Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.As a member, you'll also get unlimited access to over 70,000 lessons in math, English, science, history, and more.Plus, get practice tests, quizzes, and personalized coaching to help you succeed.This method is known as 'relative dating' and the age is called the 'relative age'.The 'absolute' geologic age of the strata which bear the index fossils has been determined by the radiometric age of igneous intrusions which are relative to the layers of sedimentary rock, according to the Law of Superposition and the Law of Horizontality.If it had happened before the layers had formed, then we wouldn't see it punching through all the layers; we would only see it going through the layers that had existed at the time that it happened. The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across.The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set.
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.In this lesson, we'll learn a few basic principles of stratigraphic succession and see whether we can find relative dates for those strange strata we found in the Grand Canyon.In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.Geologists use this type of method all the time to establish relative ages of rocks.