The names of the rock layers.
North of the park, in driving past Arizona's Vermillion Cliffs, we saw several of the same Mesozoic rock layers we had seen in southern Utah. But when we reached the park, none of these layers were present! The Grand Canyon rocks are all Paleozoic in age or older. The younger Mesozoic and even Cenozoic rocks must have existed on top of them at one time but due to uplift they have all now been eroded off.
North of the park, in driving past Arizona's Vermillion Cliffs, we saw several of the same Mesozoic rock layers we had seen in southern Utah. But when we reached the park, none of these layers were present! The Grand Canyon rocks are all Paleozoic in age or older. The younger Mesozoic and even Cenozoic rocks must have existed on top of them at one time but due to uplift they have all now been eroded off.
Before we show our last views of the Grand Canyon, here are some notes on the names and ages of the major rock layers, for those who like to put names to the things they see. (The limestones, shales, and thin-bedded sandstones were deposited in shallow seas, at various distances from shore, as the sea level fluctuated. Massive sandstones were formed on land from sand dunes.)
There are many formation names and some of them are not easy to separate visually, but those can just be viewed as groups. The main layers are quite distinctive and easy to separate.
The white stuff. At the top is a group of white or light colored rocks of Permian age (the youngest Paleozoic rocks). The rimrock is the vertical-walled Kaibab limestone. Below it is the terraced Toroweap formation, and below that the massive vertical-walled Coconino sandstone. Sloping out from the base of the Coconino is the Hermit Shale.
The red stuff. The slopes of thin-bedded red sandstones and shales belong to several formations that all together are called the Supai group, of Pennsylvanian age. The vertical red wall below them consists of the Red Wall limestone (Mississippian), the Temple Butte limestone (Devonian), and the Muav limestone (Cambrian).
Below these red rocks, forming a greenish gray apron stretching out to the inner gorge, is the Bright Angel Shale (Cambrian). The rimrock of the inner gorge is a vertical wall of Tapeats sandstone (Cambrian).
Below the rim of the inner gorge in the eastern part of the park are tilted orange-red beds of much older rock. There are several formations all belonging to the Grand Canyon Supergroup of Precambrian age (as old as 1.2 billion years).
From the western overlooks, the rocks of the Supergroup are not visible, having been eroded off before the Tapeats sandstone was deposited. Instead, the river there cuts through even older rock, known as the Vishnu Schist, which is marbled with intrusions called the Zoroaster Granite. These gray, unlayered rocks date back as far as 1.8 billion years.
Simplifying, from the top down one sees the white stuff, the red stuff, and the gray-green apron of Bright Angel Shale. In the inner gorge, the rimrock is Tapeats sandstone and below that the reddish Grand Canyon Supergroup and/or the gray Vishnu Schist.
