Table of Geological Periods
It is generally assumed that planets are formed by the accretion of gas and dust in a cosmic cloud, but there is no way of estimating the length of this process. Our Earth acquired its present size, more or less, between 4 billion and 5 billion years ago. Life on Earth originated about 2 billion years ago, but there are no good fossil remains from periods earlier than the Cambrian, which began about 550 million years ago. The largely unknown past before the Cambrian Period is referred to as the Pre-Cambrian and is subdivided into the Lower (or older) and Upper (or younger) Pre-Cambrian—also called the Archaeozoic and Proterozoic Eras.
The known geological history of Earth since the beginning of the Cambrian Period is subdivided into three eras, each of which includes a number of periods. They, in turn, are subdivided into subperiods. In a subperiod, a certain section may be especially well known because of rich fossil finds. Such a section is called a formation, and it is usually identified by a place name.
Paleozoic Era
This era began 570 million years ago and lasted for 325 million years. The name was compounded from Greek palaios (old) and zoön (animal).
Period | Duration1 | Subperiods | Events |
Cambrian (from Cambria, Latin name for Wales) | 60 | Lower Cambrian
Middle Cambrian
Upper Cambrian | Invertebrate sea life of many types, proliferating during this and the following period |
Ordovician (from Latin Ordovices, people of early Britain) | 70 | Lower Ordovician
Upper Ordovician | First known fishes |
Silurian (from Latin Silures, people of early Wales) | 30 | Lower Silurian
Upper Silurian | Gigantic sea scorpions |
Devonian (from Devonshire in England) | 50 | Lower Devonian
Upper Devonian | Proliferation of fishes and other forms of sea life; land still largely lifeless |
Carboniferous (from Latin carbo = coal + fero = to bear) | 70 | Lower or Mississippian
Upper or Pennsylvanian | Period of maximum coal formation in swampy forests; early insects and first known amphibians |
Permian (from district of Perm in Russia) | 45 | Lower Permian
Upper Permian | Early reptiles and mammals; earliest form of turtles |
1. In millions of years.Mesozoic Era
This era began 245 million years ago and lasted for 180 million years. The name was compounded from Greek mesos (middle) and zoön (animal). Popular name: Age of Reptiles.
Period | Duration1 | Subperiods | Events |
Triassic (from trias = triad) | 37 | Lower or Buntsandstein (from German bunt = colorful + sandstein = sandstone). Middle or Muschelkalk (from German muschel = shell + kalk = limestone). Upper or Keuper (old miner's term) | Early saurians (reptiles that resemble lizards) |
Jurassic (from Jura Mountains) | 62 | Lower or Black Jurassic, or Lias (from French liais = hard stone)
Middle or Brown Jurassic, or Dogger (old provincial English for ironstone)
Upper or White Jurassic, or Malm (Middle English for sand) | Many seagoing reptiles; early large dinosaurs; somewhat later, flying reptiles (pterosaurs), earliest known birds |
Cretaceous (from Latin creta = chalk) | 81 | Lower Cretaceous
Upper Cretaceous | Maximum development of dinosaurs; birds proliferating; opossumlike mammals |
1. In millions of years.Cenozoic Era
This era began 65 million years ago and includes the geological present. The name was compounded from Greek kainos (new) and zoön (animal). Popular name: Age of Mammals.
Period | Duration1 | Subperiods | Events |
Tertiary (originally thought to be the third of only three periods) | c. 65 | Paleocene (from Greek palaios = old + kainos = new). Eocene (from Greek eos = dawn + kainos = new). Oligocene (from Greek oligos = few + kainos = new). Miocene (from Greek meios = less + kainos = new). Pliocene (from Greek pleios = more + kainos = new) | First mammals other than marsupials. Formation of amber, rich insect fauna, early bats, steady increase of large mammals. Mammals closely resembling present types; protohumans |
Pleistocene (from Greek pleistos = most + kainos = new) (popular name: Ice Age) | 2.0 | Four major glaciations, named Günz, Mindel, Riss, and Würm, originally the names of rivers. Last glaciation ended 10,000 to 15,000 years ago | Various forms of early humans |
Holocene (from Greek holos = entire + kainos = new) | 0.01 | The last 10,000 years to the present | Earliest written documents c. 3200 B.C., Sumer |
1. In millions of years.
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