Relative age of rocks is defined as the geological age of a fossil organism, rock or event relative to other organisms, rocks, features, or events rather than in terms of years. Relative age is important for understanding Earth’s history. To determine the relative age fo different rocks, geologists start with the assumption that in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is known as the Rule of Superposition. This rule serves as a powerful reference point. Geologists draw on it and other basic principles to determine the relative ages of rocks or features such as faults.