Littoral Erosion.

The coastline is not a boundary, but a war zone where the kinetic energy of the ocean grinds the lithosphere into sand.

Erosion occurs primarily through Hydraulic Action—where air is trapped in rock crevices by waves and compressed until the rock shatters—and Abrasion, where the sea uses sand and pebbles as tools to sand down cliff faces. Over centuries, this creates distinctive landforms: cracks become caves, caves become arches, and arches collapse into isolated stacks.

This process is highly dependent on the "hardness" of the rock. Soft sedimentary rocks like chalk retreat by several meters a year, while hard igneous rocks like granite may remain unchanged for decades. This differential erosion results in the jagged, "discordant" coastlines seen in many parts of the world.