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|- | __toc__ |} The geological time scale is used by Geologist s and other scientists to describe the timing and relationships between events that have occurred during the History Of The Earth . The table of geologic periods presented here is in accordance with the dates and Nomenclature proposed by the International Commission On Stratigraphy , and uses the standard color codes of the United States Geological Survey . Current geological '' of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major Geological or Paleontological events, such as Mass Extinction s. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the Extinction Event that marked the demise of the Dinosaur s and of many marine Species . Graphical timelines The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. Terminology The largest defined unit of time is the Eon . Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define a system of Faunal Stage s, of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological Nomenclature , though in general there are far more recognized faunal stages than defined geological time units. Geologist s tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units -- e.g. "Upper Jurassic ", "Middle Cambrian ". ''Upper'', ''Middle'', and ''Lower'' are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while ''Late'', ''Middle'', and ''Early'' are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on Trilobites . The same timespan is split into Tommotian , Atdabanian and Botomian stages in East Asia and Siberia . It is a key aspect of the work of the International Commission on Stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world. History of the time scale The principles underlying geologic (geological) time scales were laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the Principle Of Superposition , which states that any given stratum is probably older than those above it and younger than those below it. Steno's principles were simple; applying them to real rocks proved complex. Over the course of the 18th century geologists came to realize that: 1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; 2) Strata laid down at the same time in different areas could have entirely different appearances; 3) The strata of any given area represented only part of the Earth's long history. The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner , among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" and "Quaternary" remained in use as names of geological periods well into the 20th century. The identification of strata by the fossils they contained, pioneered by William Smith , Georges Cuvier , and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more finely and precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced, between 1820 and 1850, the sequence of geological periods still used today. British geologists dominated the process, and the names of the periods reflect that dominance. The "Cambrian," "Ordovician," and "Silurian" periods were named for ancient British tribes (and defined using stratigraphic sequences from Wales). The "Devonian" was named for the British county of . British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. 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. Creationist s proposed dates of only a few thousand years, while others suggested large (and even infinite) ages. For over 100 years, the age of the Earth and of the rock strata was the subject of considerable debate until advances in the latter part of the 20th century allowed Radioactive Dating to provide relatively firm dates to geological horizons. In the intervening century and a half, geologists and paleontologists constructed time scales based solely on the relative positions of different strata and fossils. In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references ( Global Boundary Stratotype Sections And Points ) for geologic periods and faunal stages. Their most recent work is described in the 2004 geologic time scale of Gradstein et al. (ISBN 0521786738), and used as the foundation of this page. Table of geologic time References and footnotes
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