Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.
Now imagine that you come upon a formation like this: What do you think of it? How can you make any conclusions about rock layers that make such a crazy arrangement?
By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.
They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.
Numerical dating determines the actual ages of rocks through the study of radioactive decay.Crosscutting Relations are those where one rock literally cuts across another, such as for example when igneous dikes and sills are emplaced in fractures within a pile of sedimentary rocks (see picture at left).Obviously, the sedimentary rocks had to be there prior to emplacement of the igneous rocks, and thus they are older than the igneous rocks.Conversely, the igneous rocks are younger than the sedimentary rocks.Other examples of cross crutting relationships can be related to faults (fault has to be younger than the rock it is found in) and unconformities (see below).