Relative time allows geologists to determine the order of geologic events. It characterizes geologic time as a chronology, with no absolute dates. Rock formations are simply classified as older or younger relative to other formations.
Geologists determine a rock's origin by observing its physical attributes. Certain characteristics, such as a rock's chemical composition and the way its constituent minerals are arranged, give clues about the environment in which the rock was formed. For example, was the rock formed during a volcanic eruption or from materials deposited in a riverbed? Moreover, by looking at an outcrop, it is possible to determine the order of events that resulted in the rocks (or sediments) that it contains. Such determinations are made using six main principles that are central to relative geologic time.