Original horizontality relative dating
The Principle of Lateral Continuity supposes that rock layers extend for some degree over the Earth's surface. In stratigraphy terms, this allows layers to be compared against each other.
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.
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.
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.
This can indicate the rocks have been turned, twisted and moved by natural forces, such as continental shifts.