back to main menu

links to relevent geological sites

Geological Time - also known as STRATIGRAPHY.

The earth is now thought to be approximately 4.6 billion years old. Throughout this time the earth has evolved from a burning blob into what we know today. Geologists have split the age of the earth into many different divisions. They have done this mostly with the use of fossils and the principle of superposition, i.e. that within a set of layered rock the layers on top must have been formed after the layers underneath and therefore must be younger than them unless it can be shown that the layers have been inverted after formation.

After a lot of study it was realised that the same fossils could be found in different rocks around the world and that, in the sets of rocks studied, the same fossils were found in the same order. It was soon understood that different species of fossil were found during different time periods but also that at the same time in history the same fossil could be found in many different places. Therefore rocks containing the same fossils were given the same name to represent the fact that they were of a very similar age.

The groups of layers found in this way were often named after the fossils they contained. These layers were then grouped again to give larger divisions and so on until the Stratigraphic Column was built up. With the advent of new science it became possible to give actual dates (using radiometric dating) to what had previously only been a comparative dating method.

In our version of the stratigraphic column we have just outlined the names and ages of each of the geological 'periods'. The ages given on the chart represent millions of years from present. So for example the Carboniferous period lasted from 345 million years ago to 280 million years ago. Click to find out more!