Wicked find. It's a ground stone maul. DO NOT throw that away.....it could have been hafted but, I think it was prolly just used by hand. Sometimes the natives would drill a large hole in stick and secure the stone that way. Most times they chipped a groove on both sides. That way it was narrow enough to insert into the split end of a stick.
Awesome find
That is the sort of find I would absolutely treasure here in NZ; although the age of such a relic here is probably max. 1000 to 1200 AD
What age is your find likely to be? - and can you pin down a particular tribe who might have manufactured it?
Cheers, Mike