4 Chronology 
All the Neolithic settlements in Delfland belong to the Hazendonk group, a cultural unit at the end of the Swifterbant culture, restricted to the southern part of that culture’s distribution area, and extending into the Dutch and Belgian provinces of Limburg (Louwe Kooijmans 2006a; Amkreutz & Verhart 2006). The chronology, in particular that of the Hazendonk stratification, shows that this group covers a relatively short period, from 3800 until 3400 BC.
The 14C dates of the Delfland sites all fall within the aforementioned range of 3800-3400, with one possible exception. On the basis of the 14C dates, the earliest phases (2/B-3/C) of Ypenburg should coincide with the Hazendonk 2 stratum. With its S-profiled pots with widely everted rims, the Hazendonk 2 assemblage is however clearly a Swifterbant assemblage (Raemaekers 1999, 65-70), whereas Ypenburg C/3 with its straight-walled types morphologically belongs to the Hazendonk group, and must hence be later. On the basis of these considerations we may not date the beginning of occupation at Ypenburg any earlier than c. 3800.
As for the end dates, a single sherd with an everted, perforated rim at both Ypenburg and Schipluiden means that, at these sites, occupation continued just up to the Vlaardingen tradition, as indeed suggested by the 14C dates.
The chronologies of the sites can be further subdivided on the basis of the local natural stratigraphies in which the archaeological remains were embedded. At Schipluiden those stratigraphies are aquatic deposits (clay, peat) next to the dune, on the basis of which the (continuous) occupation can be subdivided into four phases and dated in absolute terms to 3630-3380 BC.
During its period of occupation, drifting sand was regularly deposited on the large dune ofYpenburg, which resulted in a succession of layers of virtually sterile dune sand alternating with soils or vegetation horizons formed in phases of vegetation development – some more distinct than others – of the kind also known from the later Older Dunes. A few finds and features show that the dune was visited by humans shortly after its formation (phase 2B). Most occupation remains however date from two main phases of stability, soil formation and vegetation development referred to as phases 3/C and 11/K. The dates of these two phases together lie between 3850 and 3450 BC, but those dates both involve a high degree of uncertainty, due largely to the wiggles in the calibration curve. Phase 3/C lies somewhere at the beginning of this range and phase 11/K entirely at the end. The succession of layers of drift sand and the vegetation horizons formed in them imply a substantial hiatus. Incidental evidence of human presence however indicates a certain continuity in the use of the site.
The two local chronologies can be linked on the basis of stratigraphically verified series of 14C dates and the calculated margins using the Oxcal program. Closer examination of this correlation reveals some chronologically sensitive variables in the pottery (fig. 4). The pottery of the Hazendonk group is simple and devoid of any pronounced stylistic features. Nevertheless, two variables can be used: temper and wall decoration. At both Ypenburg and Schipluiden the crushed shell that was initially used to temper the clay was gradually replaced by ground quartz, and the amount of wall decoration increased. We observe the same trends in the more general Hazendonk sequence.
The following general chronology, comprising two main phases, can be inferred from the data and considerations outlined above (fig. 5):
- a phase characterised by a beach plain and salt marshes with frequent marine ingressions coinciding with Ypenburg phases 2B and C/3, Schipluiden phases 1 and 2a and Wateringen 4, 3800-3500 BC,
- a phase characterised by a development towards wetter conditions and swampy grasslands coinciding with Ypenburg phase K/11 and Schipluiden phases 2b and 3, 3500-3400 BC.
Fig. 5 Chronology of the Hazendonk sites in Delfland compared with the chronologies of the Alblasserwaard region and the river area based on direct C14 dates (boxes with solid lines) or pottery typology (dashed boxes).
Now that the sites’ landscape and chronological contexts have been defined, we may turn to the question what they have in common and in what aspects they differ from one another. To what extent had these local communities continued to adhere to old traditions, and what new elements had they by this stage incorporated into their way of life?