GRAND CANYON, ARIZONA
A Mile-Deep Erosional Incision into the Earth
Rocks of the Proterozoic Era and Paleozoic Eon
It has been described as the most sublime spectacle on the Earth. Within a single glance, one can view more of the planet's history than is possible anywhere else in the world.
The first impression one has about the Grand Canyon is the immensity of the chasm. It is nearly three hundred miles long, up to twenty miles wide and, in places, more than a mile deep. The rocks exposed on the steep walls of the inner canyon are as old as 1.7 billion years. Above them are the thick horizontal layers of the much broader outer canyon. Those rocks range in age from the Tapeats Sandstone (525 million years) to the Kaibab Limestone (270 million years). Those are the rocks of the Paleozoic Era.
The Great Unconformity is on grand display at the Grand Canyon. It was first described by Major John Wesley Powell in 1869 as he and nine other men were making the first recorded passage through the canyon.
At Yavapai Point in Grand Canyon National Park one can see the Great Unconformity marking the contact between the great horizontal layers and the dark rocks of the inner canyon. The bottommost layer is the Tapeats Sandstone. Beneath it are the rocks of the Vishnu Schist and Zoroaster Granite. Those are rocks of the first two supercontinents, Columbia and Rodinia.
Rodinia split apart about a billion years ago. As it did, huge basins formed along the lines where it was splitting. Deep sediments filled those basins, exposed today as the Grand Canyon Supergroup, visible from Lipan Point, also in the national park.
View of the Grand Canyon from Lipan Point.