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.