Grand Canyon slices roughly east–west through the southwestern edge of the Colorado Plateau, an uplifted platform with an average elevation of 6,000 feet centering on the Southwest’s Four Corners area and covering about 130,000 square miles.
Ninety percent of this vast semiarid plateau is drained by the Colorado River and its tributaries. The Colorado Plateau’s colorful sedimentary rock layers, broken by faults and carved by streams, are dramatically revealed in cliffs and canyons.
Within the major physiographic province of the Colorado Plateau, shifts along fault lines have created several distinct smaller plateaus. The canyon’s north side is dominated by the Kaibab and Kanab Plateaus. On the south is the Coconino Plateau. The eastern section of Grand Canyon, known as Marble Canyon, runs roughly north–south through the lower elevations of the Marble Platform.
From the heights of the Kaibab Plateau (9,200 feet) to the Marble Canyon Airport (3,603 feet), the Grand Canyon region encompasses a number of environments. Inside the boundaries of Grand Canyon National Park (1.2 million acres), the North Rim’s Grand Canyon Lodge sits 1,180 feet higher than Grand Canyon Village on the South Rim, with 10 air miles (220 road miles) between.
Grand Canyon ends at the Grand Wash Cliffs, which form the edge of present-day Lake Mead, impounded by Hoover Dam. Here begins another major physiographic province, the alternating valleys and escarpments of the Great Basin.
The Colorado River, the canyon’s main artery, flows from north-central Colorado to the Gulf of Mexico. Glen Canyon Dam impounds the river northeast of Grand Canyon, restricting flows and changing the river’s muddy, red-brown water to cold, clear green. The distance from rim to river varies but averages 4,000 feet. The river averages about 35 feet deep and varies in width from 76 feet to more than 300 feet.
The 277-mile stretch from Lees Ferry to the Grand Wash Cliffs drops 1,900 feet in elevation and crosses countless tributary canyons. Debris from these side canyons creates the most common type of rapids, constriction rapids, where water tumbles over boulders swept into the main channel by floods. River runners encounter more than 160 rapids on the journey through the canyon, and the Colorado River drops in gradient about eight feet per mile.
Most of Grand Canyon’s tributaries flow only intermittently after rainstorms or during spring melt. About a dozen have year-round water, including the Little Colorado River, Bright Angel Creek, and Havasu Creek.
The Colorado River and Grand Canyon create a barrier between Arizona’s northwest corner and the rest of the state. Until the Navajo Bridge was completed in 1929, the Arizona Strip was relatively isolated. Even today, the corner of Grand Canyon known as Tuweep or Toroweap requires a long drive through ranch country and Bureau of Land Management (BLM) land.
The rock layers of Grand Canyon record more than one-third of the earth’s history, beginning with the Precambrian period. The oldest, deepest rock found in Grand Canyon’s Inner Gorge is Vishnu schist, a hard, fine-grained rock formed under heat and pressure nearly 2 billion years ago. The source of the schist lay at the bottom of an early Precambrian sea, where deposits of mud, silt, clay, and sand buried far below the earth’s surface consolidated under pressure.
Then, 1.7 billion years ago, the continent buckled and folded as it collided with a chain of volcanic islands. The heat and pressure metamorphosed layers of sediment and ash into schist, and volcanic intrusions solidified to form granite. The gray-to-black Vishnu schist and pinkish Zoroaster granite interweave to form the hard cliffs of the Inner Gorge. These rocks are part of the Precambrian Basement Complex on which the North American continent rests.
The high mountain range formed by the collision eventually eroded away as the continent continued to shift. About 1.2 billion years ago, the schist and granite were exposed as a coastal plain that became engulfed by the Bass Sea. Over the next 430 million years, late in the Precambrian period, 14,000 feet of marine sediments formed the nine strata collectively known as the Grand Canyon Supergroup.
