It does not look like I would expect a grind stone to look like.
Was the stone very soft? Did you try rubbing it to attempt to determine the material? If so, what occurred?
It sort looks like someone used quickcrete to form some sort of basin.
Were there any old home foundations or the like around the immediate area? What was the area?
It looks like you were in a forested area. In the surrounding area, were there any large hardwoods? It looks to be in a clearing sort of area. I don't see any large trees in the background. That may indicate its an old homestead area, or the land could have been clear cut in the past, and the workers made some homemade tool for some reason.