Mention geologic history and most people think of dinosaurs or famous landforms, especially the Grand Canyon. No doubt, many visitors have stood at the rim of that magnificent chasm and wondered about all of the dinosaur fossils that it might contain. But is the Grand Canyon older or younger than the Age of Dinosaurs? The answer is both.
The rock layers of the Grand Canyon represent an historical record that stretches from the middle of the Precambrian Era (almost 2 billion years ago) through the Paleozoic Era, which ended 225 million years ago. The oldest rocks are at the base of the Canyon while the youngest (Permian sandstone) forms its rim. On the other hand, this layer cake of geologic history has been revealed by the erosive force of the Colorado River and by freeze-thaw erosion along its walls; all of this erosive activity has occurred very "recently," within the past six million years.
The dinosaurs lived during the Mesozoic Era, which began 225 million years ago and ended 65 million years ago. Fossils of these fascinating creatures are thus found in rocks that formed from mud, sand or volcanic deposits of that Era and thus would not be found in the rocks that line the Grand Canyon; the dinosaurs lived after the rocks of the Canyon were deposited but well before the Canyon formed! The only potential source for dinosaur fossils in the Grand Canyon would be via the Colorado River, which, along with its major tributaries, erodes the Mesozoic strata of the Colorado Plateau, to the north and east (an unlikely event that is now virtually impossible due to the Glen Canyon Dam).
Too often, students of natural history forget that life evolved in concert with a changing landscape. The timing of evolutionary events determines where we expect to find fossils of a particular life form and, conversely, the presence of fossils in rock of a known age helps us understand the ecology of that specific period.
