THE HISTORY OF GEOLOGY

By noting the relationships of different rock units, Nicolaus Steno in 1669 described two basic geologic principles.

The first, "The Law of Original Horizontality", stated that sedimentary rocks are layed down in a horizontal manner.

The second, "The Law of Superposition," stated that in an undeformed sequence of sedimentary rocks, younger rock units were deposited on top of older rock units. To envision this latter principle think of the layers of paint on a wall. The oldest layer was put on first and is at the bottom, while the newest layer is at the top. Steno recognition of fossils as traces of ancient life helped him with his discoveries.

An additional concept was introduced by James Hutton in 1795, and later emphasized by Charles Lyell in the early 1800s.

"The Principle of Uniformitarianism" is the idea that natural geologic processes have been uniform in frequency and magnitude throughout time. The principle is basically stating that "the present is the key to the past."

Steno's principles allowed workers in the 1600's and early 1700's to begin to recognize rock successions. However, because rocks were locally described by the color, texture, or even smell, comparisons between rock sequences of different areas were often not possible. It was the use of fossils that provided the opportunity for workers to correlate between geographically distinct areas. This contribution was possible because fossils are widely found in areas of the earth's crust.

For the next major contribution to the geologic time scale we turn to William "Strata" Smith, a canal builder and geologist from England.

"The Principle of Faunal Succession" was validated in 1815 by Smith when he produced a geologic map of England demonstrating the principle. This principle simply stated that fossils have a very definite order in which they are found in rocks and therefore any specific time period can be recognized by its fossil content. Once established, this principle allowed geologists to identify rocks of the same age in widely separated regions. This enabled the building of the geologic time scale.

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