The man-made history of Utah is but a letter in the latest "syllable of recorded time." The geologic history of Utah is a story of mighty rivers and deserts and mountains, of simple protoplasmic things in the dark ooze of ancient seas, and of their struggle through millions of years toward consciousness and light. The geologist has divided the story of the earth into five great chapters, and every chapter can be read in the rock formations of Utah. Some pages are missing, some crumpled and torn; some words are blotted, some meanings obscure, but on the whole the record is well preserved.
Scientists, through their study of the decomposition of radioactive minerals, calculate that the Archeozoic rocks, the most ancient formations in the world, are one billion, eight hundred fifty million years old. Archeozoic rocks, exposed by folding and faulting in the Wasatch Mountains, show no fossil trace of life. If life existed in Archeozoic time, the tremendous pressure of accumulating sediments has long ago obliterated its faintest impression.
During Proterozoic time (the second chapter) water covered the greater part of eastern Utah, and the first one-celled life forms that have been definitely recognized made their appearance. Proterozoic rocks are exposed in several places throughout the State--notably in the vicinity of Big Cottonwood Canyon, east of Salt Lake City, and in the desert ranges of the Great Basin, where faulting and erosion have stripped away the younger sediments.
Seas of the Paleozoic era advanced into Utah and retreated again many times, and each advancing sea brought with it strange new forms of life. At the beginning of the Paleozoic time the little crab-like trilobite, seldom more than three inches long, was the king of beasts. As the era lengthened into millions of years, gastropods, brachiopods, cephalopods, horn corals, and even higher types of fish appeared. Huge ferns and dense forest growths lined the waterways, and sprawling vertebrates swam in the oceans and crawled awkwardly on the shores. Exposure of Paleozoic rocks is widespread throughout Utah--particularly in the Wasatch, in the Uintas, in the basin ranges, and in the high plateaus of the south. Although vertebrate fossils of the Paleozoic era are comparatively rare in the State, there is an abundance of invertebrate forms. Fossilized trilobites may be found by the thousands near Antelope Springs west of Delta. Protruding from the canyon walls, or lying in the slide-rock of the Wasatch Range, there are many fossilized specimens of corals, sea lilies, snails, clams, mussels, and similar forms. Fossil shellfish are also numerous in the Uinta Mountains, in various parts of the Great Basin, and in south central Utah.
The major part of the Great Basin was above water in Mesozoic time, and the eroded sediments were carried east and south into what is now the plateau region. Mesozoic rocks, therefore, are confined largely to the eastern and southern parts of the State; buried within them is fossil evidence of the slow progression of life. In Mesozoic waters the shellfish continued to evolve; on Mesozoic landscapes lived the first dinosaurs, birds, and mammals, and the first hardwood trees. During this period in geologic history, large areas in northern Arizona and southern Utah were covered by flood plains upon which meandering rivers and streams left stranded a great number of logs. Later deposition covered them with mud and sand, and waters bearing silica, manganese, and iron filtered through the ancient flood plains and replaced the vegetable material of the tree trunks with stone. At Capitol Reef National Monument, Zion National Park, and near Escalante, Kanab, and St. George there is petrified wood estimated to be more than a hundred million years old.
Excavations in the Uintah Basin near Jensen since 1909 have uncovered a veritable graveyard of dinosaur bones, and the site has been set aside as a national monument. In this region long ago was a flood plain across which streams meandered. Upon the banks of these rivers and in the surrounding marshes the great reptiles lived. Some of them, preying upon smaller dinosaurs, became masters of their primitive world; others, feeding upon water plants, attained a length of a hundred feet and a weight of thirty-five or forty tons. When these mighty reptiles died, their carcasses were borne away by floods and deposited on a bar, where the drifting sediments, covering them hardened into stone. The Jensen quarry (see Dinosaur National Monument) has yielded hundreds of dinosaur bones. Among them are the back and leg bones of the herbivorous brontosaurus and diplodocus; the bones of a flesh-eating allosaurus; and the complete skeleton of an immense brontosaurus, a hundred feet long and twenty feet high.