeing this from the top, and an aerial view of the coastline to the east, it might have formed on the intersection of the joint on which the cave tunnel below is formed and another joint at right angles. This hole appears to have stayed much the same for the last 150 years and the vegetation at the top shows no sign of being affected by salt water action. The view from below is so far unrecorded.

However there is a problem firstly that a finished gloup necessarily involves the removal of any prior blowhole. A second problem is that it is hard to see why/how a blowhole should get larger to the extent required to produce a gloup. It appears to require a relatively uniform widening from top to bottom. But avens are often, or mostly, wider at the bottom, narrowing upward. So it is hard to see how a progression from blowhole to gloup could be based on much more than speculation, or could exclude any of the other mechanisms suggested above. See the (upcoming) Blowhole List for a consideration of how active blowholes and blowback features may function.

Recently developed technology - such as SLAM (Simultaneous Location And Measurement) lidar scanners and drones (eg Hovermap) now make it feasible, if not yet very affordable, to make rapid surveys inside sea-caves such that if the cave is re-surveyed - say once every 5-10 years - rates of erosion could be measured. 'Rapid' (2 hrs max for the survey) is important because windows of opportunity to access a sea-cave cannot be reliably predicted more than a day or two in advance. A complementary tactic would be to re-survey after each major storm into the quadrant that the sea-cave tunnel entrance faces.

[The above, by N Feilden, is intended as a conversation starter.]

Alternative Names: None recorded.

Notes: Gloup is perhaps derived from the Norse 'gluppa' - a chasm.