What is the principle of Uniformitarianism and how is it important to the relative dating of rocks?
The principle of Uniformitarianism is the idea that all geological process have operated slowly and in the same manner as they are observed to operate today.
The principle of uniformitarianism is used to date rocks based on the assumptions of uniform process. If the process of sedimentation is observed to deposit 1 cm of soil in a year, the the age of the sedimentary layer is calculated to be the thickness of the of the sedimentary layer divided by the rate of sedimentation observed ( 1 cm/ year)
The principle of uniformitarianism is applied to the organic world as well as the geological world. Darwinian evolution uses the principle of uniformitarianism as the central idea of descent with modification that organisms have evolved by slow gradual uniform changes.
Using this principle of uniformitarianism rocks can be dated relatively. The simpler the organism the older it is assumed to be. The more complex the organisms the younger it is assumed to be.
The only chronometric scale applicable in geologic history for the stratigraphic classification of rocks and for dating geologic events exactly is furnished by the fossils. Owing to the "irreversibility" of evolution they offer an unambiguous timescale for relative age determination for world wide correlations of rock ) American Journal of Science 1957 page 395. This is the principle of uniformitarianism applied to dating rocks based on fossils.
There are valid reasons to question the truth of the principle of uniformitarianism. Even the naturalistic theory of comet impact as the reason for the extinction of the dinosaurs goes against the theory of uniformitarianism. If the theory of uniformitarianism is faulty then the dating of many rock layers is also faulty.