A brief guide to the way in which geologists divide up geological time with special reference to Fforest Fawr UNESCO Global Geopark
The Earth is thought to be about 4540 million years old. Geologists have divided that time up in many different ways over the years although increasingly there is an internationally agreed way of doing this.
Visit the geological timechart pages at the British Geological Survey (BGS).
You are likely to come across other differing dates for the start and finish of various geological periods in textbooks and websites. The times given below are largely those recommended by the BGS and the International Commission on Stratigraphy which has ratified a report on the subject published in 2012 by three experts – Gradstein, Ogg & Smith – and updated in 2022. As knowledge improves so these dates are likely to change – treat them with some caution.
The subdivisions are as set out below. Several ages make up an epoch, several of which make up a period. Several periods make up an era, several of which make up an eon. The rocks formed during a period are known as a system, similarly those formed during an epoch make up a series and so on.
The Present Eon
The last 541 million years is known as the Phanerozoic Eon. Geologists divide it into three ‘eras’ and twelve ‘periods’. Those periods within the geological timescale which are marked with an * are represented in the rocks of Fforest Fawr Geopark. We are currently living in the Quaternary Period in the Cenozoic Era within the Phanerozoic Eon.
- Quaternary Period* 2.6 – 0 million years ago (mya) – visit the Quaternary Timechart
- Neogene Period 23 – 2.6 mya
- Palaeogene Period 66 – 23 mya
- Cretaceous Period 145 – 66 mya
- Jurassic Period 201 – 145 mya
- Triassic Period 201 – 252 mya
- Permian Period 299 – 252 mya
- Carboniferous Period* 359 – 299 mya – visit the Carboniferous Timechart
- Devonian Period* 419 – 359 mya – visit the Devonian Timechart
- Silurian Period* 444 – 419 mya – visit the Silurian Timechart
- Ordovician Period* 485 – 444 mya – visit the Ordovician Timechart
- Cambrian Period 539 – 485 mya
- 4540 – 539 mya