I would never have got to see the Tarahumara without Skip McWilliams. In the early Sixties he went down to Mexico on an old Harley; he knew Ken Kesey and Timothy Leary and hung out in Oaxaca with writers, hippies and CIA spies. One winter he went exploring in northern Mexico, through Sinaloa and across the continental divide into Chihuahua, into the immense untamed ranges of the Sierra Madre Occidental, that 'ocean of mountains' as one early explorer called it.

Copper Canyon alone - it is really more than 20 canyons - is four times the size of the Grand Canyon, its deepest gorge a staggering 7,550ft. This was still an unexplored Mexico, inhabited by people who had little, if any, contact with outsiders. So Skip took his bike up to Creel and went walking with a rucksack and sleeping bag. And there he met the Tarahumara.

One of the biggest native groups in the Americas, the Tarahumara are still 60,000-strong. Subsistence farmers, they grow maize and beans; some still use wooden ploughs, live in caves and treat with the world of spirits. They speak a birdlike Uto-Aztecan tongue linked to the ancient languages of central Mexico. The Aztecs always claimed they came from somewhere up here.

In the Thirties Antonin Artaud wrote of the Tarahumara: 'These are people who live in the state before the Flood_ these are people who defy the time, who do not recognise our reality, and instead draw magical powers from the mistrust they have for our civilisation.'

In the Sixties, in the depths of canyons, Skip found the old gods still revered: Raienari the Sun God, protector of men; Mecha the moon goddess, guardian of women. Shamans still healed and exorcised spirits, consuming peyote for festivals. Skip saw the Tarahumara play their games: lung-bursting 25-mile runs up and down canyon walls, kicking a wooden ball with bare feet. And then there was the chase: relentlessly hunting down deer, barefoot and barehanded, running them to exhaustion, driving them over cliffs on to wooden stakes. Skip ran with them, and sat up all night afterwards, listening to endless woozy music on violins and drinking tesquino, their potent maize beer. During those nights round the fire they told him proudly that they had borrowed only six things from the European world: cloth, the apple tree, the axe, the metal ploughshare, the goat - and the violin.

Skip married and had children, but couldn't keep away. Twenty years ago he made the link permanent. With their permission he bought some land and built a lodge a few miles from Creel, astride the continental divide at nearly 8,000ft. Simple wooden huts, with no electricity: oil lamps and wood stoves in the rooms, solar panels to heat the water. Sparse but lovely. An hour's walk away you could begin trekking into fantastic gorges. The place was looked after by Tarahumara from the village nearby: the profits went back into the community.

In the intervening years the Tarahumara became news. On the net you will find dozens of websites. But Skip's place is still run the same way. He's still based in Michigan, but when he gets a group booking he flies down from Detroit and goes with the guests on long treks, sleeping in caves, cooking out.

He pulls his nose wryly: 'I always give them a speech at the start, so they know what they are in for. There was one group of well-heeled Bostonians: I led in some goats the first day and handed out knives. "OK, you'd better get used to it," I said. "One of the traditions here is that when you start off you cut the throat of your own animal." One woman burst into tears. When I told her I was kidding, she shrieked that I was the most objectionable person she had ever met. She did the trek though - and she's been back!'

Last winter we were filming our journey in the footsteps of Cabeza de Vaca, a shipwrecked conquistador who appeared on the Pacific coast in 1536 after living more than eight years among the Indians, having walked across the continent. In his book he calls the native populations near here 'well disposed and intelligent'. He describes clothing that sounds very similar to that worn now by the Tarahumara. The Tarahumara were the nearest living connection to the cultures here in the sixteenth century. So we phoned Skip.

'I can help, sure. But meeting people who have no contact with the outside world takes time. You'll have to trek for a few days and then sit on the edge of their land and wait for them to come to you. You can't go to them.'

'How long will that take?

'Could be two, three weeks. Maybe four or five. And then they might tell you to get lost.'

'We're a film crew, Skip. We can't spare that kind of time.'

'OK, here's a compromise. You could meet up with people I know and work with. They are prepared to take outsiders in. You won't be going deep in, but you'll still see something special.'

