Stack (geology)

[2] They are formed when part of a headland is eroded by hydraulic action, which is the force of the sea or water crashing against the rock.

The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island.

Without the constant presence of water, stacks also form when a natural arch collapses under gravity, due to sub-aerial processes like wind erosion.

)[4] The formation process usually begins when the sea attacks lines of weakness, such as steep joints or small fault zones in a cliff face.

Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast, the stack.