Dating relative rock

Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life.Steno recognized that fossils represent organisms that became buried in sediment, which later turned into rock.

Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.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.Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.Precambrian rocks must therefore be correlated by means of precise isotopic dating.

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