Idea of Stratigraphy in Geology and Archaeology
Stratigraphy relies on superimposition. This can be a easy idea stating that as layers of soil accumulate, the older deposits turn out to be progressively extra deeply buried. Due to this fact every layer in a sequence of soil is youthful than the one under it. In actuality the scenario often is far more advanced than this.
In archaeology we see that cuts (reminiscent of pits, ditches, post-holes and cellars) disturb this straightforward sequence. Nonetheless, superimposition nonetheless works. The pit, and so on is youthful than (and subsequently stratigraphically above) the layers via which it has been minimize. Its fill will probably be youthful than the pit minimize itself. A layer that covers each the pit fill and the encircling soil will probably be youthful than each of them. The scenario turns into much more advanced when a later pit is minimize via the primary pit fill. Cautious excavation is required to type out such advanced stratigraphy.
In geology, the identical idea is legitimate, as erosion (reminiscent of channels, rivers, valleys) disturb a superimposed sequence. The erosion is youthful than the layer under it. The fill of this erosion is even youthful than the erosion itself. If the erosion and the fill is roofed with new layers, these are the youngest within the sequence.
Again to our archaeology instance and let you know about different eventualities.
Typically we discover ourselves digging one thing that seems to belong to an ancient times than the layer under it. As an illustration, after a ditch has been dug, materials from exterior (i.e. stratigraphically earlier) could collapse into it as a big block. It now seems that the ditch incorporates soil that’s truly older than the ditch itself. In actuality nevertheless, the soil has been moved (‘re-deposited’) from its unique place and this makes it stratigraphically a brand new deposit. Once more cautious excavation is required to determine such re-deposited layers and keep away from acquiring inaccurate dates from them.
In geology this instance might be erosion of older rocks from some other place and transported into a brand new space with f.inst rivers and even an avalanche. These rocks are deposited on current rocks of their new place, and are subsequently superimposed on rocks deposited earlier than, nevertheless the transported rocks will be older than those they’re superimposed on. We are saying these older rocks are deposited later, nevertheless are older than the rocks they’re deposited upon.
Disentangling these relationships is a part of the problem and enjoyable of geology and archaeology. Inside geology these research type fundamental precept for exploration for oil and fuel traps, and is known as sequence stratigraphy.