THE RELATIVE TIME SCALE
Long before geologists had the means to recognize and express time in numbers of
years before the present, they developed the geologic time scale. This time scale
was developed gradually, mostly in Europe, over the eighteenth and nineteenth
centuries. Earth's history is subdivided into eons, which are subdivided into
eras, which are subdivided into periods, which are subdivided into epochs. The
names of these subdivisions, like Paleozoic or Cenozoic, may look daunting, but
to the geologist there are clues in some of the words. For example,
zoic refers to animal life, and paleo means ancient,
meso means middle, and ceno means
recent. So the relative order of the three youngest eras, first Paleoozoic, then
Mesozoic, then Cenoozoic, is straightforward.
The relative geologic time scale. The oldest time interval is at the bottom and
the youngest is at the top.
Fossils are the recognizable remains, such as bones, shells, or leaves, or other
evidence, such as tracks, burrows, or impressions, of past life on Earth.
Scientists who study fossils are called paleontologists.
Remember that paleo
means ancient; so a paleontologist studies ancient forms of life. Fossils are
fundamental to the geologic time scale. The names of most of the eons and eras
end in zoic, because these time intervals are often recognized on the basis of
animal life. Rocks formed during the Proterozoic Eon may have fossils of relative
simple organisms, such as bacteria, algae, and wormlike animals. Rocks formed
during the Phanerozoic Eon may have fossils of complex animals and plants such as
dinosaurs, mammals, and trees.
This rock sample will be taken to the laboratory where tiny
fossils will be extracted for further study.
This page is URL: http://pubs.usgs.gov/gip/fossils/scale.html
Last updated 14 August 1997 (krw)
Maintained by John Watson