I'm not an archeologist or paleontologist, but if there were other similar marks around the rock that are similar to the one end, I would think it was done by natural processes. Clearly, the piece has been there for a long time as evidenced by the smooth appearance of the concoidal fractures. They look like they've been worn down a little with wave and water action. I don't think this was an artifact that was manufactured. It may have been used as a tool of convenience to break small pieces from another rock, or to crush break small bones. Since it was found in an area where many rocks were probably available, I don't have enough education and experience to venture a reliable guess. I might have even been used by a small boy to hammer other rocks like his dad did. I really don't know.