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Relative Dating

Determining the Origin, Composition,
Distribution, and Succession of Rock Strata

Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life. Nicolas Steno, William Smith, Georges Cuvier, Alexandre Brongniart, and James Hutton developed the basic rules for the science of stratigraphy. Relative dating uses the principles or laws of stratigraphy to order sequences of rock strata. Relative dating not only determines which layers are older or younger, but also gives insight into the paleoenvironments that formed the particular sequence of rock.

A chance encounter between determined fishermen and a great white shark off the Tuscan coast in 1666 sparked a chain of events that would help change humans views of fossils and Earth’s geologic past (Cutler 2003, pp. 5-8). Nicolas Steno (1638-1686) dissected the head of this shark and realized fossil tongue stones believed to be petrified snake or dragon tongues were actually fossil shark teeth (Prothero 1998, p. 3). One problem still existed, how do fossils become embedded in solid rock? Steno recognized that fossils represent organisms that became buried in sediment, which later turned into rock. The realization that sediments turn into rock was counter to the view that all rocks on Earth formed in a single creation event. Once Steno recognized that the fossils he was contemplating (sharks teeth and sea shells) were formed in the sediments of oceans he was able to work out the basic rules of stratigraphy. Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled De solido intra solidum naturaliter contento dissertationis prodromus (Prothero 1998, p. 3).

The law of superposition is the foundation of Steno’s work on stratigraphy. The principle of superposition states that in an undisturbed sequence of strata or lava flows; each layer is older than the one above and younger than the one below. The law of original horizontality states that sedimentary strata and lava flows are deposited in horizontal sheets. If these layers are not horizontal, subsequent movements have occurred. The law of lateral continuity states that strata and lava flows extend laterally in all directions and pinch out at the edge of their deposition. Inclusions are rock fragments or fossils contained in another rock type. The principle of inclusions states that any inclusion is older than the rock that contains it. Steno's idea that fossils are older than the rock in which they are found hints at this principle, but Hutton is most often given credit for this principle. Steno developed these principles in the context water deposited sediment. It is not clear he was aware of igneous rock formed from lava flows.

The principle of faunal succession states that fossil organisms succeed one another in a definite, irreversible, and determinable order. This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p. 131) and by Georges Cuvier (1769-1832), a French anatomist, and Alexandre Brongniart (1770-1847), a French naturalist and geologist, during their work on the deposits of the Paris Basin. Brongniart was the first to use fossils to date rock strata.

James Hutton (1726-1797) is often considered the father of geology. Hutton developed the theory of uniformatarianism, which states that geologic events are caused by natural processes, many of which are operating in our own time. Put another way, the natural laws that we know about in the present have been constant over the geologic past. The concept of geologic time or deep time was a logical consequence of this theory. In 1788 John Playfair came to see Hutton’s Unconformity in Inchbonny. The unconformity consists of many vertical tilted layers of grey shale overlaid by many layers of horizontal red sandstone. Playfair later commented that, "the mind seemed to grow giddy by looking so far into the abyss of time." McPhee (1998) points out that Hutton removed humans from a specious place in time just as Copernicus had removed humans from a specious position in the universe (p. 74).

Hutton gives us three more laws to consider when seeking relative dates for rock layers, one of which, the law of inclusions was described earlier.
The law of cross-cutting states any feature that cuts across a rock or sediment must be younger than the rock or sediment through which it cuts. Examples include fractures, faults, and igneous intrusions. Igneous intrusions are sometimes referred to as a seperate principle, the principle of intrusive relationships. Unconformities represent gaps in geologic time when layers were not deposited or when erosion removed layers. This principle includes three types of unconformities. A disconformity is an unconformity between parallel layers. An angular unconformity exists when younger more parallel strata overlie tilted strata. A nonconformity is formed when sedimentary layers are deposited on igneous or metamorphic rock. Hutton’s theory of uniformatarianism and the principles of stratigraphy would be fully developed and made popular by another Scottish geologist Charles Lyell (1797-1875) with his classic three volume work, first published from 1830 to 1833, entitled Principles of Geology: being an attempt to explain the former changes of the Earth’s surface by reference to causes now in operation (Levin, 1999, p. 9).

The science of stratigraphy changed humans’ view of the world from one, which was static to one that was dynamic and changing. Not only did the rock layers indicate changing environments they also revealed that different life forms have existed in different times.

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Cutler, A. (2003). The Seashell on the Mountaintop. New York: Dutton.

Levin, H.L. (1999). The Earth Through Time [6th Ed.]. New York: Harcourt Brace College Publishers.

McPhee, J. (1998). Annals of the Former World. New York: Farrar, Straus, & Giroux

Prothero, D. R. (1998). Bringing Fossils to Live: An Introduction to Paleobiology. New York: McGraw-Hill.

Winchester, S. (2002). The Map that Changed the World: William Smith and the Birth of Modern Geology. New York: Perennial.

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