These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present.-Position in a sequence of rock- more than 1 layer is similar in rocks from different places.-Key bed- distinctive or unusual bed present in different areas (coal bed, volcanic ash layer).The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere.Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.
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.
It was first recognized by William Smith, a British Surveyor, who while working on open cuts of canals, railroads, and roads, noticed that the fossils change systematically from the older towards the younger rocks.
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.