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HOW Geologists
describe time

In day to day life we use time scales that make sense to us -  they're of human scale.  We use seconds, minutes, hours, days, weeks, months, years, decades and centuries.  As the Earth has existed for 4.5 billion years, clearly geologists need a different time scale to work to as the human scale is far too small, when describing Earth history.


Therefore geologists use larger units of time:


The largest of the geologic time units, comprising a number of eras.


The American geologist George Halcott Chadwick originally proposed two eons: the Phanerozoic (time of visible life) which incorporates the Palaeozoic, Mesozoic and Cenozoic eras; and the Cryptozoic (time of hidden life). The Phanerozoic term is used today, but commonly the time before the Phanerozoic is referred to as the Precambrian, not Chadwick's Cryptozoic.



The era is a first-order geologic time unit comprising several periods. The Phanerozoic Eon is sub-divided into three eras:  The Palaeozoic (old life) The Mesozoic Era (Middle Life) and the Cenozoic (recent life).  The Mesozoic Era, for example, is composed of the Triassic, Jurassic and Cretaceous periods.


A period is a second-order unit of geologic time.  A period is subdivided into epochs.  For example the Jurassic Period is subdivided into the Early Jurassic, Mid-Jurassic and Late Jurassic epochs.



The third-order of geologic time, the epoch is subdivided into ages.  For example the Mid-Jurassic Epoch is subdivided into the following four ages: Aelenian, Bajocian, Bathonian and Callovian.


Ages are subdivisions of epochs and may themselves be subdivided into chrons.


A small unit of geologic time, usually based on fossil zonation.



One billion years is usually abbreviated as Ga or 'giga annum'.  Likewise a million years is abbreviated as Ma and one thousand years is Ka (kilo annum).



The Quaternary Period is a subdivision of geological time which covers the last 2.6 Ma up to the present day. The Quaternary, Neogene and Paleogene periods together form the Cenozoic Era. The Quaternary is subdivided into two epochs; the Pleistocene (up to about 11 700 years ago) and the Holocene (about 11 700 years ago to the present day). The Quaternary Period has been one of extraordinary changes in global environment as well as the period during which much of human evolution took place.


NEOGENE period

The Neogene encompasses two epochs, beginning with the Miocene (23.03-5.33 Mya) and followed by the Pliocene (5.33-1.806 Mya). T

The Neogene Period started with the replacement of vast areas of forest by grasslands and savannahs. New food sources and niches on the grasslands and savannahs fostered further evolution of mammals and birds. Whales diversified in the seas, and sharks reached their largest size during the Miocene. Complex patterns of mammalian evolution resulted from changing climates and continental separations.  More modern mammals evolved as grasslands became widespread and the climate cooled and dried. Additional information about the mammals of these epochs can be found in our Prehistoric Mammals of the Cenozoic exhibits. The Neogene saw a gradual closing of the Tethys Sea as the continents moved into their modern positions. The dramatic cooling phases of the Neogene lead to more distinctive latitudinal biotic zones.

Palaeogene Period

The Palaeogene (/ˈpæliːɵdʒiːn/ or /ˈpeɪliːɵdʒiːn/; also spelled Palæogene and Paleogene; informally Lower Tertiary) is a geologic period and system that began 66 and ended 23.03 million years ago and comprises the first part of the Cenozoic Era.  The Palaeogene is divided into three epochs: Palaeocene, Eocene and Oligocene.

The beginning of the Paleogene Period was very warm and moist compared to today’s climate. Much of the earth was tropical or sub-tropical. Palm trees grew as far north as Greenland! 


By the end of the Paleogene the climate began to cool.


The Paleocene is the first epoch of the Paleogene.  During this epoch isolated continent of India moved north and collided with Asia. The end of the epoch, is marked by an an abrupt rise in Global Mean Temperatures creating a wetter climate and a rise in sea level.

The early part of the second epoch of the Palaeogene, the Eocene, is thought to have had the highest Global Mean Tempertaures of the Cenozoic Era, with mean temperatures about 30° C; relatively low temperature gradients from pole to pole; and high precipitation in a world that was essentially ice-free.


It was an important time of plate boundary rearrangement, in which the patterns of spreading centres and transform faults were changed, causing significant effects on global oceanic and atmospheric circulation. 

The Oligocene Epoch, is the third epoch of the Palaeogene and lasted from about 33.9 to 23 million years ago. During this epoch we can observe in the fossil record the appearance of the first elephants with trunks, early horses, and the appearance of many grasses.



The Cretaceous (/krɨˈteɪʃəs/, krə-TAY-shəs), derived from the Latin "creta" (chalk), usually abbreviated K for its German translation Kreide (chalk), is a geologic period and system from circa 145 ± 4 to 66 million years (Ma) ago.

The start of the period is marked by palaeontological markers which suggest a marine extinction event.


The Cretaceous ended with the most famous mass extinction in history - the one that killed off most of the dinosaurs (not the birds).

There was no great, global extinction or massive radiation of species separated the Cretaceous from the Jurassic Period. Dinosaurs continued to inhabit the forests of ferns, cycads, and conifers. Ammonites, belemnites, other molluscs, and fish were hunted by marine reptiles, and pterosaurs and early birds took to the skies. However, it was in the Cretaceous Period that the evolution of many lifeforms that would go on to play key roles in the coming Cenozoic occured.


