Of all the underground slate workings in North Wales, perhaps those around Blaenau Ffestiniog are most known. I'll list the other areas which I'm aware of.

.Corris: aka Braich Goch, Gaewern, Yr Allt Goed, Minllyn, Aberllefenni, Hen Gloddfa etc

.Bryneglwys slate mine in mid Wales

.NE Wales, Moel Fferna, Penarth, Cambrian, Wynne

more isolated underground workings for example Llanfair/Hen Gloddfa near Barmouth.

Of course another very important slate quarrying area is in the north of Snowdonia, Penrhyn, Dinorwig and Nantlle. I've been looking around aditnow for underground slate mines in this region, so far I've come across a few slate quarries around Nantlle which seem to have shafts, I'm not sure why but I'm sure someone can enlighten me 🙂 Many of the quarries, perhaps most of them in this large region, which works purple slate, use tunnels to access pit workings, many of which are impressive. I have explored many of them at Dinorwig, although I have only found one cavern/chamber at Dinorwig, which opens into the base of a quarry. The chamber was presumably formed where slate was extracted from where the tunnel enters the pit working, and not originally intended as an underground chamber.
So I'm wondering if there are any areas of actual underground slate extraction around Nantlle, Llanberis or Penrhyn?

cheers for replies people 🙂

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There are drainage levels in that region, with access shafts.

Chris.

access shafts from ventilation or for haulage?

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access shafts from ventilation or for haulage?

The angle of the vein is the largest determing factor in whether a quarry was worked as galleries, pits or underground mining.

The different techniques represent different levels of expense:

Galleries: cheapest
Pits
Underfround mining: most expensive

As i understand it:

Moderate angle or not close to surface: underground mining
Steep angle / close to surface: galleries
Steep angle / vertical: open pits

I can't remember which book, but somewhere i've seen a diagram that shows this (or something similar) very clearly.

Happy to be corrected on any of that!

There are exceptions, the obvious one being Corris region where the vein is vertcial or near vertical but was mined underground - with spectacular results in terms of the underground chambers.

The shafts I've seen in Corris mines are usually internal haulage; or haulage to surface. As typified by winch remains, rollersmetc.

http://www.aditnow.co.uk/community/viewtopic.aspx?t=2923  has some useful shippets on how Blaenau mines were worked.

http://www.aditnow.co.uk/documents/Oakeley-Slate-Mine/ctc.pdf 

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Aye, the bedding plane of slate of course makes a large difference. I suppose the bedding plane must be very steep around Nantlle then hence the pit workings. I would be very interested to hear if slate was ever extracted underground here though, if anyone knows anything.

In regard to the working of quarries though, in Cumbria things were a bit different, though the same basic principles apply. Hodge Close quarry for instance is worked on a very steep bedding plane but, like many Cumbrian slate workings it was once entirely underground before being open-topped to the surface. Another, rather old method of slate mining in Cumbria involved quarrying downwards or across, and then into the mountain from the quarry without driving an adit, these are called 'cave workings' and appear as large openings in the side of some of our mountains, most don't go in very far though

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Penyorsedd in nantlle has a drainage adit (I say has as it must still be working). It even had a water balance to uphaul from the drainage adit. Unfortunately tips have been bulldozed over that shaft, although thecounterweighting water tank can still be seen (re-used as a convential water tank) in the quarry. Take a look at archive photos on this site for how they looked.

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