Huge walls of multi-coloured sandstone tower thousands of feet above a tree-lined valley floor, and rising into a sky so deep and blue that it could have been stolen from an Andy Warhol painting, if Mr Warhol had been inclined to paint natural wonders. Up and to our left, a waterfall tumbles the length of two football pitches, into a crystal clear plunge pool surrounded by footprints from the herd of deer that came here at dawn. As the sun rises, rainbows bounce off the canyon-side, and we are blessed with one of those special moments when a landscape becomes so utterly majestic, and so epic in scale, that you feel lucky to be alive.
The God-fearing folk who named this place, in 1858, probably felt the same way. They were Mormon settlers, heading west into what eventually became Utah to escape religious persecution, and when they wandered into this 15-mile-long, half-mile-deep canyon they gawped in wonder, decided they were in a Promised Land, and christened it Zion. The rock formations which fill the surrounding wilderness got names such as Temple of Sinawava and Court of the Patriarchs.
Today, Zion Canyon is part of Zion National Park, an outrageously picturesque expanse of Navajo sandstone desert covering roughly 229 square miles of desert wilderness, three hours' drive north-east of Las Vegas. It's the first stop on the "Grand Circle", a camera-busting road trip that takes adventurous tourists through an arc of scenic national parks, which includes Bryce Canyon, Capitol Reef and Arches, before ending in the enormous, swanky Grand Canyon, about 50 miles south. Though Zion attracts something like three million visitors a year, it's still regarded, by those in the know, as one of the American outback's best-kept secrets.
Most people come here, as the old saying goes, to get away from it all. They want to hike stunning footpaths, camp by gurgling rivers and look out from well-kept viewpoints across red, orange and yellow rock faces which were laid down in beguiling geological layers. If they're more athletic, they might try to conquer the canyon's awe-inspiringly vast walls that provide some of the most famous rock-climbing challenges to be found anywhere in the world.
The bravest visitors of all, though, are the people who crawl, abseil and swim through tiny crevasses in Zion's cliffs, seeking out exotic underground chambers which look like the inside of a sea-shell, and have inspired countless National Geographic picture spreads. Their highly technical and somewhat dangerous sport is known as "canyoneering", and to its aficionados, Zion is the ne plus ultra of outdoor venues – a vast playground, tailor-made for adventure.
Presiding over the whole place is Jock Whitworth, Zion's head ranger (or "superintendent"), who kindly agreed to act as my guide last week, when I visited Zion for the first time. Among many highlights, I hiked to Emerald Pools, a collection of lakes filled by waterfalls which cascade into the canyon from dizzying heights, adding a bright green streak of lichen to the artist's palette of local colours. I wandered alone up the Virgin River at dusk, spotting herds of deer, and solitary wild turkeys. I scrambled up rock faces, peeked inside dramatic caves, and for a brief, glorious instant, gawped at a Californian condor, one of the world's rarest birds, with a 9ft wingspan.
The beauty of Zion, and indeed all of America's national parks, is their democratic remit: they cater to visitors from every background, and every corner of the planet, for a modest entrance fee (Zion levies a toll of $25/£16 per car, per week). Some visitors are suburban picnickers wearing flip-flops and jeans; others are experienced back-country adventurers who intend to haul sleeping equipment and backpacks full of climbing ropes into the park's hostile desert terrain. A few arrivals have rarely set foot outside a major city before. But all are welcome.
It's easy to forget, unless you mention it to someone like Jock, that the US actually invented national parks. The first one in the world was created by President Ulysses Grant, who in 1872 decreed that Yellowstone, with its Old Faithful geyser, would be established on 3,472 square miles of Wyoming, Idaho and Montana. Since then, the country has established a total of 58 national parks covering 84 million acres of mostly public land, from the Florida Everglades to the mountains of Yosemite. National Parks Week starts today and celebrates the concept: entrance to all of the parks is free for the duration. Like all good innovations, national parks have been widely copied. There are now thousands of examples in the world, and in Britain we have 15, preserving valuable breathing space on our small, crowded and overdeveloped island. (The newest of the lot, the green and pleasant swathe of Sussex and Hampshire known as the South Downs, was established with some fanfare at the end of last month.)
One of the most important things for an overseas visitor to understand about America's national parks is their place in the national psyche. To a relatively young country, founded by adventurers, these expanses of wilderness provide a monumental link to history. In a politically divided nation, where environmentalism is often a dirty word, their preservation manages to unite both left and right. Every proud American loves them, and if you want to understand what makes this nation tick, you must visit a few. To quote the title of film-maker Ken Burns's award-winning recent documentary series about them, parks are "America's best idea".
Jock Whitworth is deeply passionate about his role in this grand scheme, and is also anxious to share knowledge with visitors. One moment, he's telling me how rainfall seeping through porous sandstone over thousands of years created Weeping Rock, a monumental expanse of overhanging cliff which sheds what look like teardrops; the next, he's explaining to another visitor how Zion Canyon was formed (by the Virgin river cutting down through sandstone over millions of years) and why its multi-layered walls are various shades of red (because of deposits of iron ore in the rock).
