The sea attacks a weakness in the base of the cliff. For example, this could be a joint in chalk. A wave-cut notch is created by erosional processes such as hydraulic action and abrasion. As the notch becomes larger the cliff becomes unstable and collapses as the result of gravity. The cliff retreats inland. The material from the collapsed cliff face is eroded and transported away. This leaves a wave-cut platform. The process repeats over time.
Find out more about the formation of a wave-cut platform.