What are the divisions of geologic time?

Geologic time is a way to place dates on the formation of
rocks and organize the information that we have about how the Earth formed, how it has
changed, and how living things have changed along with it. In the 1600's scientists
began to be interested in rocks and rock formation, but it wasn't until 1815, when
William Smith produced the first geologic map, that geology began to become a unified
science.

From longest to shortest, the divisions currently
in use are eons, eras, periods, epochs, and ages. While the names come from localities
where discoveries were made (for example, the Cambrian is for Cambridge, England), the
lengths of the various periods and how they are divided has been dictated by the fossils
found within them. Whenever the fossils indicate that a major change in the life forms
of the planet occurred, scientists place a dividing line between one division and the
next. Really big changes, such as the appearance of the first fossils, mark eons. Lesser
changes, such as increase in diversity or range of a group of living things, are marked
by eras or periods. For example, the appearance of fish in the fossil record marks the
change from the Cambrian Period to the Ordovician Period, and the retreat of the
glaciers marks the end of the Pleistocene Epoch and the beginning of the Holocene
Epoch.