Principle of Original Horizontality
The Principle of Original Horizontality was proposed by the Danish geological pioneer Nicholas Steno (1638–1686). This principle states that layers of sediment are originally deposited horizontally under the action of gravity.
The principle is important to the analysis of folded and tilted strata. 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 of plate tectonics; that movement and collisions of large plates of the Earth's crust is the cause of folded strata.
|Folds of alternate layers of limestone and chert occur in Greece. The limestone and chert were originally deposited as flat layers on the floor of a deep sea basin. These folds were created by Alpine deformation.
As one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known that not all sedimentary layers are deposited purely horizontally.
For instance, coarser grained sediments such as sand 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 the angle of repose, and a prime example is the surface of sand dunes.
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.