Doug,
Thanks for that info, and welcome to Aditnow. I have not been here for very long myself. I only came here because I was dreaming about Murrayshall and did a Google search. Glad I did too, this site is truly fascinating!
I think you have given me something to look at on my next visit to Scotland (in about a month), as my memory seems to need refreshing. I was sure that the mine in the middle of the woods was the shaft, indeed old-maps.co.uk shows it as such on the 1922 1:10560 sheet. It was thought to be an air shaft. But if you know of an adit, that is interesting. A second air shaft where you indicate would not be surprising. Someone described that one to me once, but I have never seen it.
I don't have any definite knowledge of the base level of the dolerite, which slopes down to the east, but it is at the base of Murrayshall quarry, which as you suggest is about 125 metres on the OS map, but maybe 70 or 80 metres at the Cambusbarron quarry. Murrayshall mine was level with the top of the Murrayshall kilns, and the base of the quarry. The limestone is likely to be consistently just below that, as there is no obvious indication of major faulting. If there is a mine in the woods, it may go into the limestone, and if so it will confirm my guess that two distinct strata have been mined, as the adit by Hayford Mills would have been down at about 12 to 15 metres, and the strata do not seem to slope that steeply. Unless there is a vein of something else, which would be a complete surprise! You don't usually get major mineralisation in dolerite, although there is reputedly a lead (galena) vein in Northfield Quarry, further south in the same sill.
Thinking about this some more, the dolerite may have divided around an area of limestone, so there may have been a pocket of rather well-baked limestone in the middle, which may be what was mined in the middle of the woods. Only a guess, so far.
When I find the geology book which has some more detailed info, I may be able to clarify things.
By the way, you are right to be wary of entering the adit. I suspect the possibility of a rather deep shaft to a lower level. I don't enter Craigend adit nowadays, it was scary enough in 1967, and will not have improved. You would definitely need a hard hat and headlamp, with spare torch as backup, as a very minimum. The hard hat is not to protect against roof falls (it won't) but only to avoid banging your head. It still is unpleasant even with a hat.
Are you able to confirm that the adit by Hayford Mills is still there? There is some doubt, and I fear that it may have been reduced to a mere drain pipe in the interests of safety.
As to the old plans, if they are out of copyright (all OS maps pre 1957 are, Crown Copyright is 50 years from the end of the year of first publication, and 70 years for anything else, except where there is one identifiable author, i.e. not an organisation, which makes it 70 years from the death of the author), then I would be inclined to scan them and convert to jpeg, then post as photographs. I have not posted anything like that here yet, the "management" and regulars will be able to assist.
I see that Gillies Hill is frequented by far more people than when I was young. In those days the gate at Gillies Hill road was often locked, but some time ago, the gate had fallen off its hinges, and more recently, disappeared completely. Good thing too! There were other ways in, of course. It is important that the quarrying does not go ahead, it would be a terrible loss, and there is plenty of dolerite in other, less socially useful, places.
I think there is much yet to discover and document. It is good to see that people are actually interested. Few cared when I was young, maybe the internet has awakened interest.
Best regards,
Alan