Geologic ages

When [William Smith]? and Sir Charles Lyell first recognized that [rock strata]? represented successive time periods, there was no way to determine what time scale they represented. The age of the Earth and of the rock strata was the subject of scientific debate for over 100 years as various advances in other sciences continued to place the creation of the Earth further into the past. In the latter part of the 20th century, it became possible to assign relatively firm dates using radioactive dating. In the intervening century and a half, geologists and paleontologists devised two relative time scales. One scale -- termed the Geologic Timescale -- is used by both sciences. Geologists tend to talk in terms of Upper/Late?, Lower/Middle? and often Middle parts of periods or eras -- e.g. "Upper Jurassic", "Middle Cambrian". Paleontologist?s divide the same periods into sometimes regional faunal assembleges. For example, in North America the Late Neoproterozoic and Early Cambrian are grouped into a Waucoban? series that is then subdivided into zones based on trilobites. The same timespan is split into Tommotian?, Atdabanian? and Botomian? periods in East Asia and Siberia?.