The basic concept behind relative age dating of fossils is that the Lower Rock layers were deposited earlier than the Upper Rock layers. This is because the principle of superposition states that in an undisturbed sequence of sedimentary rocks, the oldest rocks are at the bottom, and the youngest rocks are at the top. As a result, fossils found in the lower layers are generally older than those found in the upper layers. This method of dating does not rely on measuring radioactive isotopes or other methods that require absolute dates, but rather on the relative position of the fossils within the layers of rock.