(top) Opportunistic tool example.  The natural form of this tool is angled and tapered to provide a handle. It is not, or perhaps very lightly worked, perhaps a few breaks to improve grip.  (bottom) After washing the tool in water to remove dirt, a washboard-like parallel grooved working surface was revealed.  The surface appears suitable for grinding plant material.  This tool was found on the banks of the Olentangy River, Whetstone Park, Columbus, Ohio.