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How a Landform Diagram Describes the Geological Progression of a Landscape

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  • 0:05 Landform Diagram
  • 1:24 Key Terms
  • 3:38 Practice
  • 7:07 Lesson Summary
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Lesson Transcript
Instructor: Peter Jaeger

Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

This lesson describes what a landform diagram is. We'll also discuss some basic laws of geology that can help scientists deduce both the order of events that create a landform and the geologic events that occur after a landform's formation.

Landform Diagrams

How do we learn things about the past? Usually, we study history, look for observations and evidence, and make some assumptions about what went on. Many of these ideas also hold true for scientists trying to figure out what happened to the earth in the past. In fact, one of the main principles of geology is uniformitarianism, which is the idea that the present is the key to the past. That means that natural laws have remained the same throughout the history of the earth. The same natural laws that impact the earth today had the same impact millions of years ago.

Using this idea, we can deduce the order of geologic events that created a landform, like a mountain or a plateau, to know what happened. This order of events can be shown using a landform diagram, which is a drawn cross-section of a landform that clearly shows the layers that form it.

An example of a landform diagram
Landform Diagram Example

Above is an example of a landform diagram. You can see how the different rock layers are outlined and colored. Studying this diagram can teach us many things about the landform and how it has changed over time. Keep this image in mind as we talk about some key terms that will help us understand these diagrams.

Key Terms

Besides the overarching principle of uniformitarianism, there are other key ideas or laws that need to be assumed to be in effect in order to deduce geological history. One is the law of original horizontality. This means that sedimentary rocks are originally formed in horizontal layers. This can be seen in any river or lake where sediment will settle to the bottom in layers. This means that if the layers of rock are inclined or tilted at an angle, the event that caused them to shift happened after they were deposited. This might occur when an area of land is forced to bend, usually from forces inside the earth pushing on them. The diagram below shows the layers that originally would have been laid horizontally but have been forced to bend.

Layers would originally be laid horizontally, but they have been forced to bend.
horizontality diagram

A second key idea is the law of superposition. The law of superposition states that in any undisturbed sequence of layers of rock, the oldest or first-deposited layer would be on the bottom and the youngest would be on the top.

A third key idea is the principle of crosscutting relations. This idea states that any rock or fault that cuts across other rocks is younger than those it cuts across. So, if you had layers of rock that are cut by some other event, maybe a lava flow, the original rock layers are older and the intrusion is younger.

The fourth key idea is the idea of unconformities. Unconformities are surfaces of erosion that separate younger rocks from older ones. For instance, if a sedimentary rock forms and is still exposed to the elements, it would begin to wear away. Later, new sediment can begin to pile up on that eroded surface and eventually form new rock. In an unconformity, existing rock is lost and cannot be recovered. It is very difficult to determine how much erosion took place or what kinds of rocks were worn away. Unconformities can occur because of wind, water, or friction when other rocks scrape along them, like when a glacier slides over the landform.

Unconformity occurs when an older layer of rock erodes and is covered by a new layer.
Unconformity Rock Layers Image

Practice 1

Okay, so using these four key ideas, we can start to take a look at some landform diagrams. These diagrams are simply sketched cross-sections of a landform and usually numbered to identify the different types of rock. As we practice, we should be able to deduce the order of layers laid down by looking at erosion and intrusion events.

Here is a simple example:

Diagram for practice 1
diagram for first practice

In this picture, the layers at the bottom of the formation must form before the layers at the top. This is the law of superposition. So, following this law, layer 1 is the oldest, followed by layer 2, then layer 3, and so forth, with layer 9 being the youngest. In looking at a picture of the Grand Canyon, we would guess that the layers at the bottom of the formation would be older than those near the top. From all our experiences, this makes perfect sense.

The law of superposition states that the oldest rocks are on the bottom and the youngest on the top.
Law of Superposition Sample

Practice 2

Here is another basic example:

Diagram for practice 2
practice diagram for 2

Remember to use the key ideas we highlighted at the beginning to determine what happened first. Give yourself a chance to answer it.

This picture shows some rock layers and a large canyon or valley on the left side. What happened first and in what order? First, the layers of rock were laid down, layers 1-6. They were laid down in order and horizontally. Next, a big unconformity, an erosion event, occurred, destroying the rock that originally was on the left. This could have happened through glacial movement, a river causing erosion over time, or a cataclysmic event, such as a flood or a lava flow. In any case, the layers formed first, and then they were eroded away by some kind of event.

This landform diagram might have been drawn by looking at an actual cross section of a natural formation, like this one:

Cross section of a natural rock formation
natural rock formation

Practice 3

Okay, here's a trickier example:

Diagram for practice 3
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