We arranged to meet in Creel. There are several ways to get to there. One is the greatest railway journey in the Americas: the Copper Canyon Railway, which runs from the US border for nearly 1,000 km to the Pacific at Los Moches. It's a magical journey: the train is comfortable, the staff couldn't be more helpful; the food is good and the views wonderful. Planned back in the 1870s, the railway was finally finished in 1961. It's an amazing piece of engineering: you trundle at a sedate pace over 36 bridges and through 96 tunnels. There are delightful stopovers too: my favourite is the old Spanish colonial town of El Fuerte, which lies in a gorgeous landscape and has one of the nicest hotels you could wish for, the Hotel Posada de Hidalgo. It was somewhere near Fuerte, or so I imagined, that Cabeza de Vaca had come to 'a countryside so rich, beautiful and fertile it seemed to us there could be nowhere so beautiful in all the world'.

We came overland from the north-east, past the spectacular native city of Casas Grandes. This is one of the great archeological sites in the Americas: a vast honeycomb of adobe, fabulous curves like abstract sculpture in the pale wintry light. Then we took the train to Creel, a little alpine town overlooked by thick pine forests and jumbled rock formations. Tarahumara were in traditional dress, men in smocks and coloured headbands, women in bright cotton skirts, cardigans and hats. There on the platform was Skip.

It was dark by the time we reached the lodge. The December night closed in fast and chill. 'It'll soon be time for snow,' said Skip. A stove filled the room with the scent of pine. I fell asleep dreaming of snow drifts.

We woke at dawn to a magnificent view of crags crowned with lightning-split pines and weirdly balanced boulders. After breakfast we began the trek - up all morning, down all afternoon, 3,000ft into the Cusarare canyon. As dusk settled, we struggled through huge pines down a rubble scree to a terrace above the river, cliffs towering 500ft the other side. We made our camp in front of a blackened cave. I went along the river to gather driftwood for the fire. We washed in a hot spring close by, where sometimes Tarahumara men go on their own to pass the night and reflect on the world. The women prepared the food, and night fell to the comforting slip-slap of tortillas being made.

After the meal the coffee pots were placed in the hot embers and the tequila opened. The fireside chat came round to the pros and cons of city life. 'I've never been to Cuautemoc,' said Reyas, our Tarahumara translator.

'I have,' said Chaparequi, the old man with the violin.

'Did you like it?'

He laughed shyly. 'It was very nice.'

'Would you live there?'

'It would be too expensive to buy land to grow corn.'

'So what's so good about life here?'

'Goatshit.' Laughter all round.

'You see we use it to manure the corn. The corn grows - all plants come from God - and from it we make tortilla, pinole and tesquino. With that we have a fiesta - we dance all night and come to dawn praying in this dance - so God will benefit us.'

They call themselves 'Raramuri', Reyes explained, which means not as usually translated 'The ones who run fast' but 'Those who walk straight'.

Skip says this has a moral connotation. 'For them to walk straight means in all aspects of life to be fully conscious and aware of the consequences of what they do. This is a very experiential culture and as with many native American societies, some of our categories are simply meaningless to them, just as a lot of theirs are untranslatable to us. They say people who leave the canyons and go to the city are the mistaken ones because to join Western consumer society is stupid, wandering around in a daze, not understanding the point of life. To become obsessed with owning things, even naming things, is not to be masters of our own destiny.'

'So we've forgotten how to walk straight?' I asked.

'I'm sorry,' said Reyes, as if gently chastising an errant younger brother. 'You seem to us to be crashing about all over the place.'

That night we all curled up together under blankets around the fire at the mouth of the cave; or inside on a thick layer of earth and ash: a chilly winter night, but we were warm under a starry sky. Chaparequi stayed half awake and fed the fire with resinous pine, whose scent wafted to the cave mouth, strong as incense.

Over a century ago Carl Lumholtz wrote in his book Unknown Mexico that such people are becoming 'scarce on the globe' as they are 'crushed beneath the heels of civilisation'. Government plans for Copper Canyon include roads, hotels and camper sites for a projected half a million US tourists. It is inevitable that the six gifts of the chabochis - or the bearded ones, as they call us - will be followed by others, less useful. Let's hope at least that the changes are on their terms.

And perhaps if any of us are lucky enough to spend time with the Tarahumara, even if only on the outer margin, as we did, it might help us to see better who we are. And to remember what that goatshit really means.

More information

www.tarahumara.org and www.visitmexico.com. For further information on holidays to Mexico phone: 0870 900 9866. Skip McWilliams: Copper Canyon Hiking, Sierra Lodge, Creel, Chihuahua. Creel Lodge (Jose): 00 52 145 600 36. US reservation office: 001 248 3407230; fax: 00 1 248 340 7212

• Michael Wood's BBC2 series 'Conquistadors' is on Fridays at 9pm