Process of relative dating
The most obvious feature of sedimentary rock is its layering.
This feature is produced by changes in deposition over time.
The ten strata systems that compose the “standard geologic column” are the familiar Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Tertiary periods.
Although no absolute methods were available to establish actual dates, Lyell needed to assign very old dates to the strata to make them consistent with the long eons of time that would be necessary to meet the new uniformitarianism theory developed by James Hutton and himself.
The relative dating methods themselves are generally sound when used with good assumptions.
However, when scientists apply relative dating to a preconceived uniformitarianism model, the dating methods are only as good as the model.
Some types of relative dating techniques include climate chronology, dendrochronology, ice core sampling, stratigraphy, and seriation.
In the example above, Sandstone B is younger than Sandstone A.
Because relative dating doesn't give you an absolute age, errors are much less common in comparison to absolute dating.
This theory held that the past was the key to the future and that processes that formed the layers were the very slow processes that we see forming layers at the bottom of the ocean today. Until Lyell successfully convinced scientists that uniformitarianism was the correct theory, it was believed that the worldwide flood and other catastrophic events were primarily responsible for the formation of the geologic layers and that they didn’t represent long ages.
Later, when radiometric absolute dating methods were developed, they still were not applicable to sedimentary layers.