A stack, or sea stack, is a steep, often vertical column or columns of rock standing in the sea near a coast, formed over time by wave erosion acting on a headland. Hydraulic action from crashing water widens cracks in the headland until part of it collapses, leaving a natural arch, and the further collapse of that arch leaves the freestanding pillar of harder rock that remains as the stack; continued erosion eventually topples the stack itself, leaving only a stump. Stacks provide nesting habitat for seabirds and, in places such as Ireland, are a popular destination for rock climbing.
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