A brief guide to the way in which geologists divide up geological time with special reference to Fforest Fawr 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. 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 Era
The last 542 million years is known as the Phanerozoic Eon. Geologists divide it into three ‘eras’ and twelve ‘periods’. Those periods 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.
- Cenozoic/Cainozoic Era
- Quaternary Period* 0-2.6 million years ago (mya)
- Neogene Period 2.6 – 23 mya
- Palaeogene Period 23 – 65.5 mya
- Mesozoic Era
- Cretaceous Period 65.5 - 145 mya
- Jurassic Period 145 - 201 mya
- Triassic Period 201 – 252 mya
- Palaeozoic Era
- Permian Period 252 -299 mya
- Carboniferous Period* 299 – 359 mya
- Devonian Period* 359 – 419 mya
- Silurian Period* 419 – 443 mya
- Ordovician Period* 443- 485 mya
- Cambrian Period 485 – 541 mya
- Precambrian ‘era’ 541 – 4540 mya
Further subdivisions of each of those periods which are represented by rocks or deposits within Fforest Fawr Geopark are shown below: