However, relative methods are , which is that if there are layers of deposits, those laid down first will be on the bottom and those laid down last will be on the top. However, geological strata are not always found to be in a neat chronological order.
Wind and water erode strata and some areas are uplifted or even tilted.
In addition, because of its particular relation with past human presence or past human activity, archaeology uses almost all the dating methods that it shares with the other sciences, but with some particular variations, like the following: Seriation is a relative dating method (see, above, the list of relative dating methods).
In other words, they may no longer be in their primary context.
When the bones of our early ancestors are found in the same geological strata as those of other animals that are known to have lived only during a specific time period in the past, we assume that these ancestors must also have come from that time.
Dating material drawn from the archaeological record can be made by a direct study of an artifact, or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence relative to datable contexts.
Dating is carried out mainly post excavation, but to support good practice, some preliminary dating work called "spot dating" is usually run in tandem with excavation.
Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history, archaeology, geology, paleontology, astronomy and even forensic science, since in the latter it is sometimes necessary to investigate the moment in the past in which the death of a cadaver occurred.
Relative dating methods are unable to determine the absolute age of an object or event, but can determine the impossibility of a particular event happening before or after another event of which the absolute date is well known.
It was the case of an 18th-century sloop whose excavation was led in South Carolina (United States) in 1992.
Thus, from the oldest to the youngest, all archaeological sites are likely to be dated by an appropriate method.
However, the assumption of contemporaneity may not always be correct.