it looks like a tool to me,thats a type of granite from michigan i think and does look like a axe,or war club.can you tell if it is really smooth around the notches going across the rock as it was halfted and should show signs of wear.thats a really hard rock and to me it shows some type of wear on the tip.thanks for sharing as i grew up in michigan and never looked for artifacts,i think you have something there its just so hard to tell by a picture.or it could of been a woodworking tool to carve out canoes,its not a fine example but looks like a really hard rock to work with.just check it out for smooth spots around the bit end if they used it alot it would smooth up pretty well