Single-celled bacteria formed clumps, preserved in the Bass limestone as stromatolites, the oldest fossils in Grand Canyon. Gray or reddish Bass limestone, 120–340 feet thick, is interbedded with sandstone and siltstone, indicating a shifting coastal environment. In areas, Bass limestone has been partly metamorphosed, resulting in asbestos, which was mined by Grand Canyon pioneers link William Bass and John Hance.
Subsequent layers, primarily sandstone and shale, are cross-bedded, cracked, and rippled, some even bearing raindrop impressions, as the land alternated between marine and dry coastal environments. Overlying the Bass limestone is bright orange-red Hakatai shale, 430–830 feet thick. Above it, Shinumo quartzite varies 1,060–1,500 feet thick, with marbled patterns suggesting earthquakes or tremors. Reddish Dox sandstone, deposited about 1.2–1.1 billion years ago, is soft and easily eroded.
Volcanic activity occurring just over 1 billion years ago is recorded by Cardenas lava, forming intrusions and basalt cliffs nearly 1,000 feet thick in places. The volcanic activity was part of a greater collision and uplift, the Grenville Orogeny, which formed mountain ranges along a supercontinent.
At the center of this supercontinent, the Grand Canyon region was a low area of trapped seawater. Deep sediments from this time are represented by the Nankoweap, Galeros, Kwagunt, and Sixtymile Formations. The Nankoweap Formation, 1 billion years old, is a purplish sandstone 370 feet thick, visible primarily in Nankoweap Canyon in the eastern canyon, as are the Galeros and Kwagunt Formations, shale and siltstone deposited 900 million years ago. The Sixtymile Formation, exposed on top of Nankoweap Butte and in Sixtymile Canyon, is sandstone and conglomerate deposited 820 million years ago as widespread geologic unrest began to break the supercontinent apart.
During this period, 820–770 million years ago, the Grand Canyon Supergroup layers were tilted and uplifted along fault lines. The exposed layers eroded as much as 15,000 feet during the Great Unconformity, a period that lasted 250 million years and left only colorful remnants of the Supergroup layers in wedges and folds near Unkar Delta, Phantom Ranch, Bass Camp, and downriver between miles 130 and 138. In most of the canyon, the Supergroup has completely eroded away, and the contact line between schist and Tapeats sandstone—the edge of the Tonto Platform—represents 1 billion years of missing geology.
Following the period of rifting and erosion, the Grand Canyon region lay along a relatively tranquil coastline. Over the next 325 million years, throughout the Paleozoic era, sediments 3,500–6,500 feet deep formed 15 rock layers. These horizontal strata are the canyon’s most visible layers, recording a proliferation of life forms in fossils—the most complete record of the Paleozoic era on the planet.
The Cambrian period began with beaches and tidal flats, preserved as coarse-grained Tapeats sandstone, a dark-brown, stratified, 150–250-foot-thick cliff that lies directly on top of the basement rocks in many areas of the canyon. Above and intermingling with the Tapeats are the green and purple mud, silt, and sand of the Bright Angel shale, up to 450 feet thick, formed in a shallow marine environment. Both the Tapeats sandstone and Bright Angel shale are riddled with worm burrows and trilobite fossils. Deeper waters deposited the Muav limestone 535 million years ago. These three layers, known collectively as the Tonto Group, represent a shifting coastline.
The next 130 million years, the Ordovician and Silurian periods, are completely missing from Grand Canyon. The Devonian period is only partially represented by Temple Butte limestone, more predominant in the western canyon. In the eastern canyon, Redwall limestone lies directly on top of the Cambrian rocks of the Tonto Group.
The Redwall formation dates to the Mississippian period, 360–320 million years ago, when ocean covered all of western North America. Redwall cliffs rise 500–800 feet above the Tonto Platform in the central canyon. Redwall limestone is naturally grayish, but has been stained red by overlying rock. The limestone is often pocked with caves and alcoves where softer deposits have been dissolved by seeps. Sheer Redwall cliffs are often draped with tapestries of desert varnish, dark mineralized stains. Nodules of chert (fossilized sponge), along with nautiloids, brachiopods, crinoids, and other fossils indicate a rich marine life.
