7.3 Folds

Can you see the anticline at Anticline Overlook?

Moving around the desert Southwest, we see a lot of folds. This view is from the Anticline Overlook at Canyonlands National Park. Look up what an anticline is below and then see if you can spot this one. Remember you may only be able to see part of it in the photo. All of the folds (not the basin) pictured below are found in the arid Southwest.

Folds

Rocks deforming plastically under compressive stresses crumple into folds. They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture.

You can see three types of folds.

Monocline

A monocline is a simple bend in the rock layers so that they are no longer horizontal (see Figure below for an example).

What you see in the image appears to be a monocline. Are you certain it is a monocline? What else might it be? What would you have to do to figure it out?

Anticline

Anticline: An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure below). The oldest rocks are at the center of an anticline and the youngest are draped over them.

When rocks arch upward to form a circular structure, that structure is called a dome. If the top of the dome is sliced off, where are the oldest rocks located?

Syncline

A syncline is a fold that bends downward. The youngest rocks are at the center and the oldest are at the outside (Figure below).

When rocks bend downward in a circular structure, that structure is called a basin (Figure below). If the rocks are exposed at the surface, where are the oldest rocks located?

Summary

• Rocks deform by compressive stress into folds.
• A monocline is a simple bend.
• In anticline, rocks arch upward. A three-dimensional anticline is a dome.
• In a syncline, rocks arch downward. A three-dimensional syncline is a basin.

Review

1. Draw a picture to show how compressive stresses lead to the formation of anticlines and synclines.
2. Do you think that anticlines and synclines are ordinarily found separately or adjacent to each other?
3. If you found a bulls-eye of rock on the flat ground with no structure to guide you, how could you tell if the structure had been a syncline or an anticline?

4. What folds can you find in this photo of Monument Valley in Arizona? Notice the rock layers at the top of the ridge. What is the geologic history of this region?

Explore More

Use this resource to answer the questions that follow.

1. What causes folds?
2. What are the folds called?
3. What is dip? What is it measured from?
4. What is strike?
5. What does a block diagram show you?
6. What is the strike and dip symbol?
7. What do the arrows on the diagram tell you?
8. Describe the effects of erosion on how a rock layer looks on a map.