The Grand Canyon is primarily composed of layered sedimentary rock, which forms the vast majority of the visible canyon walls. This monumental gorge acts as a geological cross-section, exposing nearly two billion years of Earth’s history in a dramatic series of colorful strata. The full geological picture, however, includes older, non-sedimentary foundations.
The Dominant Rock Type
Sedimentary rock forms when fragments of pre-existing rock, minerals, or organic material accumulate and are cemented together under pressure. These materials, known as sediment, are commonly deposited by water, wind, or ice. In the Grand Canyon, this process created rock types such as sandstone, shale, and limestone. Sedimentary rocks are recognized by their distinct layering, called bedding, and often contain fossils.
The flat-lying, “stair-step” layers making up the upper two-thirds of the canyon walls are known as the Layered Paleozoic Rocks. These strata are between 3,000 and 5,000 feet thick, representing a period from about 530 to 270 million years ago. These layers were primarily formed by ancient seas and deserts. Their composition—sandstone, shale, and limestone—confirms the dominance of sedimentary formation.
Stratigraphy: The Layered History
The stratigraphy, or sequential build-up of sedimentary layers, is central to the Grand Canyon’s geology. The layers were deposited horizontally, meaning the oldest rock is found at the bottom and the youngest at the top. This sequence records numerous shifts in the regional environment, from deep ocean basins to sprawling desert dunes. The Paleozoic layers are divided into distinct groups based on age and composition, starting with the Tonto Group at the base.
The Tonto Group, deposited during rising sea levels in the Cambrian era (525 to 505 million years ago), includes the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. As the sea advanced eastward, it first deposited beach sands that became the Tapeats Sandstone. Deeper, calmer waters led to the deposition of fine muds and silts forming the Bright Angel Shale. Finally, calcium carbonate skeletons of marine life formed the Muav Limestone in the farthest offshore environment.
Higher up, the Supai Group (315 to 285 million years ago) is characterized by reddish sandstones and siltstones. It represents a transition between marine and terrestrial environments, showing evidence of river floodplains and coastal areas when sea levels fluctuated. Above this, the striking white Coconino Sandstone formed about 275 million years ago from a massive field of wind-blown sand dunes. The sweeping cross-stratification visible within this layer signifies an ancient desert.
The uppermost layers, including the Toroweap Formation and the Kaibab Formation, show a return to a shallow marine environment during the Permian period. The Kaibab Formation forms the resistant rim of the canyon. It is a light-colored limestone rich in fossils of ancient marine creatures like brachiopods and mollusks. The distinct composition of each layer provides a clear record of the specific depositional environment that existed at the time of its formation.
The Non-Sedimentary Foundations
While the spectacular layers defining the Grand Canyon are sedimentary, the deepest part of the Inner Gorge exposes rocks of a different origin. These are the non-sedimentary Vishnu Basement Rocks, which are significantly older than the overlying strata. Estimated at around 1.7 billion years, they form the crystalline foundation upon which the entire canyon rests. These rocks are only visible at the very bottom of the canyon, near the Colorado River.
The Vishnu Schist is the most prominent metamorphic rock in this foundation. It formed from ancient volcanic and sedimentary rocks subjected to intense heat and pressure deep within the Earth’s crust. This process, called metamorphism, transformed the original materials into a hard, dark, foliated rock. Intruding into the schist is the Zoroaster Granite, an igneous rock that formed when magma pushed into the Vishnu Schist and slowly cooled beneath the surface.
The formation of the Vishnu Schist and Zoroaster Granite involved mountain-building events and deep crustal processes, contrasting sharply with the surface deposition that created the sedimentary layers. These basement rocks predate the layered rocks by over a billion years. They are separated from the overlying strata by a vast gap in the geological record known as the Great Unconformity. The difference in their formation highlights the diverse geological forces that contributed to the region’s history.
Erosion: Exposing the Layers
Although the visible layers were deposited horizontally, the canyon itself began to form relatively recently in geological time. This dramatic landscape resulted from massive regional uplift followed by relentless erosion. The entire Colorado Plateau began to rise about 75 million years ago during the Laramide Orogeny, a mountain-building event affecting much of western North America.
This regional uplift raised the layers of rock thousands of feet above sea level, giving the Colorado River the necessary gradient to begin its deep carving action. The river started cutting down through the elevated rock layers approximately five to six million years ago. The sediment-laden water acted like a powerful abrasive, grinding away the rock as the river flowed.
The downcutting action of the Colorado River was the primary force establishing the canyon’s depth, while other forces contributed to its width. Weathering, such as freezing and thawing, along with the erosive action of tributary streams, caused the canyon walls to retreat. The combination of the Colorado Plateau’s high elevation and the river’s persistent flow created the massive gorge, exposing the ancient layers of rock.