caves

cliffs

cliff top storm deposits

geos

gloups

headlands

stacks

Giant's Leg, Bard, Bressay. Gently-dipping Devonian sandstone. A natural diving platform for young Shetlanders.

 

Dore Holm in 1774

David Parker takes a different view of stacks and arches

David Parker

 

Warie Gill (HU 240833). An arch developed in andesitic tuff. The Burn of Tingon falls to the sea in a waterfall. Note the loss of material from the key stone of the arch, probably during storms in the early 1990s.

Arches at Brei Holm, Papa Stour. The best example of a partially-collapsed sea cave in Britain?

Arches

A sea arch is a natural opening eroded out of a  cliff face by marine processes. Some arches appear to have developed from surge channels, which are created by wave refraction causing the focussing of wave fronts on the side of a headland. More generally, arches develop where waves attack a plane of weakness which cross-cuts a promontory. Caves produced on either side of a promontory may become joined over time to become a tunnel and, finally, an arch.

The supporting roof of the arch is known as the keystone. The architecture of an arch is a reflection of its lithology and structure. Sea arches have been regarded as ephemeral forms tending to survive over periods of just few decades or centuries. The term sea tunnel can be used to describe a hole in the cliff line where the arch itself is considerably longer than the width of the entrance.

Arches are fragile, ephemeral forms. The Horn (left), a former arch off the NW coast of Papa Stour popular with 19th century sightseers, collapsed into the sea during the great storm of 31/1/1953 leaving only a stack that disappeared later. The Brough Stack on Foula also supported an arch topped by a Bronze Age broch until the arch collapsed during a storm in 1965.