Here's how it works:

1. Wave action: Waves constantly crash against the coastline, eroding the cliffs and headlands.

2. Headland erosion: The softer rock in the headland is eroded faster than the harder rock, creating caves and arches.

3. Arch collapse: The arch eventually collapses, leaving an isolated rock column standing – a stack.

4. Further erosion: The stack continues to be eroded by waves, eventually collapsing into the sea.

So, while deposition plays a role in building up coastlines, stacks are a product of the erosive power of waves.