The geological time scale is based on the the geological rock record, which includes erosion, mountain building and other geological events. Over hundreds to thousands of millions of years, continents, oceans and mountain ranges have moved vast distances both vertically and horizontally. For example, areas that were once deep oceans hundreds of millions of years ago are now mountainous desert regions.
How is geological time measured?
The earliest geological time scales simply used the order of rocks laid down in a sedimentary rock sequence (stratum) with the oldest at the bottom. However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata. After Charles Darwin’s publication Origin of Species (Darwin himself was also a geologist) in 1859, geologists realised that particular fossils were restricted to particular layers of rock. This built up the first generalised geological time scale.
Once formations and stratigraphic sequences were mapped around the world, sequences could be matched from the faunal successions. These sequences apply from the beginning of the Cambrian period, which contains the first evidence of macro-fossils. Fossil assemblages ‘fingerprint’ formations, even though some species may range through several different formations. This feature allowed William Smith (an engineer and surveyor who worked in the coal mines of England in the late 1700s) to order the fossils he started to collect in south-eastern England in 1793. He noted that different formations contained different fossils and he could map one formation from another by the differences in the fossils. As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels.
By matching similar fossils in different regions throughout the world, correlations were built up over many years. Only when radioactive isotopes were developed in the early 1900s did stratigraphic correlations become less important as igneous and metamorphic rocks could be dated for the first time.
Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms. For example, the Devonian Period is known as the ‘Age of Fishes’, as fish began to flourish at this stage. However, the end of the Devonian was marked by the predominance of a different life form, plants, which in turn denotes the beginning of the Carboniferous Period. The different periods can be further subdivided (e.g. Early Cambrian, Middle Cambrian and Late Cambrian).
This is the latest version of the time scale, as revised and published in 2012.
4.56 – 2.5 billion years ago
Era: Archaean
2.5 billion – 541 million year
Era: Proterozoic
541 – 485 million years ago
Period: Cambrian
Era: Palaeozoic
485 – 444 million years ago
Period: Ordovician
Era: Palaeozoic
444 – 419 million years ago
Period: Silurian
Era: Palaeozoic
419 – 359 million years ago
Period: Devonian
Era: Palaeozoic
359 – 298 million years ago
Period: Carboniferous
Era: Palaeozoic
298 – 252 million years ago
Period: Permian
Era: Palaeozoic
252 – 201 million years ago
Period: Triassic
Era: Mesozoic
201 – 145 million years ago
Period: Jurassic
Era: Mesozoic
145 – 65 million years ago
Period: Cretaceous
Era: Mesozoic
66 – 56 million years ago
Epoch: Palaeocene
Era: Cenozoic
56 – 34 million years ago
Epoch: Eocene
Era: Cenozoic
34 – 23 million years ago
Epoch: Oligocene
Era: Cenozoic
23 – 5.3 million years ago
Epoch: Miocene
Era: Cenozoic
5.3 -2.6 million years ago
Epoch: Pliocene
Era: Cenozoic
2.6 million -10,000 years ago
Epoch: Pleistocene
Period: Quaternary
10,000 years ago to the presen
Epoch: Holocene
Glossary of Terms
Faunal succession: is the time arrangement of fossils in the geological record.
Formations: are stratigraphic successions containing rocks of related geological age that formed within the same geological setting.
Ga: is an abbrevia