Looks somewhat like a huge block plane blade, which instead of that being a split would have been a slot, except that a chunk broke off of that end. Does it look like a chunk might have broken off?... would be good to know, even if that ID is incorrect (which it very well may be).
If it's iron, which it appears to be, then whacking some of the rust off won't hurt it any, and may reveal some makings on it?