HOME PAGE
MASTER INDEX

GEOLOGICAL TIME SCALE:
EONS, ERAS, PERIODS, EPOCHS
and TIME-ROCK UNITS

© 2005 T. W. Proctor, J.D.

To find a given geological age, go to the bottom of this page
and click on the geological age you want to visit.

These sections are under construction so you may receive a message on a given age site, that it is not yet completed in content and ready for viewing. Thank you for your patience as the construction of these geological age sites will take considerable time.


What is the Geological Time Scale and what does it consist of?
What are Eras, Periods Epochs and Aeons (Eons)?
What does Time-Rock Units mean?

[NOTE: Further down on this page, you will find a Geological Time Scale Card]



Scientists who have done extensive testing, over many years, using various means (many of which overlap and therefore double check each other) and have determined that the Earth is approximately 4,600,000,000 years old. For additional information on telling the age of things, see "Showing Your Age" (an award-winning article by your Curator). Since 4.6 billion years is a very long period of time, earth scientists have broken this 4.6 billion years down into units, called geological ages. These geological ages are determined by a number of factors, not just arbitrary numbers of years.

Geological time is divided into various units of time.

EONs: There are two (2) Eons. The Precambrian Eon, which commenced at the formation of the Earth and ended when fossils became abundant in rocks about 570,000,000 years ago (570 MYBP). The other eon is the Phanerozoic Eon, which commenced at the close of the Precambrian Eon and has lasted to the present.

ERAs: There are five (5) major categories called Eras. Starting at the formation of the Earth are the two earliest eras. They are the Archezoic (or Archean) and Proterozoic. These two are called the Precambrian Eras.

The other three eras are the Paleozoic, Mesozoic and Cenozoic, which are the Cambrian and the post-Cambrian eras. We presently live in the Cenozoic Era, which has been around for a very long time. These five eras take us from the formation of the Earth to the present.

Eras are usually separated from one another by intervals of mountain building called geologic revolutions and included great forces in and upon Earth which result in land elevation, erosion, the folding of land and faults (slippage in one section from another). These periods of activity (called diastrophic activity) are usually accompanied by vulcanism or by igneous intrusion. In short by magma and sub-crustal activity resulting in crustal changes.

During these diastrophic activities, the land rises at such time. Then the crust undergoes erosion by water, wind and other forces upon the surface of the earth, and sediments (i.e. broken down portions of the surface by such forces) are carried away by water, wind and other forces and redeposited elsewhere. A good example is the Mississippi River where sediments from a good portion of the United States is washed gradually down the river and out into the Gulf in which is called an alluvial fan (i.e. the sediment makes a fan shaped deposition into the Gulf). This forms new land areas and can also wipe out and carry away old land areas. These breaks or gaps in the deposition of the strata are called unconformities.

There is a great amount of stability of the Earth's crust and the flora (plant life) and fauna (animal life) types during Eras. However, there are new forms of plant and animal life which evolve from older species and others become extinct during an Era.

The eras are further divided into Periods.

PERIODs: The following are the periods within the Cambrian and the post-Cambrian eras.

Paleozoic:
     Cambrian
     Ordovician
      Silurian
      Devonian
      Mississippian
      Pennsylvanian
      Permian
It should be noted that the Mississippian and Pennsylvanian are jointly called the the Carboniferous Period (see additional information below)

Mesozoic:
     Triassic
     Jurassic
     Cretaceous

Cenozoic:
     Tertiary
     Quaternary

These Periods are time frames or systems of rock formations, in which minor or localized earth movement occur. In short, they are less dramatic than the Eras.

Each Period is characterized by certain fossils which come into being during that Period, which are distinctive and are distributed widely in many places on Earth or assembled worldwide. This makes possible a comparison or correlation of these identical or very similar fossils, from place to place, around the Earth. Hence scientist may determine the age of certain geological formations because of the similarities or virtual identity of these fossils. See the description of Index Fossils, which are also called by some as Marker Fossils and/or Guide Fossils.

EPOCHs:

Each of the above Periods is divided into categories called Epochs. During these epochs, deposition of individual rock formations or groups of formations, including any fossil remains, takes place on the land surface or on the floor of seas, lakes and other bodies of water.

The Periods of the Paleozoic and Mesozoic are divided into epochs simply called "Early", "Middle" and "Late", or simply into "Early" and "Late". The Cenozoic Periods are divided into epochs with distinctive names:

Tertiary:
     Paleocene
     Eocene
     Oligocene
     Miocene
     Pliocene

Quaternary:
     Pleistocene
     Holocene (or recent)


Ages & Stages: Epochs may be broken down further into ages and stages, but we will not herein go into these smaller units at this time.


