29 Folds

Elizabeth Johnson

Anatomy of a Fold

A fold is a geologic structure that is formed by layers or beds of rock being bent or folded.  The plane that marks the center of the fold is called the axial plane.  The line which marks where the axial plane intersects the surface of Earth is called the hinge line.  The areas on either side of the curved hinge zone stick out like arms or legs, and are appropriately called limbs.

Figure 21. Fold Anatomy. From https://commons.wikimedia.org/wiki/File:Flank_%26_hinge.PNG.


Types of Folds

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


An anticline is a fold that is convex: it curves like a rainbow.  “A” is for “anticline,” and the capital letter “A” represents the shape of the fold.

Figure 22. Anticline showing fold hinge line and strike and dip symbols. Older layers appear in the center of anticlines in map view.

3D interactive model of Figure 22: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYic3JmAEM


A syncline is a fold that is concave: it forms a “U” shape.  “S” is the first letter of “syncline,” and a syncline looks like a Smile. 

Figure 23. Syncline showing fold hinge and strike and dip symbols. Younger layers appear in the center of synclines in map view.

3D interactive model of Figure 23: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYmZvKmAEM



A monocline is a special type of fold in which both limbs are parallel but offset to each other.  The limbs are horizontal, or nearly so.



By Kilom691Own work, CC BY-SA 3.0, Link



Question 10. What type of fold is this? Click on this link -> https://skfb.ly/WU6y

  • Anticline
  • Syncline
  • Monocline


Figure 24. Fold for Questions 11 and 12.

3D interactive model of Figure 24: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYm5vKmAEM

Question 11. Which type of fold is shown in Figure 24?

  • Anticline
  • Syncline
  • Monocline


Question 12. What is the azimuth of the fold axis in Figure 24?

  • 0 degrees
  • 71 degrees
  • 161 degrees
  • 341 degrees
Figure 25. Folds for Question 13.

3D interactive model of Figure 25: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYqb2smQEM

Question 13. Which strike and dip symbol is correct in Figure 25?

  • A
  • B
  • C
  • D





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Physical Geology Laboratory by Elizabeth Johnson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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