2 Schokkerhaven-E170: geological context and stratigraphy
The geomorphology of the Central Netherlands changed considerably over the course of the Saalian through to the Weichselian (de Mulder et al. 2003). The Saalian glaciation stands at the basis of the formation of ice-pushed ridges which partly surround the Flevoland polders and deep basins in the northern half of the Netherlands. One such basin is present in the Flevoland area. The basins became filled with fluvioglacial, marine and organic terrestrial sediments at the end of the Saalian and throughout the Eemian interglacial, while the Weichselian cold stage was characterized by the deposition of aeolian cover sands over vast areas.
During the early Holocene the geomorphology of the landscape changed under the influence of processes induced by sea-level rise and fluvial dynamics. The Noordoostpolder area was increasingly influenced by rising (ground) water tables, in particular during the Middle and Late Atlantic (Gotjé 1993; Van de Plassche et al. 2005). The growth of basal peat gradually followed the upsloping Pleistocene surface from west to east. At the same time, the area was increasingly influenced by changes in the tidal patterns due to the encroaching shoreline. However, tidal influence halted when the coastal barrier along the western coast closed during the Subboreal, resulting in the formation of an extensive freshwater lake (Almere) in the area and erosion of peat deposits due to surges in Subboreal and Subatlantic times. The re-opening of the northern coastal barrier in the Subatlantic resulted in the formation of an inland sea (Zuiderzee) and almost complete inundation of the area. Only some small islands formed on outcrops of glacial till remained dry land, until systematic land reclamation (between 1942 and 1968) subsequent to the construction of a dam (Afsluitdijk) in 1932 altered the entire nature of the area.
The site of Schokkerhaven-E170 (Dutch coordinates 179.811/514.625) is located southwest of the former island of Schokland (the base of which is formed by an outcrop of glacial till), on a west-east oriented aeolian river dune that stretches over at least 600 m and is some 200 m wide. The dune is flanked by a gully in the south. The highest parts of the dune, above roughly 4 m below NAP[1], were eroded due to natural processes and anthropogenic activities. Below this level the stratigraphy is undisturbed and represents the natural accumulation of peat with clastic sediment and sandy and anthropogenic components. The local stratigraphic sequence on the slopes of the river dune at Schokkerhaven-E170 is up to several meters thick. The upper part consists of marine Zuiderzee deposits of which the top is ploughed. The stratigraphic sequence in general terms, as depicted in Figure 2 (from top to bottom), consists of the following layers:
- locally undisturbed lacustrine Almere deposits (silty and humic clay) with locally reworked sand at the bottom, originating from the top of the dune
- brook peat deposits (the upper part is desiccated due to the presently low water table)
- detritus/gyttja with some clay
- humic sand (A-horizon)
- river dune sand (C-horizon)
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Figure 2 Profile of the trench at Schokkerhaven-E170 and position of the sample boxes.
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The thickness of layers 3 and 4 varies strongly and this is related to the surface level of the river dune sand. The timing of the start of peat growth along the dune slope at Schokkerhaven was related to the elevation of the sandy substrate and local (ground) water level. Peat layers formed a natural archive of the local and regional vegetation by embedding microfossils (among which pollen and fungal spores), seeds and other macrofossils.
At the site, archaeological materials were found in the upper layer of the sand dune and on the slopes in the layers 3, 4 and 5. The great majority of finds consists of knapped flint and pottery fragments, the latter are principally assigned to the late Swifterbant Culture (Hogestijn 1990; Raemaekers 1999, 2005). Three radiocarbon dates on hazelnut shells, charcoal and the food crust from a pottery sherd (collected in situ from layer 4 on the dune's southern slope) produced results between c. 4000 and 3700 cal BC (Lanting & Van der Plicht 2002) and these dates are synchronous with the late Swifterbant Culture. Furthermore, three wooden posts (possibly belonging to a palisade) sampled from layer 3 produced dates between c. 3500 and 3100 cal BC (Lanting & Van der Plicht 2002). These dates correspond to the Funnel Beaker Culture, which succeeded the Swifterbant Culture in the region. In addition to remains of the Swifterbant and Funnel Beaker cultures, there is also evidence for Mesolithic flint artifacts and Neolithic pottery remains.