Arch

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Arches – sometimes also referred to as sea arches or natural arches are a natural openings eroded out of a cliff face. Generally, arches develop where waves attack a plane of weakness which cross-cuts a promontory. Sometimes caves may be produced on either side of a promontory by the waves. These caves may become joined over time to become a tunnel which will then be enlarged to an arch.

The roof of the arch is known as the keystone. If the keystone collapses a stack or a smaller stump may be left.

The best known example of a sea arch is the one on Dore Holm off the south coast of Eshaness; another wellknown and often photographed arch is next to Bressay Lighthouse, Bressay. The series of arches at Brei Holm, Papa Stour probably represent a partially-collapsed sea cave which once must have been one of the biggest sea caves in Britain.