How are cross cutting relationships used in relative dating
Inclusions are always older than the rock they are found in.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.Underlying assumptions are 1) that Faunal Succession is based on the observation that animals and animal communities that are preserved in sedimentary rocks change noticeably as geologic time passes (evolution).
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.
James Hutton’s observations also serve as the basis for the important geologic principle, cross-cutting relationships, which represent another technique used in relative dating.
In short, an intrusive rock body is younger than the rocks it intrudes.
Superposition of rock units is a very simple and straightforward method of relative age determination.The principle states that in a sequence of undeformed sedimentary rocks the oldest beds are at the bottom and the youngest ones are at the top.