National Geographic : 1955 May
602 Grand Canyon in Profile: The Finest Open Record of Earth's History Here the reader peers, as might a visitor near El Tovar Hotel, into the violent convulsive past of his planet, laid bare by the remorseless gouging of the Colorado River. After millions of years of digging, the torrent now cuts into earth's primeval crust, formed about a billion and a half years ago (page 604). Exposed rock layers, sweeping up from Kaibab Suspension Bridge to Yaki Point, reveal to the geologist's trained eye a story of mighty cataclysms: of mountains buckling skyward and wearing away; of seas sweeping over the land and laying down vast deposits, then disappearing. Buried deep in the multicolored strata, fossils trace life's development from primitive algae and shells to amphibians and reptiles. earth's history rests directly upon the first chapter. By skipping the second chapter, the rocks show a gap of millions of years. We spanned it merely by placing a finger tip across the line where the strata meet. Geologists call that gap in time the "great unconformity," and people come from all parts of the world to see it. First Amphibians Left Tracks in Stone Animal life flourished in the Paleozoic Era, earth's Chapter III. Our trail revealed the fossil remains of trilobites, extinct sea crea tures resembling horseshoe crabs. And as we rode up the canyon walls, the fossils disclosed life becoming increasingly complex. Land areas in those times grew ferns and primitive conifers. Going a little higher, we found telltale signs of animals emerging from the sea: amphib ians, descendants of fish, had climbed onto land and left their tracks in mud now turned to stone. We did not see the dinosaur graveyards of Chapter IV, the Mesozoic, or Age of Reptiles, for they have been worn off the top of Grand Canyon. However, they are represented in the Painted Desert and Zion National Park, both fairly close. Chapter V, the Cenozoic, or Age of Mam mals in which we live, has left no sedimentary deposits in the canyon. It is represented, however, by geologically recent lava flows in Grand Canyon National Monument (page 608). Man stepped on the geological scene an estimated million years ago, though not in the Grand Canyon. If we could condense earth's story into a movie lasting 24 hours, man's role would occupy the screen only the last 58 seconds, and the canyon's role in history from its discovery four centuries ago would last less than a fortieth of a second. As the lion-hunting cowboy said, "Some thing sure happened," and many of today's visitors want to know just what happened. "Did the earth crack open?" they ask. "Did glaciers cut it?"