The most obvious feature of sedimentary rock is its layering. This feature is produced by changes in deposition over time. With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?
Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another. This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago. Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments. With out individual time stamps the process of dating these structures could become extremely difficult.
To deal with many of these problems geologists utilize two types of geologic time: relative time and absolute time. Relative time places events or formations in order based on their position within the rock record relative to one another using six principles of relative dating. Relative time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.
This process lead to a system of time containing eons, eras, periods, and epochs all determined by their position in the rock record. For example, rocks of the Phanerozoic eon are found on top of rocks from the Proterozoic eons therefore rocks of the Phanerozoic are younger than rocks of the Proterozoic.
Unlike relative time, absolute time assigns specific ages to events or formations and is typically recorded in years before present. This process requires much more sophisticated chemical analysis and, although other processes have been developed, often utilizes the decay rates of radioactive isotopes to determine the age of a given material. Using this process geologists are able to assign actual ages with known degrees of error to specific geologic events. By combining knowledge gained using both relative and absolute dating processes geologists have been able to produce the geologic time scale.