Zion is run on an environmentally aware basis. Jock has aspired V C to carbon neutrality "since before it was cool to do that". A decade ago, he decided to ban cars from the main canyon, where the vast majority of visitors spend most of their time, and force them to instead hop aboard propane-powered shuttle buses to gain access to the area's best-known sightseeing locations and walking trails. "Each bus we send into the park replaces 28 cars, so it totally eliminated traffic jams that had been really awful here in summer months. It also vastly reduced noise. Suddenly, you could see and hear more wildlife," he says.
More recently, the park banned the sale of bottled water within its boundaries, forcing visitors to buy flasks that can be refilled from taps dotted around the canyon ("that saved 60,000 pieces of plastic from being trashed each year"). Jock also gave Zion Lodge, which was built by President Roosevelt's New Dealers in the 1930s, a green makeover. It's the only accommodation in the park (other overnighters must camp or billet in guesthouses in the pleasant local village of Springdale, which has nice gift shops and restaurants). Rooms there now boast recycled carpets and furniture, water-preserving showers, and energy-saving light switches.
This may not sound like a big deal, but context is everything. In most of the world, Jock's brand of environmentalism would be run-of- the-mill. But in the US, it's rare indeed: telling a nation of car-lovers to use a shuttle bus is verging on the revolutionary. Weaning visitors off a bottled-water habit which sees the average American throw away 160 empty plastic containers a year may very well upset them. And doing it in the name of global warming, which roughly half the US electorate is in denial about, is like waving a red piece of cloth at an angry bovine.
Jock, though, is evangelical about his mission. "Living out here, and working in the Park, I know climate change is happening," he says. "I can see it. We're getting invasions of non-natural species and seeing different birds coming in big numbers, at odd times of year. I wouldn't be doing my job if we weren't teaching people about this, and trying to stop it."
Most people don't come to be lectured, of course. Instead, they visit to experience the incredible scenery and hike Zion's famous trails, which range from the serene and flat to the deeply challenging. Perhaps the best-known of all, which has several websites run by fans, is The Narrows, a 16-mile hike (and wade) upstream through the sometimes waist-deep Virgin River. It takes you through a narrow, winding canyon surrounded by sheer walls that pauses at spectacular caverns that you'll recognise from guide books and motivational posters.
Another favourite destination is Angels Landing, a hilltop that was originally called he Temple of Aeolus and boasts perhaps the most spectacular views in the entire park. To get there, you must scramble for hours up a narrow pathway cut into solid rock, passing sheer drops that become particularly dangerous when wet and slippery. Although chains line the final mile or so of the path for hikers to grip hold of, five people have fallen to their deaths there in the past decade; it's not a route for those prone to vertigo.
The rangers at Zion step into the breech when things go wrong. They conduct countless rescue missions each year, and the walls of their park headquarters, which are covered with bravery citations and thank-you letters, bear witness to the efficiency of their operation.
They also act as the local law-keepers: in keeping with the American love of bearing arms, they carry a 25lb belt that holds handcuffs, a baton, a pistol and pepper spray. Their vans contain a shotgun and a rifle.
Spend some time with people like Jock, though, and you realise that the badge-carrying role of the ranger is at best an occasional diversion. Their real priority is to preserve the incredible scenery that surrounds them, and to help everyone who uses the park to leave it refreshed and enlightened about the American landscape and the forces that shaped it.
On my way out of the gates, I stop at the visitors' centre, and witness one of Jock's co-workers giving a talk about the geology of Zion. She talks about the many fossils that have been found there, and describes how dinosaurs came here and were later wiped out. A crowd of around 50 children and adults hang on her every word.
Later, I realise an important fact: she is educating them about evolution, a basic scientific theory, but one which polls suggest that between 35 and 50 per cent of Americans are (for mostly religious reasons) deeply sceptical about. Here, in a nutshell, lies the most important role of the national parks. They do not just feed the soul, they provide nutrition for the mind. And that is really what makes Zion, along with 57 parks like it, one of America's best ideas.
Travel essentials: Utah
* The writer travelled to Zion National Park as a guest of Merrell and was kitted out in Merrell's latest highly acclaimed gear, which included Chameleon3 Ventilator Gore-Tex shoes, the lightweight waterproof Carbon jacket and Bision convertible trousers ( merrell.com ).
* For the chance to win a range of Merrell products as well as Garmin and Olympus gear, see independent.co.uk/merrell
* The closest airports are Las Vegas and Salt Lake City. Las Vegas is served by Virgin Atlantic (0844 874 7747; virgin-atlantic.com) from Gatwick and British Airways (0844 493 0787; ba.com ) from Heathrow. Salt Lake City is served by Delta Airlines (0845 600 0950; delta.com ) from Heathrow via Minneapolis, Atlanta or Detroit.
* Best Western Zion Park Inn, 1215 Zion Park Boulevard, Springdale, Utah (001 435 772 3200; zionparkinn.com ). Doubles start at $112 (£75), room only.
* Zion National Park Lodge, 1 Zion National Park, Springdale (001 435 772 7700; zionlodge.com ). Doubles start at $181 (£121), room only.
* Zion National Park: 001 435 772 3256; nps.gov/zion