Arizona has three landform regions: the Colorado Plateau across the north, where the Colorado River has cut the Grand Canyon 446 km (277 mi) long and up to 1,857 m (6,093 ft) deep; a rugged transition zone of mountains and canyons that runs diagonally through the centre of the state below the Mogollon Rim; and the Basin and Range country of the south and west, where low desert basins such as the Salt River Valley alternate with isolated ranges. The highest point is Humphreys Peak, a dormant volcano north of Flagstaff, at 3,852 m (12,637 ft); the lowest is the Colorado River at the Mexican border near Yuma, at 22 m (72 ft). Across 295,254 km² (113,998 sq mi), the sixth-largest state, those regions line up almost perfectly from north to south, which means one drive from Page to Tucson crosses every one of them. Here is that trip, stop by stop.
Page and U.S. Route 89: Glen Canyon Dam, Horseshoe Bend and Marble Canyon
The route starts on the Colorado Plateau at Page, a few miles south of the Utah line, beside the Glen Canyon Dam. Built between 1956 and 1966, the 220 m (710 ft) concrete arch dam holds back Lake Powell, which flooded the deep sandstone gorges of Glen Canyon. Just 8 km (5 mi) downstream, and about 6 km (4 mi) southwest of town, a short trail off U.S. 89 leads to the rim of Horseshoe Bend. From an overlook at about 1,300 m (4,200 ft), the drop to the river at about 1,000 m (3,200 ft) is roughly 300 m (1,000 ft).
Horseshoe Bend is an entrenched meander, and it explains a great deal about northern Arizona. About six million years ago this region stood much closer to sea level, and the Colorado wound lazily across a nearly flat floodplain. When the plateau began to rise, between six and five million years ago, the river was trapped in its looping bed and cut straight down, preserving the old curves in solid rock. Whether that uplift came from part of the plate’s base sinking into the mantle or from heating at the base of the crust is still debated.
South of Page, a short detour west on U.S. 89A crosses the river at Navajo Bridge, high above Marble Canyon. This is the stretch of the Colorado from Lees Ferry, where river trips through the Grand Canyon launch, down to the confluence with the Little Colorado River. John Wesley Powell named it knowing there is no marble there; he thought the polished limestone looked like it. The uppermost walls are Kaibab Limestone resting on the Toroweap Formation and Coconino Sandstone. West of the bridge, the Vermilion Cliffs rise as much as 910 m (3,000 ft) in eroded sandstone, siltstone, limestone and shale, fronting the Paria Plateau in Vermilion Cliffs National Monument, established in 2000.
State Route 64 to Desert View and the Grand Canyon
Back on U.S. 89, the road runs south to Cameron, where State Route 64 turns west and climbs toward Desert View, the eastern viewpoint of the South Rim. Here the Grand Canyon opens up below Mary Colter’s 21 m (70 ft) stone watchtower, completed in 1932. The canyon runs 446 km (277 mi) and is up to 29 km (18 mi) wide. The Colorado River set its course through the area about 5 to 6 million years ago, and as the plateau rose the river and its tributaries sliced through layer after layer, exposing nearly two billion years of rock, from Proterozoic schist near the river to the Paleozoic limestones and sandstones in the upper walls.
The plateau’s uneven tilt shapes the canyon too. The North Rim stands more than 300 m (1,000 ft) higher than the South Rim, at an average of about 2,400 m (8,000 ft), and almost all runoff from the north side drains into the canyon, while much of the water behind the South Rim flows away from it. That is why long, deep side canyons branch off to the north and short, steep ones to the south. The first European known to have seen the canyon, García López de Cárdenas, arrived in 1540; Powell’s 1869 river expedition made the first recorded passage through the whole canyon.
U.S. 180 to Flagstaff: Humphreys Peak and the San Francisco volcanoes
From Grand Canyon Village, State Route 64 and U.S. 180 run south across the plateau through ponderosa pine; Arizona holds the world’s largest stand of the tree. The skyline ahead is the San Francisco Peaks, the remains of a stratovolcano complex at the heart of the San Francisco Volcanic Field. The field covers 4,700 km² (1,800 sq mi) on the southern edge of the Colorado Plateau and contains more than 600 volcanoes. The oldest erupted nearly 6 million years ago near Williams, and the eruptions have migrated eastward over time, possibly as the North American Plate drifted west over a fixed source of melting in the mantle.
