Cemented oak.

Never seen that before do you know the reason ,they look very old
If it had been in the UK I would have said the contractor had cement left and did not know what to do with it.

:)
These oaks have around 400 years and are under protection of a special organization. Oaks are very often struck by lightning, often causing the tree to split, from which they sometimes do not recover. Such trees are called in America "widow makers" :)
This plomb is of course not a cement which you use for buildings. But it contracts the trunk together, prevents parasites to attack the weakened trunk and consists from absolutely indifferent material to the tree tissues.

That's interesting have never seen this method carried out in the UK .That of course does not mean it is not done,just I have never seen it.

In Slovakia we have about 10 people who are called tree doctors. They are real specialists in trees, they mostly had education from our University for Wood (funny, I do not know how to translate it) but must be also good climbers (usually they are sportsman as well) as they make checks and intervention in big heights sometimes.
Three years ago I called to one, as I had something with a large pine, unfortunately they were very busy with saving of old historical trees in Central Slovakia so they could not come to me.

That is a shame missed out on having some big strong Hunky men in your garden. lol

They are not hunky, they are slim, although well trained. But what can I have from a man, who is permanently on the tree?

We have seen it done somewhere in this country Brian , OH and I have been trying to remember where but our poor old brains are refusing to tell us ,it might come to us later ..............

Hi Ami, how are you? Yes, I am sure, you have them as well, but maybe they are too "out of the madding crowd", e.g. invisible for people :)