Nineteenth
century geologists determined the relative ages of rock
by studying changes in fossil assemblages. Geologist
came to realize that rocks reveal an amazing story of life on
Earth; different organisms have lived at different times and
environments change. Furthermore, this history of life
is irreversible.
Thus, a relative time scale could be developed from boundaries
defined by the appearance and disappearance of life forms as
well as changes in rock types. The history of life on Earth was
a revelation from the fossil record. This revelation would
throw light on extinction and evolution. To learn more
about human's evolving views of fossils and the patterns
they
reveal,
visit the fossil
section of our website.
You can use the drop down menus or the Geologic Time Scale below
to navigate through different periods and epochs in our museum.
Each period and epoch has an introduction that outlines some
of the
important
biological patterns revealed by fossils. In addition to the introduction,
galleries containing example fossils representative of the period
or epoch may be explored.
There
has been an on-going debate as to whether or not the Quaternary
should be included in the timescale. Many
marine geologists have argued that the Quaternary is a
climato-stratigraphic unit, not a chronostratigraphic unit
defined by clear markers
in the rock record. Geologists who find this time period
useful have come to view the Quaternary and Pleistocene
as marked by the start of global cooling and glaciation
2.6
million years ago.
In
2008 the International Commission on Stratigraphy (ICS)
published a timescale with the Quaternary as a full period
starting at 2.6 million years ago. The Pleistocene epoch
was pushed back from 1.8 to 2.6 million years ago to
align with
the Quaternary. The International Union of Geological Sciences
(IUGS) ratified the ICS recommendation in June 2009 (Gramling,
2009, p. 13). For now, the debate as to whether or not
the Quaternary should be included in the timescale seems
to be
settled. The following websites can be used to explore
geologic time in more detail. Each site will open as a
separate window.
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