"better than a potato chip Madonna"
Man, I ought to keep my 2 cents in my pocket on this one, but what the heck.
How about both, natural erosion with a little utilization.
In the first pic, on the left side, you can see vertical ridges running thru the horizontal grooves
that could only remain from natural erosion. But on the right side of the rock you don't see
those ridges, as much. In the second pic the right side of the rock is up and the lower portion
of the grooves seem to show some grinding or wear.
What if someone found this rock in the river, with the grooves allready naturally eroded, and
decided it wouldn't take too much effort to haft it and use it to quarry/spall some rock up the
hill there? Maybe he was lucky enough to get a few hours of use out of it, before the ends got
broken into what we see now.
Just a thought about a really strange looking rock found in an area of obvious activity that makes
it suspicious.
You gotta wonder.............
Joe