It was a relatively warm period with no ice caps at the poles. Southern England and the midwest of the USA was underwater as sea levels reached their highest levels in geologic history during this time.


The Atlantic Ocean grew much wider as North and South America drew apart from Europe and Africa. The Indian Ocean was formed at this time, and the island that was India began its journey north towards Asia.

Jurassic Period

The Jurassic (/dʒuːˈræsɪk/; from Jura Mountains of the Alps) is a geologic period and system that extends from 201.3± 0.6 Ma (million years ago) to 145± 4 Ma.

  • The start of the period is marked by the major end Triassic extinction event.


  • The end of the period marked more by a marine extinction event possibly due to the opening of the Tethys, creating new global ocean circulation.  Changes in salinity and the release of ocean hydrates may also have played a part.

The Jurassic was the most diverse period in Earth history.


The largest dinosaurs of the time — in fact, the largest land animals of all time — were the gigantic sauropods, such as the famous Diplodocus, Brachiosaurus and Apatosaurus. Other herbivorous dinosaurs of the Jurassic included the plated stegosaurs.


The Jurassic also saw the origination of the first birds, including the well-known Archaeopteryx, probably from coelurosaurian ancestors.


Ichthyosaurs were at their height, sharing the oceans with the plesiosaurs, giant marine crocodiles, and modern-looking sharks and rays. Also prominent in the seas were cephalopods such as ammonites.


triassic period

The Triassic /traɪˈæsɪk/ is a geologic period and system that extends from about 250 to 200 Mya (252.2 ± 0.5 to 201.3 ± 0.2 million years ago).


  • The Triassic began in the wake of the Permian–Triassic extinction event.


  • The end of the period was marked by yet another major mass extinction, the Triassic-Jurassic extinction event.

The Triassic marked the rise of the reptiles; notably the archosaurs (ruling lizards) and therapsids (mammal-like reptiles).


The archosaurs held the evolutionary edge, over the therapsids and evolving by the middle Triassic into the dinosaurs.

Image: *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey

Permian period

During the Permian Period, Britain lay on the supercontinent of Pangaea and experienced hot, dry conditions. Sandstones formed from desert sand dunes. Limestones and salts were deposited in a nearby inland sea (the Zechstein Sea). Many groups of animals became extinct at the end of the Permian.

Image: *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey


carboniferous period

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 358.9 ± 0.4 million years ago, to the beginning of the Permian Period, about 298.9 ± 0.15 Ma.

  • The Carboniferous began following the Late Devonian mass extinction event.

  • The formation of Pangea marked the end of the Carboniferous Period.

A new group of tetrapods, the diapsids, enters the scene. These are the ancestors of reptiles, and actually the diapsids divides into two groups:

  1. marine reptiles, snakes, and lizards and

  2. the archosuars, which includes crocodiles, dinosaurs and birds.


Amphibians also flourish during this period. The major ancient amphibian groups appear in the geological record.

Image:  *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey


The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 419.2 ± 3.2 Mya (million years ago), to the beginning of the Carboniferous Period, about 358.9 ± 0.4.

  • The Devonian Period started with a major marine regression.

  • A mass extinction event marked the end of the Devonian Period.

The Rhynie Chert in Scotland is a Devonian age deposit containing fossils of both zosterophylls and trimerophytes, some of the earliest vascular plants. This indicates that prior to the start of the Devonian, the first major radiations of plants had already happened. The oldest known vascular plants in the Northern Hemisphere are from the Devonian Period.

Image: *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey



The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.4 ± 1.5 million years ago (mya), to the beginning of the Devonian Period, about 419.2 ± 3.2 mya.

  • The Silurian Period started a mass extinction.

  • A marine transgression marked the end of the Silurian Period.

The Silurian is a time when many biologically significant events occurred. In the oceans, there was a widespread radiation of crinoids, a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs. As mentioned earlier, this time period also marks the wide and rapid spread of jawless fish, along with the important appearances of both the first known freshwater fish and the appearance of jawed fish. Other marine fossils commonly found throughout the Silurian record include trilobites, graptolites, conodonts, corals, stromatoporoids, and molluscs.

Image: Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey


In the Ordovician Period, Britain lay south of the equator and had a cool climate. Seas covered Britain and there was dramatic volcanic activity as the ocean separating England and Wales from Scotland started to close.

Image: *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey



In the Cambrian Period, England and Wales lay near the south pole and experienced a cold climate. They were separate from Scotland, which was joined to North America. A shallow sea covered much of the area and animals such as trilobites, graptolites and molluscs first appeared.

Image: *Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions. Credit: British Geological Survey


The Precambrian is the largest span of time in Earth's history (seven eighths) before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale. It spans from the formation of Earth about 4.6 Gya to the beginning of the Cambrian Period, about 541.0 ± 1.0 Mya.

Image: The 'Precambrian' is not used here, although it is often in use informally. Geochronologically the Archaean and Proterozoic are more useful.


*Based on 'The Geologic Time Scale 2012' by F M Gradstein, J G Ogg, M Schmitz & G Ogg (2012), with additions by the BGS.

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