Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers.
Although most attention in today's world focuses on dinosaurs and why they became extinct, the world of paleontology includes many other interesting organisms which tell us about Earth's past history.
For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.
Return to top The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place.
If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata.
By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale.