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Using these key or index fossils as markers, Smith could identify a particular layer of rock wherever it was exposed.Because fossils actually record the slow but progressive development of life, scientists use them to identify rocks of the same age throughout the world.Dating is not necessary to demonstrate that evolution is a fact.Chronological sequence is all that is really required.
Information about Simon Winchester's delightful biography of Smith, The Map That Changed the World is available at Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect variations in seasonal and annual rainfall.
Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology.
A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.
A short but clear explanation about radioactive isotopes commonly used for determining ages of rocks (with graphics) and putting numbers on the geologic time scale, extending it back before the occurance of abundant index fossils.
This is a relatively new method intended to to improve the precision of uranium and thorium istopy methods.