Continental drift and the art of choosing slate

What’s the connection between Canadian slate mines and Snowdonia National Park in Wales? The answer is that 500 million years ago, before continental drift formed the Atlantic Ocean, they were in much the same place.

What has this got to do with extending a Grade II listed house, you may well ask? Quite a lot, it turned out: by last week we had obtained all the listed building consents required before breaking ground on the extension foundations, bar one; the remaining condition was the source of the roofing slate. The strong preference of our local authority was for Welsh slate, because that was what used to be used in East Anglia. Our request to use Spanish slate did not go down well.

Welsh slate is, however, hard to get and very expensive – five to six times the price of Standard Spanish slate, with long waiting times for delivery. Furthermore, a visit to a specialist supplier established that one of the very few easily available Welsh slates, from the Penrhyn quarry, has a blueish tinge. So the supplier, Cembrit, suggested a Canadian slate called Glendyne, which is approved for use by the Snowdonia National Park authority for use within its own borders as a substitute for the well known dark grey Ffestiniog slate, which is no longer mined. If it can be used in Snowdonia, why not Suffolk? It is pricey, but still less than half the cost of Welsh slate.

We delivered samples of Glendyne to the planners. We were taken aback by the reply: it was acceptable if we could confirm that it came from the same geological formations as Welsh slate.

Now it so happens that many years ago I did Part I of my science degree in geology and mineralogy, before switching disciplines. It was so long ago that our lecturers were the very same scientists who had finally proved the reality of continental drift, more than a century after the theory was first proposed (in 1858).

With a combination of geological and slate producer websites, not to forget an old university geology textbook that I had kept for decades on a bottom shelf, I was able to confirm that the Glendyne slate quarry, on the Quebec-New Brunswick border, now 3,000 miles from Wales, was from the same Ordovician formations as Ffestiniog. They drifted apart hundreds of millions of years ago. (If you’re after a substitute for Penrhyn, that turns out to be from the even older Cambrian era, and is a match for Newfoundland slate. But that does not seem to be sold in the UK).

The answer was given, the permission came, and the foundations were started.

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