Fossils and relative dating
New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great ‘tree of life’.Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since 1859, nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.The realization that sediments turn into rock was counter to the view that all rocks on Earth formed in a single creation event.
An absolute age is one determined usually by mass-spectrometry where an isotope is measured and then an age can be calculated (a very very basic explanation).
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.
Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.
This optimistic hope, however, must be tempered by the realization that much of the record—older than 541 million years—is missing.
The idea of radioactive decay and half lives, a type of absolute dating, is shown through an activity using M&M's candy and graph paper. Sequencing Time, University of California, Berkeley. This 5-12-grade activity lets students place parts of their own life story into a time line so that they can better understand how geologic time is reconstructed by scientists.