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Undisturbed iron industry sites in the Sonian Forest, Belgium
Journal of Archaeological Science: Reports 29 (2020) 102113
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Undisturbed iron industry sites in the Sonian Forest, Belgium Florias Mees a b
a,b,⁎
, Roger Langohr
T
b
Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium Department of Geology, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: Metallurgy Iron production Loess Sonian Forest
In the Sonian Forest of Middle Belgium, undisturbed remnants of ancient metallurgic activity, dating from the Early Middle Ages, are found. They occur in an area with iron-rich geological formations at shallow depth below a loess cover. Iron-industry traces include scoria mounds, which always occur in pairs. Associated with each pair is a single large zone with a reddish-pink colour of the upper part of the soil. These zones were formed by heating of the soil from the surface, most likely during roasting of the iron sandstone ore as pretreatment. The surface horizon was later homogenised by biological activity, down to the top of the Bt horizon or the compacted part of the E horizon. Charcoal mounds, whose distribution is not clearly related to that of other archaeological structures, date at least partly from more recent periods. Another associated feature is an area with a surface cover of yellowish sands, representing a site where the sandstone was unloaded or cleaned before further treatment. The preservation of these various features related to ancient metallurgic activity is due to the unique land-use history of the Sonian Forest, where a centuries-long continuous prohibition of agricultural practices has prevented the destruction of shallow surface structures.
1. Introduction In comparison with land under agriculture, forest stands have a good potential for conservation of archaeological sites, because they can be largely unaffected by processes such as ploughing, land-levelling and man-induced erosion. In the loess belt of Middle Belgium, a prime example is the Sonian Forest; near Brussels, where no agricultural activities have taken place for several centuries and where acid soil conditions preclude strong bioturbation (Langohr, 2009a, 2009b). Archaeological structures that are preserved in part of the forest include traces of ancient metallurgic activity. The latter were first reported by Vincent and Vincent (1910), in the form of small mounds composed of scoria. Since their discovery, these and other iron-industry remnants in the Sonian Forest have received only little attention. They were considered in a forest-wide survey of anthropogenic features by Verboom and Langohr (1982), followed by some profile observations (Sanders et al., 1985) and by a more extensive study of one representative locality (Mees, 1989). Partial results of the survey by Mees (1989) have been made available (e.g. Langohr and Pieters, 1996; Langohr, 2009b, p. 18-19), but a full report has never been published. Following some occasional later studies (e.g. Bonenfant and Defosse, 1994; Metalidis et al., 2008), new research initiatives have recently been launched, starting with an age-determination feasibility study (Ghyselbrecht,
⁎
2018). In this paper, we present the results of the 1988–1989 survey by Mees (1989), as a baseline study providing information about spatial distribution patterns and soil properties that have not been duplicated by later investigations. It provides further evidence of the uniqueness of the Sonian Forest as an area with exceptional conditions of soil preservation. 2. Setting The Sonian Forest is located south-east of Brussels, in the central part of the Belgian loess belt. The area selected for a detailed survey is situated in the southern part of the forest, near Groenendaal (Fig. 1). It is delimited by a shallow man-made ditch in the east, a 6 m deep flatbottomed valley in the west, and a 4 to 6 m deep V-shaped gully in the south. The area was selected based on its plateau position and on the near-absence of anthropogenic features unrelated to iron-industry activity. Throughout the forest, Pleistocene loess deposits cover a substrate of marine Paleogene sediments. The loess cover, dating from the Pleniglacial B of the Weichsel glaciation (25,000 to 20,000 years BP; Haesaerts, 1984), has a total thickness of several metres. In the study area, the loess cover overlies fine iron-rich sands with sandstone layers (Lede Formation, Middle Eocene). Below these deposits (10 m
Corresponding author at: Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium. E-mail address: fl[email protected] (F. Mees).
