Why can it be difficult to assign dates to the division of the geologic time scale?
1 Answer
the geologic time scale is based on fossil evidence, as interpreted by Darwinian evolution.
Explanation:
Fossils occur in sedimentary rocks. Sedimentary rocks can not be dated using radioactivity. This means that the main method for dating the geological record is the fossils.
Early on efforts were made to set up a time scale based on the thickness of the layers of sediment . Using the present rates of sedimentation on the ocean floor calculations were made as the the length of time it would require to form the thicknesses of the observed sedimentary layers in places like the grand canyon.
This calculations proved to be inconsistent. The assumptions of uniform processes do not hold the same throughout the world. Some layers in places seem to be laid down fairly rapidly. (See the results of Mount St. Helens). Other places seem to have very slow rates of sedimentation.
Modern plate tectonics have thrown a monkey wrench into these calculations. The ocean sediments rather than being collected on the continents are usually destroyed at subduction zones. How vast deep sedimentary layers were formed on the continents is a mystery given the evidence of Modern plate tectonics.
The fossils can not be dated directly by any methods making the divisions of the geological time scale very difficult and somewhat subjective.
