<P> A stack or sea stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion . Stacks are formed over time by wind and water, processes of coastal geomorphology . 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 . Erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast--the stack . Eventually, erosion will cause the stack to collapse, leaving a stump . Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing . </P> <P> Isolated steep - sided, rocky oceanic islets, typically of volcanic origin, are also loosely called "stacks" or "volcanic stacks". </P> <P> Stacks typically form in horizontally - bedded sedimentary or volcanic rocks, particularly on limestone cliffs . The medium hardness of these rocks means medium resistance to abrasive and attritive erosion . A more resistant layer may form a capstone . (Cliffs with weaker rock--such as clay--tend to slump and erode too quickly to form stacks, while harder rocks such as granite erode in different ways .) </P> <P> The formation process usually begins when the sea attacks small cracks in a headland and opens them . The cracks then gradually get larger and turn into a small cave . When the cave wears through the headland, an arch forms . Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast--the stack . Eventually, erosion will cause the stack to collapse, leaving a stump . This stump usually forms a small rock island, low enough for a high tide to submerge . </P>

Explain the formation of a sea stack aqa