Method of dating fossils by radioactive decay on line dating toledo area
So it serves as a instant in time around the world and can be used to correlate ages of rock around the globe.
We know which rocks anywhere around the world were deposited before and after the event.
Well we do this pretty easily because bed forms form in sand and silts allowing us to tell if the rock has been folded or turned over.
We can look at grain size, larger grains usually settle first, followed by smaller and then finer grains so we look at the particle size distribution.
Basically from the Geologic Law of Superposition which states, the sedimentary bed on the bottom is the oldest.
This comes from the fact that sediments are laid down in water or on the surface of the earth in layers so the first is the oldest and next is younger.
If these same fossils are found elsewhere in different types of rocks we know too that that rock was deposited in the same general range of time.
They could burrow into an underlying layer and become fossilized there, but in nearly all cases this would be accounted for in the "age range" assigned to the fossils of the creatures.
So this allows geologists and paleontologists to give rocks relative dates based on fossils and visa versa.