I have recently finished my Geography course, I have just taken my final exams.
This is how a wave-cut platform is made. Erosion from the sea takes place between the "high" and "low" "water-mark". When a large notch is formed at the base of a cliff the cliff collapses and is taken out the sea by the tide.
Crack, cave, arch, stack, stump. It's Geography to me.
Here is the pebble being carried along the beach by the longshore drift.