Permanence and change: together they provide the pleasing paradox of a walk in the Himalayas.
Here in the Langtang Valley, in the lee of changeless mountains and timeless horizons, the scene shifts hourly, each hundred metres of elevation peeling back another layer of this country's character, and each stage of the trek a novelty in every way but one: the direction - up.
The goal is Kyanjin Gompa, the last village on this three-day plod to the valley's end, at 3,900 metres above sea level. Beyond it lie unclimbed peaks, holy lakes and an impassable wilderness of ice and rock.
That stark landscape is difficult to imagine as you strike out from the cheerful hill town of Syabrubesi, situ-ated at a manageable 1,500 metres and a day's drive north of Kathmandu, the Nepalese capital.
The trail begins as alpine forest, with green paddy fields etching the valley sides and little villages huddling on the level spaces. Troupes of langur monkeys peer quizzically through the foliage and bright pheasants - Nepal's national bird - flee clumsily through the undergrowth. Great beehives hang from the eaves of cliffs while the Langtang Khola river rumbles grandly below. The air is warm, the going easy.
But the gradient soon steepens and each upward step brings change. The flora alters, turning hardier: the forest thins with the air and the trees retreat to sheltered recesses, where they cling on grimly. Soon the valley widens and boulders, the wreckage of an ancient glacial advance, start to outnumber plants and shrubs. The fauna also changes: easy-living cows cede this tougher ground to yaks and the flashy lowland birds give way to ravens and choughs, which hang stubbornly in the stiff, cold breeze.
Most striking of all is how the identity of the land itself changes. Having set off in Nepal, you soon find yourself in a different place entirely: Tibet - at least in spirit, if not in the strict geographical sense (the Tibetan border lies a few miles north).
Most of Langtang's inhabitants are Tibetans descended from settlers who crossed the obscure border generations ago. Over the years, their Buddhist culture has shaped the place as surely as the wind and ice: prayer wheels built upon streams spin in the water's currents, unspooling their infinite vows to the wind; mani walls carved with mantras and mystic symbols bless the route; and everywhere prayer flags flutter brightly on the mountainsides.
These five-coloured flags seem to mimic the surrounding landscape, their bold stripes matching its contrasting shades: the muted earth, the red sprays of blooming rhododendrons, the brilliant white of snow-topped mountains and the stratospheric blue of the sky.
The peaks pile ever higher as you advance up the valley.
If the terrain is now characteristically Himalayan, the human geography of this other Tibet is quite distinct from that of the land to the north. These Nepali Tibetans enjoy freedoms their cousins lack: culture and religion are unrestricted; a picture of the Dalai Lama hangs on every wall; and superstitions endure, with strange talismans set along the trail to ward off evil forces. But the migration to Nepal has hardly freed these people from the Himalayas' crippling remoteness.
"It doesn't really make any difference that we are in Nepal," explains Lhakpa, who runs a bakery catering to foreign trekkers. Nepal's dysfunctional government has forgotten all about them, he says. "For a hundred years, we've been hoping for the same things: education, development. But tourism is the only good thing that has happened here."
So he focuses on perfecting his chocolate cake, for which none of the ingredients are available locally (being that there is nothing yak-based in the recipe). A British visitor recently offered him some pointers. "She said it was the worst chocolate cake she'd ever tasted," Lhakpa shrugs. It's better now, he says - and I have indeed had worse.
So the trekkers are welcome, for all that they have indelibly altered these villages' character, with homes now doubling up as rough-and-ready hotels. But at least the visitors pass through on their own two feet (even if they don't necessarily carry their own bags). Everything else that makes these communities function - sacks of rice, masonry, gas canisters - must be hauled up by porters.
One of them staggers past laden with a huge metal vat; it must weigh more than he does. He trudges up the mountain, chanting the Buddhist mantra under his breath to ease his gruelling ascent: " Om mani padme hum."
In the empty air of the upper valley, it's hard enough transporting yourself. But the outrageous beauty of the place is a distraction from the effects of the altitude - dragging feet and heaving lungs - as you arrive at Kyanjin Gompa. Here, finally, the valley flattens out, and the village has room to spread. The river, which has roared angrily for the past three days, relaxes; white water softens into an opalescent blue stream which gently polishes bizarre artworks of rock shaped through the ages.
Of course, all this beauty comes at a price. Resources are meagre and the weather ominous. Sleep at altitude is poor and the nights are bitingly cold. It's amazing that anything can survive up here without down jackets and all-season sleeping bags.
The following day a hearty lunch of yak cheese fuels one final exertion: the testing hike up Kyanjin Ri, the 4,700-metre peak that overlooks the village. It's a brutal climb; the air now contains half the oxygen it does at sea level. Om - inhale - mani - exhale - padme - inhale - hum - exhale.
Near the peak, a raven glides past without so much as a wing flap, mocking my struggle, and a lammergeyer, the majestic Himalayan vulture, patrols overhead, reading me like a menu.
I make it, however, and after days of gazing up, I can finally look down and around - at the Langtang Lirung glacier, a rough tongue of ice lolling down a mountainside untouched by man; at the vast moraine that sweeps over the valley floor; at the yak trails that score the otherwise perfect snow; at the miniature village below; and, in all directions, at the splendid Himalayas.
The summit is no place to linger but I pause to look down the valley before retreating to the richer air below. My Tibetan host in the next village down - she was 30, maybe more - told me she had only once been to Kyanjin Gompa, four hours' walk up the valley from her home; and that she had never been to Syabrubesi, eight hours down. She smiled acceptingly. Her horizons were narrow, but at least they were special.
Getting there: Dragonair (www.dragonair.com) flies from Hong Kong to Kathmandu, Nepal. Syabrubesi, where the Langtang trail begins, can be reached from the Nepalese capital by public bus (seven to nine hours) or private bus (four to six hours).