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. Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing.
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Stacks typically form in horizontally bedded sedimentary or volcanic rocks particularly on limestone cliffs. These rock types' medium hardness means medium resistance to 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.
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 is usually a small rock island, small enough to be submerged by high tide.
The term kekur (Russian: кекур) is used for the stacks in the Arctic and Pacific oceans, in the Russian Far East, and for stone columns in the watersheds of the Lena, Yana and Indigirka Rivers.[3]
The Swedish rauk refers to a stone column created by natural erosion. Examples are found on the islands of Gotland, Fårö and Öland.