Hi AJG.
That must be what Ian called it.
I worked at Wardley Colliery early 1960s so know how it works.Fresh air for all the mines went underground via a shaft simply called the Downcast shaft.After the air circulated miles underground it was expelled via another shaft simply called the Upcast shaft.
Every mine had at least two shafts.
The workers accessed the mine via the Downcast shaft.
Monkton only had that one Shaft but Ive no idea why it was created and then closed.