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rocks. For example, natural processes can give different sediments that all form at the same time. Thus, a river will carry all kinds of sediment to the coast, but the coarser and heavier sands will be deposited to make a delta, while the finer and lighter muds will travel into the sea and be deposited as a layer of mud farther from the coast. These different sediments, all forming at the same time, are known as facies. They show that rocks of a similar age can be a sandstone (a sandy facies) in one place, while elsewhere they may form a shale (a muddy facies).
Fossils can be used to relate rocks over large areas, even when the rocks have very different textures. Thus, for example, a shale rock in one place might contain the same fossils as a sandstone or a limestone somewhere else.
There was also no doubt that fossils changed over time, with some species fading away and being replaced
Heavier sands are deposited nearer the coast.
Same species found in both depositional environments
Sand facies
Shale facies
Fine muds carried farther out to sea and deposited on the ocean floor
(Below) The same species of fossil can be found in different deposits because, despite their differences, the depositional process took place at the same time.