Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.
In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating which provided a means of absolute dating in the early 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques to determine the geological events.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes. Relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167–69). The Law of Superposition was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith. While digging the Somerset Coal 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.
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Relative dating methods in archaeology are similar to some of those applied in geology. The principles of typology can be compared to the biostratigraphic approach described above.
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 for which we have no surface samples. Many of the same principles are applied. For example, if a valley is formed inside an impact 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)
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