Geologic time relative age dating
This time scale, from the Decade of North American Geology, is widely used in North America.
As we improve our ability to date rocks using radiometric dating methods, the time scale is amended.
They had no way of knowing the ages of individual rock layers in years (radiometric dates), but they could often tell the correct sequence of their formation by using relative dating principles and fossils.
Geologists studied the rates of processes they could observe first hand, such as filling of lakes and ponds by sediment, to estimate the time it took to deposit sedimentary rock layers.
A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.
Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.
The 4.55 billion-year geologic time scale is subdivided into different time periods of varying lengths.
These ages, usually called radiometric ages, are used in conjunction with relative dating principles to determine at least an approximate age for most of the world 's major rock formations.When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages.Relative-age time periods are what make up the Geologic Time Scale.This image from Lake Mead NRA shows a sequence of sedimentary rock layers.The oldest layers, Cambrian (570-505 million years ago) are at the base, with younger layers piled on top.