This land is their land: over one third of the state of Colorado is public property, a boundless wilderness with some of the finest mountain scenery in North America. The backbone of the state – indeed, of the whole continent – is the Rocky Mountains, which stretch some 2,700 miles from Alaska to Mexico. The present mountains were formed by the great forces of plate tectonics 70 million years ago, when the dinosaurs still roamed the earth. In fact, they are the third set of Rocky Mountains: two earlier and much older formations were thrust skywards and then eroded over many tens of millions of years.
Today, the high spots of Colorado are easily accessible thanks to a daily non-stop flight from Heathrow to Denver, which itself lies 5,130ft above sea level. It's a good place to start acclimatising: an (almost) mile-high city where the flat expanse of the American Mid-West ends and the Rocky Mountains begin. The Denver Art Museum downtown is worth seeing if only for its architecture – its new titanium and glass extension is a geometric artwork in itself – but Denver's hidden cultural gem is the Museum of Contemporary Art (MCA), tucked away on Delgany Street, within sight of the Coors Field stadium (home of the Colorado Rockies baseball team). The MCA's current exhibition, "Another Victory Over the Sun", explores the relationship between theatre and museum. In one darkened room comes the frightening roar of a panther while, in another, light shines from a mouse hole in a skirting board, accompanied by the soundtrack of a Tom and Jerry cartoon.
They say it's possible to cross the continental divide 11 times when journeying north to south through Colorado. This is not the divide that splits the haves from the have-nots, the liberals from the neo-cons, or those who think South Park is funny and those who don't. No, this is the continental divide, the geographical watershed that separates the rivers that flow to the Pacific Ocean in the West from those that flow, ultimately, to the Atlantic in the East.
Two hours north of Denver is Estes Park, an impressive 7,522ft above sea level and the entry point for the Rocky Mountain National Park. Among its more beautiful natural treasures, the park boasts the highest major road in the US. At one point Highway 34 reaches 12,183ft above sea level. (Understandably, it is closed in winter.)
The park is the jewel in Colorado's vertiginous crown; no trip to this part of America is complete without at least some time walking on the many trails that take you through this most spectacular chunk of wilderness. If you want to do some serious back-country trails, there are many to choose from, provided you have the $20 permit required between May and October. Easier day-trip trails come at all altitudes and gradients, and some are accessible in a wheelchair.
Although the Rocky Mountain National Park is only one-ninth the size of Wyoming's better-known Yellowstone, it receives roughly the same number of visitors. Nevertheless, it is not difficult to find yourself in virtual solitude, even on some of the easier trails leading off from the road. On one unforgettable evening on a relatively horizontal walk through the glaciated U-shaped valley of Moraine Park, I found myself alone with a grazing moose, a family of playful chipmunks, a sleepy snowshoe hare in its summer colours and an operatic bullfrog singing his monotonous refrain with repetitive gusto as hummingbirds trilled falsetto overhead.
There are three distinct ecosystems in the park, delineated by altitude. The highest is the alpine tundra above the tree line between 11,500ft and 14,000ft. Here, small, waxy-leaved plants are adapted to the fierce drying winds, bitter cold and intense UV-rich sunlight. Below comes the sub-alpine with its wind-driven huckleberry and juniper shrubs. The lowest, and in my opinion the most beautiful, is the montane region with its tall ponderosa pine and wispy aspen cloaked in shimmering, summer-green foliage. The overpowering scent is from older ponderosa, which develop a cinnamon-red bark that warms in the sunshine to fill the air with a volatile, sweet pine fragrance.
Breathlessness is a problem in Colorado's mountains, and not just because of the magnificence of the landscape. These summits are seriously high. The highest peak in the entire Rockies is Colorado's Mount Elbert, at 14,440ft. In the Rocky Mountain National Park, the highest is Longs Peak at 14,259ft, the northernmost of the state's 53 "fourteeners", peaks rising to above 14,000ft. Even at the "lower" altitudes, anyone coming from sea level will soon feel the effects: fatigue, shortness of breath, headaches and a raking thirst.
Outside the city of Colorado Springs, it is possible to see the curiously shaped remnants of the Rockies' ancient geology, in the form of a stunning rock formation in a city park called the Garden of the Gods. Some of these rocks date back 300 million years to the time of the first formation: the Ancient Rocky Mountains. Over a period of many hundreds of thousands of years, the red rocks here have formed vertical stone pillars and slabs that bear witness to the tortuously slow processes of geological time.
