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In time, this repeated pounding action begins to loosen blocks of rock, and they fall into the sea. This leaves the upper part of the cliff overhanging.
When the overhang becomes large, the upper part of the cliff collapses (picture 2).
So, by simply breaking against the bottom of the cliff, the waves cause the whole cliff to wear back (picture 3).
 2 When waves break against a cliff, the force of the water is like a hammer beating on the rock (A). When the wave is spent, any loosened blocks can fall out (B). Then the next wave approaches, hammering the cliff once more (C). (Note: it may take many years for waves to prise a block loose.)
ABC
Cliff is undermined here.
G 3 Waves attack the bottom of cliffs wearing them away. From time to time, the cliffs become so undermined that they simply collapse. (Because rockfalls of this kind can happen without warning, it is unsafe to go near the foot of tall cliffs.)
Cliff falls down.
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