One of the most obvious differences in layers is thickness. Layers of sedimentary rock may range in thickness from less than an inch to many feet.
Sedimentary layers that are identifiably different from each other are called beds or strata. In many places on Earth's surface, dozens of strata are stacked one on top of each other. Strata are often separated from each other by relatively well-defined surfaces known as bedding planes.
In 1669, the Danish physician and theologian Nicolaus Steno (1638–1686) made a seemingly obvious assertion about the nature of sedimentary strata. Steno stated that in any sequence of sedimentary rocks, any one layer (stratum) is younger than the layer below it and older than the layer above it. Steno's discovery is now known as the law of superposition.
The law of superposition applies only to sedimentary rocks that have not been overturned by geologic forces. Igneous rocks, by comparison, may form in any horizontal sequence whatsoever.
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