In Arizona a three-hundred-kilometre step splits the state in two
The Mogollon Rim runs diagonally across Arizona and drops 300 to 600 metres at once. Above, the largest continuous pine forest in the world and 229 centimetres of snow a year; below, a desert with 183 millimetres of rain. Sixteen degrees of average temperature separate the two halves.
The image almost everyone has of Arizona is a desert of cactus and red rock. It is accurate for roughly half the state and completely wrong for the other half, where there are pine forests, lakes and well over two metres of snow a year.
The interesting part is that there is no gradual transition between the two. There is a line, and you can see it from the air and from the road.
01 The edge of the plateau
The step is called the Mogollon Rim and it is the southwestern edge of the Colorado Plateau, the same geological province of horizontal layers that produces the Grand Canyon, the canyons of Utah and the northern mesas. It crosses Arizona diagonally for more than three hundred kilometres.
What makes the Rim different from a mountain range is that it has no two flanks. It is a cut: on the north side you step out onto a more or less flat plateau, and on the south the ground drops between 300 and 600 metres at once towards the ranges and basins of the southern state.
Hence it does not work as a political border but it does as the border of everything else: of vegetation, of temperature, of snow, of the kind of farming and of where people live.
02 Sixteen degrees of difference
The quickest way to measure the step is to compare the two cities on either side. Flagstaff sits on the plateau, at 2,135 metres; Phoenix, in the basin, at about 337. Some 230 kilometres of road separate them.
Flagstaff's annual average temperature is 8.2 degrees and Phoenix's is 24.2. That is sixteen degrees of difference inside the same state, and in January the gap opens wider still: an average of −0.8 degrees up top against 13.8 below.
The extremes finish the job. Flagstaff has recorded −34.4 degrees; Phoenix, 50.0. And where Phoenix sees no snow, Flagstaff accumulates some 229 centimetres a year over 28.3 days.
03 The largest continuous pine forest in the world
That difference in altitude produces a forest, and not just any forest. Above 2,100 metres, on the plateau and along the Rim, grows the largest continuous ponderosa pine forest in the world: some 10,500 square kilometres running from the New Mexico state line to the south rim of the Grand Canyon.
The reason is a combination of height and water. At that altitude the air is cool enough, and 521.2 millimetres of rain a year reach Flagstaff, almost three times Phoenix's, spread over 82.2 days against 33.4.
The result baffles anyone arriving expecting desert: roads between thirty-metre pines, sawmills, cabins and ski areas. All of that is in Arizona, and it is a bit over an hour from the cactus.
04 And below, a desert with trees
South of the Rim another geological province begins, basin and range: wide flat valleys separated by narrow, elongated ranges. This is the ground where Phoenix and Tucson sit, and where most of the state's population lives.
The desert occupying those basins is the Sonoran, and it has a peculiarity that sets it apart from almost any other: it has tall vegetation. The saguaro, the columnar cactus that gives Arizona its silhouette, can exceed ten metres.
The explanation is water again, but distributed differently. Here it rains twice a year: in winter, with Pacific fronts, and in high summer, with monsoon storms. Two separate wet seasons allow a desert far more thickly planted than one with only a single season.
05 The step distributes the water too
There is a third consequence of the Rim, less visible and more decisive: it is a watershed. The rain and snow falling on the plateau run north and west, into the Colorado River system; what falls to the south feeds the Salt and Verde rivers.
That second family of rivers is what makes the southern half of the state possible. They rise in the forest zone, where it genuinely rains and snows, and drop into basins where 183.4 millimetres fall a year.
In short: the dry part of Arizona depends on the water the high part collects. The step does not only separate two landscapes, it connects them in one direction.
06 How to travel it
The first practical consequence concerns packing and the calendar. An itinerary crossing the Rim goes from 40 degrees to night frost on the same day, so there is no good month for the whole state: there is a good month for up top and another for down below.
The second is that it pays to cross the step by road at least once, and slowly. The transition between basin scrub and plateau pine forest happens over a few kilometres of climbing and is one of the most striking things the state has.
And the third reorders the whole trip. If you stop thinking of Arizona as a desert with some mountains and start thinking of it as two countries separated by a ledge, the map explains itself: where the water is, where the forests are, where the cities are and why.
