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Relative dating
Relative dating
Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of the relative dating techniques. It estimates the order of prehistoric and geological events determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, often in horizontal, stratified bands of rocks present throughout the world. Though relative dating can determine the sequential order in which a series of events occurred, not when they occur, it is in no way inferior to radiometric dating; in fact, relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167-9). The Law of Superposition was the summary outcome of ’relative dating’ as observed in geology from the 1600s to the 1800s. Relative dating was observed by some in prehistory, but most recently it was discovered around 1800 by a British canal surveyor named William Smith. While digging up the Somerset Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor, he found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. In 1831, Smith received recognition for his work in the form of the Wollaston Medal, the highest honor of the Geological Society of London ("Biostratigraphy: William Smith")
Planetological use
Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the evolution of bodies in the Solar System, particularly in the vast majority of cases in which we have no surface samples. Many of the same principles are used; for instance, if a valley on Mars cuts across a crater, the valley must be younger than the crater. Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)
See also
• Key bed • Stratigraphy • Structural geology
References
• "Biostratigraphy: William Smith". Understanding Evolution. 2009. University of California Museum of Paleontology. 23 January 2009 • Hartmann, William K. Moons & Planets, 4th ed. Belmont: Wadsworth Publishing Company, 1999. ISBN 0-534-54630-7 • Monroe, James S., and Reed Wicander. The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0-314-09577-2 • Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
Relative dating in archaeology
Relative dating methods in archaeology are similar to some of those applied in geology. Form a partially ordered set so that the true chronological sequence cannot be reconstructed by stratigraphic means. The principles of typology can be compared to the biostratigraphic approach described above.
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