to the east as Pangaea forms, 320 million to 250 million years ago: Around 320 million years ago the supercontinent Pangaea, a landmass composed of all continents with North America along its western shore, began to form. The continental margin west of the Grand Canyon was still passive, but to the east, an ancestral mountain range, the Ancestral Rockies, rose. As this mountain range was uplifted, large quantities of sediment were eroded, first filling large basins immediately west of the mountain range, and later spilling westward to the Grand Canyon region. It was a desert environment all the way to the coast.
The Supai Group is composed of sediments from 320 to 285 million years ago. It is primarily red desert sands from the eroding mountains to the east. This sediment was deposited in an extensive coastal plain with enormous river deltas. During this time period Pangaea was centered over the South Pole and recurring glaciations (due to changes in sun strength) tied up vast quantities of water. As a result, sea level fluctuated more than 400 feet every 100,000 years, leading to repeated incursions of the sea, creating thin deposits of shale and limestone between the layers of desert sand.
The environment was similar when the next stratum, the Hermit Formation, was deposited, except that sea levels were lower and the formation is exclusively terrestrial. Large rivers continued to carry red mud and fine sand from the deserts to the east. As the continent became ever drier and the shoreline retreated farther west, large dunes spread across the Grand Canyon area. These giant sand dunes are preserved as the Coconino Sandstone (275 million years ago).
While the inland drought was continuing, the shoreline crept east again: By 273 million years ago, a very shallow sea again covered the area, leading to the deposition of intertidal deposits (the Toroweap Formation) and then deeper water where calcite accumulated (the Kaibab Formation; 265 million years ago). The Toroweap Formation contains gypsum and salt crystals, minerals formed by the evaporation of water.
This marks the end of the sequence of sedimentation that is preserved today—the 4000-foot-thick sequence of multicolored strata that makes the Grand Canyon so spectacular. However, the subsequent 250 million years are important as well: Those rocks needed to be lifted far above sea level and carved by flowing water. The remainder of the geologic description is therefore focused on the sequence of events that led to the creation of the Grand Canyon.
Initial breakup of Pangaea, 250 million to 145 million years ago: The Grand Canyon region was an arid terrestrial environment throughout this time. The 4000 feet of sediment, mostly desert sand, deposited atop the Paleozoic strata have since eroded. No sediments from this time period, the Mesozoic, survive in the Grand Canyon, but they are visible farther north on the Colorado Plateau.
Subduction of the Pacific Plate begins, 145 million to 70 million years ago: During this period, Pangaea continued to breakup, causing North America to be pushed westward, and the Pacific Plate to begin subducting beneath it. This new subduction zone created the beginnings of California’s Sierra Nevada and thickened the continental crust west of the Grand Canyon, causing the Colorado Plateau to be depressed. An internal sea, the Mancos Sea, flooded the central parts of North America, including much of the Colorado Plateau. The water retreated toward the end of this period. Around 70 million years ago, the strata that are now on the rim of the Grand Canyon were still at sea level.
Subduction expands eastward, 70 million to 18 million years ago: Around 70 million years ago the effect of the subduction zone at the western edge of North America began to be felt much farther east, leading to rapid uplift inland. The uplift includes the Laramide Orogeny (the uplift of the Rocky Mountains), the formation of the Mogollon Highlands in southeastern Arizona, and the uplift of the Colorado Plateau. In the Grand Canyon, the outside pressures caused the buried rock strata to be uplifted, raising the rocks we see today far above sea level. In order for the Colorado River to later carve such a deep canyon, the strata needed to be elevated; an ocean-bound stream can, after all, only carve down to sea level. The strata were also compressed, leading to the formation of the East Kaibab Monocline, the arching of the rock layers into an elongate dome in the central Grand Canyon region. This episode of uplift was completed by 40 million years ago.
The Colorado Plateau, although raised far above sea level, remained lower than the surrounding mountain ranges. During this period, rivers in the Grand Canyon region flowed to the northeast, as sediment and water were transported out of the Mogollon Highlands, into giant lakes in northern Arizona and Utah. This drainage pattern persisted until at least 30 million years ago and possibly as recently as 18 million years ago.
Sometime between 30 and 18 million years ago drainage patterns may have begun to change, but very few details are known. The Colorado River system did not exist. Some water may have drained southward, but most stream systems still headed north. Much water may have drained into internal basins that did not connect to the ocean.
WHY ARE THE RIMS DIFFERENT ELEVATIONS—AND DOES IT MATTER?
Both the South Rim and North Rim sit atop the Kaibab Formation. Although the layers appear flat from either vantage point, they are slightly slanted, and the North Rim is approximately 1000 feet higher than the South Rim, for these layers are bent into a broad arch, the East Kaibab Monocline. The apex of the arch is 12 miles north of the North Rim. The southern side of the arch slants gently downward from that point and drops 1000 feet between the North and South rims.
This difference in elevation has other implications: The angle of the rock layers means that water falling on the rims flows away from the South Rim, but is funneled toward the North Rim. Therefore, relatively little water flows down creeks from the South Rim, but considerable amounts flow down drainages beginning on the North Rim. As a result, less side-stream erosion has occurred on the south side of the canyon and the descent from the canyon rim to the Colorado River is much shorter and steeper than from the North Rim. The landscape is accordingly less complex; there are far more buttes and intricate side canyons on the north side of the river.
Extension to the west, 18 million to 6 million years ago: During this time period, stretching and thinning of western North America caused the Basin and Range Province in Nevada to form and the land surrounding the Colorado Plateau to decrease in elevation. Once the regions to the south and west were lower than the plateau, runoff began to etch south-flowing drainages, the direction of today’s Colorado River.
However, sediment deposits suggest that the Colorado River itself did not exist until approximately 6 million years ago. Instead, there were still large inland lakes and shorter waterways that were not connected. Water likely flowed along some of the Colorado River’s present path and through some of its major tributaries, although possibly opposite from the direction it does today. For instance, water may have flowed down the Little Colorado and then up Marble Canyon, but not through the main Inner Gorge of the Grand Canyon to the west of the confluence of these two drainages.
What caused these many rivers to coalesce into a single large drainage system is not definitively known, but two theories rise to the forefront. One is that the inland lakes suddenly joined together, possibly because of a catastrophic overflow of one of them, providing the force to cut through topographic barriers and integrate previously disconnected drainage systems. A second theory is headward erosion, whereby erosion causes waterways to move progressively upstream, cutting into slopes. Eventually this process destroys a ridge that had previously divided two drainages and connects the two stream systems. A combination of these two processes likely integrated the Colorado River system.
The final big event required for the creation of the modern Colorado River course is the creation of the Gulf of California, which occurred about 6 million years ago, and provided the Colorado River’s outlet to the ocean.
Glaciations and fault movements, 6 million years ago to present: By 6 million years ago, the Colorado River followed its current course to the Gulf of California. Events during the past 2 million years contributed to significant downward erosion, allowing the canyon to achieve its depth. First, the past
