What is stratigraphy dating
When we think of fossils, we often visualize dinosaurs or other large animals.
Those are generally too rare to be of much stratigraphic use.
An intervening ocean or a plate boundary can destroy the physical continuity of the sedimentary layers, making them impossible to compare by inspection alone.
Consequently, some other point of comparison is needed, some attribute that spreads more even more widely than the particular color and composition of a particular seam of rock.
Instead, stratigraphers normally use small animals or (more often these days) microfossils.
Thus, stratigraphic relative dates are considerably easier to get.
Animals, partizan those in the seas, are found over vastly wider areas than the sediment from one particular rock seam.
So, using these biological markers, rock layers across the world can be correlated with one another.
Each layer is compressed by the weight of those above it and becomes sedimentary rock. That is, wherever the layers of rock are exposed -- in a road cut, a steep river valley, a cliff face -- we can be fairly certain that the sediments get older as we go down the rock column and younger as we go up.
So, looking at any given exposure, we can be quite sure about relative age.
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That works for one cliff face, but how do we compare different exposures?