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Why are younger sedimentary beds usually found above older beds?

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Faux amphibolite — geology A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.

In the field of geology, the principle that younger sedimentary beds are found above older ones is known as the law of superposition.

This fundamental concept of stratigraphy states that in an undisturbed sequence of sedimentary rock, deposition occurs from the bottom to the top. Consequently, each successive layer is deposited upon the debris or surface of the previous one, making the base of the sequence the oldest and the layers progressively younger as you move upward.123

While this principle is a cornerstone of geologic dating, it assumes the sequence remains in its original orientation. In some cases, geological processes such as crustal deformation can tilt or overturn rock layers, and thrust faults can occasionally displace older units on top of younger ones. Geologists use various visual indicators—such as ripple marks, cross-bedding, or the orientation of fossils—to confirm the original top and bottom of these layers when the sequence is not clearly undisturbed.13