To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.Using this process geologists are able to assign actual ages with known degrees of error to specific geologic events.By combining knowledge gained using both relative and absolute dating processes geologists have been able to produce the geologic time scale.Unlike people, you can’t really guess the age of a rock from looking at it.Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
There are two basic approaches: relative age dating, and absolute age dating.The most obvious feature of sedimentary rock is its layering.This feature is produced by changes in deposition over time.It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.After all, a dinosaur wouldn’t be caught dead next to a trilobite.
This process lead to a system of time containing eons, eras, periods, and epochs all determined by their position in the rock record.