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Why can tilted or overturned rock layers complicate the law of superposition?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

The law of superposition states that in an undisturbed sequence of sedimentary rock, the oldest layers are at the base and the youngest are at the top. Tilted or overturned rock layers complicate this principle because they disrupt the original horizontal orientation of the strata.123

When crustal deformation causes rocks to tilt or overturn, the standard vertical sequence is altered, making it difficult to visually determine which edge of a layer represents the original top and which represents the bottom. This challenge is further compounded if erosion has removed significant portions of the deformed rock, effectively blurring the geological record. In such cases, geologists must rely on other indicators—such as cross-beds, ripple marks, or fossil orientations—to identify the original depositional top and correctly interpret the relative ages of the units.14