English: Deformed rocks are common in mountain belts. Deformation includes folding, faulting, and metamorphism. Seen here are structurally-tilted sedimentary rocks in western Colorado. All layered rocks rocks were originally oriented more or less horizontal - this is the Principle of Original Horizontality. The orientations of layered rocks that are tilted or folded cannot be original - they are the consequence of orogenic deformation. The Principle of Original Horizontality was first defined in 1669 by Nicholas Steno: “As regards form, it is certain that at the time when any given stratum was being produced its lower surface, as also it lateral surfaces, corresponded to the surfaces of the lower substance and lateral substances, but that the upper surface was parallel to the horizon, so far as possible; and that all strata, therefore, except the lowest, were bounded by two planes parallel to the horizon. Hence it follows that strata either perpendicular to the horizon or inclined toward it, were at one time parallel to the horizon.” (from a 1916 English translation of Steno, 1669)