Stratigraphy is the study of the order and position of layers of archaeological remains. These layers are laid down in a sequence and this can provide important clues about the succession of events as they occurred at the site. This is why stratigraphy is a vital archaeological tool: it helps us to understand the chronology of events and occupation at archaeological sites.
When people inhabit a site the various activities they engage in result in the formation of deposits of soil and sediment at that site. When the first phase of occupation is followed by a second phase, and many more phases after that, this results in a sequence of deposits that are stratified, they are ordered with the earliest layers at the bottom and more recent layers at the top.
As a site is being excavated, the archaeologist’s job is to identify the different deposits that remain from human activity in the past. The next step is to remove these deposits, starting with the most recent ones, that is, the ones closest to the top, and gradually working through the layers until the earliest deposits are revealed and excavated.
This means that archaeological sites are excavated in the exact reverse of the order in which the deposits were laid down; the earliest deposits are the last to be excavated. Think of the many layers that can be removed from around the heart of an onion.
Excavation is like this, gradually peeling away all the layers that represent the sequence of human activity at any given site. The aim of this type of excavation is to respect the stratigraphic sequence of the layers at the site.
Stratigraphy can also be examined in section. Think of the onion again, only this time instead of peeling away the layers one by one, take a knife and cut straight through the heart of the onion, then examine the surface that you have revealed. You can see all the layers of the onion at one time.
This is why archaeologists look at sections through large sites, to understand the sequence of events at the site on the large scale. This way they can identify the earliest, the middle and the latest deposits all in one go.
Once the stratigraphy of the site is understood, archaeologists will usually try to get a date from the earliest layers at the site, either through scientific dating methods such as radiocarbon dating, or through the identification of diagnostic artifact types such as pottery.
Once the archaeologist has this date, they can then identify the earliest phase of activity at the site with some certainty. A date from the most recent archaeological layers would then give them an idea of the entire duration of occupation of the site.
You can see from this explanation how fundamentally important stratigraphy is in understanding chronology and the sequence of events at any archaeological site!