Typically, the geologic time scale is displayed
as a vertical bar, with the youngest age units at the top, and the
oldest at bottom. This structure parallels the tendency of sediments
to accumulate one on top of another, with the older layers of rock
being covered over by new sediments.
(click on legend item to view map)
Here, we've divided the time scale into its four major components:
the Cenozoic era, the Mesozoic
era, the Paleozoic era and
the Precambrian. Since Cenozoic
is the most recent of these it is at the top-left. Precambrian,
being the oldest, is at the bottom.
The time scale is further divided. For example, the
Mesozoic, the age of the dinosaurs, is made up of the Cretaceous,
Jurassic, and Triassic periods.
The numbers on the scale indicate commonly accepted
age boundaries of these eras. The Precambrian stretches from the
origin of the earth, more than 4 billion years ago, to 540 million
years ago (or mega anna, MA). The Paleozoic from 540 MA to 248 MA.
Mesozoic covers the interval between 248 and 66 MA. Cenozoic covers
the span from 66 MA to the present.
As you can see, the time scale is not drawn linearly,
which is why the Precambrian, which lasts over 3.5 billion years,
appears shorter than the Paleozoic, which lasted a little more than
290 million years.