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USGS CMG InfoBank: Original Horizontality

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Comment: 01:49 - 03:08 (01:19)
Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures
Keywords: "Nicholas Steno", "Principle of Original Horizontality", deformation, strata, sedimentation

Our transcription: Geologic structures are patterns in the arrangement of rock inside the Earth.
Among the most common patterns is parallel layering seen in sedimentary strata and some volcanic deposits.
One of the key insights leading to the birth of Geology as a modern science concerned the nature of this layering.
In the early Seventeenth Century, Nicholas Steno, a Danish military engineer living in Italy, published an important observation.
He noted that in most places at the bottom of water bodies sediment settles to form continuous flat lying layers.
This explains why young sedimentary strata tend to be horizontal with the youngest layer on top and the oldest at the bottom.
Steno's observation became known as the "Principle of Original Horizontality."
Geologists find this principle useful as a basis for measuring how much deformation has occurred in ancient strata.
If layers are folded, geologists assume that they layers were once nearly horizontal, and that the folding came later.

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