This matching process is called correlation, which has been an important process in constructing geological timescales.Some fossils, called index fossils, are particularly useful in correlating rocks.
Many of these organisms have left their remains as fossils in sedimentary rocks.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.
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.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.The most obvious feature of sedimentary rock is its layering.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.