The assumption that the layers represent time periods is just an assumption...
Layers, wildswanderer?
It is radiometric dating that determine time periods of fossils or the rocks surrounding the fossils.
There are other dating methods to dating rocks or fossils that verify radiometric dating. Have you ever heard of “thermoluminescence” dating?
There are several methods of using thermoluminescence dating, but they all amount to the same result, which is date when any object, like rocks, minerals, fossils were last exposed to direct sunlight, more specifically to ultraviolet.
“Why ultraviolet radiation?” ...you may ask...
Ultraviolet radiation, like x-ray and gamma-ray, move photons (particles) at higher speed, and EM waves have much more energy than the other end of EM spectrum, non-ionizing radiation, eg infrared, radio waves, etc.
Such higher energy will cause objects that are hit by ultraviolet, will cause the objects to lose some particles, like electrons, thereby ionizing object.
If the objects are ionized, and then buried, then thermoluminescence techniques can be used to measure how long ago it have been buried for, like the last time the objects being exposed to ultraviolet radiation.
Thermoluminescence can be used to verify or refute the radiometric methods.