Introduction: Have you ever been to the Grand Canyon? Perhaps you have seen it in photographs or videos. Imagine climbing a trail from the top of the canyon down to its base. As you walk, you pass distinct rock layers, each with its own unique colors and textures. In a way, you are traveling back through time as you descend into the canyon! 

The earth is a dynamic system and it is constantly changing, even when we can't see those changes on a daily basis. Over the course of hundreds, thousands, and millions of years, the landscape of the earth changes dramatically. New rock layers form. Faults break apart the crust. Mountains uplift and valleys form. Geologists work to piece together the story and timeline of these changes using a combination of methods. One method is absolute dating, which uses the half-life of radioactive isotopes to date the age of each rock sample. A second method is relative dating, which places the order of geologic events by comparing rock layer positions without setting a specific time of formation. 

Geologists use a collection of guiding principles when determining relative sequencing of geological formations. Although each principle may make common sense, using them together to unravel the history of ancient rock formations can prove very complex. Major principles of relative dating include:

Superposition - in undisturbed sedimentary rock layers, younger layers form on top of older layers

Original Horizontality - sedimentary deposits form as flat (horizontal) layers or beds

Cross-Cutting Relationships - a geologic feature that cuts across existing rock is younger than the rock it cuts across

Inclusions - pieces of rock that are within another rock are older than the rock that contains them 

Faunal Succession - fossils occur in a set order that can be used to correlate relative ages of the rocks they located within

Directions for This Lesson: In this lesson, you will be introduced to the principles of geologic relative dating. First, try the practice questions to determine what you already know. Then, watch the video lesson to learn more. Finally, apply what you learned in the activity and practice sections.

 

 

Apply what you learned by trying this interactive activity. 

Practice what you have learned by completing the post-lesson worksheets.