Free 5-day trial Discover how geologists study the layers in sedimentary rock to establish relative age.
Learn how inclusions and unconformities can tell us stories about the geologic past.
Unlike people, you can’t really guess the age of a rock from looking at it.
Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range.
The most obvious feature of sedimentary rock is its layering.
This feature is produced by changes in deposition over time.
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 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.
Using relative dating the fossil is compared to something for which an age is already known.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.