Law of Superposition
The law of superposition states that in undisturbed layers of sedimentary rock, each layer is older than the one above it and younger than the one below.
Definition
The law of superposition says that in a stack of rock layers that has not been flipped over, the bottom layer is the oldest and the top layer is the youngest. Stated by Nicolas Steno in 1669, it holds that in an undeformed sequence of sedimentary rocks or lava flows, each layer is younger than the layer beneath it and older than the layer above it, and it applies only where layers have not been overturned by folding or faulting, which geologists check using features such as ripple marks, mud cracks, and graded bedding. Superposition is one of Steno's three stratigraphic principles, with original horizontality and lateral continuity. It provides the ordinal framework for relative dating but says nothing about elapsed time, since deposition rates vary and unconformities may represent more missing time than the preserved rock records. In tectonically deformed terranes, overturned limbs of recumbent folds and thrust sheets place older rock over younger, so "way-up" indicators (cross-bed truncation, sole marks, geopetal fills) must be checked before applying the law.
Example
Think of a stack of newspapers on a porch: the one on the bottom was delivered first. In the Grand Canyon, the Vishnu Schist at the bottom is nearly $1.7$ billion years old, the Kaibab Limestone at the rim is about $270$ million years old, and every layer between falls in order. In the Appalachian Valley and Ridge, Alleghanian thrust faults stacked Cambrian carbonates on top of younger Devonian shale. Applying superposition naively there would invert the sequence; the geopetal structures show the thrust sheets are right-side-up but displaced.
Key Insight
It sounds obvious, but this simple rule is the foundation for reading Earth's whole history from rock layers. Superposition works for anything that accumulates, from snow layers to trash in a landfill to sediment in Lake Norman, and archaeologists rely on it as much as geologists do. It is the reason the geologic time scale could be assembled before radiometric dating existed. Absolute ages later confirmed every major ordering it had produced.