The Supai Group of shale, limestone, and sandstone was laid down in a coastal environment during the late Pennsylvanian and early Permian periods, 310–285 million years ago. The topmost member, Esplanade sandstone, forms a hard shelf in western Grand Canyon, and the sculpted reddish bedrock of eastern tributaries like North and South Canyons. Reptile tracks can be seen in the upper members of the Supai Group, which forms a band of alternating cliffs and slopes 950–1,350 feet thick.
The transition from the Pennsylvania to Permian periods was marked by a continental collision that formed steep mountain ranges. During the Permian period, drainage from the ancestral Rockies reached the Grand Canyon region, depositing muddy sediments in a delta-like environment, rich with plant life. In Grand Canyon, 35 types of ferns and other plants were fossilized in Hermit Shale, laid down 286–245 million years ago. It’s easy to spot the 250–1,000-foot-think Hermit Shale, which forms dark red slopes.
When the ancient rivers ceased flowing, the mud left behind began to crack and dry out, and by 270 million years ago, the canyon region was a desert environment. Windblown sand dunes reached up to 1,000 feet high and covered a vast area, all the way to present-day Montana. Buff-colored Coconino sandstone forms eolian (wind-deposited) cross-bedded cliffs 350 feet high in the eastern canyon, pinching out to the west.
Some 265 million years ago, seawater returned, evaporating quickly in the tidal flats of an arid environment. The Toroweap Formation, silty limestone 250–450 feet thick, left steep slopes of pale yellow, usually vegetated by trees and shrubs.
Over the next 5 million years, seawater continued to engulf the canyon, laying down the Kaibab Formation, limestone rich with marine invertebrate fossils and even fish. This layer, 290–500 feet thick, forms the canyon’s rims. By this time, the Grand Canyon area was just north of the equator, part of the supercontinent Pangaea formed as the planet’s landmasses collided and slowly coalesced.
At the end of the Permian period, 245 million years ago, the rocks that make up Grand Canyon’s gorgeous walls had been laid down, with the Kaibab formation at sea level.
The canyon itself had yet to be formed. The canyon’s predominant Paleozoic layers can be memorized, top to bottom, by the acronym formed from the first letters of the phrase “Know The Canyon’s History, Study Rocks Made By Time”: Kaibab, Toroweap, Coconino, Hermit, Supai, Redwall, Muav, Bright Angel, Tapeats.
Mesozoic geology is visible in areas outside the park’s boundaries. On the drive to Lees Ferry, you can see Mesozoic rocks: the Chinle Formation of the Painted Desert, the Moenave and Kayenta Formations at the base of the Echo and Vermilion Cliffs, and the Navajo sandstone upstream in Glen Canyon. During the Mesozoic Era, Pangaea broke up during uplifts, earthquakes, and volcanoes. About 65 million years ago, at the beginning of the Cenozoic era, a mountain-building period known as the Laramide orogeny raised the Colorado Plateau region, creating the series of monoclines known as the Grand Staircase. Mesozoic rocks eroded away from the heights of the Kaibab Plateau, stripping Grand Canyon back to its Permian layers and setting the stage for canyon cutting.
Several forces have combined to create Grand Canyon, and many geological theories about the canyon’s formation have been advanced during the hundred-plus years that geologists have studied the region. Most agree that although its rocks are very old, the canyon itself is relatively young. But exactly when and how the canyon was formed is still under debate.
Many geologists believe that sometime between 5 and 6 million years ago, the ancestral Colorado River began cutting through rock layers that had been laid down over billions of years. Recent research suggests that canyon formation may have begun as long as 16 million years ago. Although canyon-cutting theories conflict in regard to timing, scientists agree that the ancestral Colorado River was a powerful erosional force.