Humphreys Peak, about 18 km (11 mi) north of Flagstaff, is the highest summit on the rim of what was once a single stratovolcano, San Francisco Mountain. Around 200,000 years ago that mountain is estimated to have stood about 4,900 m (16,000 ft) high. Since then much of it has been removed to leave the Inner Basin, a collapse caldera inside the ring of summits. Geologists are unsure whether the missing rock went in one lateral blast like the 1980 eruption of Mount St. Helens, or gradually through landslides, running water and ice-age glaciers. The youngest volcano in the field is Sunset Crater, a 340 m (1,120 ft) cinder cone north of Flagstaff that erupted around AD 1085, spreading ash and lapilli over more than 2,100 km² (810 sq mi) and forcing the local Sinagua people to abandon their settlements for a time. East along Interstate 40, about 29 km (18 mi) west of Winslow, lies a landform of a different origin entirely: Meteor Crater, an impact crater about 1,200 m (3,900 ft) across and some 170 m (560 ft) deep, formed around 50,000 years ago.
State Route 89A down Oak Creek Canyon and off the Mogollon Rim
South of Flagstaff, State Route 89A drops off the edge of the plateau in a series of hairpin bends and then follows the floor of Oak Creek Canyon for about 21 km (13 mi) to Sedona. This is the Mogollon Rim, the escarpment that marks the southwestern edge of the Colorado Plateau and runs about 320 km (200 mi) across the state from northern Yavapai County toward New Mexico. Its cliffs of Kaibab Limestone and Coconino Sandstone rise to about 2,400 m (8,000 ft), and much of the land south of the rim lies 1,200 to 1,500 m (4,000 to 5,000 ft) above sea level. The rim is also a biological boundary, with Rocky Mountain species on top and species of Mexico’s Sierra Madre Occidental on the slopes below.
Oak Creek Canyon is about 19 km (12 mi) long and 240 to 610 m (800 to 2,000 ft) deep. It follows the Oak Creek Fault, a 48 km (30 mi) north–south fault, and because the land east of the fault dropped, the west rim at about 2,200 m (7,200 ft) stands 210 m (700 ft) higher than the east rim. An ancestral canyon formed here about 8 to 10 million years ago, was filled by gravel and lava flows, and was then cut open again by the creek. Its walls show pale, cross-bedded Coconino Sandstone above the red Permian Schnebly Hill Formation, a layer that does not appear at the Grand Canyon and gives Sedona’s buttes their colour. Below Sedona, Oak Creek joins the Verde River in the Verde Valley, a broad basin overlooked by Mingus Mountain and the rim itself.
Interstate 17 and I-10 through the Sonoran Desert to Tucson
From the Verde Valley, Interstate 17 continues south through the transition zone, the diagonal belt of mountains and canyons between the high plateau and the low desert, and descends into the Salt River Valley. Phoenix sits in the northern Sonoran Desert at a mean elevation of just 331 m (1,086 ft). The valley floor is flat enough for a strict street grid, but low ranges ring it: the McDowell Mountains, the White Tank Mountains, South Mountain, the Sierra Estrella and, farther east, the Superstition Mountains, with Camelback Mountain and Piestewa Peak inside the city. That pattern of flat basins broken by isolated ranges is the Basin and Range, the product of the crust stretching and breaking into faulted blocks from about 17 million years ago.
Interstate 10 then runs southeast across the desert. Between Casa Grande and Tucson, the spire of Picacho Peak rises beside the highway to 1,028 m (3,374 ft). It looks like a volcanic neck but is now thought to be a tilted and eroded block of rock capped by a lava flow. Its slopes carry the saguaro, the signature cactus of the Sonoran Desert, which covers about 260,000 km² (100,000 sq mi) of Arizona, California and northwestern Mexico.
Tucson, 806 m (2,643 ft) above sea level on an alluvial plain, is ringed by five ranges: the Santa Catalina and Tortolita mountains to the north, the Santa Rita Mountains to the south, the Rincons to the east and the Tucson Mountains to the west. The Santa Catalinas climb to Mount Lemmon at 2,795 m (9,171 ft), home to the southernmost ski area in the continental United States. They are one of the Madrean Sky Islands, roughly 27 ranges in the U.S. whose pine–oak woodlands have been cut off from Mexico’s Sierra Madre since the ice ages by the desert around them.
From Tucson the lowest ground in Arizona is still a long way west, where the Colorado River leaves the state at 22 m (72 ft) on the Mexican border near Yuma, some 3,830 m below the summit of Humphreys Peak. For more on the canyon at the centre of this trip, read our Grand Canyon geography guide, and for the Sonoran Desert and the state’s other dry lands, see the deserts in Arizona.