Geological periods of time are blocks of historical time to each of which block, a name is assigned by earth scientists. These periods of time are not just cut into even pieces of time, but are arranged by intervals of dramatic changes upon Earth. This groupings by name has a purpose and each covers a large period of historical time of the Earth. The units of time usually run for millions of years each. In each historical time grouping, there are some similarities and/or a period of time between great changes on Earth. Hence the start of a historical time unit starts with some change and ends with another change in the condition of the Earth. These may be mass extinctions; changes in the geological make-up of the Earth (such as massive land movements, i.e. tectonic plate movements and/or rising of mountains); changes in the temperature and/or water-earth relationship; and/or other factors which dramatically change the environment of the Earth).

Units of time, set out in the Geological Time Scale are shown with a "margin of error", which varies with different units of historical time, depending upon earth scientists ability to obtain accurate readings of the antiquity of things in that Geological age unit.



You may want to read your Curator's award-winning article "Showing Your Age", which deals with how scientists determine the age of various formations and materials. (just click on the title above to go to this article).

Various events, as stated above, create these divisions in geological time.

For instance at the dividing line between the Cretaceous Period and Tertiary Period (called the K-T boundary), which was about 66,400,000 million years ago (66.4 MYA) there was a worldwide incident which led to the extinction of a large share of the species on Earth, land and sea. There are various theories, but the most widely accepted by virtually by scientist around the World, is that at least one of the major causations for that extinction was a huge meteorite which fell to Earth, landing near the Yucatan, Peninsula of Mexico. The theory is that this caused a violent explosion, tremendous heat in an extensive area, followed by a worldwide dust cloud, which circled the Earth for some time. This lead to a Nuclear winter and the devastating results from that dust cloud and other effects of the collision of the meteorite with Earth. The dust cloud would have killed vegetation upon which plant eating animals lived, including plant eating dinosaurs. This in turn resulted in the death of meat eating animals, also including dinosaurs, which had no plant eating animals to live upon. Some things survived, but a large percentage of the species on Earth at that time, because extinct.

To support this theory, there is an element called iridium which is fairly rare on Earth, but which is found in some meteorites. All over the Earth, at around 65 MYBP, there is an iridium layer present. The following is a Hadrosaurus femur (leg bone) found and dug out by PMNS President Terry Brawner and PMNS Curator Terry Proctor in Montana, in August, 2005. Just above this femur is what is believed to be the iridium layer of the K-T boundary. The finding of this Hadrosaurus femur just below what we believe is the K-T boundary layer, certainly adds authenticity to this widely accepted theory, if that is what this dark layer all over the search area is determined to be. See following picture.

Hadrosaurus (duck-billed dinosaur) femur is the rust-colored fragmented bone to the left. Just above it are several darker horizontal lines, which we believe is the iridium layer of the K-T boundary (which many paleontologist believe is what caused the mass extinction of dinosaurs about 66 MYBP).

Bone was extracted from private property in the Hell Creek formation of Eastern Montana in August, 2005 by Terry Brawner and Terry Proctor of the Proctor Museum of Natural Science

Throughout the Earth, there are various scales of Geological Time used by various scientists. For instance in England the Geological Time Scale most often used shows one period called the Carboniferous Era (from 360 to 286 MYA). In the United States our most commonly used Geological Time Scale chart shows that same period broken into two Periods: 1. the Mississippian Period (360 to 320 MYA) and 2. the Pennsylvanian Period (320 to 286 MYA).

Therefore, you may see the same time periods called different things on different charts. However, most charts are fairly similar. In the very ancient periods of the Earth, the Geological Time Scales are fairly recent. Some charts are very detailed, but most show only the broader time periods, which is what we will do here.

The oldest Geological Time period is the Eon called the "Precambrian". This consists of three Eras known as: 1. Hadean Era, which is the time frame dating to the formation of the Earth, Moon and other Solar planets. It starts at 4,600,000,000 (4.6 BYA) to 3.8 BYA; 2. Archaean Era 3.8 BYA to 2.4 BYA; and 3. Proterozoic Era 2.4 BYA to 570 MYA.

Next is the Phanerozoic Eon, which is everything since the Precambrian Eon. It includes three era, known as: 1. Paleozoic Era 570 MYA to 248.2 MYA; 2. Mesozoic Era 248.2 MYA to 65 MYA; and 3. Cenozoic Era 65 MYA to the present.

These Eras are then broken down into Periods, which are sometimes further broken down into Sub-Periods. Periods are broken down into Epochs and finally into Ages. The most commonly referred to Geological Time Scales are probably the Periods and Epochs.

What is the importance of these breakdowns of time periods? They reference certain things which were going on at the time, on our Earth. The starting and ending of certain life forms and conditions. For instance the dinosaurs lived during part of the Triassic, the Jurassic and the Cretaceous Periods, then they died out at the K-T Boundary about 66.4 MYA. At the time that the meteorite hit in the Yucatan Peninsula in Mexico, the element Iridium was found in a layer, at that Geological Age, worldwide. Or to put it another way, there was found worldwide to be a layer of the element Iridium. Since this tied in with other factors, similar worldwide, i.e. certain index fossils, it verified that SOMETHING happened worldwide at that time, which causes or contributed to the extinctions at that time.

