The sculptured beauty and brilliant colors of the Colorado Plateau's sedimentary rock layers have captured the imaginations of countless geologists. This is a vast region of plateaus, mesas, and deep canyons whose walls expose rocks ranging in age from billions to just a few hundred years old.
List of National Parks exhibiting Colorado Plateau Geology
Building the basement
Ancient Precambrian rocks, exposed only in the deepest canyons, make up the basement of the Colorado Plateau. Most are metamorphic rocks formed deep within the Earth while continental collision on a grand scale produced the nucleus of the North American continent well over a billion years ago. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement.
Reflection in pothole. Cedar Mesa Sandstone, Canyonlands National Park. Photo by Marli Miller.
These deeply-formed rocks were uplifted, eroded, and exposed for eons. By 600 million years ago North America had been beveled off to a remarkably smooth surface. It is on this crystalline rock surface that the younger, more familiar layered rocks of the Colorado Plateau were deposited.
Lithic layer cake
Throughout the Paleozoic Era, the Colorado Plateau region was periodically inundated by tropical seas. Thick layers of limestone, sandstone, siltstone, and shale were laid down in the shallow marine waters. During times when the seas retreated, stream deposits and dune sands were deposited or older layers were removed by erosion. Over 300 million years passed as layer upon layer of sediment accumulated.
It was not until the upheavals that coincided with the formation of the supercontinent Pangea began about 250 million years ago that deposits of marine sediment waned and terrestrial deposits dominate. The Mesozoic Era sedimentary deposits are striking. Great accumulations of dune sand hardened to form sweeping arcs in cross-bedded sandstone. Eruptions from volcanic mountain ranges to the west buried vast regions beneath ashy debris. Short-lived rivers, lakes, and inland seas left sedimentary records of their passage.
Crossbedded sandstone. Jurassic Aztec Sandstone Formation. Zion National Park. Photo by Marli Miller.
Rising crust and downcutting streams
One of the most geologically intriguing features of the Colorado Plateau is its remarkable stability. Relatively little rock deformation (ex. faulting and folding) has affected this high, thick crustal block within the last 600 million years or so. In contrast, the plateau is surrounded by provinces that have suffered severe deformation. Mountain building thrust up the Rocky Mountains to the north and east and tremendous, earth-stretching tension created the Basin and Range Province to the west and south.
Although the Basin and Range and Colorado Plateau may seem to have little in common, their geological stories are intimately intertwined. In the early part of this Era (Paleogene Period, Cenozoic Era), both regions had low elevations of probably less than 1 kilometer. Geologists are still gathering evidence and vigorously debating what came next.
Confluence of Green and Colorado Rivers, looking south, southeast Utah. Photo by Marli Miller.
Beginning about 20 million years ago, during the Miocene Epoch both the Basin and Range and Colorado Plateau regions were uplifted as much as 3 kilometers. Great tension developed in the crust, probably related to changing plate motions far to the west. As the crust stretched, the Basin and Range Province broke up into a multitude of down-dropped valleys and elongate mountains. Yet for some reason not fully understood, the neighboring Colorado Plateau was able to preserve its structural integrity and remained a single tectonic block. Eventually, the great block of Colorado Plateau crust rose a kilometer higher than the Basin and Range.
As the land rose, the streams responded by cutting ever deeper stream channels. The most well-known of these streams, the Colorado River, began to carve the Grand Canyon less than 6 million years ago. The forces of erosion have exposed the vivid kaleidoscope of rock layers that make the Colorado Plateau a mecca for rock lovers.