Holes of Scraada (Old Norse Old Scratch or the Devil), a geo formed by partial collapse of a former cavern roof. The chasm is 30 m deep and 120 m long and the surviving cave is over 100 m long. The Holes were once two; a natural arch spanned the chasm until its collapse on 9th October 1873.
Villians of Ure. Marine erosion exploits former lava tubes to create caves. Image courtesy of Ronnie Johnston.
Sea kayakers' high jinks at Brei Holm, a partially collapsed cavern with arches on Papa Stour
"You come to Francie's Hole (and) enter through a perfectly arched entrance cut in the face of a cliff perhaps forty feet or so in height, and can almost fancy you are in a cave in fairyland, so exquisite is the colouring of the roof and sides, and so pellucid is the water. What the length, or breadth, or height may be the writer cannot say, so overpowered with the beauty of the place was he, that he utterly forgot to estimate them. The rocks forming the sides and the roof, apparently porphyritic, is partly green from seaweed and slime, and partly red of many shades, and in places glistens like mica. The roof is studded with bosses of a deep rich purple, like the bloom on a grape, and resembling in form and regularity what are to be seen on the roof of cathedral crypts and cloisters. Several caves branch off on the left, and at the head is a beautiful pink beach, at the top of which are alcoves like the stalls in a church."
John Tudor, Victorian traveller, on Papa Stour
Image gallery for the sea caves of Papa Stour
Fogla Skerry, (HU140613) sea cave videos filmed from a RIB by Ryan Leith.
Caves form in response to the structural control of local geology on cliff form. The exploitation of joints, faults, cracks and other irregularities all lead to the opening of slot caves. The caves can be either tunnel or dome shaped, reflecting the type and inclination of the geological structures. Caves are most common where massive layers overlie weaker beds with resistant ribs.
Fogla Skerry photographed by David Parker. The cave is formed in rhyolite.
Small openings may be enlarged by abrasion and by hydraulic action. On exposed coasts in Shetland the considerable height range over which wave action can attack the base of the cliff, due to swell and the frequency of high seas, together with the way that many cliffs plunge into deep water means that abrasion is often insignificant. The compression of air in rock crevices is probably the most important process in enlarging caves. High shock pressures are generated against a cave roof when a lens of the air becomes trapped between the incoming waves and the roofs. Similarly, very low air pressures may be generated for an instant as the water leaves the cave. These extreme pressure variations promote the removal of joint blocks from the roof and progressive enlargement of the cave. On more sheltered coasts, caves are shorter and often form slots above the level of the high water mark.
Slot cave at Ollinsgarth (HU 445 308)
On Papa Stour, the Holl o Boardie passes through the north-west tip of the island. The cave is ~300 m long and wide enough to row through, making it one of the largest sea caves in the world. See a photo tour. Fogla and Lyra Skerries are also pierced through by several large caves.
Jonathan Swales adds: "As marked on the 1:10000 map, Bordie Holl works out at about 360 metres, but they have the eastern entrance in the wrong place - it's actually at grid ref HU 1526 6238, not 1530 6230 as shown. Also the bend in the middle is more acute. I reckon the real length is nearer 330 metres. The caves through Fogla Skerry would also count - the longest is about 250 metres but there's an intersecting grid of them so you can probably do about twice that without seeing the sky."
Caves may be connected to the surface through a joint or fault-controlled shaft, known as a blowhole or gloup. We lack detailed surveys of the form of Shetland caves due to the difficulties and dangers in access but the adventurousness of sea kayakers and divers is increasing knowledge of sea cave interiors. Submerged and semi-submerged sea caves occur on Papa Stour, drowned by sea level rise. Shetland and Orkney are perhaps some of the best locations in the world to study cave formation.
The beauty of the interior of Fogla Skerry revealed by David Parker