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The Earth is very old -- 4.5 billion years or more -- according to recent estimates. This vast span
of time, called geologic time by earth scientists, is difficult to comprehend in the familiar time units of months and years,
or even centuries. How then do scientists reckon geologic time, and why do they believe the Earth is so old? A great part
of the secret of the Earth's age is locked up in its rocks, and our centuries-old search for the key led to the beginning
and nourished the growth of geologic science. The rocks are not all the same age -- or even nearly
so -- but, like the pages in a long and complicated history, they record the Earthshaping events and life of the past. The
record, however, is incomplete. Many pages, especially in the early parts, are missing and many others are tattered, torn,
and difficult to decipher. But enough of the pages are preserved to reward the reader with accounts of astounding episodes
which certify that the Earth is billions of years old.
From http://pubs.usgs.gov/gip/geotime/geotime.html
The possible topics for geologic history are:
1. The geologic time scale
2. Relative age dating
3. Absolute age dating
4. Fossils
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