Sea stack

A sea stack is an isolated column of rock offshore, left behind as waves erode a coastal headland or cliff.

Headland: A projecting area of land that extends into a body of water. Stacks often begin as parts of headlands exposed to wave attack.
Old Harry Rocks: A group of chalk stacks and stumps on the Dorset coast of England. These chalk formations show how stacks change as softer rock erodes.
Joint (geology): A natural fracture in rock along which there has been little or no displacement. Waves exploit joints to widen cracks into caves and passages.
Sea stump: A low remnant of a sea stack, usually close to or submerged beneath the water surface. Further erosion and collapse can reduce a stack to a stump.
Wave erosion: The wearing away of rock or sediment by the motion and impact of water waves. Wave erosion drives the formation and eventual collapse of coastal rock features.
The Old Man of Hoy: A sandstone sea stack on the island of Hoy in Scotland's Orkney archipelago. Its sheer profile makes it a striking example of a tall, isolated stack.
Differential erosion: The uneven wearing away of materials with different resistance to erosion. Differences in rock strength help isolate a resistant column from its surroundings.
Rockfall: The rapid descent of detached rock fragments down a steep slope or cliff. Rockfalls remove material from stack faces and can precede collapse.
Sea cave: A cavity in a coastal cliff formed when waves erode weaker rock. A cave can deepen through a headland before its roof breaks through.
Dun Briste: A sea stack beside Downpatrick Head on the coast of County Mayo, Ireland. Its layered cliffs expose the structure of a stack cut off from shore.

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