A sea arch is a natural opening ero

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.