Lateral Continuity, Superposition, and Inclusions

The deposition of rock layers, particularly sedimentary layers, in a sequence where one layer is atop the other layer and so on leads to a sequence of rock layers that geologists can interpret. A geologist can look at these layers and determine which was deposited first, which layer is the youngest, what type of environment existed at the time to form the rock layer, and where there is missing time in the sequence due to weathering and erosion, and so on. Interpreting the stacks upon stacks of rock layers in the Geologic Record is one of the fundamental things geologists can do to establish the history of our planet and the life that has lived on it. How do they do it? Let’s start by learning fundamental rules for “reading” a stack of rock layers!

First, the principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous. Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin. As long as sediment is transported to an area, it will eventually be deposited. However, as the amount of material lessens from the source, the layer of that material will become thinner.

Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a stratum is known as sedimentary facies. If sufficient sedimentary material is available, it will be deposited up to the limits of the sedimentary basin. Often, the sedimentary basin is within rocks very different from the sediments being deposited. In those cases, the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.

Principle of Superposition

Most sediment is laid down in layers where it eventually gets buried, compacted, and cemented to form a sedimentary rock. If the sedimentary rocks are no longer horizontal, then we can assume the tilting event happened after the layers originally formed.

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 law was formulated in the 17th century by the Danish scientist Nicolas Steno. When combined with the related principle of faunal succession, the law of superposition provides a very powerful tool for dating rocks and strata.

Figure Above: Younger units are deposited on older units. Tan sediment is deposited over older gray rock, and red layers are deposited over tan rock.

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, determining their place in the layers. Since the oldest was deposited first, it is on the bottom and vice versa.

Principle of Inclusions

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 shape. Steno could show by this reasoning that fossils and crystals must have solidified before the host rock that contains them was formed.