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How do we know the age of the rocks?
Some originated as layers of sediment on the sea bed or lake floors (so they are called sedimentary rocks) and may contain the fossilised remains of animals or plants. These fossils can give a relative indication of age, with different fossil assemblages appearing in vertical succession (oldest at the bottom, youngest at the top) and each one having a lateral correlation with the same assemblage in other areas. Absolute, or numerical, age determinations are obtained by careful measurement of the proportions of minor radioactive components in igneous rocks (that were once molten), or metamorphic rocks (those much changed by great heat and high pressure). These proportions are ‘frozen-in’ when the molten or reheated rock cools and solidifies, and thereafter the radioactive elements decay steadily at a known rate. By integrating both kinds of evidence (Figure P785797) we can build up an accurate timetable of when things happened.