That is a very weird decay pattern, not at all what would be expected. Any idea what damaged it ?
Is that the the old pumping station on the Cauldon Canal ?
It isn't decay, the dark parts are sound, solid timber with no indication of rot.
The whole tree was healthy and vigorous, in fact too vigorous for its own good, as one of the reasons my wife wanted it gone is that she didn't like me clambering around at height to trim the crown every year. To be fair, I wasn't too keen either.
She has no excuse, however, for uprooting a 40cm high cutting that I'd established about 2m away from the parent tree!