The rock layers of Grand Canyon record more than a third of 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 two 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 mined by William Bass, John Hance, and others.

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 from 1,060–1,500 feet thick, with marbled patterns suggesting earthquakes or tremors. Reddish Dox sandstone, deposited about 1.2 to 1.1 billion years ago, is soft and easily eroded.

Volcanic activity occurring just over a 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, a 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 a 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-to-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 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 deltalike 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 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. Wind-blown sand dunes reached up to a thousand feet high and covered a vast area, all the way to present-day Montana. Buff-colored Coconino sandstone forms aeolian (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 five 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. By 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. Though 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 a 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, though today most sediments are trapped behind Glen Canyon Dam, and the 30–40,000 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.