The geologic history of our earth has been recorded primarily in marine
sedimentary rocks, and this record indicates that our earth is very old
and that life has been present for many millions of years. The earth is
not only extremely old (more than 3½ billion years of age), but it has
also undergone many changes which have taken place slowly but steadily
and have greatly affected both the earth and its inhabitants. The
earth’s physical features have not always been as they are seen today.
Geologic research has shown that mountains now occupy the sites of
ancient seas, and that coal is being mined where swamps existed millions
of years ago. Furthermore, there is much evidence to indicate that
plants and animals have also undergone great change. The trend of this
organic change is, in general, toward more complex and advanced forms of
life, but some forms have remained virtually unchanged and others have
become extinct.
In order to interpret geologic history, the earth scientist must attempt
to gather evidence of the great changes in climate, geography, and life
that took place in the geologic past. The record of these changes can be
found in the rocks, and here is found the story of the various events in
earth history.