Headlands are usually made of resistant rocks. Waves crash into these headlands and enlarge the cracks, through hydraulic action, this repeated erosion causes a cave to be formed.
Continued erosion deepens the cave and causes it to form an arch. E.g. Durdle door. Erosion continues to wear away the rock until a stack is formed. E.g. Old Harry
Stacks are then eventually worn away to give a stump. E.g. Old Harry's wife.