Just outside the park, in Manitou Springs, stands a row of mock-Tudor Edwardian mansions built by English artisans in the 1900s. In one of them is probably the best restaurant in Colorado Springs, the Craftwood Inn, which takes its name from the Arts and Crafts Movement – the Englishman who built the house was part of it. The timber-framed interior with its leaded glass windows is reminiscent of the Liberty store in London. The menu, meanwhile, is famous for its game meat.
The drive from Colorado Springs to my final stop, the mountain resort of Vail, was a classic crossing of the continental divide. As I climbed the first peaks, I could have been forgiven for thinking I was almost there. But then the landscape fell away again to a vast high plain, a sort of lost world where cattle grazed in lush meadows watered by fast-flowing streams and surrounded on all sides by snow-capped mountains, even in the heat of mid-summer. This vast grassland basin is set in a region known as South Park and, yes, here lies the town of Fairplay, upon which the animated sitcom is said to be based.
After passing through Fairplay, a nondescript place with timber and adobe houses, a stone courthouse and a few scattered bars with rows of Harley Davidsons parked outside, I began the slow climb towards the continent's geographical division.
As the gradient became steeper, the sky began to look as if it was mirroring the schism in the landscape; the brilliant blue of the east was divided by a sharp, sunlit line that marked the edge of the black thunderous clouds of the west. I stopped the car, taken aback by the sheer, breathless magnificence of this apogee moment – half in brilliant sunshine, half in looming shade. It was only when I looked at the map that I realised I had paused right on the continental divide.
Vail was no anti-climax. It was built in the 1960s as a skiing destination, in a crazy mix of European Alpine styles. I especially enjoyed the luxurious experience of The Sebastian, a tasteful hotel where the mood was of carefully cultivated chic. Its Mexican owners evidently have a deep interest in art and culture: the hotel has its own library stocked with rare, old books. And at 8,150ft, if altitude sickness is still a problem, the hotel even offers reviving oxygen treatments, provided by probably the friendliest spa staff in America.
The journey back to Denver was long and contemplative. Because the roads are so good, it was easy to cover distances that would have taken days or weeks when these mountains were first explored by Europeans. There was the chance for one final ethereal climb before I crossed my final divide: from land to air, on the journey home.
Less than an hour's drive away from Denver, and standing amid the "flatirons" rocks 5,430ft above sea level, Boulder is like no other town in Colorado, or even America. It is said to be the brainiest place in the US – home to a major university, the University of Colorado at Boulder and a clutch of government research centres, such as the National Centre for Atmospheric Research and the National Oceanic and Atmospheric Adminstration.
It also has a decidedly "alternative" edge, exemplified by Pearl Street, which is a bit like London's Camden Market, yet is bathed in seemingly perpetual sunshine.
There are few other places in Colorado – or in the US – where you can watch street performers play practically every instrument imaginable, from the didgeridoo to the classical violin, while you munch on delicious corn tamales from an open-air farmer's market.
I stayed in the stately Boulderado Hotel, which dates back to 1909 and has retained the atmosphere of pre-space-age America. Louis Armstrong once stayed here at the height of the jazz era and, even though the stained glass ceiling of the hotel's main hall is a replica of the real thing, the mood is still authentically retro. The hotel's restaurant, Q's, has the feel of Mad Men chic, a cool style matched by a flawlessly executed menu.
Boulder seems to stir mixed emotions among native Coloradans. People outside Boulder refer to it being in a bubble of its own. Those who live there talk half seriously of the "People's Republic of Boulder", somewhere that is self-consciously liberal– especially in comparison with more conservative-minded places lower down on the plains, such as Colorado Springs, a centre for Christian evangelism.
* The writer flew from Heathrow with British Airways (0844 493 0787; ba.com), which offers the only non-stop flights from the UK to Denver; return fares start at £520. You can also get there with a change of planes on a range of other carriers, such as United Airlines (0845 8444 777; unitedairlines.co.uk) via Chicago.
* Brown Palace Hotel, 321 17th Street, Denver (001 303 297 3111; brownpalace.com). Doubles start at $309 (£206), room only.
* Aspen Lodge Ranch Resort, 6120 Hwy 7, Estes Park (001 970 586 8133; aspenlodge.net). Doubles start at $147 (£98), room only.
* Garden of the Gods Club, 3320 Mesa Road, Colorado Springs (001 719 632 5541; gardenofthegodsclub.com). Doubles start at $198 (£132), room only.
* The Sebastian Hotel, 16 Vail Road, Vail (001 970 477 8000; thesebastianvail.com). Doubles start at $206 (£137), room only.
* Boulderado Hotel, 2115 13th Street, Boulder (001 303 442 4344; boulderado.com). Doubles start at $229 (£153), room only.
* Colorado Tourism: colorado.com