Journal of Archaeology in the Low Countries 2-2 (November 2010)Bart Vanmontfort; Marijn Van Gils; Etienne Paulissen; Jan Bastiaens; Marc De Bie; Els Meirsman: Human occupation of the Late and Early Post-Glacial environments in the Liereman Landscape (Campine, Belgium)

4 Geomorphology

4.1 Data

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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).

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.

7 Peat.

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).

4.2 Chronology

Optical dating results combined with field evidence (Derese et al. in press) show that the upper 1.5 m aeolian sand sequence was deposited from the final phase of the Late Pleniglacial up to the Younger Dryas. A more detailed chronology is based on five AMS radiocarbon dates: one sample of uncharred seeds from the very top of the peat in pit B and four samples from charcoal, two in the lower and two in the upper bleached horizon in pit C (table 1 and fig. 9). The end of the peat growth is AMS dated on uncharred seeds at 11,000 ± 60 BP (Poz-28168), or 13,065-13,010 (5.1%) and 12,985-12,672 (90.3%) cal BP according to IntCal09 (Reimer et al. 2009). As no charred seeds were recovered from the bleached horizons, charred wood remains were dated instead. Radiocarbon dating of a sample containing different charcoal fragments implicitly risks producing a mean age of several fire events. This is especially true for samples, which have been radiocarbon dated by the conventional method, needing significantly higher amounts of carbon and thus more charcoal pieces. Therefore single small charcoal fragments have been submitted for AMS dating. Fragments of small branches were selected, as evidenced by their form and growth rings, in order to largely eliminate old wood effect.

Sample number	Locus	Context	Material	Remarks	Conventional Age	2 Sigma Calibration in Cal BP	1 Sigma Calibration in Cal BP
Poz-28168	Korhaan pit B	top peat layer	uncharred seads		11000 ± 60 BP	13084 (95,4%) 12688	12965 (68,2%) 12735
Poz-28515	Korhaan pit C	upper bleached layer	one single charcoal piece		10480 ± 60 BP	12581 (88,9%) 12204	12558 (59,8%) 12380
						12197 (6,5%) 12140	12263 (8,4%) 12223
Poz-28516	Korhaan pit C	upper bleached layer	one single charcoal piece	0,8mg Carbon	10880 ± 60 BP	1290 (95,4%) 12610	12835 (68,2%) 12655
Poz-28517	Korhaan pit C	lower bleached layer	one single charcoal piece		11010 ± 50 BP	13085 (95,4%) 12699	13049 (3,5%) 13,033
							12967 (64,7%) 12752
Poz-28518	Korhaan pit C	lower bleached layer	one single charcoal piece	0,4mg Carbon	11240 ± 120 BP	13358 (95,4%) 12797	13276 (58,5%) 13058
							13023 (9,7%) 12970
Weigthed mean age samples 28516, 28517 and 28518					10983 ± 37 BP	13065 (5,1%) 13010	12925 (68,2%) 12742
						12985 (90,3% 12672

Table 1 Details of the 14C age determinations. All ages have been calibrated with IntCal09 ( Reimer et al. 2009 ) – Oxcal v4.1.5 ( Bronk Ramsey 2009 ).

For three charcoal pieces (Poz-28516, Poz-28517 and Poz-28518) the possibility that they represent different fire events cannot be excluded on the level of a single standard deviation, but on the 95% confidence level all three dates overlap. The question arises whether or not the four charcoal dates belong to the same normal distribution. As a result of the calculation and for a 1% probability, Poz-28515 is significantly different from the other three samples (Poz-28516, Poz-28517, and Poz-28518), which, although originating from both the upper and lower bleached horizons, may belong to the same normal distribution. The weighted mean of these samples is 10,983 ± 37 BP or 13,065-13,010 (85.1%) and 12,985-12,672 (90.3%) cal BP (fig. 9).

4.3 Environmental reconstruction

The bleached horizon with charcoal that covers the cross-bedded sands of layer 9 (fig. 6), interpreted as the Usselo horizon, is a key horizon in the stratigraphy of the Korhaan sand ridge. The weighted mean of the three individual charcoal pieces that were sampled from this layer is 10,983 ± 37 BP, which is in agreement with the ages of around 10,950 BP on charcoal fragments from pine found in the Usselo horizon at many sites (van der Hammen & Van Geel 2008). The Korhaan data, however, suggest fire events not only at the transition between Allerød and Younger Dryas (YD) but also during the YD (Poz-28515). A disruption of the local vegetation during a very short period with an influx of sands is suggested to explain the split of the Usselo horizon at this particular spot. Its timing remains however unclear: did it occur at 10,983 ± 37 BP or during the YD? The presence of YD charcoal in the upper whitish layer of the Usselo horizon in any case confirms that this soil surfaced during the earlier part of the YD.

The presence in pit B of small but deep frost wedges suggests that the Usselo horizon is buried under a cover of YD aeolian sands, with a maximum thickness of 140 cm and a mean of 80 cm for all observations where the Usselo horizon is attested. In the transect in figure 7 it is on average 75 cm with a maximum of 100 cm. It cannot be excluded that the Usselo horizon has been obliterated elsewhere by the later podzolisation as a result of a thinner YD sand cover. The YD cover thus slightly influenced the height of the sand ridge, but barely its morphology. This is mainly an inherited (pre-)Allerød morphology, which is mostly determined by the unidirectional cross-laminated sands deposited by northwestern winds and prograding towards the southeast.

The top of the peat allows a precise environmental reconstruction for the transect between pit A and the Luifgoor depression at the transition Allerød-Younger Dryas. The Final Palaeolithic artefact scatter (see below) at the top of the Allerød horizon in pit A is situated only 0.5 m above and 35 m from the border of the wet depression (Fig. 8).

A simulation of the wider environment is based on the present-day DEM and the maximum elevation of the peat (fig. 10). It shows an extended marshy area towards the east, possibly with open water in its lower parts. This image is considered to be an underestimation of its real extent because of the later sediment deposition in the area, with the YD cover probably the most important. An interdisciplinary study is in progress to specify the chronology of the ridge building and the paleoenvironment based on investigations of both pollen and macroremains.

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