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A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.However, by itself a fossil has little meaning unless it is placed within some context.Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).Younger layers are deposited on top of older layers (principle of superposition).Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.

Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.This is the principle of cross-cutting relationships.

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