Moor House was one of the original eight National Nature Reserves designated in 1952, when the new postwar conservation movement in Britain was still drawing up its list of places worth protecting forever. Upper Teesdale was a separate reserve, declared later, set up specifically to protect the rare arctic-alpine flora that had survived along the upper River Tees since the last Ice Age. At the end of the 20th century the two reserves were combined into a single 7,400-hectare unit straddling the boundary between Cumbria and County Durham. The combined reserve includes Great Dun Fell at 848 metres, Little Dun Fell, Cross Fell at 893 metres - the highest summit in England outside the Lake District - and the long, lonely watershed where the Tees rises.

In 1976, while most of Britain was preoccupied with the Cod Wars and the IMF crisis, UNESCO designated Moor House-Upper Teesdale a biosphere reserve. The biosphere programme exists to recognise places where conservation science can be done at landscape scale: not just a single rare site, but a whole functioning ecosystem with people working in it. Moor House qualifies because the reserve is one of the most intensively studied uplands in Europe. Researchers have been measuring peat depths, water chemistry, plant communities, and weather at Moor House since the 1950s, building one of the longest continuous environmental data sets in Britain. When climate scientists need a baseline for how northern English peatland has been changing, this is where they look.

The Pennine Way, England's first long-distance national trail, passes through both halves of the reserve - which means walkers tackling the 268-mile route from Edale to Kirk Yetholm get to spend two of their hardest days here. The route crosses Upper Teesdale around High Force and Cauldron Snout, climbs over the watershed, and runs along the edge of Cross Fell. In bad weather it is one of the most exposed sections of the entire trail; the high tops have no shelter, and Cross Fell is notorious for its own local wind, the Helm Wind, the only named wind in the British Isles - a fierce easterly that pours down the west-facing escarpment when conditions align. Walkers who have been blown off their feet by it tend to remember the experience.

The biological reason for the reserve's importance is not obvious at a glance. The high tops look bare - cotton grass, sedges, lichens, the dark hummocks of blanket peat. But hidden among the more common plants are species that have been here since the tundra retreated at the end of the last Ice Age: spring gentian, with its astonishing intense-blue flowers; Teesdale violet; bird's-eye primrose; Scottish asphodel and several others. Some grow on outcrops of so-called sugar limestone, a sugary, friable rock found in only two places in Britain, which weathers into a thin alkaline soil that suits these arctic-alpine relicts. The reserve also supports breeding populations of upland waders - golden plover, dunlin, curlew - whose calls in spring are the soundtrack of these moors. It is a place that asks for patience. The reward is the realisation that you are standing in a piece of ecological deep time.