Radiometric dating speed of light
The depth at which either is found can vary dramatically.
In the Grand Canyon area the Cambrian lies beneath a huge column of strata; in California's Mojave Desert portions of the Cambrian are exposed at the surface.
Studies done on the European continent soon demonstrated the universal validity of index fossils.
While we're on this subject, you might wish to know the odds of arranging the Precambrian era, the seven geologic periods of the Paleozoic (Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian), the three periods of the Mesozoic (Triassic, Jurassic, Cretaceous), and the two periods of the Cenozoic (Paleogene, Neogene or Tertiary, Quaternary) in their proper order by pure chance.
Once the worth of index fossils had been established on the basis of stratification studies, they could logically be used to extend the correlation of rock formations to other continents.
At this point in time they were simply a useful tool for correlating rock formations.
Noah's flood might just as easily deposit the same crystal in one place as another. Pressure has nothing to do with it, and zircon crystals all have about the same density as their total lead content is small.
Just what is it that a Cambrian stratum has which a Cretaceous stratum lacks?