Oak tree callussed branches

Whilst doing some gardening at a customers house. I noticed a very large common oak tree on the boundary line, always dropping branches, acorns and pigeon mess. This time something caught my eye.

A couple of the large branches had crossed over each other, and had rubbed against each other. This must have occured 75 odd years ago when young. The rubbing had broken the bark down to the cambian layer, whilst nice warm still weather had initiated callusing and probablt the 2 branches have now joind each other.