III.

Geology of the Grand Canyon

Although the canyon itself may be of comparatively recent origin, the rocks exposed in its walls are not. Most of the rock strata (layers) were originally deposited as marine sediment, indicating that for long periods of time the canyon area was the floor of a shallow sea.

In a typical section of the canyon, toward its eastern end, nine separate rock layers can be seen, piled vertically like a stack of pancakes. The topmost layer is a limestone, the Kaibab limestone. Below this layer is a thick deposit of sandstone, called the Coconino sandstone, and below that a layer of soft, shaly rock known as the Hermit shale. Still lower is a series of shales and sandstones interbedded with each other, collectively termed the Supai formation. The fossils found in the Supai and the rocks above it suggest that these rocks were all deposited in the Permian Period at the end of the Paleozoic Era. However, the Supai may be slightly older still. Next comes a deposit of light gray-blue limestone, the Redwall limestone, which in many places has been colored red by seepage from the Supai beds above. The Redwall is 152 m (500 ft) thick and is easily identified because of the prominent sheer cliffs that it forms in the canyon walls. This layer has been identified as belonging to the Mississippian Period. A thin layer of sandstone, the Temple Butte, beneath the Redwall, gives evidence of having originated in the Devonian Period. The next three rock layers, consisting of the brown Muav limestone, the green Bright Angel shale, and the Tapeats sandstone, all belong to the Cambrian Period, at the dawn of the Paleozoic Era. Beneath these layers, at the bottom of the canyon, are the most ancient rocks of all, Precambrian schists and gneisses, from half a billion to a billion years old.