Imagine how many years that little rock had to be in the perfect current to sit there and vibrate on the larger one in order to wear into it like that. Not a geologist, so I'm assuming that is what happened lol. Awesome find.
That is awesome! I found a piece of wonderstone in a river I fish here, that had a dished out portion with a pebble in it very similar circumstance, now I need to go find what I did with it! LoL!