| Education Resources Center
Geologic Time Chart History
In the late 1790's, an English canal builder, William Smith (1769 - 1839), discovered the most important tool for relative age dating. Smith noted that fossils did not occur in a haphazard pattern in the rocks, but rather in a definite and determinable, systematic sequence. Hence the age of a layer of rock could be determined by its characteristic faunal content.
The Geologic Time Chart was not developed in the sequential order as it is known today with the oldest interval units of time appearing at the bottom and the most recent interval units appearing at the top of the chart. Actually, each unit of the Time chart was developed only as early geologists identified the layers of rock and the fossils captured within them.
As shown in the image above of the Grand Canyon, each layer of rock needed to be studied, researched, identified, and named in order for geologists to devise the Geologic Time Chart.
Each rock layer that corresponded to a unit of the Geologic Time Chart was identified in a somewhat haphazard manner. The Paleozoic Era was not identified and included in the developing time chart until 1838, however the Devonian Period of the Paleozoic Era was identified a year earlier in 1837. Other Eras and Periods of the Geologic Time Chart were identified in the following order:
Throughout the 1800's geologists continued to add to their knowledge about the rock layers and the fossil remains found within them. Slowly, the Geologic Time Chart took shape, however, in the course of devising the chart geologists also found that the rock layers offered some challenges and mysteries. For example, older rocks would sometimes be captured in younger rocks, causing confusion about the relative date of the rock layer. Consequently, Geologic Principles were developed to explain those curious events.
Click here to Learn More About Geologic Principles