Headlands which stick out into the sea are exposed to erosion from destructive waves
Waves erode the weakest parts of the headland - a joint, crack, fault which becomes larger, wider and deeper and forms a Cave
Hydraulic action - the force of the waves compresses air in cracks causing rock to weaken and shatter
Abrasion - sand/pebbles are thrown at the headland scraping off pieces of rock
Solution - chemicals in the seawater dissolve and weaken rocks
Waves erode the sides and back of the cave and eventually cut through to the other side of the headland to form an Arch
If there is a vertical weakness and strong hydraulic action you can get a blowhole
The arch is eroded and the arch roof becomes unstable and collapses
It leaves a pillar of rock separated from the headland, called a Stack
In time further wave action will result in the stack collapsing to leave a Stump