Stacks

• Hydraulic action is a process where pounding waves compress trapped air in the rocks, creating an explosive blast which weakens and loosens rock fragments.
• Abrasion is a process where rock fragments are thrown against the headland, breaking down the cliff face and wearing away the rock.
• Solution is a process where salts in sea water dissolves rock.
• Attrition is a process where rock fragments are slowly ground down by friction from wave action into smaller and rounder pieces which can then be used as an abrasive material.
• Rocks contain weak points that are more easily eroded.
• Over time, lines of weakness are enlarged and develop into small sea caves.
• In some cases, a blowhole can form in the roof of the cave as compressed air is pushed upwards by the power of the waves, causing vertical erosion.
• There may be erosion on both sides of the headland due to wave refraction until eventually the sea cuts through the back walls of the caves, forming an arch.
• Continued erosion enlarges the arch.
• The base of the arch is also undercut by the erosion, causing the arch to become unsupported.
• Eventually the arch collapses due to gravity to form a stack.
• Old Harry is an example of a stack.