This is from my memory as I'm not an expert. I just served my engineering apprenticeship & then because I didnt like the three shift system, I moved to Sigmund Pumps Service Dept on the Team Valley in Gateshead..
All coal mines in our NE area were in what was known as the 'Great Northern Coalfield' and the same coal seams were there underneath. A bit like a massive cake with numerous layers of jam and cream.
Hebburn & Wallsend Colliery were linked underground and worked the same coal seams.
I can only speak of my memory from Wardley Colliery where the deepest seam was 1480ft down, then another above it at the 1260ft, then a seam called 'The Hutton' at about 1100 ft & one called 'The Maudlin' at about 10,000ft.
There was other another coal seam about 100 ft down called 'The Monkton seam' but I dont know if they ever worked that. I know the Monks at Jarrow worked that one using the 'Bell Pit' method described in my article.
Miners in the 40s and 50s would have retired at 65
like every one else Carol. Because of their age they'd probably have been put on lighter work.