Relative Time

Fossils and their associated rocks provide much of the data by which relative time, or the relative sequence of events, is calculated. Study of the evolution and interrelationships of fossils and of the nature, position, continuity, similarity, and alteration of the rocks, interpreted in the light of the assumption that conditions in the past were little different from those of today, is the basis of the system of eras, periods, epochs, and ages which is the framework of relative geologic time.

Relative geologic time is divided into five eras, beginning with the Archeozoic ("beginning life") and the Proterozoic ("primitive life") eras. These two eras occupy nearly 4 billion of the 4/:! billion years of the estimated existence of the earth. Their fossils are few, faint, and often problematical; they do not interest the amateur. But out of this dimly understood void, life sprang abundantly with the dawn of the next era, the Paleozoic ("ancient life") era. Like subsequent eras, the Paleozoic is divided into shorter units, the periods, which themselves are divided into epochs and ages. The first Paleozoic period is the Cambrian, and in fossil literature the term "Pre-Cambrian" is often used to designate time before the Paleozoic.

The Paleozoic era was followed by the Mesozoic ("middle life") era, often referred to as the Age of Reptiles, then by the Cenozoic ("recent life") era, the Age of Man, in which we are living. The Cenozoic is divided into the Tertiary and Quaternary periods, but is more commonly described by reference to epochs, which are subdivisions of the periods.

The Grand Canyon of the Colorado river is a monument to relative time. Here deposition built a remarkably complete series of strata lying on ancient metamorphic schists. As earth forces slowly raised the plateau formed by these strata, a dauntless river, fighting for its existence, cut through the noble pile of rocks, exposing, like the sheets of a desk calendar, the strata that record by their thickness, composition, and fossils the geological history of this region for the last 2 billion years.

Some strata are missing altogether or appear at only one or a few places in the Canyon. But these parts of the whole history can be tied into one chronology by tracing a few key formations, such as the Redwall limestone, and with it as a fixed point determining what is lacking. This is the way a historian works, gathering his documents where he can, fitting them into place, and then drawing conclusions. Similarly, the geologist studies the composition and texture of the stone, studies the comparative development and character of the fossils in it, and perhaps supplies a missing fact from study of comparable formations elsewhere that may be more complete in some detail. From these data he arrives at a place and a time for the formation and its fossils in the larger context of facts, marshaled by all geological and paleontological research.

The Grand Canyon is a magnificent record of such events in time because of the long vertical exposure of strata going back into the Pre-Cambrian even though the Mesozoic record must be supplied from nearby formations because it has been eroded from the path of the Canyon. Furthermore, even the Pre-Cambrian Grand Canyon system of sedimentary rocks resting on the metamorphosed schists carries algae fossils, and all the rocks above it contain fossils, which makes the work of correlating them much easier. Certain fossils characteristic of a widely distributed rock formation and present only in that formation are used to identify that formation wherever it may appear. These are called index fossils. Their presence in a formation helps geologists to tie together the evidence of identity of formations even though they may be broken or disguised by erosion, by geological faults, or by differing conditions.

One of the most useful tools used by the geologist to date formations is correlation, which is the placing together of the relationships of unlike rocks even though they may have been formed at the same time and in adjacent areas. An ocean, a shore, and a brackish swamp may have been geographical neighbors at some time far in the past, just as they are today. Yet each would spawn its own particular fossils because conditions in each made life possible for specific organisms.

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