Stack (geology)

Stack (geology)

A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast. Stacks 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 a stacks and small island. They 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 typically form in horizontally bedded sedimentary rocks particularly on limestone 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. The sea attacks small cracks and opens them. The cracks then gradually get larger and turn into a small cave. When the cave wears right through the headland, an arch forms. Further erosion causes the arch to collapse. This causes a pillar of hard rock standing away from the coast. This is called a stack.

Stacks are often important sites for nesting seabirds.

Some stacks are famous for rock climbing.


* Flowerpot Island, Lake Huron, Canada.
* Three Sisters, Eatonville, Nova Scotia.
* Haystack Rock, Cannon Beach, Oregon, USA.
* Goat Rock, California, USA.
* Old Harry Rocks, Dorset, England.
* The Needles, Isle of Wight, England.
* Am Buachaille, Sutherland, Scotland.
* Old Man of Hoy, Orkney, Scotland.
* The Twelve Apostles, Victoria on the Great Ocean Road, Victoria, Australia
* Percé Rock, Percé Quebec, Canada
* Hopewell Rocks, Hopewell Cape New Brunswick, Canada
* [ Po Pin Chau] , High Island, Hong Kong
* Étretat, Seine-Maritime, France

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