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 4560 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).
The times given below are largely those recommended by the BGS (following a report by two experts - Gradstein & Ogg - in 1996). You are likely to come across other differing dates in textbooks and websites such as those published by the International Commission on Stratigraphy in 2004, though these have not yet been ratified. 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 last 545 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.
Further subdivisions of each of those periods which are represented by rocks or deposits within Fforest Fawr Geopark are shown below: