In archaeology the sequence of the vertically formed layers, features and artefacts within a site are termed ‘stratigraphy’. The term was borrowed from geology, which uses it to refer to sequences of rock strata. Generally, the lower a layer the older it is. Therefore Roman artefacts and features are likely to be found lower than Saxon ones.
However, archaeological stratigraphy gets more complicated than geological, because humans tend to disturb or dig through earlier layers and mix them with later ones. Instead of having just a series of layers one on top of the other what you end up with, especially on urban sites, are silted up pits and post-holes cutting through ditches, which cut through wall foundations, and so on.