It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.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.Learn how inclusions and unconformities can tell us stories about the geologic past.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.
Inclusions are always older than the sedimentary rock within which they are found.Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.Topics include rock layer sequencing, fossil correlation, and radiometric dating.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.Try It risk-free Discover how geologists study the layers in sedimentary rock to establish relative age.The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.How do we use the Law of Superposition to establish relative dates?Let's look at these rock strata here: We have five layers total.Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.