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A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events.When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced.In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.Any feature, including colour variations, textures, fossil content, mineralogy, or any unusual combinations of these can be used.

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In addition, because sediment deposition is not continuous and much rock material has been removed by erosion, the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.Continents move, carried on huge slabs, or plates, of dense rock about 100 km (62 miles) thick over a low-friction, partially melted zone (the asthenosphere) below.In the oceans, new seafloor, created at the globe-circling oceanic ridges, moves away, cools, and sinks back into the mantle in what are known as subduction zones (i.e., long, narrow belts at which one plate descends beneath another).Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute isotopes has been improved to the point that for rocks 3 billion years old geologically meaningful errors of less than ±1 million years can be obtained.

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