That means they don't really know how old their rocks actually are.
Sort of an offshoot of stratigraphic succession is fossil succession, or a method in which scientists compare fossils in different rock strata to determine the relative ages of each.
Scientists know that the layers they see in sedimentary rock were built up in a certain order, from bottom to top.
When they find a section of rock that has a lot of different strata, they can assume that the bottom-most layer is the oldest and the top-most layer is the youngest.
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