When the roof of a natural arch collapses, the resulting rock pillar left standing in the sea is called_____?

Analyze the question: It asks what a rock pillar is called when it is left standing in the sea after the roof of a natural arch collapses. This is about coastal erosion landforms.

Explain the concept: Waves attack a coastline through hydraulic action, abrasion (corrasion), and attrition. Over time, a sequence of landforms develops:

1. Crack/joint: Waves exploit weaknesses in a headland.
2. Cave: Repeated wave attack enlarges the crack.
3. Natural arch: The cave is eroded right through the headland, leaving a rock "bridge."
4. Stack: The roof of the arch becomes unsupported and collapses, leaving an isolated pillar of rock in the sea.
5. Stump: The stack is further eroded and reduced to a low rock remnant.

Check the options:

• A. Headland: A piece of land jutting into the sea, made of resistant rock. It is where the arch forms, not what is left after the collapse.
• B. Stack: A detached rock pillar standing in the sea after an arch collapses. ✔
• C. Tombolo: A depositional feature, a sand or shingle bar connecting an island to the mainland. It is not erosional.
• D. Wave-cut platform: A flat rock surface at the base of a cliff, formed as the cliff retreats. It is not a pillar.

Answer: The correct option is B. Stack.

Exam tip: Remember the sequence "Crack → Cave → Arch → Stack → Stump" (C-C-A-S-S). Whenever a question mentions an arch collapsing, the answer is a stack. Also remember that tombolos are depositional, while stacks are erosional.