This is the case in rural environments, but in the case of urban conditions, where the layers have been disturbed for building, it is not always so clear-cut.
Often it’s necessary to know the date of a piece of evidence. In forensic archaeology, this can be approximated by Stratigraphy. The principle behind stratigraphy is that deposits are laid down in layers. Newer deposits lay over the top of materials, such that the deeper the hole is dug the older any finds are likely to be. This is the case in rural environments, but in the case of urban conditions, where the layers have been disturbed for building, it is not always so clear-cut. Each deposition layer is carefully recorded and removed before digging any deeper (Future Learn).