In 1815, William Smith, a surveyor, canal builder, and amateur geologist from England, produced a geologic map of England and in doing so demonstrated the validity of the principle of faunal succession. This principle simply stated that fossils are found in rocks in a distinctive order. This principle led others to use fossils to define increments within a relative time scale.
The history of the earth is divided into a hierarchical set of age groups. As increasingly shorter units of time, the generally accepted divisions are eon, era, period, epoch, and age. These time periods reflect the history found in the geological record.
In more recent times, geologists have attempted to place absolute dates on the boundaries between the time periods. There are a variety of reasons for doing so, but they include better insight into biological evolution, adaptive radiations, extinctions and recoveries, climate change, and catastrophes.
There are diagrams for
three timescales on this page. The
first shows how absolute
ages for part of the
timescale have varied
through time. The second
is the timescale of the
and the third is
the timescale of the
Geological Society of
America, developed for
their successful Decade
of North American Geology
Nick Christe-Blick has some interesting thoughts on Geological Time Conventions and Symbols.
More detailed timescales: Devonian timescale (pdf): A paper (1013 kb .pdf) describing the methodology for the revised calibration is available here.