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.
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.Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.Geologists use this type of method all the time to establish relative ages of rocks.Now, what if instead of being horizontal, this rock layer was found in a tilted position?What could a geologist say about that section of rock?Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata.There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.These interfaces between discontinuous layers of rock are called unconformities.