My father planted this tree 25-30 years ago in his Blue Mountains garden. It's now about 40 ft tall and I have just found fallen fruits for the first time. I have looked through several books and done some image googling and am left rather uncertain about what species it might be. The nut surface looks like some pictures of J. nigra but leaves are perhaps more like J. cinerea, though leaflet bases seem more cordate than any. I'm sure there are people here who know a lot more about walnuts than I do. p.s. I am using feet and inches in the hope of getting more sympathetic attention from your U.S. members (one in particular!?).
Of those two, black walnut. Butternut has larger leaflets, less numerous, and nuts closer to an egg in shape rather than round.
I'd agree with Juglans nigra. J. cinerea has broader leaflets, and always has a terminal leaflet (which J. nigra often doesn't, as here)
Butternut looks more like Asian walnuts than it does black walnut. In fact, large Asian walnut trees in my area are sometimes mistaken for butternuts.
