Wow - I know nothing about it, but it looks like a LOT of work went into it. I bet a lot of people are going to get excited about this one. Did you post it in Native American? And in "What Is It?"
Really nice find. I think this is a stone spud preform. Size is pretty big. If you google stone spud Cahokia you will find similar. They were mostly ceremonial, but the experts are not sure what they represent or used for. If not ceremonial mostly likely this is a type of celt. Find some more!
Nice find! Based on just the one picture I'd say it's a particular style of a large hoe. More pictures are needed of both sides and edgewise for a more certain I.D., because it could also possibly be an adze or a Celt. Whatever it was do to its size it was most likely "long handle" halfted, that is, attached to a long handle like a modern day garden hoe Many large hoes of that general shape and size have been documented.
Again, NICE find!