Caves, arches and stacks

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Q1: How does a stack form?

[Answer]

Waves can pick out even the tiniest weaknesses in rocks. For example, waves can enlarge hairline cracks until a block falls away.

All rocks have weaknesses like this and the sea carries away the weakened rock, forming caves.

In a headland a cave can form on both sides. When the backs of two caves meet you get an arch. When the roof of the arch falls in, a pillar called a stack is left.