rvc[8-22] - erosion in sight

Fascinating ...

When the tide comes in, the waves washing across and into this hole will swirl the trapped stone around. This will grind the stone surfaces together, the resulting abrasion will make the hole bigger, at the same time, the entrapped stone will get smaller - and eventually, it will be rinsed out of the hole.
Bad weather - ie more violent waves - will probably push another stone into the hole, and the process repeats ...