Relative dating

Relative dating is the kind of geochronology that determines the relative order of past events. The idea is to find the age of an object or event by comparing it to another object or event, or by using clues from the environment or circumstances where it was found. Relative dating is not about finding the absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Before the discovery of radiometric dating in the early 20th century, archaeologists and geologists used relative dating to determine ages of materials. 
Radiometric dating, on the other hand, is a means of absolute dating. Relative dating can only determine the "order" in which a series of events occurred, not "when" they occurred. Even so, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. Steno's Law of Superposition states that older layers will be deeper in a site than more recent layers. This law was the 'relative dating' method of choice in geology from the 17th century to the early 20th century