What defines a coastal arch?

A coastal arch is specifically defined as a natural opening formed through a headland, typically as a result of erosional processes. This occurs when waves erode soft rock at the base of cliffs, creating a tunnel-like feature that can eventually evolve into an arch. The structure is characterized by its formation where the rock has been hollowed out on both sides, eventually leading to a vaulted shape that is open to the sea.

The other options do not accurately describe a coastal arch. A vertical column of rock near a coast refers more to a sea stack or pillar, while a narrow flat area of rock at the base of a sea cliff describes a wave-cut platform, which is formed by the erosion of the cliff face. An indentation of the shoreline could refer to various coastal features but does not specifically correspond to the unique structure of an arch. Thus, defining a coastal arch as a hole through a headland aligns correctly with the geological concept of how these formations occur in coastal environments.