Old Harry Rocks
A stack is a geological landform. Stacks are steep, often vertical, sided columns of rock in the sea, near coasts. Stacks are formed when part of a headland is eroded leaving a small island. These also form when a natural arch collapses due to sub-aerial processes and gravity. A stack may collapse or be eroded leaving a stump. Stacks form most commonly on chalk cliffs, because of the medium resistance to erosion. 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.
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