The coast of the normandie consists of calkstone and thus it is not as solid as i.e.grantie or basalt. Sun heats it up during the day, while it cools down during the night. These changings in temperature bring cracks in the rock, where the water (i.e. the rain) can attack the stone furthermore. The Atlantic ocean also attacks the cliffs and carves impressing monuments out of the chalkstone. This process isn’t finished, yet. All these processes are still in place and are constantly modifying the coastal line.
Currently you can find stone needles, gates and windows. Have a look and see. It’s really impressive.