I went back to the site to look around and there are a few other objects that look interesting. The one I brought back to my studio looks like a small grinding surface. I have to look up how to tell if a surface has been worn by human activity and not some other way, like by water, because I never did know very much about it.
This surface wasn't struck in the same way as the sharpened edge of the other piece. Given the pock marks on the top surface, it looks like someone took a tool about 1/2 inch in diameter to pound with -- about the size of a nail sink. I don't know why you'd do that unless you were crushing small seeds or something.
But there are three other places that show smoother wear, where there aren't pock marks but more like gouges made over a longer period of time by bigger, rounder tools. And those places aren't symmetrical or consistent with the sheen or smoothness you'd get from water. So. 
Fun 
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