You see, this is why a local person's knowledge beats all. This massive rock has influenced the whole area. From initially being a safe place to cross the river to:-

This is most likely the rock that the Rock River and Rock County were named after.

Looking at the rock more closely it looks like sedimentary rock. It was once the sea bed or a lake bed and you are right.

In 500 years the graffitti will be looked at in wonder and awe. Who wre these artists/humans? What message were they trying to convey?

Or is it as simple as some people wanting to leave their mark to show others that they existed?