The law of superposition (or the principle of superposition) is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:

The law was formulated in the 17th century by the Danish scientist Nicolas Steno.

While discussing the origins of mountains in The Book of Healing in 1027, Avicenna first outlined the principle of the superposition of strata as follows:

Assuming that all rocks and minerals had once been fluid, Nicolas Steno reasoned that rock strata were formed when particles in a fluid such as water fell to the bottom. This process would leave horizontal layers. Thus Steno's principle of original horizontality states that rock layers form in the horizontal position, and any deviations from this horizontal position are due to the rocks being disturbed later.

There are exceptions to this case, because sediments may be deposited on slopes or gradients. These may be steep, locally, and can be up to several degrees. Nevertheless, the principle is essentially true.