Stratigraphic Time Chart
Structure of Stratigraphic Time
Nicholas Steno first developed the underlying principles of the stratigraphic time chart in the 17 th century. These principles argue that rock layers (or strata) are laid down from oldest to youngest and represent different slices of time. Since then, geologists have used these principles to compile a stratigraphic time chart that divides Earth’s history into units of time. The largest defined unit of time is the supereon, which is composed of eons. Eons are divided into eras, which are then divided into periods, epochs, and ages.
The Frasnian Age lasted from 385.3 +/- 2.6 million years ago to 374.5 +/- 2.6 million years ago during the Late Devonian period. During this time, major reef-building organisms dominanted, especially in western Canada and Australia. The first forests in Earth’s history were also beginning to form. The Alamo Impact Event in southeastern Nevada also took place during the Frasnian, about 382 million years ago.