Nicolaus Steno first proposed the law or principle of original horizontality in the mid-17th century. It states that sediments are always deposited in horizontal (or flat) strata under the action of gravity. Any deformation (tilting or folding) of sedimentary rock layers occurs after formation, i.e., sedimentation and lithification, resulting from Earth’s movements.
This principle is important as it helps in relative dating together with other laws of geology, especially in giving a sequence of events. Also, it can provide an idea of the actual event and likely led to plate tectonic theory.
However, it does have its shortcomings, such as the existence of inclined beds on cross-bedding, which it cannot explain. Therefore, it is not universally applied by stratigraphers, sedimentologists, or structural geologies.
Today, we will discuss the original horizontality principle or law, giving examples, significance, and where it may not apply.
Historial background
Nicholas Steno (Nicholas Stenonis) (1638-1686), born Niels Stensen, is the man who proposed the law or principle of original horizontality. He was a Danish scientist who pioneered geology and medicine before becoming a Catholic bishop.
Also, he proposed three other laws or principles, i.e., the law of superposition, lateral continuity, and cross-cutting relationship. These principles laid the basis of stratigraphy and geology.
Steno’s ideas or laws were published in Prodromus or De Solido Intra Solidum Naturaliter Contento Dissertationis Prodromus (the Prodromus of Nicolaus Steno’s Dissertation Concerning a Solid Body Enclosed by Processes of Nature Within a Solid) in 1669. However, not many accepted his ideas until many years later. Today, his work is still important to geology.
Lastly, did you know that he also worked on paleontology and crystallography? Perhaps Steno deserves more credit than he gets.
What is the principle of original horizontality?
The law or principle of original horizontal states that sediments are always deposited in horizontal or near horizontal layers or strata under gravity action. Any folding or steep tilting of these rock layers happens after formation due to the Earth’s movements.
Some of the Earth’s movements that could make nearly or horizontal/flat rock layers fold, incline or deform rocks include mountain building, earthquakes, faulting, or tectonic plate movements. These forces will break, uplift, or buckle layers, making some younger one goes beneath older ones, steep contours, or inclination.
Steno observed that since sediments settle from water or wind due to gravity, they will be deposited horizontally on the sediment basin or Earth’s surface and then cement with time. Otherwise, the sediments would have slithered under gravity if on a steeply inclined surface to the lowest point.
Subsequent deposition of sediments will also be flat on top of the previous ones before they lithify. Consequently, you will have sedimentary rock layers nearly horizontal with different lithologies.
In the case of Earth or tectonic movements, including mountain building, faulting, or earthquakes, the initially horizontal sedimentary strata may deform (fold or incline). Therefore, if you find any tilted or folded sedimentary rock layers, these deformations occur after their formation, not before.
Example to explain the original horizontality
Since fluids carry them (water or wind), consider sediments a set of liquids with different colors, say red, blue, yellow, green, etc. Let us assume each solidifies when you let it dry after some time, just like sediments lithify.
If you pour the first red liquid into a glass, it will have a flat top surface due to gravity and solidify after some time. Next, add the second blue liquid on top solidified red; it will also form a flat surface on top of the first red and dry, leaving a flat upper surface.
Now, continue with the process until you have several layers of different colors, all dry and one lying flat on each the other. This arrangement is exactly how sediments form, going by the law of original horizontally, and will remain undeformed.
Video
Here is a video to further help you understand this law or principle of original horizontality.
Significance
How does the original horizontality help geologists? You may wonder. The answer is straightforward. It is important in relative dating, i.e., putting past events in the correct sequence they happened, together with other laws, i.e., cross-cutting relationships, inclusions, and superposition.
Furthermore, by studying the deformation that happened, it is possible to tell which forces were acting or what event happened. Did it happen from mountain building, earthquakes, faulting, etc., or if plate tectonic movements were convergent or transformed boundaries?
Lastly, from Steno’s original horizontality, it was clear that the Earth was not static, and some forces had been acting on it for a very long time and were the ones that caused deformation (folding, inclination, etc.). This realization may have contributed to plate tectonics, where collisions can cause folding.
Are there some exceptions?
Original horizontality remains an important law or principle in geology. That is not in doubt. However, it does have shortcomings, i.e., not all sedimentary rock layers are horizontal, and deformation isn’t the reason.
For instance, if you consider cross-bedding, it is possible to have rock layers inclined to overall bedrock. It happens because of internal friction to hold coarse sediments and prevent them from slithering or plummeting if it doesn’t exceed the angle of repose.
Here, the angle of repose is the highest level the sediments will reach before they avalanche unless there is some effort or reworking, and cross beds are common on dunes in deserts or beaches.
Also, conformable sediment deposition on some existing inclined surfaces can result in a tilted rock layer. It occurs if sediments drape instead of slithering to the lowest point. Such a case doesn’t indicate some disturbance as assumed original horizontality.
That is not all. The pinching out of sedimentary strata along the strike indicates slight angles at deposition. This further
These shortcomings make the law of original horizontality not universally applicable in stratigraphy, structural geology, or sedimentary studies. Nonetheless, it is widely applicable.
What do other Steno’s laws say?
Perhaps one of Steno’s most popular laws or principles is superposition, which states that in an undeformed sedimentary rock sequence, a rock strata or layer is older than the one above it and younger than the one below it.
The other law is lateral continuity, which states that sediment layers extend outwards in all directions on the depositional area unless interrupted. Therefore, you can assume those broken by a valley or other eroding agents were originally continuous.
Lastly, there is the law of cross-cutting relationships. It states that a geological feature that cuts another is younger than the one it cuts.
Illustrations/images
References
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill Education.
- Principle of original horizontality. (2022, October 21). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Principle_of_original_horizontality&oldid=1117436897
- Wicander, R., & Monroe, J. S. (2010). Historical geology: Evolution of the Earth and life through time (6th ed.). Books-Cole.