A wave-cut notch is formed when sea waves undercut the base of a cliff. As the force of the waves continually weaken the base, the overhanging rock or the area above the notch eventually caves in.
The formation of a wave-cut notch is part of the cycle of cliff erosion. A cliff along a coastal region develops from the mechanical breakdown of rocks, such as sand, clay or chalk, due to varied weather conditions. When the base of an outcropping is composed of more resistant materials, the wave-cut notch may take the shape of a sea stack, sea arch or sea cave.