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Why are the oldest sedimentary layers usually found at the bottom of a sequence?
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Layered strata in an outcropping of the Morrison Formation on the west side of Dinosaur Ridge, near Denver, Colorado.
The principle that the oldest sedimentary layers are found at the bottom of a sequence is known as the law of superposition.
This fundamental concept of stratigraphy and geochronology dictates that in any undisturbed sequence of sedimentary rock, each layer is deposited upon the debris of the previous one. Consequently, the layers are progressively younger as you move upward in the sequence.123
While this principle is a cornerstone of geological study, it assumes the sequence remains undisturbed. In some cases, geological forces such as crustal deformation can tilt or overturn rock layers, and erosion may remove portions of the record, which can complicate the interpretation of which side of a layer was originally the top.1