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Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared.There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).
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.
The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).
The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).
Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.
In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).