It looked, absurdly, as if we were walking straight into the Garden of Eden itself. There we were in a pink and rocky wilderness in the heights of the High Atlas Mountains in southern Morocco. We followed a trickle of running water and, as it got broader, wound our way down into a sudden miracle of greenery: a valley bursting with fig trees, pomegranates and tiny fields of maize. We were accompanied by a constant gurgle from a clever combination of irrigation ditches bringing life to a barren place. Apart from the odd feud over water rights, the local people say that little disturbs the tranquillity here: there's practically no crime, not much violence (certainly no drunkenness; this is a good Muslim area), and, so they claim, few serpents.
But perhaps they're reckoning without the tourists.
We had just finished peeling off our walking boots and examining our blisters, trophies of the day's challenge, when the boy and girl racers of the adventure travel world swept up. From about 6.30 that evening the mountain bikers straggled in, all Lycra, sweat and Oakley sunshades, collapsing exhausted into the gite in the little Berber village of Wawriykt. Hikers and bikers, we quickly discovered, create their own uneasy culture clash. Particularly when they are squeezed into a confined space in a remote Moroccan village whose population they have just swollen by nearly a quarter.
Space was one thing, attitude quite another.
"Chicken and children are fair game," Baz announced later that evening. "You don't have to give way to them. But you shouldn't," he added, "slap the rump of any mules. That would be antisocial."
Baz (Sebastian) was the British tour guide to the group of 16 mountain bikers, and was briefing his team for day two of their 10-day trip. Meanwhile, squashed together, sitting on the floor at the far end of the gite's overcrowded dining- and sleeping-area, we walkers listened and looked on with indignation. Baz, we appreciated, was speaking in slight jest, but we couldn't help feeling that beneath this attempt at humour there was a jarring lack of respect for the local community. And, what was worse, privately we realised that we ourselves were not above reproach.
Outwardly, however, we clung precariously to the moral high ground. We had booked with the same tour operator as the cyclists. But as walkers we liked to think of ourselves as socially and environmentally sensitive travellers: a group of nine who had signed up for five days' hiking in a starkly spectacular landscape, leaving (as green campaigners urge) only footprints.
It was to be, so the Exodus Travels brochure and itinerary had implied, a holiday on the comfortable side of basic: four nights sleeping inside, or on the roof - under the stars - of a Berber home turned gite.
This had the comparative luxury of two Asian-style loos, two cold showers and, wonderfully unexpected, its own home-made hammam - a Moroccan version of a Turkish bath-house, with a hot-water barrel heated by charcoal lit beneath the floor.
Numbers would, the itinerary had stated, be limited to fewer than 12. We were to spend several days getting right away from it all, staying in an area without roads or electricity and walking five to seven hours a day around the contours of one of northern Africa's most geologically stunning areas.
Trekking (and indeed cycling) in the High Atlas Mountains has become northern Africa's hybrid answer to the Himalayan hike and the Tuscan walking tour. And with reason. Not only is this a cheap and fairly convenient area to reach (air fares to Marrakesh, a two-hour bus ride from the mountain range, are a lot lower than those to Asia), it also offers a staggering landscape of awe-inspiring panoramas; strange, vividly coloured rock formations; and, of course, magically green valleys above which nestle little mud-brick Berber villages.
All of which sound like great ingredients for a challenging fresh-air holiday. The reality, though, can be gruelling for those who, literally, don't have the stomach or time to acclimatise to some rough going.
For a start, the local Berber people rightly think you're nuts as you clomp off, walking through the heat of the day to arid, empty spaces. It's tough terrain, too: lose your concentration as to where you're placing each foot and you could stumble badly over the rocks, doing yourself a serious injury in the middle of nowhere. And then there's the harsh, unrelenting sun: our group went down like ninepins as heatstroke exacerbated any queasy unfamiliarity with the local food.
But if you survive all this in an upright position, you get a tremendous sense of satisfaction. Those of us who retained the energy were taken over lunar-like landscapes to far-flung desert lakes and waterfalls. We walked for miles through scenery untamed by mankind, clambering through great canyons of boulders, weaving our way across aromatic plains of wild thyme where swooping house martins and silky lizards provided rare signs of life. And, just when we thought we'd sweated our way well beyond reach of civilisation, lunch could come trotting past on the back of a mule. About half-an-hour further on we would find a meal impeccably laid out on a rug, with the mule driver brewing up peppermint tea which he would ceremoniously serve in a polished pewter teapot. This exercise in neat planning and delicate politeness was, we rapidly appreciated, one of the advantages of joining an organised tour.
In fact, it might all have been a happy experience were it not for the 16 mountain bikers and the feel-bad factor that came in their wake. Back at the gite we snarled at them with a dog-in-the-manger-like attitude as they tripped over our walking boots and encroached on our jealously guarded sleeping spaces on the roof. We blenched at the sanitary implications of such numbers, and we sneered at their shiny Lycra thighs - how inappropriate in a cover-up country like Morocco.
And then we realised that, probably, they were just like us: tourists in search of a bit of adventure, wanting some understanding of the people and the places they were travelling through.
It was simply that, owing to the oversight of a tour operator back in Britain, there were too many of us. Lumped together we became an invading army, threatening the finely balanced ecology of an idyllic valley - one that generations of gentle Berber people had created out of the same dramatic, unforgiving landscape that lay beyond.
Harriet O'Brien paid pounds 535 for a seven-day trip to Morocco through Exodus Travels (0181-675 5550). The price included flights, three nights' accommodation in Marrakesh and a five-day, all-inclusive trek in the High Atlas Mountains. Exodus Travels also offers two-week treks in the High Atlas, as do Explore Worldwide (01252 344 161); Headwater Holidays (01606 48699); Sherpa Expeditions (0181-577 2717); and Worldwide Journeys (0171-381 8638), which also has seven-day trips. Best of Morocco (01380 828533) organises three-day hikes in the High Atlas.
For independent travellers, Marrakesh is the best gateway. In October, Royal Air Maroc (0171-439 4361) has daily flights from Heathrow via Casablanca for pounds 360 including tax. For the same fare, GB Airways (a British Airways affiliate, 0345 222111) flies from Gatwick on Tuesday and Friday via Gibraltar.
Moroccan Information and Advice Centre, 61 Golborne Road, London W10 5NR (0181-960 6654).