The river carved a languid path through the crisp, parched grassland and bordering pine forest. Beyond were the piercing, crystalline peaks of distant, snow-laden mountains. A herd of buffalo, perhaps thirty-strong, made their sluggish, ill-tempered way along the water's edge before plunging into the flow, the current barely troubling them as they guided their calves to the other side. Some way behind, a rogue male loitered in hope of a thunderous clash with the herd's leader - but, for now, peace reigned.
Until the first cars pulled over. Initially, a handful of devoted wildlife-watchers turned up, wielding giant zoom lenses. Then the amateurs, who stop only when someone else does, arrived. The narrow road was soon clogged with camper vans, jeeps and assorted people carriers, creatures and chrome mingling into a single logjam. Next came the park ranger, trapped between tedium and fury as she tried to save the wilfully suicidal: "Ma'am, please step away, that's a wild animal. Sir, I think your child is better off in the car. No, please don't try to feed the buffalo . . ."
Yellowstone National Park bewitches and befuddles America in equal measure. Trapped on a spacious plateau where the Rocky Mountain states of Idaho, Wyoming and Montana meet, the Yellowstone high country is an implausibly beautiful magnum opus of creation, the alpine drama of the landscape given an extra air of magic by the steam and sulphur that drift on the breeze, simmering from volcanic mud pools and roaring geysers. This unearthly place has always inspired contrasting responses (it's known both as heaven on earth and "the place where hell bubbles up") but the greatest confusion surrounds Yellowstone's enduring symbolic purpose - as the crucible in which America's eternal national dilemma demands to be finally decided: How, on this most bountiful continent, can people and nature share the same space?
The first white people to set eyes on Yellowstone were so awestruck that their imaginations were rattled loose and they reported that they had seen valleys in which the animals had been turned to stone, and others in which the creatures had shrunk to miniature. But these pioneers did not find what they were really after - gold - and Yellowstone remained largely unspoilt when the first official surveyors arrived. They lobbied for the pristine plateau to be protected, and in 1872 the world's first national park was created. It was a visionary decision - the famous British diplomat James Bryce declared national parklands to be the best idea America ever had. But the significant complicating factor, that the brand new park was actually a very old Native American hunting ground and homeland, was cheerfully ignored. The tribes were removed and barred, behind the smokescreen of a hastily concocted myth that they were scared of geysers.
Not that Yellowstone would be an inhuman place. Far from it - in those "frontier" days, the wilderness was perceived as an affront and a challenge, and to tame it was man's triumph. The park was mandated to exist for the "benefit and enjoyment" of American citizens. It would, in essence, be a zoo. Tourists duly flocked, but it soon emerged that running an alpine plateau as a theme park was a risky environmental endeavour. The early park-keepers encouraged the most popular fauna, such as elk, to flourish, and they packed the rivers with the fish most likely to take a fly. Yellowstone's star attractions, the bears, were fed from car windows - one bear called Jesse James, clearly smarter than the average, learned to block traffic until he was fed.
This all made for a wonderful holiday, but by the 1950s, predictably, the Yellowstone ecology had gone quite haywire. Animal populations steepled then plummeted; monocultures of plant life sprang up; species disappeared entirely; while the increasingly panicky park-keepers didn't know what to feed, what to plant and what to shoot. The vision of America that Yellowstone was supposed to embody, of the land entirely tailored to man's needs, wasn't going well. Another mythology took its place.
Then, in 1963, a near-complete U-turn was announced: the park would now be "a vignette of primitive America . . . free from man's spoliation". This reverse vision, of a natural perfection that people could only pollute, was expressed still more fervently the following year by the law that protected America's "wilderness", defined in statute as "an area where the earth and its community of life are untrammelled by man". Once again, the complicating factor of at least 12,000 years of human occupation of the continent could not be accommodated: as the geographer Paul F Starrs observed: "A geographer's assessment of all landscapes as cultural - no land is realistically 'natural' without counting its human residents - would find no argument almost any place else in the world outside the United States."
So, today, Yellowstone is torn between these two all-or-nothing, and equally all-American, positions - between those who would close every road, ban every bus and curtail all human activity bar the campfire song, and those who would cheerfully open a Wal-Mart next to Old Faithful. On the one side, conservationists seriously debate introducing elephants and tigers into the park to replicate the missing ecological impact of their long-extinct ancestors; on the other, the Bush administration has lobbied the United Nations to remove Yellowstone from its list of at-risk World Heritage Sites to facilitate oil and gas drilling on the edge of the plateau.
Extricating myself from the bumper-to-buffalo logjam, I parked at the nearest peaceful spot and hiked away from the road. In just a few minutes, silence fell, the path tailed off, and, as snow began to fall, I had the splendour and mystery of a Yellowstone valley all to myself. Whether I was master of all I surveyed, or an alien pollutant of the pristine, the modern keepers of this place have yet to decide. When they are asked, the people who were here before, now based on res ervations around the park, have some answers. As Chief Luther Standing Bear of the Oglala Sioux said: "We did not think of the great open plains, the beautiful rolling hills and winding streams with tangled growth as 'wild'. Only to the white man was nature a 'wilderness'. To us it was tame. When the very animals of the forest began to flee his approach - then it was for us that the 'Wild West' began."
Brian Schofield is the author of "Selling Your Father's Bones: the Epic Fate of the American West" (HarperPress, £17.99), out now