Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
Unlike people, you can’t really guess the age of a rock from looking at it.
Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age, (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.
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.
Some very straightforward principles are used to determine the age of fossils.