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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.
Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.
The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.
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.
Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.
Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.
Try it risk-free Discover how geologists study the layers in sedimentary rock to establish relative age.
Learn how inclusions and unconformities can tell us stories about the geologic past.
Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.
We could assume that this igneous intrusion must have happened after the formation of the strata.What could a geologist say about that section of rock?