We know that before the completion of Glen Canyon Dam in 1963, the Colorado River carried 380,000 tons of sediment daily through Grand Canyon, giving it a tremendous cutting power that probably pales in comparison to that of the ancestral river. During a volcanic period 1 million years ago, the river was powerful enough to grind through several lava dams formed in western Grand Canyon. And as ice age glaciers advanced and retreated, the ancient river carried debris-laden floods from the Rockies.
Erosion continues, although today most sediments are trapped behind Glen Canyon Dam, and the 30,000–40,000 tons of sediment that flow through the canyon daily are from tributaries such as the Little Colorado or Paria Rivers. Yet the canyon continues to change. Though its glowing cliffs and temples may appear to be frozen in time, the canyon continues to be shaped by water and wind at a pace humans are barely able to comprehend.
The Grand Canyon region is semiarid, with great variation in temperature and rainfall due to elevation. Average annual precipitation is less than 10 inches at Phantom Ranch, 15 inches on the South Rim, and more than 20 inches on the North Rim. Most of the moisture arrives during the late summer monsoon season or during the winter. Temperatures vary from below 0°F to higher than 100°F in the inner canyon. Rims are often windy, and the inner canyon “breathes,” with upstream winds during daylight and downstream winds at night.
The seasonal descriptions below are general and hardly a guarantee. Expect the unexpected: snowstorms in June, 90°F temperatures in October, dry winters when the North Rim is accessible for stretches at a time, and long, cool springs when wildflowers seem to be in constant bloom.
In late March, when the calendar says it’s spring, the North Rim will still be tucked under a blanket of snow. On the South Rim it may be cold and windy, but if you find a sunny, sheltered spot, you can already feel the power of the sun’s rays. During the next couple of months, spring creeps up from the inner canyon as wildflowers bloom along trails and birds go about the business of establishing territories and nests.
Lingering Pacific storm patterns may dump inches of snow on the South Rim into April, but the snow melts quickly as the sun gains strength. By May, inner canyon temperatures are already reaching into the 90s, and winter loses its grip on the North Rim.
May and June are dry and cloudless. While the North Rim begins to experience spring, inner canyon temperatures edge toward the 100°F mark as the dry desert foresummer tightens its grip. The South Rim is pleasant, with highs in the 70s and 80s, perfect for hiking. The intense sun quickly dries the forests surrounding the rims, and by the end of June, fire danger may trigger camping and hiking restrictions in the national forest lands surrounding the park.
As southern Arizona deserts heat up and weather patterns shift, pulling in moisture from the Gulf of Mexico, clouds begin to build, heralding the arrival of the annual monsoon. Hot, dry air may evaporate moisture before it reaches the earth, creating dry storms that spawn dangerous lightning.
By mid-July, the rains arrive at last in the form of brief, powerful thundershowers that sweep across the canyon, usually in the afternoon. The monsoon pattern lasts through August, the month when the inner canyon receives the most rain. A second bloom of wildflowers begins with the rains.
By mid-September the monsoon retreats and cloudless skies return. Inner canyon temperatures moderate. Birds begin to migrate south, and on the North Rim the aspens turn gold as October approaches. Sometime in November, the first heavy snowfall may close the road to the North Rim.
In December, Pacific storms again march their way west, bringing winter moisture to the canyon in the form of rain or snow. The North Rim can receive as much as 120 inches of snow; the South Rim, 65 inches. Ground squirrels hibernate, while deer and elk sink into winter sluggishness, conserving energy by browsing the piñon-juniper woodlands near the village and West Rim.
Snowpack is important in the West, providing a slow release of moisture as the weather warms, recharging springs and maximizing Colorado River flows. Glimpses of spring begin as early as February in the inner canyon, when brittlebush sends up its golden blooms and the cycle of seasons turns again.
© Kathleen Bryant from Moon Grand Canyon, 5th Edition