[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]).
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” QUATERNARY0-1 MILLION YEARS Recent [Pleistocene] TERTIARY62 MILLION YEARS [Pliocene] Miocene Oligocene Eocene Paleocene [MESOZOIC]“MIDDLE LIFE” CRETACEOUS72 MILLION YEARS JURASSIC46 MILLION YEARS TRIASSIC49 MILLION YEARS [PALEOZOIC]“ANCIENT LIFE” [PERMIAN]50 MILLION YEARS CARBONIFEROUS PENNSYLVANIAN30 MILLION YEARS MISSISSIPPIAN35 MILLION YEARS DEVONIAN60 MILLION YEARS SILURIAN20 MILLION YEARS ORDOVICIAN75 MILLION YEARS CAMBRIAN100 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.
Much of the basic information which the geologist uses to reconstruct the geologic history of a region comes from his examination and interpretation of bedrock [outcrops]. Bedrock is the solid unweathered [rock] which underlies loose earth material such as soil, sand, and gravel. An outcrop, or exposure, is a place where bedrock is exposed at the surface.
The first chapter of earth history begins with the most ancient [rocks] known. Because they were formed early in [geologic time], these rocks are normally found deeply buried beneath younger rocks which have been deposited on top of them. It is for this reason that earth history is read from the bottom up, for the earliest formed rock layers correspond to the opening chapter in our earthen history book. The later chapters are found in the upper younger rocks which are located nearer the surface. Thus, in “reading” the geologic history of Palo Duro Canyon we start with the oldest “chapter” which is recorded in the Quartermaster [Formation] ([p. 17]) of [Permian] age, for these are the oldest rocks exposed in the canyon.