Fossils and geologic time Fossils provide important clues to the ages of the rocks in which they are found. The slow evolution of living things through geologic time can be traced by a systematic study of fossils. The fossils are then used to determine the relative ages of the rocks that contain them and to establish a geologic time scale that can be applied to fossil-bearing rocks throughout the world. Figure 18 shows the major subdivisions of the last 600 million years of geologic time and some forms of life that dominated the scene during each of these intervals. Strata containing closely related fossils are grouped into _systems_; the time interval during which the strata comprising a particular system were deposited is termed a _period_. The periods are subdivisions of larger time units called _eras_ and some are split into smaller time units called _epochs_. Strata deposited during an epoch comprise a _series_. Series are in turn subdivided into rock units called _groups_ and formations. Expressed in tabular form these divisions are: Subdivisions of Time-rock units Rock units geologic time Era Period System Epoch Series Group Formation The time scale based on the study of fossil-bearing sedimentary rocks is called the stratigraphic time scale; it is given in table 1. The subdivisions are arranged in the same order in which they were deposited, with the oldest at the bottom and the youngest at the top. All rocks older than Cambrian (the first period in the Paleozoic Era) are classed as Precambrian. These rocks are so old that fossils are rare and therefore cannot be conveniently used as a basis for subdivision. The stratigraphic time scale is extremely useful, but it has serious drawbacks. It can be applied only to fossil-bearing strata or to rocks whose ages are determined by their relation to those containing fossils. It cannot be used directly for rocks that lack fossils, such as igneous rocks, or metamorphic rocks in which fossils have been destroyed by heat or pressure. It is used to establish the relative ages of sedimentary strata throughout the world, but it gives no information as to how long ago a particular layer was deposited or how many years a given period or era lasted. Radioactive clocks