Index Fossils are fossils which appear in only certain layers of Geological Time periods and are found throughout the world or at least in many places. Hence, when you find these index fossils, it tells you what Geological Time period you are in. (as with all science, all scientific findings and presumptions are constantly challenged, which is good, because it leads to ever more dependable results and conclusions.

One of the friends of the Proctor Museum of Natural Science and the Houston Gem & Mineral Society is the Geochron Laboratories a division of Krueger Enterprises, Inc. Geochron Laboratories for a number of years has provided me, your PMNS Curator, with a plastic pocket card entitled "Geologic Time Scale" which I distribute to both PMNS andHGMS-Paleo members each year. We appreciate Geochron Laboratories and Krueger Enterprises, Inc. for their generosity in making these card free. Below is the front and back of the card which they provide and the contact information on how to get in touch with them. These folks did testing on the age of materials, but no longer do so. They were generous to turn over to the curator of the PMNS all remaining cards for distribution. We have those available as long as they last. Please no longer contact this firm as they no longer do this work. Our appreciation of them continues however.

If you are interested in learning a small amount of information on any given geological age, simply click on that Geological Age in the following Table. This site is still under construction, so for now, all or many of these pages at this time will not bring up the page to which they eventually will or many simply show the name of that age, but not yet the content of the page. We will be working on filling these in as quickly as we can do so. Thanks for your patience.

Read from left (most recent) down, then to the top of the
middle column down, then to the top of the right column down.

Holocene Cretaceous Silurian
Pleistocene Jurassic Ordovician
Pliocene Triassic Cambrian
Miocene Permian Proterozoic
Oligocene Pennsylvanian Archaean
Eocene Mississippian Hadean
Paleocene Devonian Pre-Solar System

Color Codes on above Geological Time periods, showing what
colors indicate other than is typed in the boxes above:

Cenozoic Era-Quaternary Period Paleozoic Era-Permian Period Precambrian Eon
Cenozoic Era-Tertiary Period Paleozoic Era-Carboniferous Period Universe Pre-solar System
Mesozoic Era Paleozoic Era-Periods as shown


TIME-ROCK UNITS

The rocks deposited and/or created during a Period are known as a System. Hence a Period is a historical time unit, whereas a System is a time-rock unit. This means it is a unit of rock deposited during a certain period of geological time

One may refer to the Pennsylvanian Period (of time) and the Pennsylvanian System (i.e. the the rocks that were deposited during that Period of time). The names of most Periods and Systems have been derived from the names of the areas where the rocks were first studied and described. It should be noted however, that some of these names vary in different parts of the World.

For instance, the Carboniferous Period occurred from about 354 to 290 million years ago during the late Paleozoic Era. The term "Carboniferous" comes from England, in reference to the rich deposits of coal that occur there. These deposits of coal occur throughout northern Europe, Asia, and midwestern and eastern North America. The term "Carboniferous" is used throughout the world to describe this period, although this period has been separated into the Mississippian (Lower Carboniferous) and the Pennsylvanian (Upper Carboniferous) in the United States. This system was adopted to distinguish the coal-bearing layers of the Pennsylvanian from the mostly limestone Mississippian, and is a result of differing stratigraphy on the different continents.

Just as geological time units are broken down into smaller units, so are Time-Rock Units.

Formations: The basic rock unit is the Formation. A formation may be defined as a recognizable unit of similar rocks useful for mapping. The names given to formations are commonly derived from the locale where such formation is first observed and recorded. This may also include the name of the predominant rock which is in that formation. In our area for instance, if you collect on McFadden Beach (from High Island on the West to Sabine Pass on the East--which is just south of Beaumont, Texas) of Galveston Bay, you learn that fossils are found in the Beaumont Clay. You also learn to recognize this muddy layer and that it is there that you may find Pleistocene vertebrate fossils. In Eastern Montana you find the Pierre Shale and know that here you can find Cretaceous invertebrate fossil.

Members: are smaller units of formations. These may also be given geographical related names and/or rock type names.

Lentils: these are smaller lens-shaped rock bodies within the formation.

Tongues: these are interfingering or intertonguing bodies of different lithology (rock types).

Beds: there are individual rock layers.

Group: This is not a breakdown, but rather the largest rock unit recognized. If several formations have certain definite characteristics in common, they may be referred to as a group.

Location: This is not necessarily a scientific term, but may be used in common language to describe a particular place where one can dig certain types of rocks, minerals and/or fossils. An example is when a trip is planned to the "Whiskey Bridge" location. There one can find one of the most fossiliferous places in Texas. The fossils there, for the most part, are Middle Eocene fossils of the Stone City Formation, Claiborne Group.