Stacks are landforms located in seas or near coastlines. They have steep, vertical columns made of rock that are formed by wind and water over a period of time.
The creation of stacks (also known as sea stacks) begins at headlands. Headlands are high coastal landforms with sheer drops that extend into the sea. As sea water crashes against the headlands, cracks within them weaken, causing them to collapse. Their collapse is what forms the free-standing rocks known as stacks.
Ball's Pyramid is the world's tallest sea stack. It stands 551 meters tall, and is located near Lord Howe Island in Australia.