An arch is the result of the wearing away of a weak part of a cliff in a headland.

A coastal arch will develop when waves have worn a tunnel right through the headland. In time, the roof of the arch will collapse, and the end of the headland will be left as a tiny island, known as a stack.

A cliff arch inland will develop in rocks made of different hardness. Rain seepage and or frost will attack the weaker band of rock more quickly than the harder band, and this will gradually make holes in the weaker rock, producing spectaclar arches.