Here's why:

* Erosion: Waves crashing against the coastline erode the rock, wearing it down over time.

* Differential Erosion: Softer rock erodes at a faster rate than harder rock. This creates an uneven surface.

* Sea Stack Formation: As the softer rock is eroded away, the harder rock above it is left standing as an isolated pillar or stack.

Let me know if you want to know more about coastal landforms!