2 Geological time-scales – a brief review
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 (Figure 2).
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.