| Geologic Time | Paleozoic Era | Mesozoic Era | Cenozoic Era |

The Geologic Time Scale

The study of the Earth from the beginning of time to the present has been the task of geologists who attempt to unravel the events that have shaped our planet as it is today. The Earth carries the history of geological events in its rock layers. It follows that the oldest layers are at the bottom and the youngest are at the top. With this understanding geologists are able to determine the ages of rock relative to one another. By assembling all these layers together, scientists have worked out what is known as the stratigraphic column or record of the various ages of rock. This record spans the 4.6 billion year record of Earth's history.

In order to simplify the huge amount of geological information, geologists have broken Earth's history down into sections which are called geological eras, periods, and epochs. Fossils records have shown that life existed for about 3,800 million years, but complex life emerged only about 600 million years. Over time, life forms change. Their fossil record allows geologists to date and compare rocks across geological time. For example, dinosaur fossils are only found during the Mesozoic era some 245 to 65 million years ago.

The following table shows the geologic time scale.

Phanerozoic Eon (544 million years ago - Present) The period of time, also known as an eon, between the end of the Precambrian and today, The Phanerozoic begins with the start of the Cambrian period, 544 million years ago. It encompasses the period of abundant, complex life on the Earth.
Era	Period or System	Epoch or Series
Cenozoic (65 million years ago - Present) "Age of Recent Life" An era of geologic time from the beginning of the Tertiary period to the present. Its name is from Greek and means "new life."	Quaternary (1.8 million years ago - Present) The second period of the Cenozoic era. It contains two epochs: the Pleistocene and the Holocene. It is named after the Latin word "quatern" (four at a time). The several geologic eras were originally named Primary, Secondary, Tertiary, and Quaternary. The first two names are no longer used. Tertiary and Quaternary have been retained but used as period designations.	Holocene (8,000 years ago - Present) An epoch of the Quaternary period. It is named after the Greek words "holos" (entire) and "ceno" (new).
		Pleistocene (1.8 million - 8,000 years ago) "The Great Ice Age" An epoch of the Quaternary period. It is named after the Greek words "pleistos" (most) and "ceno" (new).
	Tertiary (65 - 1.8 million years ago) The first period of the Cenozoic era (after the Mesozoic era and before the Quaternary period).	Pliocene (5.3 - 1.8 million years ago) Final epoch of the Tertiary period. It is named after the Greek words "pleion" (more) and "ceno" (new).
		Miocene (23.8 - 5.3 million years ago) A epoch of the upper Tertiary period. It is named after the Greek words "meion" (less) and "ceno" (new).
		Oligocene (33.7 - 23.8 million years ago) An epoch of the early Tertiary period. It is named after the Greek words "oligos" (little, few) and "ceno" (new).
		Eocene (55.5 - 33.7 million years ago) An epoch of the lower Tertiary period. Its name is from the Greek words "eos" (dawn) and "ceno" (new).
		Paleocene (65 - 55.5 million years ago) Earliest epoch of the Tertiary period. It is named after the Greek words "palaois" (old) and "ceno" (new).

Era	Period or System	Epoch or Series
Mesozoic (248 - 65 million years ago) "Age of Medieval Life" An era of geologic time between the Paleozoic and the Cenozoic. The word Mesozoic is from Greek and means "middle life."	Cretaceous (145 - 65 million years ago) "The Age of Dinosaurs" The final period of the Mesozoic era. The name is derived from the Latin word for chalk ("creta") and was first applied to extensive deposits of this age that form white cliffs along the English Channel between Great Britain and France.	Late or Upper
		Early or Lower
	Jurassic (213 - 145 million years ago) The middle period of the Mesozoic era. It is named after the Jura Mountains between France and Switzerland, where rocks of this age were first studied.	Late or Upper
		Middle
		Early or Lower
	Triassic (248 - 213 million years ago) The earliest period of the Mesozoic era. The name Triassic refers to the threefold division of rocks of this age in Germany. The Break-up of the continent Pangea ... MORE	Late or Upper
		Middle
		Early or Lower

Era	Period or System		Epoch or Series
Paleozoic (544 - 248 million years ago) "Age of Ancient Life" An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic. The word Paleozoic is from Greek and means "old life."	Permian (286 - 248 million years ago) The final period of the Paleozoic era. It is named after the province of Perm, Russia, where rocks of this age were first studied.		Late or Upper
			Early or Lower
	Carboniferous (360 - 286 million years ago) A period of time in the Paleozoic era that includes the Pennsylvanian and Mississippian periods.	Pennsylvanian* (325 - 286 million years ago) "The Coal Age" A period of the Paleozoic era. It is named after the state of Pennsylvania where rocks of this age are widespread.	Late or Upper
			Middle
			Early or Lower
		Mississippian* (360 - 325 million years ago) A period of the Paleozoic era. It is named after the Mississippi River valley, which contains good exposures of rocks of this age.	Late or Upper
			Early or Lower
	Devonian (410 - 360 million years ago) A period of the Paleozoic era. It is named after Devonshire, England, where rocks of this age were first studied.		Late or Upper
			Middle
			Early or Lower
	Silurian (440 - 410 million years ago) A period of the Paleozoic. It is named after a Celtic tribe called the Silures.		Late or Upper
			Middle
			Early or Lower
	Ordovician (505 - 440 million years ago) The second earliest period of the Paleozoic era. It is named after a Celtic tribe called the Ordovices.		Late or Upper
			Middle
			Early or Lower
	Cambrian (544 - 505 million years ago) The earliest period of the Paleozoic era. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied.		Late or Upper
			Middle
			Early or Lower

Precambrian (Beginning of earth - 544 million years ago) All geologic time before the beginning of the Paleozoic era. This includes about 90% of all geologic time and spans the time from the beginning of the earth, about 4.5 billion years ago, to 544 million years ago. Its name means "before Cambrian."