|Education Resources Center|
The Structure of Geologic Time
The Geologic Flow of Time
The Geologic Time Chart describes and maps the flow of Time, and organizes it into meaningful, manageable units. The units of Time represent layers of sedimentary rocks and fossils that have been identified and described by geologists over the past two hundred years.
The Geologic Time Chart is read from the bottom to the top in accordance with the Law of Superposition. Also, the chart reads from left to right, with the largest units of Time, the Eons, shown first, and smaller units of Time extending out from the left to the right. To see the entire Geologic Time Chart for a reference, click on the Geologic Chart button on the left.
The Eons are the largest units of Time, the next largest units are the Eras, then the Periods, and then the Epochs. Each represents a distinct layer of sedimentary rocks and fossils or a related group of rock layers. There are incrementally smaller units of Time in highly technical Geologic Time Charts, such as Stages. However, the typical chart includes just the Eons, Eras, Periods, and Epochs.