Unlike relative time, absolute time assigns specific ages to events or formations and is typically recorded in years before present.

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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.

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.