Initially the producers of the DVD had been keen to include slate walls, but I wasn't and anyway you can't cover everything. As a contractor I've worked north of Inverness and South of London, on researching "Dry Stone Walling" I travelled as far afield as Thurso in Caithness, to Lizard Point in Cornwall. I've given talks in Yorkshire, and competed from the Cotswolds to Perthshire. Whenever I say I come from North Wales apart from the comments on my obvious London accent the most common responses would be along the lines of 'lot of slate walls around there'. NO, NO, NOOOO!!! This annoys me even more than the inevitable 'it's just like a jigsaw' you get at demonstrations etc. (Whoever does jigsaws where half a dozen puzzles, with half the bits missing, have been mixed up; come out of a box with another picture completely; and the only way you can get the pieces to fit is to hit them very hard?).
As I write this week I have 2 projects on the go, both at quarrymen's cottages within spitting distance of slate quarries in Rhosgadfan and Mynydd Llandegai. Outside of the quarries there's hardly a slate in sight- excepting the slate pillar fences at Mynydd Llandegai - and certainly not in 'my' walls.
Walls are built where stone is plentiful, (cloddiau perhaps where it is less so), field clearance and perhaps highly localised quarrying and borrow pits supplementing this. I openly admit that my geological knowledge is a little lacking but as I understand it slate isn`t exactly hard, in the very least it disintegrates fairly easily. So you wouldn't expect to see slate field clearance walls as there wouldn't be much of it lying around. Walls from local quarrying/borrow pits lead to local variations/styles except where the outcrops are widespread and of a similar rock type. Seems to me that any significant areas of slate, have been quarried for purposes other than walling. Outside of the actual quarries active/dis-used there is actually very little slate available for walling.
There are of course huge mounds of waste slate so why haven`t these been used. I`m not overly familiar with all slate areas but around here (Bethesda/Llanberis) there`s plenty of alternative stone, available much more locally than a quarry even when it is less than a mile away, especially if it was produced by field clearance. Local is the key, they didn't have 16 tonne wagons, and didn't want to transport anything any further than they really had to. (You also have to wonder if the quarry owners, not renowned for their largesse, would have been overly keen on their material disappearing en masse!) Why use slate when you have a more durable alternative to hand? (There is also the consideration that numerous walls will pre-date the quarrying.) The fact that suitable coping is not always 'to hand' would also explain the greater use of slate for this purpose. It comes down to cost/benefit analysis, the more useful the stone, the further you would be willing to transport it. If you are rich and can afford to move stone then such restrictions ought to be removed, but then again the alternatives which you can also afford are likely to be more durable and arguably more aesthetically pleasing, so once again slate is the exception rather than the rule.
I have spent my whole walling career living virtually alongside slate quarries; a significant proportion of my work has been within a few miles of slate quarries. I haven't built a slate wall for around 15 years. I've built far more limestone wall (not counting the grey/black Benllech stuff) in North Wales than slate, which if taken at face value would suggest we have much in common with Derbyshire. As far as I'm concerned slate walls play only a minor role in the mosaic of boundaries that makeup North Wales. There are some highly specialised and notable examples but to present them in a way that would be likely construed as representative of the area would only serve to re-inforce a myth. In North Wales slate is for roofs not for walls!