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, leaving a 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 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.

Stacks are often important sites for nesting seabirds.

Some stacks are famous for rock climbing.

Examples

Flowerpot Island, Lake Huron, Canada.
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, Sutherland, 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
"Lange Anna" on Heligoland