Explain the formation of a stack

The formation of a stack occurs at a high energy coastline involving many forms of erosion. These include weathering , wave and wind erosion. Examples include freezethaw, hydraulic action and abrasion.
Over time a headland shall erode via these processes forming a small cave. Continued erosion over time weakens the top of the cave creating more lines of weakness causing it to collapse. As the headland is eroded on both sides a stack begins to form which stands alone away from the headland.