Why can it be difficult to assign dates to the division of the geologic time scale?

Since sedimentary rocks cannot be dated by radioactivity, fossils are the primary source of information for dating the geological record. Fossils are found in sedimentary rocks.

Early on, attempts were made to establish a time scale based on the sediment layer thicknesses. Calculations were made regarding the amount of time needed to form the observed sedimentary layer thicknesses in locations such as the Grand Canyon, utilizing the current rates of sedimentation on the ocean floor.

The results of this calculation proved to be inconsistent; the assumptions of uniform processes do not hold true everywhere in the world. Certain places appear to have very slow rates of sedimentation, while other places appear to have fairly rapid layer formation (see the results of Mount St. Helens).

The evidence of modern plate tectonics has thrown a monkey wrench into these calculations, as the ocean sediments are typically destroyed at subduction zones rather than being collected on the continents. It remains unclear how vast deep sedimentary layers were formed on the continents.

Since there is no direct way to date fossils, the divisions of the geological time scale are highly subjective and challenging.

The difficulty arises due to incomplete and fragmented geological records, varying preservation conditions, and the gradual nature of some geological processes.