Excerpt from Beyond the 100th Meridian, by Wallace Stegner:
The Plateau Province:
“Here the earth has had a slow, regular pulse. It rose and fell for millions of years under Carboniferous, Permian, Triassic oceans, under Cretaceous seas, under the fresh-water lakes of the Eocene, before it was heaved up and exposed to rain and frost and running water and the sandblast winds. Mountains were carved out of its great tables and domes, river systems cut into it and formed canyons, elevations were weathered and carried away. What had accumulated pebble by pebble and grain by grain, cemented with lime and silica, folding into itself the shells of sea life, scales of fishes, the compacted houses of corals, began to disintegrate again. Vast cyclic changes have left only traces.
Though the geological record in the Plateau Province is probably as clear as it is anywhere on earth, the boundary between ignorance and knowledge, between speculation and certainty, often no more than a line of ancient fracture almost obliterated, or an enigmatic unconformity between two layers of rock, or a slight but significant change from salt water to brackish water fossils.”
The photo below, taken from the air (credit P. E. Hare) shows the Tapeats sandstone (Cambrian, ~500 million years old) overlying ~2 billion-year old metamorphic Vishnu schist. View is to the west, looking downstream. The “line” between the Tapeats and the Vishnu is the “Great Unconformity” – it can be traced over ~100 miles within the Canyon and represents ~1.5 billion years missing from the physical record.
