Here's how it happens:

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

2. Hydraulic action: The force of the water compresses air in cracks, which eventually widens them.

3. Abrasion: Sand and pebbles carried by the waves grind against the rock, wearing it away.

4. Formation of a cave: Over time, erosion creates a cave in the cliff.

5. Arch formation: The cave continues to erode, eventually forming an arch.

6. Sea stack formation: The arch collapses, leaving behind an isolated rock formation called a sea stack.

So, sea stacks are the result of the relentless erosion of the coastline by waves, not the deposition of sediment.