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We must answer this question if we want to add numerical ages to the geologic column.Geologists obtain dates for sedimentary rocks by studying cross-cutting relationships between sedimentary rocks and datable igneous or metamorphic rocks.By isotopically dating the igneous rocks, they have been able to provide numerical ages for the boundaries between all geologic periods.For example, work from around the world shows that the Cretaceous Period began about 145 million years ago and ended 65 million years ago.And if a datable volcanic ash buried the strata, then the strata must be older than the and sedimentary rocks or for layers of datable volcanic rocks inter-bedded with sedimentary rocks.They argued that physical processes that shape the Earth and form its rocks, as well as the process of natural selection that yields the diversity of species, all take a very long time.

The discovery of new data may cause the numbers defining the boundaries of periods to change, which is why the term numerical age is preferred to absolute age.For example, if we find a sequence of sedimentary strata deposited unconformably on a datable granite, the strata must be younger than the granite (figure above).If a datable basalt dike cuts the strata, the strata must be older than the dike.Another way to grasp the immensity of geologic time is to equate the entire 4.57 billion years to a single calendar year.On this scale, the oldest rocks preserved on Earth date from early February, and the first bacteria appear in the ocean on February 21.

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