It is, however, important not to confuse geochronologic and chronostratigraphic units.In the same way, it is entirely possible to visit an Upper Cretaceous Series deposit—such as the Egyptian mangrove deposit where the Tyrannosaurus fossils were found—but it is naturally impossible to visit the Late Cretaceous Epoch, as that is a period of time.It is based on the the changing sequence of life on Earth, with different species evolving, dispersing and becoming extinct through time.Sedimentary rocks that have been deformed, tilted, or even overturned can be correctly understood through knowledge of biostratigraphy, even though they may now be upside down (see Superposition).A geologic map or report typically is only a summary of investigations that frequently involve the collecting and processing of hundreds of rock samples, followed by the evaluation and interpretation of data from a variety of analytical techniques.A relative age is the age of a fossil organism, rock, or geologic feature or event defined relative to other organisms, rocks, or features or events rather than in terms of years.Planktic species (that float and live in the surface water of the oceans) are best because they are widely distributed and independent of the type of seafloor sediment.
The aim of chronostratigraphy is to give a meaningful age date to fossil assemblage intervals and interfaces, as well as to determine the geologic history of the Earth and extraterrestrial bodies.
Darwin’s most direct geological contribution was his explanation of the origin of atolls, the annular coral islands that dot the western Pacific Ocean.
He observed many different kinds of islands and realized each represented a developmental sequence—from the earliest stages as reef-ringed, extinct volcanic peaks, to the latest stages of barely submerged coral reefs with central lagoons.
While the biostratigraphic age of an Upper Permian bed may be shown to be Lopingian, the true date of the bed could be anywhere from 270 to 251 Ma.
On the other hand, a granite which is dated at 259.5 /- 0.5 Ma can reasonably be called "Permian," or most properly, to have intruded in the Permian.