27 Tilted Beds and Strike and Dip

Elizabeth Johnson

Strike and Dip

Geologists use a special symbol called strike and dip to represent inclined beds (Figure 2)Strike and dip map symbols look like the capital letter T, with a short trunk and extra-wide top line. The short trunk represents the dip and the top line represents the strikeDip is the angle that a bed or layer plunges into the Earth from the horizontal. A number next to the symbol represents dip angle.

Figure 2. Strike and dip on a map. Protractor modified from: Autiwa – Own work, CC BY-SA 3.0.

One way to visualize the strike is to think about a line made by standing water on the inclined layer. That line is horizontal and lies on a compass direction that has some angle with respect to true north (see Figure 3). The strike is indicated by azimuth (orientation) on the map – see Azimuth. In Figure 2, the strike is measured as either 37 degrees, or 37 + 180 = 217 degrees.  Convention is to use the smallest angle (in this case, 37 degrees), but either is correct.

The dip of the inclined layer represents the direction the layer or bed is tilting into the Earth . The direction of dip would be the direction a ball would roll if set on the layer and released.  In Figure 2, the layers are dipping to the SE.  In this case, the dip is labelled “27 degrees,” so we know it is tilted exactly 27 degrees into the Earth.  Sometimes dip angles are not labeled, but we can still determine the dip direction (but not the magnitude) from the dip symbol.

A horizontal rock bed has a dip of 0° and a vertical bed has a dip of 90°.

Figure 3: Strike and Dip 3D model (click on link):

https://skfb.ly/6GEHr

Rules of Strike and Dip

  1. Strike is always parallel to the bedding direction at that location.
  2. The dip is always drawn perpendicular to strike in map view.  It may be drawn at an angle to show perspective in a 3D block diagram.
  3. The dip CAN be labelled with the dip angle, or only the direction can be indicated.  The dip always shows which way layers are tilting into the Earth.
  4. Special symbols are used for horizontal beds and vertical beds.

 

Figure 4. Symbols for horizontal and vertical beds.

 

Tilted Beds

Tilted beds or layers occur when plate tectonic forces cause horizontal layers to be pushed up or dropped down unevenly.  This results in a tilting or incline of the original horizontal beds.

Figure 5. A 3D virtual outcrop of tilted beds (click on link): https://sketchfab.com/3d-models/vom-2-65d74feafc1f4e99a87d0c9d17070fa5

Examples

Here are some examples of tilted beds:

Figure 6. Tilted beds- 15 degrees.

3D interactive model of Figure 6: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYqcLLnAEM

Figure 7. Tilted beds – 60 degrees.

3D interactive model of Figure 7: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYyeKJpAEM

Figure 8. Tilted beds – vertical (90 degrees).

3D interactive model of Figure 8: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYqezDmgEM

Questions

Figure 9. Block diagram and map view for Question 2.

3D interactive model of Figure 9: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYyaOkogEM

 

 

Figure 10. Block diagram and map view for Question 3.

3D interactive model of Figure 10: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwY-cORowEM

Figure 11. Block diagram and map view for Question 5.

3D interactive model of Figure 11: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYibyKoQEM

Some questions, like Questions 5-6, are intentionally left as non-interactive.

Question 5: Which strike and dip symbol is correct in Figure 11?

  • A
  • B
  • C
  • D
Figure 12. Block diagram and map view for Question 6.

3D interactive model of Figure 12: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYmp7DmgEM

Question 6: Which strike and dip symbol is correct in Figure 12?

  • A
  • B
  • C
  • D

References

Text modified from http://opengeology.org/textbook/9-crustal-deformation-and-earthquakes/ CC-BY-SA.

Protractor modified from: https://commons.wikimedia.org/w/index.php?curid=19070259 By Autiwa – Own workCC BY-SA 3.0Link

License

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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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