Dates from ditches 3 and 4 from Perdigões. They are Chalcolithic ditches, dated from the 2nd quarter and middle 3th millennium BC. But in a top layer of ditch 4 we got a Late Neolithic date (a ditch that has to middle 3rd millennium dates for bottom deposits). A typical situation of incorporation of earlier material. In fact, just 3 meters away there was a Late Neolithic pit burial that was disturbed in Chalcolithic times. Taken from Valera & Silva, 2011.
Dating the fillings of ditches and interpret the results is a tricky issue.
First, we must be aware that we never date the opening of a ditch, only the beginning of its filling. And we do not know the time between the opening and the beginning of the last filling process of the ditch (I say last because we must considerer the possibilities of re-openings).
Second, what is integrated in the ditch filling may be older than the filling process or even older than the excavation of the ditch. In fact, especially in sites with long living periods (like Perdigões, for instance), where earlier materials can be about or the excavation activity is so intense that earlier deposits are constantly being remobilized, it is natural that some of the material that integrates a filling deposit inside a ditch is actually from earlier occupations. By dating this material we will not date what we intent to. We will be dating earlier material that has nothing to do with the time the ditch building or its filling moments. That is why sometimes we have earlier dates in top deposits and later ones in the bottom deposits.
So, how can we avoid these problems?
Dating ditch fillings implies dating series of samples, where outliers can be detected. It is not just the need of dating the sequence of deposits to determine the filling rhythm (which is, of course, important). Even if there are just one or two deposits, we must have more than just one date, precisely because of the problems raised by the typical dynamics and activities that take place in this kind of sites.
It is more expensive, I know. But one date in a ditch, except if you have a well defined and closed context (like a burial, for instance), is far from enough. And every date or series of dates need a serious critique, regarding the dating procedures, dated material, relation to context, nature of the context formation, post depositional events, and so on. Or we will just adding smoke to an already foggy area.
Valera, A. C. & Silva, A. M. (2011), “Datações de radiocarbono para os Perdigões (1): contextos com restos humanos nos Sectores I e Q”, Apontamentos de Arqueologia e Património, 7, Lisboa, NIA-ERA, p.7-14.