Dating and correlating rock layers
Throughout the history of life, different organisms have appeared, flourished and become extinct.Many of these organisms have left their remains as fossils in sedimentary rocks.Suppose you find a fossil at one place that cannot be dated using absolute methods.That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.At a particular location, the rocks never fully represent the entire geologic rock column due to extensive erosion or periods of non-deposition or erosion.For example, rocks named Juras (for the Juras Mountains) in France and Switzerland were traced northward and found to overlie a group of rocks in Germany named Trias.Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.This matching process is called correlation, which has been an important process in constructing geological timescales.
Fossils are important for working out the relative ages of sedimentary rocks.
Rock divisions, such as the Cambrian System, can be correlated worldwide based on fossils.
In contrast, rock units such as groups, formations, and members are localized subsets of systems.
Rock units depend on the environment of deposition, which varies from one location to another.
The main rock unit of stratigraphy is the formation, a localized and distinctive (easily recognizable) geologic feature (i.e., the Chinle Formation of Late Triassic lake and river deposits in Arizona, Nevada, Utah, and New Mexico).