T he vital moment comes a couple of hours into the walk. I drop down from a tussocky hill towards the village of Y Fron. There’s an abandoned slate-quarrying machine and a grassy track that suddenly emerges on the rim of a deep lake-filled crater. The water is the colour of an old bruise. Around the rim is a fence of lichen-crusty slates like gravestones. It’s a dark, brooding spectacle and totally unexpected. At that moment, I know that whoever designed this route knew what they were doing.

This is the Llwybr Llechi Eryri (Snowdonia Slate Trail), an 83-mile hike that follows a great loop around the national park, starting in Bangor and ending in Bethesda. It strings together a series of abandoned slate quarries with some of the finest riverside and mountain walking in the UK. I’m doing just three days, starting in Llanberis and heading to Betws-y-Coed via stops in Beddgelert and Llan Ffestiniog.

Beyond Y Fron village, I descend into my first quarry, and it proves to be a classic.…