Water jacket gunk?

Evenin,

I stripped the engine down this afternoon, when I removed the head I discovered one of the water jackets is completely blocked on cylinder 4



I dug the stuff out with a small screwdriver



Any idea what's caused this?  Under the head gasket a blob of liquid gasket was wedged in another smaller water way, same sealant as used around this housing



Wondering if this what caused the problem on the blocked jacket?

Gaz

possibly, not a big fan of use of sealants as seen too many problems due to excessive sealant.
Seen similar on a 7mgte I had displeasure of working on & that had clogged couple water ways due to sealant fragment partially blocking then further debris builds up round restriction.
Some coolant chemicals or leak additives can make interesting gunk & chemically react too .
You doing a full strip & clean ?

Of course Mr B full clean to match the car

Noticed one of the other liquid sealant joints had excess on from Nissan too! Hopefully not done any damage!