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Proposed by the Danish geological pioneer Nicholas Steno (1638-1686). This principle states that layers of sediment are originally deposited horizontally. The principle is important to the observation of folded and tilted strata where these are exposed by erosion.

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 collision of large plates of the earth's crust is the cause of folded strata.

As one of Steno's Laws, the Principle of 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.

Similarly, sediments may drape over a pre-existing inclined surface, and so not all sediments in a section may be deposited entirely flat. Similarly, sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition.

Thus the Principle of Horizontality is widely, but not universally, applicable in the study of sedimentology and structural geology and one should always bear the above caveats in mind before accepting original horizontality as a fact.

The proponents of creation science attempt to use the violation of Steno's Laws as proof that geological science is founded on erroneous facts and thinking and that therefore higher reasoning in geology, namely Evolution and the Age of the Earth are invalid.

• Law of superposition
• Principle of lateral continuity
• Stratigraphy