What is stratigraphy dating
When we think of fossils, we often visualize dinosaurs or other large animals.
Those are generally too rare to be of much stratigraphic use.
Why do we bother with all these weird names for different time slices?
Actually, almost all of the time periods in the Precambrian are based on radiometric dates. As soon as stratigraphers can find enough information, they will change the simple date ranges to more complex entities defined in some other way.
Actually there are three primary reasons for using this system. The science of stratigraphy was born at the same time as geology, early in the Nineteenth Century.
However, they soon developed quite sophisticated techniques for relative dating.
These techniques were developed using sedimentary successions.
Each layer is compressed by the weight of those above it and becomes sedimentary rock. That is, wherever the layers of rock are exposed -- in a road cut, a steep river valley, a cliff face -- we can be fairly certain that the sediments get older as we go down the rock column and younger as we go up.
So, looking at any given exposure, we can be quite sure about relative age.
Radioactivity was not discovered until a century later (by Roentgen).