The rocks along this popular scenic stretch of the Pennine Way, upstream from Wynch suspension footbridge, have been worn smooth by hiking boots. In summer, many visitors come to admire Teesdale’s famous assemblage of rare wild flowers; in autumn and winter, after heavy rain, spectacular waterfalls beckon.

Steady drizzle all the way today, weather that can dampen spirits but suits mosses and lichens so well, adding sparkle to the emerald green cushions on fence posts, reviving encrusting circles, in pastel shades of grey, on drystone walls. Tactile, vibrant details in the landscape, among the fading russets and yellows of decaying summer leaves.

Just as we reach the top of High Force waterfall, sunlight breaks through the pearly layer of cloud. Drama, pure exhilaration. Upstream, the River Tees, shallow and fast, clatters over its rocky bed, but beside us, funnelling through a channel worn through sedimentary rock, it squeezes between hard, igneous dolerite, accelerating with awe-inspiring energy. Raging water, a turmoil of spume and spray, hurtles over the precipice, into the foaming plunge pool 70ft below. Then, its fury spent, the river dissolves into mist trapped in its deep, wooded gorge.

A moment to stand and contemplate the climatic and geological forces that have shaped this landscape. Limestone and sandstone laid down in ancient warm seas, fiery intrusions of volcanic rock, monstrous glacial scouring, torrents of water released from melting ice: the sheer scale of it all is so hard to comprehend.

Three miles to the east, on the fellside 750ft above us, sunlight plays across Coldberry Gutter, a notch in the horizon above Middleton-in-Teesdale, as though hacked out with a giant knife. It was widely believed to be the work of hush miners who released torrents of dammed water downhill, scouring away loose rock, exposing lead ore veins. Recently, Durham University geologists concluded that the chasm was primarily the work of glacial meltwater. The magnificent High Force waterfall under our feet suddenly seems puny, compared with those elemental forces released from a rapidly melting ice sheet, 10,000 years ago.

On the way back we pause again to admire the mosses and lichens, the first plant life to colonise bare rock after the ice retreated. Such grandeur, such beauty, such tenacity.

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