“The slate quarries are beautiful,” a friend told me, having returned from a holiday in Gwynedd, where I grew up. “They are not so beautiful when your ancestors died in them,” I said, half-joking just to see her face. The windows of my childhood home looked out at that landscape, the mountain a series of gradated ledges, like a staircase built for a giant. The workers were long gone, but I used to imagine them, hanging from thin bits of rope hundreds of feet in the air, risking their lives for a pittance, when I watched the rock climbers who flock to the area from all over the world. Now, after a long local campaign led by bid leader Dafydd Wigley, the slate landscapes of north Wales have won world heritage status in recognition of 1,800 years of slate mining; the people, culture and language, and how the slate from these quarries, as is often said, used to roof the world. The skills of the workers, most of them Welsh-speaking, are now consigned only to museum demonstrations. I was quite small when I first watched a man split a slate with a chisel and mallet and saw the purplish sheet become thinner and thinner beneath his hands. In my father’s house there is a slate fan made by one of our ancestors as an apprentice. There was great pride in those skills, and there continues to be. World heritage status feels like a recognition of that. Unesco has acknowledged the figure of the quarryman in Welsh culture and literature and how he has been lauded “for his progressive politics, loyalty to the Welsh language and cheery nature despite the difficulties his labour inflicted upon his health”. But it’s complicated. The history of the quarry is also a history of enclosure, colonialism and exploitation. As children we used to go up to the quarry hospital and stand in the mortuary, looking at the thick slab of slate where they’d lay the men out, and I’d wonder, if my own ancestor had been taken there, how his loved ones must have felt. As it turned out, he had been. My great-great-great-grandfather was crushed in 1892 by a massive block of slate. The cause of death was listed as heart failure. Where other schools studied the Romans, we did the quarries. This was essentially a socialist education that emphasised the importance of organised labour, though we didn’t realise it at the time. The “bargain”, as it was called, was exploitative: the quarrymen were essentially contractors working sections of rock of varying quality. We were told of the lockout at Penrhyn quarry, which lasted from 1900-1903, after Lord Penrhyn, the aristocratic owner whose fortune derived from the slave trade, tried to break the union. People would place cards in their windows that read: “Nid oes bradwr yn y tŷ hwn”(“There is no traitor in this house”). Whole terraces of houses in the area are still known as “Tai bradwr” – the houses of the traitors – after those who crossed the picket line. Trade union groups from all over Britain made contributions; support came from boilermen, from bookbinders, bakers and confectioners in London, and blastfurnacemen in Cleveland and Durham. “The lavish houses and parklands of the quarry owners convey the levels of capital achievable from the extraction of this resource,” the Unesco website says, prompting a wry smile. There are still people locally who refuse to set foot in Penrhyn Castle. When our local quarry closed in the 1960s, a void was left in the community (another ancestor was one of the last to die there, in 1961). As children, the quarries were our playgrounds. We climbed through holes in fences and waded into caves, scratched our names into blocks of slate where people who came before had carved theirs. On the National Trust website there’s a photo of some graffiti. “Am y llech’ yma slafiodd ein Teidia … Bobolbach!” (For these slates our granddads slaved … good grief!”) Now, the slate quarries are part of the Welsh tourist economy. People flock there to climb the mountains, dive and swim in the former quarry holes, the waters in which, thanks to some strange combination of minerals, are a vivid turquoise colour. For adrenaline junkies, Zip World – which has the fastest zipline in the world – occupies the former Penrhyn quarry. World heritage status will hopefully provide more jobs and infrastructure. This is crucial, as is Unesco’s emphasis on the importance of the Welsh language; at least 65% of people living within the site are Welsh speakers (this proportion is higher in certain communities). It’s wonderful to see the area acknowledged as the heartland of the Welsh language, which is so frequently belittled or ignored by others within these isles. The quarries are a lesson in the beauty of post-industrial landscapes, in all their complexity and history. Rhiannon Lucy Cosslett is a Guardian columnist and author