Cave in Stroma

Sea 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 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.

On the Devonian sandstones of Caithness, cave development can be seen in perhaps its simplest form. The rocks often have low angles of dip and crossing horizontal and vertical fracture systems which allow waves to attack cliffs effectively formed of tightly stacked cuboidal rock blocks. The caves develop stepped, rectilinear shapes, with many sockets from where box-shaped blocks have been removed.

Box caves mark the first stage of development, where groups of fracture-bounded blocks are removed. Block removal can occur not only at sea level but also at various heights on the cliff face, a reflection of the way in which the level of wave attack on the face changes with wave height, itself a function of coastal configuration, exposure, and storm intensity. The box caves are most commonly found at sea level and may join together in time to form a marked notch or slot at the cliff base.

Caves on Stroma

Small openings may be enlarged by abrasion and by hydraulic action. In Caithness, wave action can attack not only the base of the cliff but also well above sea level, due to swell and the frequency of high seas. 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 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 are endlessly repeated and so promote the opening and extension of joints. The hydraulic forces responsible for liberating blocks are unclear but require wave water first to enter joints at high pressure. As a cavity opens, it can enlarged by block collapse from the roof. Caves may eventually connect to the surface through a joint or fault-controlled shaft, known as a blowhole or gloup. Fine examples occur on Stroma.

Main image: Sea caves and cliffs at Lybster (ACredit: George Maciver / Alamy Stock Photo / 2RC8KDR)

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