Arch – A wave-eroded passage through a small headland. This begins as a cave formed in the headland, which is gradually widened and deepened until it is cut right through

The best way to learn about arches is to see how one develops, as below:

The beginning of an arch is a crack in a rock that is further opened by hydraulic action caused by powerful waves. Other forms of erosion, notably abrasion, then make the opening bigger. Eventually, it becomes a cave, then the cave breaks through to the other side of the rock to become an arch.

As the rock supporting the arch is further eroded and weakened, the roof of the arch collapses, leaving behind a stack. Yet more erosion reduces the stack to a stump.