This is because new sediments are always laid down on top of sediments that have already been deposited.So, when looking at the history of a cliff face, it is important to read the story it tells from the bottom layer up.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.Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.
In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.