Geological timescale

Geological time can be divided into a number of Eons, Eras and Periods, with further
subdivisions into sub-Periods or series and epochs. These are arranged chronologically, with
the oldest at the bottom, younging upwards to form the stratigraphic column.
The stratigraphic column can be looked at in two ways. The first deals with the order of rock
units. This order has been established using the Principle of Superposition and the Principle
of Faunal Succession, and produces the lithostratigraphic (‘rock-stratigraphic’) column, based
simply on the relative ages for rock successions. The second aspect of the stratigraphic
column relates to the geochronological dating of rocks using a variety of radiogenic isotopes.
This forms the chronostratigraphic (‘time-stratigraphic’) column and allows geologists to
apply absolute ages to rock successions.
The stratigraphic column for the whole of geological time: (a) to true scale and (b) expanded scale
for the Phanerozoic. (The Cryptozoic Eon represents ~90% of all geological time.) The scale is divided
into Eons, Eras and Periods, which form the lithostratigraphic column. The column of ages of each
Period, in millions of years (Ma), forms the chronostratigraphic column.