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Theprinciple of original horizontality states that layers ofsediment are originally deposited horizontally under the action ofgravity.[1] It is arelative dating technique. The principle is important to the analysis offolded and tiltedstrata. It was first proposed by the Danish geological pioneerNicholas Steno (1638–1686).
From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science ofplate tectonics; that movement andcollisions of large plates of the Earth'scrust is the cause offolded strata.[2]
As one of Steno's Laws, the principle of original horizontality served well in the nascent days ofgeological science.[3] However, it is now known that not allsedimentary layers are deposited purely horizontally. For instance, coarser grained sediments such assand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort. This is known as theangle of repose, and a prime example is the surface ofsand dunes.
Similarly, sediments may drape over a pre-existing inclined surface: these sediments are usually deposited conformably to the pre-existing surface. Also, sedimentary beds may pinch out alongstrike, implying that slight angles existed during their deposition. Thus the principle of original horizontality is widely, but not universally, applicable in the study ofsedimentology,stratigraphy, andstructural geology.
Geologic principles and processes | ||
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| Stratigraphic principles |  | |
| Petrologic principles | ||
| Geomorphologic processes | ||
| Sediment transport | ||