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.
Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct. These are: Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.
Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.
Precambrian rocks must therefore be correlated by means of precise isotopic dating.
This principle has in the meanwhile been established to be true for all sediments worldwide, and is the basis of worldwide correlation of sedimentary rock units and one of the underpinnings of the theory of evolution. In location A we have rock layers that successively have different types and combinations (assemblages) of fossils.
If in location B we find the same fossil assemblage (Assemblage 2) in a rock unit, we may assume that they are of essentially the same age as in location A.