Principles of Geology

Lesson 1: Determining Relative Age of Rocks

Made by T. Bart

To the right you see a typical rockformation. What can you tell by just looking at this profile?

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​How Old is That Rock Layer?

By the end of this lesson, you’ll be able to:

- Explain the principles geologists use to determine the age of rocks.

- Apply these principles to identify the relative age of rock layers.

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Lesson Objectives

Uniformitarianism states that the geological processes we see today, like erosion and sedimentation, also happened in the past—over millions of years!

Example: "Think about a river carving through rock. Over time, this same river can create canyons, just like the Grand Canyon.

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What is Uniformitarianism?

Imagine an old building made of stone. Rain and wind have slowly worn down the outer layers over decades—similar to how mountains and rock layers change over time.

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Real-Life Connection: Uniformitarianism

This law tells us that in undisturbed rock layers, the youngest layers are at the top, while the oldest layers are at the bottom.

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Principle 2 – Law of Superposition (Intro)

Multiple Choice

According to the law of superposition, which layer is the youngest?

• The bottommost layer is the youngest.
• The second layer from the top is the youngest.
• The layer in the middle is the youngest.
• The topmost layer is the youngest.

Think of stacking books on a shelf. The book at the bottom is the first you put there (the oldest), and the last book you stack on top is the newest.

Principle 2 – Everyday Example

Sedimentary rock layers are generally deposited in flat, horizontal layers. If we see layers that are tilted, something must have disturbed them after they were formed.

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Principle of Original Horizontality

Multiple Choice

What does the Principle of Original Horizontality state?

• Layers of sediment are originally deposited horizontally.
• Sediment layers are deposited in a random order.
• Sediment layers can only be deposited in water.
• Sediment layers are always deposited vertically.

Review: The Three Principles

- Uniformitarianism: Past processes are similar to present ones.

- Superposition: Older layers are at the bottom.

- Original Horizontality: Layers start flat and can be tilted over time.

​Ancient Clues in Rocks

There are many clues about how life was in the past hidden in the rock layers.

Although layers vary wildly in different area, they are catogorized in 3 main rock types.

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​Main types of rock

Multiple Choice

If a layer is tilted or folded, which principle would we use to understand its original position?

• Principle of Original Horizontality
• Principle of Superposition
• Principle of Lateral Continuity
• Principle of Cross-Cutting Relationships

​The grand canyon is a classical example of how rock formation happens.

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Case study: Grand Canyon

Multiple Choice

Why is the Grand Canyon a good example of the law of superposition?

• The Grand Canyon is a good example of the law of superposition because its layered rock formations.

• The Grand Canyon has no visible rock layers.
• The Grand Canyon is formed entirely from volcanic activity.
• The Grand Canyon's rocks are all the same age.

Summary: What We Learned

In today’s lesson, we learned three ways to determine the age of rock layers.

- uniformitarianism: Geological processes have happened in the past as they do now.

- Superposition: If undistrurbed the oldest layers are at the bottom and the newest at the top

- Original Horizontally: Layers are flat and accumulate very slowly over time

• Charles Lyell

" The present is key to the past"