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Geologic Principles

Beginning in the 1600's, with a renaissance in scientific investigation, early geologists began to investigate the rock layers that were evident on the land. Their early observations eventually proved to hold true and were described as Principles. Four of these Principles are important in the understanding of the Geologic Time Chart.

Law of Superposition

Law of Crosscutting Relationships

Law of Inclusions

Law of Faunal Succession




























































Law of Superposition

Nicolaus Steno, a Danish anatomist, geologist, and priest (1636 - 1686) observed the changes in a sequence of rock layers while working in the mountains of Italy. Steno's observations became known as the Law of Superposition which simply stated that in a sequence of sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it. The Law of Superposition also applied to other geologic events on the surface, such as lava flows and ash layers from volcanic eruptions.

The Law of Superposition meant that the Coconino Sandstone is older than the Toroweap Formation and younger than the Hermit Shale as seen in the image above. As it pertained to the Geologic Time Chart, the Law of Superposition meant that the Jurassic Period was older than the Cretaceous Period and younger than the Triassic Period.

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Law of Crosscutting Relationships

Described by Scotsman James Hutton (1726 - 1997), the Law of Crosscutting Relationships stated that if a fault or other body of rock cuts through another body of rock then it must be younger in age than the rock through which it cuts and displaces.

In the image above a Dike of igneous rock cuts through three layers of previously deposited sedimentary rocks. In this instance is the Dike the Oldest, Next Oldest, or the Youngest layer of rock?

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Law of Inclusions

The Law of Inclusions was also described by James Hutton and stated that if a rock body (Rock B) contained fragments of another rock body (Rock A), it must be younger than the fragments of rock it contained. The intruding rock (Rock A) must have been there first to provide the fragments.


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Law of Faunal Succession

In 1790, while engineering canals to link Britain's looming industrial age together, William Smith observed that fossils of invertebrate animals found in the rock layers appeared in a predictable sequence. From this observation the Law of Faunal Succession was developed and stated that fossils occur in a definite, invariable sequence in the geologic record.

As you can see in this image the fossil remains of living things are present in the rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?



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