Cave, Arch, Stack

• Cave: Rocks have weak points that are easily eroded into large cracks. Overtime these cracks are eroded to become small sea caves. Sometimes a blowhole can form in a caves roof as compressed air is pushed upwards by the power of the waves causing vertical erosion.
• Cave example: Tilly whim.
• Arch: There may be erosion on both sides of headland due to wave refraction until eventually cuts through the back walls of the caves forming an Arch, continued erosion enlarges the arch.
• Arch example: Durdle door
• Stack: The base of the arch is also undercut by erosion, causing the arch to become unsupported. Eventually the arch collapses due to gravity to form a stack.
• Stack example: Old Harry