The three pits and the coring transect in zone 4 together provide a section with key information on the formation history of the Korhaan sand ridge. The complex history of the sand ridge is reflected by a series of lithological and pedological units (fig. 6: 10-3).
A coring in the bottom of pit A at 3.4 m below the current surface revealed (fig. 6: 13-11) a c. 10 cm thick organic sand layer containing macroscopic plant remains. They most likely represent a short interruption in aeolian activity prior to the ridge formation.
Crucial to the understanding of the ridge building processes is the 1.3 m thick sand deposit with unidirectional cross-bedding obliquely to the ridge below the Usselo horizon in pit A (fig. 6: 9). This structure is identical to sedimentary facies 4 as defined by Kasse (2002, and references therein), of which only a few examples have been attested in the Late Pleistocene aeolian deposits of Western Europe. This facies 4 is truncated by a veneer of coarse sands (fig. 6: 2) and covered by horizontally stratified sands (fig. 6: 8).
Layers 9, 8 and 2, which contain no artefacts, are interpreted as a single unit that reflects the dune building processes. The cross-bedded sands, which were built up by northwestern winds, correspond with the lee side of a dune progressing towards the southeast. As the dune progressed, windblown sands were transported over the windward side of the dune which was eroded, resulting in a lag formed by coarser grains on its top. The horizontally stratified sands were deposited at the end of the dune building process.
1 Lithic artefacts.
2 Deflation level, characterized by coarse sands.
3 Disturbed podzol horizons in aeolian sands.
4 In situ compact humic and iron B horizon of the podzol soil in aeolian sands.
5 BC and C horizon of the podzol soil in aeolian sands.
6 Whitish silty sand upper part of the Usselo horizon.
7 Whitish sand lower part of the Usselo horizon.
8 Horizontally stratified aeolian sands.
9 Unidirectional cross-laminated aeolian sands.
10 Sands with no visible layering.
11 Organic layer with macroscopic plant remains.
12 Sands with organic layers.
13 Reddish brown humic sands.
In pit A, the 10 to 15 cm thick bleached horizon with few, very small and dispersed charcoal fragments on top of these sands (fig. 6: 7-6) is interpreted as the Usselo horizon. It is subdivided into two sub-units: a very pale sub-unit of pure sand at the base and a more greyish sub-unit containing about 5% silt at the top. The texture differences are based on detailed grain size analysis (not presented here). This profile, with low amounts of fine material, was sampled for micromorphological analysis (Derese et al. in press). The analysis was not conclusive on the processes that resulted in the whitish horizon. Illuvial textural features, composed of coarse or fine clay, are absent below the Usselo horizon but two lateral samples of the layered deposits directly underlying the Usselo horizon do include horizontal bands with brown limpid illuvial clay, recording the occurrence of clay illuviation in the area at some stage. Final Palaeolithic artefacts have been recovered from this horizon in pit A and in the test pits in zone 2 (fig. 6: 1).
Fig. 7 Pit C, northeastern section. The bleached horizon splits towards the left in two distinct levels, each containing charcoal (
Van Gils et al. 2009
Charcoal fragments are abundant in the Usselo horizon in pit C, 25 m towards the southwest. This horizon is locally subdivided into two distinct bleached levels containing charcoal, the top of which is 0.2 m apart (fig. 7). The charcoal is concentrated in the top of both, but with a denser concentration for the upper level. No artefacts were recovered from the bleached horizons in this pit. A core transect between pits A and C with an average core interval of 3 m confirmed that the horizons in both pits are interconnected and represent the same stratigraphical level. It is not clear whether the absence of charcoal on top of the bleached horizon in pit A is a primary phenomenon or the result of later deflation.
The core transect from pit A towards the southeastern depression (fig. 8) shows the continuous presence of the Usselo horizon. From core 65 on, a thin peat layer covers its top, laterally rapidly evolving into a 40 cm thick, well-preserved and stratified peat layer (at core 67). A 2x2 m large pit was dug in the latter location (fig. 5b and 8: Pit B). In the entire transect both the Usselo horizon and the peat have been covered by a thin bed of yellowish gray aeolian sands in the top of which small but deep frost wedges and a podzol soil have developed (fig. 6: 5-3 and fig. 8: 4-1). The Usselo horizon was systematically prospected by drilling over a surface of 0.5 ha in zone 4 (fig. 5b: red line), as mentioned above. It was clearly separated from the podzol soil by yellowish grey sands in 44% of the cores (fig. 5b: white polygons).
1 Disturbed podzol horizons.
2 E-horizon of podzol.
3 B-horizon of podzol.
4 Yellowish-gray sands (C-horizon of podzol).
5 Sands with organic layers.
6 Clayey sands.
8 Usselo horizon.
9 Dark brown sands.
10 Yellowish-gray sands.
11 Sands with organic layers.
12 Location and level of the border of the Allerød swamp.
13 Location and level of the Final Palaeolithic occupation in pit A ( fig. 6 : 1).