https://doi.org/10.1016/j.jasrep.2019.102113 Received 14 February 2019; Received in revised form 7 November 2019; Accepted 18 November 2019 2352-409X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Archaeological Science: Reports 29 (2020) 102113
F. Mees and R. Langohr
Fig. 1. Location of the Sonian Forest, SE of Brussels, and location of areas with iron industry remnants in southern half of the forest, near Groenendaal, investigated by this study and others. Survey area as inset, with indication of valley margins (dashed lines), man-made ditch (full line), and road (double line).
raised to the melting point of iron, but most impurities will be melted. The melt flows into a pit next to the furnace, forming scoria upon cooling. These are still quite rich in iron, due to the low efficiency of the process. Numerous scoria mounds in a region around Groenendaal bear witness of ancient metallurgic activity in the Sonian Forest. The production of crude iron is not mentioned in the ‘Cuerboeck van Zoniën’ of 1371 or in later editions of the same document, which describe and regulate all economic activity in the forest. Metallurgic activity therefore certainly predates the period covered by these reports. In the past, it has been attributed to Roman times (Thoen, 1983) and to the Iron Age (D'Hondt, 1983), but radiocarbon dating of charcoal found in association with iron-processing features has subsequently established that it dates from the Early Middle Ages (Metalidis et al., 2008; Ghyselbrecht, 2018), corroborated by Optically Stimulated Luminescence dating results for associated heated soil material (Ghyselbrecht, 2018). Iron-rich sandstone from the Lede and Brussels Formations, which occur at shallow depths around Groenendaal, was used as ore. Zones where the ore was extracted have a characteristic microrelief, with many small depressions. Scoria mounds are usually present in the vicinity of these zones, but they also occur at greater distances. The Sonian Forest has always been important for the supply of charcoal, from the Roman period till the end of the 18th century. Charcoal was produced by placing wood in a pile, lighting the fire and then covering it with soil taken from around the pile. Remnants of this activity within the forest are the many small mounds with black earth, containing small charcoal fragments. A shallow, sickle-shaped depression often lines part of the mound, marking the place where soil was removed to cover the wood pile. Unlike scoria mounds, charcoal mounds occur throughout the forest.
thickness), a 50 m thick layer of sands of the Brussels Formation occurs (Middle Eocene) (Buffel and Matthijs, 2002, 2009). Soils of plateau areas of the Sonian Forest are loamy and well drained, with a clay-illuvial horizon that is penetrated by clay- and iron-depleted vertical planes. Parts of the E and Bt horizons are pedogenetically consolidated. These soils are Dystric Glossic Fragic Retisols (IUSS Working Group WRB, 2015) or Fraglossudalfs (Soil Survey Staff, 1999, 2014), recording physical changes during periglacial stages (Langohr and Vermeire, 1982; Langohr and Pajares, 1983; Langohr and Sanders, 1985). The influence of man on the soil and the landscape is minimal in the Sonian Forest, because the area has never been under agriculture since at least the eleventh century (Langohr, 2009a). In soils of more open areas of the forest, surface gleying often occurs, which is linked to compaction of surface layers, caused by circulation of forestry engines and trampling by horses.
3. Metallurgic activity During the Roman period (75 BC to 450 CE), metallurgic activity was quite extensive in southern Belgium, with Belgian Lorraine and the region between the rivers Sambre and Meuse as the most important centres. Remnants of low furnaces were also discovered in most of the Roman settlements in the sandy area of northern Belgium (Thoen, 1983). According to descriptions of the production process by Thoen (1983) and Laban et al. (1988), the ore was first crushed, washed, and then roasted to make it more reactive. The low furnace, consisting of a shallow pit with a chimney built over it, was filled with alternating layers of charcoal and pretreated iron ore. Large amounts of charcoal are required, because the required ratio between iron ore and charcoal volumes is about 1/10. In a low furnace, the temperature can not be 2
Journal of Archaeological Science: Reports 29 (2020) 102113
F. Mees and R. Langohr
Fig. 2. Distribution of archaeological structures in the study area.
4. Methods
polyester resin.
