When geologists refer to events that occurred 200 million years ago, they are referring to dates on the geological time scale that have been added since the discovery in the late 19th century of the use of radioactive decay as a tool for determining the age of geologic materials.
The geological time scale is an evolving document, and radiometric dates are subject to refinement as technology advances.
But the geological time scale has always been an evolving document, even before dates were added. The names of the geologic eras, periods, and epochs on the geologic time scale we use today were not written in stone (so to speak!) but represent the latest version of a document that has its origins in principles enunciated almost 400 years ago.
Figuring out the order in which rock layers were deposited or emplaced was the first step to developing a time scale.
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