This is a photo of the grinding rock on my friend's property. The depression in the rock was made by Native Americans grinding acorns into meal. Nearby, we found two stone tools that we think were used to crack acorns open and then grind them on the large stone. There's a second depression about a foot away from this one, but my photo of it didn't come out very clear.
I visited the rock several times during my week there. What fascinated me was the feeling that if I stood there long enough, I would actually see the people using the tools. It never happened, but somehow, I never lost the feeling that it could.