The fairytale, lichen-encrusted oaks of Wistman's Wood are a bit of a mystery. Ecologists are still puzzled over why its wizened and gnarled trees, some of which are over 300 years old, are all of the lowland, pedunculate variety, whereas all those in the well-wooded valleys on the edge of the moor are the more hardy sessile oaks, usually found in the uplands of Britain.

Another mystery is that this incredibly ancient-looking wood, which feels as though it has been there since the beginning of time, is on the move. Not only are its trees twice the height they used to be 80 years ago, but the wood itself has doubled in size over the past 60 years, and is slowly but surely creeping up the slopes of neighbouring Longaford Tor.

This part of the mystery, however, is easier to solve. As the plentiful new acorns germinate, they are sheltered by the older trees from the prevailing westerly winds which are therefore responsible for gradually pushing the wood uphill. So despite its venerable and unchanging appearance, Wistman's Wood is what the ecologists call a "dynamic ecosystem". This arboreal enigma lies hidden away in the desolate valley of the West Dart river in the moorland heart of Dartmoor, near the National Park's epicentre at Two Bridges. For many it represents the essence of Dartmoor.

While it is usually heralded by guidebooks as "the last great wilderness of southern Britain", Dartmoor's spirit is not necessarily to be found on the open, tor-topped moors, described by their most famous chronicler, Arthur Conan Doyle, as "so vast, and so barren, and so mysterious".

Wistman's is thought to take its name from the old West Country word "wisht" which means sad or uncanny, so "Wistman" may mean "Wisht-man", and the wood may be named after a local, other-worldly spirit. Certainly, early Dartmoor antiquarians were keen to link this ancient oakwood with the activities of Celtic Druids, who were known to favour oak groves for their pagan ceremonies.

But perhaps the most remarkable feature of Wistman's Wood, apart from its exposed position at 400m (1,300ft) above the sea, is its luxurious mantle of mosses, ferns, liverworts and lichens. They drip from the branches of the weirdly-contorted branches like the epiphytes and bromeliads of an Amazonian rain forest, and in fact represent a habitat which is every bit as rare and threatened.

The loss of the rare and beautiful moss antitrichia curtipendula, which formerly hung in liana-like fronds from the branches of many of the trees in Victorian times, is blamed on the increase in airborne pollution, and the gradual opening-up of the wood has in turn caused a loss of humidity which has affected the moisture-loving ferns and liverworts.

Wistman's Wood is now a National Nature Reserve managed by English Nature under agreement with the Duchy of Cornwall, and access is strictly by written permission only. However, a public footpath runs conveniently past the wood, and you can peep into it along this easy, three-mile stroll from Two Bridges.

Start from the quarry car park on the side of the B3357, opposite the Two Bridges Hotel. The way leads clearly north along a track towards Crockern farmhouse, where a stile takes you out into open country and the first sighting of Wistman's Wood, low-lying like a hedge on the slopes of the valley ahead.

The prominent rocks on the hillside to your left are Beardown Tors, parts of which make up the Merrivale Firing Range, still in use by the Army despite the fact it is now situated in a National Park supposedly set aside for the quiet enjoyment of the public. When a red flag is seen flying from the tors, firing is taking place and walking is forbidden on these hills.

The winding artificial waterway that can just be seen contouring beneath the tors is the Devonport Leat, cut between 1793-96 to take water from the West Dart river, whose natural course flows down in the valley below, to the Burrator Reservoir and thence to the then fast-growing dockyard town of Devonport.

As you step out across the rock-strewn moorland heading due north towards the approaching wind-stunted "hedge" of Wistman's Wood, you may notice some broken down walls and mounds emerging from the moorland grass. The map of Dartmoor is scattered with the words "settlement" and "hut circle" in that Gothic lettering the Ordnance Survey reserves for ancient monuments. These humps are all that remains of medieval and earlier villages and farms that once made Dartmoor one of the most populous places in the south west.

Indeed, it is said to contain, "fossilised" under the moor grass, the most important and complete prehistoric landscape in north-west Europe.

Walk around the top edge of the wood, looking inside at what one author has described as "the ultimate in arboreal deformity" and "a phantasmagoria of greenleaf and granite". Wistman's Wood covers about four hectares (8.5 acres), and its amazing survival is thought to be due to the fact that the trees have grown in the shelter of the rocky crevices of the "clitter" (the Dartmoor name for loose rocks). These create a favourable micro- climate, protecting the trees from gales and frost and, above all, from being nibbled by local sheep.

When you reach the northern end of the wood, turn right and head steeply uphill to reach the rocky summit of Longaford Tor, from where a splendid view opens up to the north towards the 1,978ft (603m) Cut Hill, perhaps the most remote of the Dartmoor summits. The nearer tor is Higher White Tor, and between that and Longaford Tor are the remains of one of Dartmoor's enigmatic prehistoric stone rows.

Take the ridge due south, crossing the strange, masonry-like blocks of Littaford Tors and down through gaps in the walls towards Crockern Tor.

It is now a short step due west and downhill, passing some more pillow mounds, to reach Crockern again and your outward route back to Two Bridges.

The practicals

Wistman's Wood is near Two Bridges in the centre of Dartmoor, off the B3357 Dartmeet-Tavistock road. The walk is approximately three miles in length and should take about 2 hours. It is over rough moorland, so appropriate footwear should be worn. Indispensable for any exploration of Dartmoor is the Ordnance Survey's 1:25,000 Outdoor Leisure Map No. 28, covering the National Park. The Two Bridges Hotel (tel: 01822 890581) has a beautiful riverside setting near the junction of the West Dart river with the Cowsic, and is very popular with walkers. The nearest National Park Information Centre is the splendid High Moorland Centre occupying the former Old Duchy Hotel in the centre of Princetown (tel: 01822 890414).