Relative dating notes
But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: Relative age dating has to do with determining the temporal ordering of events in Earth's past.
Geologists employ a handful of simple principles in relative age dating; two of the most important of these are are the principles of Just as uniformitarianism is the key underlying assumption of geology, the science's most fundamental principle is superposition, developed by Danish anatomist Nicholas Steno (1638-1686) in the 17th century.
The regular order of the 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.
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An imaginary cross-section, showing a series of rock layers and geological events (A-I).
The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horizontality and the Law of Superposition, two important ideas used in relative dating.
Imagine cutting a slice of bread from a whole loaf.
Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.