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Geological Eons, Eras and Periods

Geological time has been divided up into many named divisions, getting progressively more narrowly defined.
'Eons' are divided into 'eras', which are divided into 'periods', which are further divided into 'epochs'. These divisions are global, while on a yet-smaller scale (usually regional) stages may be divided into 'ages' and 'chrons'. These latter divisions are not considered hereafter.

The following discusses geologic time units rather then chronostratigraphic ones, the difference between which is described in the geological time pages.
(The equivalent series are as follows:

geologic time:eon --> era --> period --> epoch --> age --> chron
chronostratigraphic:eonothem --> erathem --> system --> series --> stage --> chronozone

The following discussion covers:

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Eons

The whole of Earth's history was initially divided into 2 eons:

  • The Phanerozoic, from 543Ma ago to the present day.
    The term means "obvious (or visible) life", and starts from about when the first shelled macro-fossils made an appearance - although the date is now fixed, despite more recent discoveries of earlier life.
  • The Cryptozoic, from when the Earth formed about 4.6 billion years ago, to the start of the Phanerozoic, around 543Ma ago.
    The term means "hidden life", during which time (when it was named, at least) that there was no evidence of life.
    The Cryptozoic has also variously been termed the PreCambrian, Azoic ("time of no life"), Archaean or Archaeozoic ("time of ancient life").
    Since this first devision (in 1930), 3 eons have been proposed for the Precambrian, being the
    • Priscoan (before 4000Ma ago)
    • Archaean for 4000-2500 Ma ago
    • Proterozoic for 2500-590 Ma ago

    However; "Precambrian" is still used informally and seems favoured.

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Eras

The Phanerozoic is then divided into 3 eras, which are the "first-order units of geological time".

The 3 eras are as follows:

The Palaeozoic

The Palaeozoic ('old life') is the era from 543Ma to 248 Ma ago.
It lasted from the start of the Phanerozoic for over 320 million years. It started with visible life and was characterised by many invertebrate marine animals, such as the now-extinct trilobites, graptolites, brachiopods and cephalopods, and also corals.
By the end of the Palaeozoic era, reptiles and amphibians had evolved, and extensive forests were populated by giant tree-ferns, cycads and horsetails.
The Palaeozoic era contains 6 periods:

  • Cambrian, Ordovician, Silurian (constituting the Lower Palaeozoic sub-era) and the
  • Devonian, Carboniferous, Permian (constituting the Upper Palaeozoic sub-era)

The Mesozoic

The Mesozoic ('middle life') lasted from 248Ma to 65 Ma ago.
The Mesozoic era contains 3 periods:

  • Triassic, Jurassic and Cretaceous periods.

The Cenozoic

The Cenozoic ('recent life') lasted from 65Ma to the present day.
The Cenozoic era contains 2 periods (sometimes referred to as sub-eras), the Tertiary and Quaternary.

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Periods

Each era is sub-divided into a number of periods, which are the "second-order units of geological time".

The 11 periods, and the PreCambrian, are listed below.
The table also gives a rough indication of the UK latitude during the period, to show how the area that is now the UK has moved up from the Southern Hemisphere over time.
The derivations of the period names are also given. These period names are now globally accepted, and reflect the importance of the British and European pioneering geologists.

Period Million years ago UK Latitude name derivation
Pre-Cambrian 4,600 - 543 ?? Before the Cambrian
Cambrian 543 - 505 60 S "Cambria" was the Roman's name for Wales
Ordovician 505 - 438 70 S The "Ordovicas" were an ancient Welsh border tribe
Silurian 438 - 408 30 S The "Silures" were an ancient British tribe
Devonian 408 - 360 20 S Marine rocks of this age found in Devon
Carboniferous 360 - 286 Equatorial The (carbon!) coal measures of UK and Europe formed then
Permian 286 - 248 10-20 N Outcrops in the area of Perm in the (old) USSR
Triassic 248 - 208 10-20 N Named for the 3-fold divisions in Germany
Jurassic 208 - 144 30 - 40 N Named for the Jura mountains
Cretaceous 144 - 65 51 - 58 N From the Latin "Creta" for CHALK
Tertiary 65 - 2 40 - 50 N left over from 1759
Quaternary 2Ma - Present Day 51 N left over from 1759

Remember the sequence:
Pregnant Camels Ought Sit Down Carefully. Perhaps Their Joints Creak?

Note that the Americans do not include the Carboniferous, but divide that time period into:

Pennsylvannian 323-290 Ma named for the state of Pennsylvannia where these rocks have produced much coal
Mississippian 354-323 Ma named for the Mississippi River valley where rocks of this time are well exposed.

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Each period is further sub-divided into epochs, the "third-order units of geological time".
Only the younger divisions of the Cretaceous Periods and Cenozoic Epochs are discussed in further detail, as this is the age of the Portsdown Hill geology. To read more about these, please go on to Cretaceous Stages.






For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk