Caves, arches, stacks and stumps are erosional features that are commonly found on a headland.
Cracks are formed in the headland through the erosional processes of hydraulic action and abrasion.
As the waves continue to grind away at the crack, it begins to open up to form a cave.
The cave becomes larger and eventually breaks through the headland to form an arch.
The base of the arch continually becomes wider through further erosion, until its roof becomes too heavy and collapses into the sea. This leaves a stack (an isolated column of rock). An example of a Welsh stack can be found at Stack Rocks, Pembrokeshire.
The stack is undercut at the base until it collapses to form a stump.
The Green Bridge of Wales in Pembrokeshire is a fantastic example of an arch