Principle of Original Horizontality

The principle of original horizontality holds that layers of sediment are originally deposited in essentially horizontal, flat-lying beds under the influence of gravity, so that any layers now found tilted, folded or otherwise deformed must have been disturbed by some later geological process after their original deposition. Danish scientist Nicolas Steno first articulated the principle in his 1669 work on the geology of Tuscany, alongside two companion principles concerning the sequence and lateral extent of rock layers, work that is widely regarded as founding the modern discipline of stratigraphy. The principle gives geologists a crucial reference baseline for interpreting rock formations in the field, since any deviation from horizontal layering observed in sedimentary rock can be read as direct physical evidence of subsequent tectonic deformation, such as folding or faulting, that occurred after the sediment was originally laid down. Together with Steno's related principles of superposition and lateral continuity, the principle of original horizontality remains one of the basic working assumptions geologists still rely on today when reading the sequence and history recorded in layered rock.