nice find whether it's natural or manmade. It kind of looks worked imo, pecking marks and fractures. It's hard to tell from just a pic though. water, weather and time will do some amazing things though. Any idea what the material is? sort of looks like sandstone. If it was found on an ancient man site, I'd personally have an expert look at it. Have you found any artifacts, arrowheads in the same location as this find?