Article from the Backbenders Gazette of May, 2006
(now in competition in Adult Advanced Articles 2006 competition
South Central Federation of Mineral Societies & American Federation of Mineralogical Societies)

The following is an article by the PMNS Curator


© 2006 Terrell William "Terry" Proctor, J.D., Curator, PMNS

Member Houston Gem & Mineral Society
Curator Proctor Museum of Natural Science

(There are photos in connection with this article, which appear at the bottom of the article)

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

Earth scientists have done extensive study and 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. See "Showing Your Age" at https://proctormuseum.us/Articles/articleshowage.htmfor information on how to tell the age of formations, an award winning article by this author. 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 (or Aeons): 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, i.e. million years before present). The other eon is the Phanerozoic Eon, which commenced at the close of the Precambrian Eon and has existed to the present.

ERAs: There are five major geological categories called Eras in some tables and six in others. Starting at the time of the formation of the Earth are the two or three three earliest eras, called the Precambrian Eras, as follows:

  1. Hadean Era, which is the time frame dating from the formation of the Earth, Moon and other Solar planets, starting at 4,600,000,000 (4.6 BYBP) to 3.8 BYBP;
  2. Archezoic Era (or Archean) from 3.8 BYBP to 2.4 BYBP
  3. Proterozoic Era, from 2.4 BYBP to 570 MYBP (up to the Cambrian Era)

The other three eras are the Paleozoic, Mesozoic and Cenozoic, which are the Cambrian and post-Cambrian eras. We presently live in the Cenozoic Era, which has existed for a very long period of time. These five or six eras also 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. Magma and sub-crustal activity result in crustal changes. During these diastrophic activities, the land rises.

Then the crust undergoes erosion by water, wind and other forces upon the surface of the earth, and sediments (i.e. broken up portions of the crust 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 are washed gradually down the river and out into the Gulf of Mexico, in what 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. During Eras, there is a great amount of stability of the Earth's crust and of the flora (plant life) and fauna (animal life) types. However, there are new forms of plant and animal life which evolve from older species, while other species of each, become extinct during an Era. The Eras are further divided into Periods.

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

These Periods are time frames or systems of rock formations, in which minor or localized earth movement occur. In short, overall, 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. These are called index fossils, marker fossils and/or guide fossils.

See the description of index fossils at https://proctormuseum.us/Fossils/Index-fossils/indexfossils.htm.

EPOCHs: Each of the above Periods is divided into smaller geological 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 into "Early" and "Late". The Cenozoic Periods are divided into epochs with distinctive names:

Ages & Stages: Epochs may be broken down further into ages and stages, but we will not 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 changes 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 various earth scientists' ability to obtain accurate readings of the antiquity of things in that Geological age unit.

Various events, as stated herein, 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 years before the present (66.4 MYBP) there was a worldwide incident which led to the extinction of a large share of the species on Earth, land and sea. 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 MYBP.

There are various theories of what caused this. Virtually all scientist around the World, accept at least in part, the theory that 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 then had no plant eating animals to eat and live upon. Some things survived, but a large percentage of the species on Earth at that time, because extinct.

Support for this theory comes from an analysis of 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. Pictured with this article is a Triceratops metatarsal (foot bone) and a femur (leg bone) found by Proctor Museum of Natural Science (PMNS) President Terry Brawner and PMNS Curator Terry Proctor in Montana, in August, 2005.

Around the dig area where these dinosaur bones were found in the Hell Creek formation, in Eastern Montana, is signs of a dark layer in the otherwise light colored matrix. It is unclear at this writing whether this layer has anything to do with the K-T boundary or not.

It has taken several hundred years and untold hours of work and sweat by scientists around the World, to develop these geological age designations and determine the basis for each and what happened during that time.

The names of geological ages varies somewhat in different countries and regions on the Earth. For instance in England the Geological Time Scale from 360 to 286 MYBP is most often called the Carboniferous Era. In the United States that same period is broken into two Periods:
1. the Mississippian Period (360 to 320 MYBP) and
2. the Pennsylvanian Period (320 to 286 MYBP).
Other geological scales show the Carboniferous from 354 to 290 MYBP.

Therefore, you may see the same time periods called different things on different charts. Most charts are fairly similar throughout the Earth. The naming of the most ancient geological periods were done most recently. Also charts vary in the amount of detail, some showing broad groupings and others much more detailed.

Not included with the article in the BBG (left out because of limited length that the article could be for the BBG) was the following portion:

What does Time-Rock Units mean?


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 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.

As stated above, some of the geological names vary in different parts of the World. The Carboniferous Period occurred from about 354/360 MYBP to 286/290 MYBP. 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, as previously stated.

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 of Galveston Bay (from High Island on the West to Sabine Pass on the East--which is just south of Beaumont, Texas), you learn that fossils are found in the Beaumont Clay formation. You also learn to recognize this muddy layer and that it is in that formation that you may find Pleistocene vertebrate fossils and some artifacts. In Eastern Montana you find the Pierre Shale and know that here you can find Cretaceous invertebrate fossils in that formation.

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.

Photos which were included with this article in the BBG, which are now shown below:

Earth crust folded by forces of nature on the rocks. An example of what tectonic plate movement and other crustal forces can cause to hard rock #1
photo by Terry Proctor
Triceratops Right foot, fourth metatarsal (foot bone) #2
eArt Scan by Terry Proctor
Hadrosaurus femur (leg bone) in situ (i.e. in the matrix) from which it was extricated and will then have to be prepped (i.e. the pieces restored to one bone, as in life) #3
photo by Terry Proctor
Curator Terry Proctor with cap rock which protected the matrix pedestal below it from erosion #4
photo by Terry Brawner, Pres. PMNS-06
Dinosaur fossil terrain in the
Hell River formation of Eastern Montana #5
photo by Terry Brawner
Cap rock on top of pedestal, protecting the
pedestal from the extensive erosion otherwise #6
photo by Terry Proctor

Contact: Terrell William "Terry" Proctor, J.D. c/o T. W. Proctor & Associates
630 Uvalde Road, Houston, Texas 77015-3766
Phone: 713) 453-8338 FAX (713) 453-3232 Email: auraman@swbell.net
Other Websites: https://terryco.us and http://www.terrylaw.us.