6.3: Folds

What folds can you see?

Moving around the desert Southwest, we see a lot of folds. This view is near the Valley of the Gods in Utah. Can you see all of the types of folds mentioned below? All of the folds mentioned in the text below are found in the arid Southwestern U.S.

Folds

Deep within the crust, as plates collide, rocks crumple into folds. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. Your hands moving toward each other creates compressive stress. In sedimentary rocks, you can easily trace the folding of the layers (Figure below). In the image below, the rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. Once rocks are folded, they do not return to their original shape.

This is a geologic cross section of the Grand Staircase in Utah. Where the rocks fold up, at the left of the diagram, is a small syncline.

There are three types of folds: monoclines, anticlines, and synclines.

Monocline

A monocline is a simple “one step“ bend in the rock layers (Figure below). In a monocline, the oldest rocks are at the bottom, and the youngest are at the top.

The rock layers in the center left are tilted in one direction, forming a monocline.

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 found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks are exposed at the center.

(A) An anticline is a convex upward fold. (B) This anticline is at Calico Ghost Town, California.

Syncline

A syncline is a fold that bends downward (Figure below). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a basin. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin.

(A) A syncline is a concave downward fold. (B) This syncline is seen at Calico Ghost Town near Barstow, California.

Summary

• Rocks deform by compressive stress into folds.
• A monocline is a simple bend in one-direction.
• In an 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 folds. Can these stresses create anticlines and synclines?
2. Do you think that anticlines and synclines are ordinarily found separately or next to each other?
3. Describe domes and basins. Where are the youngest rocks in each?

Explore More

Use the resource below to answer the questions that follow.

1. What is a geologic fold and what causes one to form?
2. What types of forces create a set of folds?
3. What is a monocline? Where are the older rocks found?
4. What is an anticline? Where are the older rocks found?
5. What is a syncline? Where are the older rocks found?