A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, isolated by erosion.[1] Stacks are formed through processes of coastal geomorphology, which are entirely natural. Time, wind and water are the only factors involved in the formation of a stack.[2] 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.
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