Stratigraphic Law

Rock formations showing rock layers.

Geologists use many tools to study the Earth’s origin, history and composition. Among these tools are the stratigraphic laws which are used to determine the relative age of rock layers.

The first official statement of these stratigraphic laws is attributed to Nicholaus Steno, a 17th century Danish geologist, priest and anatomist. His original laws on superposition, original horizontality, lateral continuity and cross-cutting relationships are the main stratigraphic laws used today.

Law of Superposition

Nicholaus Steno‘s first law of stratigraphy, superposition (super in this case means “above” and position means “to place”) is based on his study of the Amo River valley in Italy. He noted in 1669 that when the lower layers had already formed, any upper layers were “…fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed.”

This ultimately means that the lowest level of sediment is the oldest, and is covered with younger layers. The law assumes that this concept holds true except when the settled order has been changed.

Law of Original Horizontality

Steno wrote, “Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon.” Layers that are flat are assumed to be undisturbed. Assuming rock layers are formed horizontally was necessary in order to analyze any later change.

Law of Lateral Continuity

This law covers how far a continuous layer (stratum) is spread. According to Steno, “Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way.” When the rock was deposited it occupied distance in every direction of an area. However, if there is something that separates the deposit then the cause occurred after the original deposit of the rock.

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Law of Cross-Cutting Relationships

When Steno saw a break in an otherwise continuous stratum, he concluded that the interruption (such as magma that had broken through the rock layer and crystallized, or when the stratum continuity was broken) is younger than the surrounding stratum.

He states, “If a body or discontinuity cuts across a stratum, it must have formed after that stratum.”

Considerations

Steno’s work was based on the theory of uniformitarianism. This theory’s fundamental principle is the assumption that the geological processes at work in the past are identical to the geological processes observed today. As a result, geological structures and formations can be explained through observing current geological activity.