Geologic Timescale

Initial Publication Date: December 2, 2005
A faunal rogues gallery of the Phanerozoic Eon

Topics can be covered in chronological order, from the formation of the Solar System to the current Industrial Period. Most courses only focus on part of Earth's history.

This structure has the advantage of simplicity, and helps students to connect sequential events through cause and effect.

Suggestions for Teaching

  • Millions (let alone billions) of years is a little hard for many students to imagine, so the magnitude of time itself may require a separate discussion.
  • Instructors employing this paradigm may need some time to introduce principles like evolutionary biology, plate tectonics, or biogeochemical cycling before discussing them in the context of Earth history. They can either:
    • take a week to teach these before launching into the sequence of events that formed the world of today, or
    • work them in at various points between mass extinctions.
  • Likewise, the types of evidence and the rules used for making deductions will need to be laid out. This can either be done separately, as described above, or done in context, using details of a particular event or time period as an example.



Not all time units will receive equal coverage. Some may not be covered at all.

  • Paleobiology and paleontology courses are likely to stress the last 540-odd million years (since which we have a substantial fossil record for that time period).
  • An Age of the Dinosaurs course focuses on the Mesozoic and doesn't bother with most of the Triassic. Within a course, it is possible to manage topics chronologically.
  • The Precambrian: we've recently learned amazing new things about the first four billion years of Earth's history.


Below are web resources that will prove very useful to anyone teaching the geologic timescale, even if the course is not organized around it.

  • GSA Geologic Time Scale. This Geological Society of America (GSA) site contains a detailed geologic time scale as an educational resource. It may be downloaded to a larger size, and includes all Eras, Eons, Periods, Epochs and ages as well as magnetic polarity information. (more info)
  • The Geologic Time Scale in Historical Perspective. This brief discussion of the development of the Geologic Time Scale begins with Nicolas Steno in 1669 whose ideas have become known as the principles of original horizontal deposition and superposition. Next are James Hutton in 1795 and Charles Lyell in the early 1800s who supported the principle of uniformitarianism. The work of William Smith and the principle of faunal succession is also noted. The site goes on to explain how and why the scale is divided as it is. ( This site may be offline. )
  • University of California's Museum of Paleontology: Geologic Timeline. University of California's Museum of Paleontology has created a hyperlinked Geologic Timeline with all sorts of details about each time unit that may be useful later in the course. Each hyperlink contains a variety of information including stratigraphy, ancient life, localities and tectonics associated with that specific time period. Users can also link to an Introduction to Geology page and a description of the Museum's geology wing. (more info)
  • What is Geologic Time?. This webpage of the National Park Service (NPS) and United States Geological Survey (USGS) discusses geologic time and what it represents. Beginning about 4.6 billion years ago and ending in the present day, this site exhibits (to scale) the various eras, periods, eons, and epochs of Earth's history with a downloadable geologic time scale available. Links provide maps of what the Earth looked like at various times in its history, as well as a description of how scientists developed the time scale and how they know the age of the Earth. (more info)