Sedimentary rocks are typically _stratified_, that is, they occur in layers or beds called _strata_. In addition, sedimentary rocks—especially those of marine origin—commonly contain _fossils_. These fossils are traces or evidence of prehistoric plants and animals that have been preserved in the rocks, and they may provide clues as to the age of rocks and the manner in which they were formed. Fossil remains have been found at a number of places in the park and these are discussed later. UNRAVELING EARTH HISTORY In order to understand better the geologic history and development of the canyon, one should also have some knowledge of the basic principles of earth history and should be familiar with the geologic time scale (fig. 6). [Illustration: Fig. 6. Geologic time scale. Reproduced from _FOSSILS: An Introduction to Prehistoric Life_, William H. Matthews III, Barnes and Noble, Inc., 1962.] GEOLOGIC TIME SCALE ERA PERIOD EPOCH SUCCESSION OF LIFE CENOZOIC “RECENT LIFE” QUATERNARY 0-1 MILLION YEARS Recent Pleistocene TERTIARY 62 MILLION YEARS Pliocene Miocene Oligocene Eocene Paleocene MESOZOIC “MIDDLE LIFE” CRETACEOUS 72 MILLION YEARS JURASSIC 46 MILLION YEARS TRIASSIC 49 MILLION YEARS PALEOZOIC “ANCIENT LIFE” PERMIAN 50 MILLION YEARS CARBONIFEROUS PENNSYLVANIAN 30 MILLION YEARS MISSISSIPPIAN 35 MILLION YEARS DEVONIAN 60 MILLION YEARS SILURIAN 20 MILLION YEARS ORDOVICIAN 75 MILLION YEARS CAMBRIAN 100 MILLION YEARS PRECAMBRIAN ERAS PROTEROZOIC ERA ARCHEOZOIC ERA APPROXIMATE AGE OF THE EARTH MORE THAN 4 BILLION 550 MILLION YEARS The geologist has learned that the earth’s physical features have not always been as they are today. It is known, for example, that mountains now occupy the sites of ancient seas. Coal is now being mined where swamps existed many millions of years ago. Furthermore, the earth’s plants and animals have also been subject to great change. The trend of this organic change is, in general, toward more complex and advanced forms of life. However, some forms have remained virtually unaltered while others have become extinct at different points in geologic time. In order to interpret earth history, the earth scientist gathers evidence of the great changes in climate, geography, and life that took place in the geologic past. He does this by studying the rock formations, the structural relationships of these formations, and the landforms of the area. The record of ancient events is pieced together by studying the stony layers of the earth as one might study a giant history book. Indeed, the sedimentary rocks are the rocky “pages” of earth history, for in them we find the tracks and trails, and bones and stones, which reveal the intriguing story of life long ago.