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:
Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.
The principle was first proposed in the 11th century by the Persian geologist, Avicenna (Ibn Sina), and the law was later formulated more clearly in the 17th century by the Danish scientist Nicolas Steno.
It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimantary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains.
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.
Steno stated another, more general principle in this way:
If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface.
In other words: a solid object will cause any solids that form around it later to conform to its own shape.
Steno was able to show by this reasoning that fossils and crystals must have solidified before the host rock that contains them was formed. If a "tongue stone" had grown within a rock, it would have been distorted by the surrounding rock, in much the same way that a tree root is distorted by growing into a crack in the earth. Instead, the "tongue stone" must have been buried in soft sediments which hardened later. Veins (mineral-filled cracks) and many crystals, on the other hand, must have formed after the surrounding rock was a solid, because they often did show irregularities of form caused by having to conform to the surrounding solid rock.
Finally, in the case of strata, layers on top of a set of strata conform to the shape of lower layers. . . and therefore, in a set of strata, the youngest layers must be those of the top layer, and the oldest must lie on the bottom. This is because the youngest layer was deposited after the oldest layers, which determines their place in the layers. Since the oldest was deposited first it is on the bottom and vice versa.
From Steno's observation that rock strata form when particles fall out of suspension in a fluid, it then follows that the youngest stratum is on the top of a sequence. However, this principle also applies to other types of rocks that do not form with water, such as volcanic rocks which spread on older flows, by flow banding.
Steno realized that other geological processes could create apparent exceptions to his laws of superposition and horizontality . He reasoned that the formation of caves might remove part of a lower layer, and that the collapse of a cave might transport large pieces of an upper layer downwards. He recognized that rocks might be uplifted by subterranean forces. Geologists now recognize that tilting, folding, and faulting may also complicate the analysis of a stratigraphic sequence. Molten rock may force its way through surrounding rocks and may sometimes squeeze between older rock layers, also forming an exception to Steno's law. However, such anomalies leave physical evidence in the disturbed rocks; for example, faulted rock layers may be cracked, broken, or metamorphosed along the fault lines.
Steno's law is a statement of relative time, not absolute time: two rock layers, in principle, could form millions of years apart, or days apart.
Thrust faults were unknown to Steno and his contemporaries and were not described until the late 19th Century and early 20th century by Peach and Horne at Knockan Crag, Scotland, on the Moine Thrust Fault. Thrust faults can cause confusion with the Law of Superposition because they occur parallel to bedding and can be difficult to detect, thus creating situations where inexplicably, older strata can overlay younger.