Stratigraphy is based on the 'law of superimposition'. This is a simple rule stating that as layers of soil
accumulate, the older deposits become progressively more deeply buried. Therefore each layer in a sequence is
younger than the one below it. In reality the situation is much more complex than this.
Cuts (such as pits, ditches, post-holes and cellars) disturb this simple sequence. However, superimposition still
works. The pit, etc is younger than (and therefore stratigraphically above) the layers through which it has been cut.
Its fill will be younger than the pit cut itself. A layer that covers both the pit fill and the surrounding soil will
be younger than both of them. The situation becomes even more complex when a later pit is cut through the first pit
fill. Careful excavation is needed to sort out such complex stratigraphy.
Sometimes we find ourselves digging something that appears to belong to an earlier period than the layer below it.
For instance, after a ditch has been dug, material from outside (i.e. stratigraphically earlier) may collapse into
it as a large block. It now appears that the ditch contains soil that is actually older than the ditch itself.
In reality however, the soil has been moved ('redeposited') from its original position and this makes it
stratigraphically a new deposit. Again careful excavation is needed to identify such redeposited layers and avoid
obtaining erroneous dates from them. Disentangling these relationships is part of the challenge and fun of
Return to Stratigraphy.