Around 295 million years ago, upwelling magma forced its way through fissures in the older Carboniferous Limestone of what would become Upper Teesdale. The intrusion - geologists call it the Whin Sill - cooled over an estimated fifty years, contracting as it solidified and splitting into the vertical columns of dolerite you can still see at High Force. The heat from the sill cooked the limestone above it, recrystallising it into a crumbly white marble that mineralogists named Sugar Limestone. Then, much more recently, Pleistocene glaciers scraped the dale into its present shape, burying the old river course under glacial drift. The Whin Sill is the reason Teesdale's waterfalls exist. Over its hard ledges the Tees drops at High Force, runs in tiered cataracts at Low Force, and falls in a single thundering plume at Cauldron Snout.

When the glaciers retreated, Arctic and Alpine plants colonised most of Britain - then, as the climate warmed, they were crowded out by trees and scrub almost everywhere. Almost. On the impervious dolerite of Upper Teesdale, with its thin acid soils, scrub and trees never took hold, and the Sugar Limestone above happens to provide the alkaline, mineral-rich conditions some of those Arctic-Alpine species need. So they stayed. The Teesdale Violet survives nowhere else in England together with its companion species. The blue Spring Gentian flowers in May. Mountain pansy, bird's-eye primrose, butterwort, spring sandwort, rockrose - common Pennine flowers mingle with relicts of the post-glacial tundra. The hay meadows above High Force are managed to preserve globe flower, wood cranesbill and Early Purple Orchid. And on the south bank near High Force grows the largest surviving juniper wood in England. Part of Upper Teesdale, around Cow Green Reservoir, is a National Nature Reserve precisely because of these plants.

People have lived here for as long as plants have. The Bronze Age burial site of Kirkcarrion crowns a hill on the south side of the dale. The Romans built roads across the high passes. By the medieval period the Honour of Richmond ran the upper dale and the Bishops of Durham held the mineral rights below - rights that funded centuries of lead mining whose traces still scar the slopes and side-valleys. The dale's principal town is Barnard Castle, the market town that gives the surrounding country its centre; further upstream sit Middleton-in-Teesdale and the villages of Mickleton, Eggleston, Romaldkirk and Cotherstone. The boundaries have shifted - the Cumbrian end once divided between Cumberland and Westmorland, the County Durham stretch between the former Durham and the North Riding of Yorkshire. Most of the dale is now within the North Pennines National Landscape, and Upper Teesdale is a Site of Special Scientific Interest. The Tees still falls over the Whin Sill, the juniper still grows, and the violet still blooms each spring under sub-Arctic skies.