A map was created by mapping squares of 10 by 10 m at the time (scale 1/250), recording surface features and gauge auger observations (30 cm depth, 2 m spacing). Five types of archaeological structures were recognized and indicated on the field maps: (i) scoria mounds, (ii) charcoal mounds, (iii) mounds with mixed material, (iv) areas with a reddish-pink surface horizon, and (v) surface occurrences of yellowish sand (see Fig. 2). In addition, microrelief characteristics (breaks in the gradient of the slope, isolated depressions), vegetation (trees, ground vegetation), and tracks were mapped. Five profile pits were dug, within a short distance from each other (Fig. 2). Two pits are located in an area with a reddish-pink surface horizon (one in the centre of the area, one at the edge). The three other profiles represent the centre of a charcoal mound, a zone with yellowish more sandy material along the surface, and an area with normal profile development for soils on loess in depressions. Physico-chemical properties were determined by standard methods: grain size by wet sieving and pipette-method analysis, organic carbon and nitrogen contents by Walkley and Black and Kjeldahl procedures, pH using 1:1 soil:solution ratios, cation exchange capacity determination with ammonium acetate at pH 7, and measurement of exchangeable cations and dithionite-extractable Fe and Al with atomic absorption spectroscopy. Thin sections (mostly 24 by 36 mm large) were prepared after impregnation of undisturbed oriented samples with a
5. Results 5.1. Distribution patterns of archaeological structures Figure 2 presents a simplified version of the original base map, at a reduced scale, showing only archaeological structures and tracks. The survey demonstrated that scoria mounds always occur in pairs and that a zone with a reddish-pink surface horizon is associated with each pair, which was not noticed during earlier studies. Seven of these units have been recognised within the study area. One unit, in the north-eastern corner of the study area, is separated from the others, but the other six are grouped close together and appear to be arranged along two lines that are at a right angle. They define a rectangular area, delimited by natural boundaries to the west and to the south. More or less parallel tracks, coming from the main road, seem to lead directly towards some of the scoria mounds. Small isolated patches with reddish material (< 2 m diameter) are scattered all over the area that was mapped. The distribution of charcoal mounds seems to be unrelated to the distribution of scoria mound pairs. The few mounds with mixed material (scoria, charcoal, and fragments of soil horizons) also do not show a clear pattern. The morphology of one of these structures is similar to that of charcoal mounds, with a shallow depression partially 3
Journal of Archaeological Science: Reports 29 (2020) 102113
F. Mees and R. Langohr
Fig. 3. Soil profiles in areas with (a) a reddish-pink surface horizon, (b) a yellowish sand cover, (c) scoria (Fig. 3c after Sanders et al., 1985). Horizon symbols – pi reddish-pink colour, g surface gley development, c compaction, mn manganese enrichment, t clay illuviation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
expressed in the micromass of the iron-eluvial surface gley horizon that locally developed within the reddish-pink interval. In the transitional horizons and in deep vertical structures, patches with a darker micromass and fragments of clay-eluvial and Bt horizons are observed. Many of the sand grains in the reddish-pink surface horizon are covered with dark reddish-brown to opaque iron oxide coatings, which are often thin and not always continuous. The material of the coatings is similar to that of the matrix of the unaltered iron sandstone, but this sandstone has a markedly smaller grain size than the coated grains. The sandstone fragments in the reddish-pink layers always have an opaque matrix. Physico-chemical analysis confirms that the reddish-pink surface horizon closely resembles the normal unconsolidated clay-eluvial E horizon. The free iron content is about twice the normal value, and the sand content is 2–3% higher, but organic carbon content, nitrogen content, C/N ratio, pH, CEC, base saturation and the relative proportions of exchangeable cations are all the same as for natural soils (Table 1; cf. B.pi of Profile 2 vs E.bi of Profile 4).
surrounding it. An accumulation of yellow sands was observed in one place, at the southern end of a zone with anthropogenic structures, bordered by a clear break in slope gradient. 5.2. Soil profiles 5.2.1. Soils with reddish-pink surface horizon Soil profiles with a reddish-pink surface horizon (B.pi) are quite similar to undisturbed soils of the area in most respects. The same thin A horizon is present, the top of the Bt horizon is on average at the usual depth (30 to 40 cm) (Fig. 3a), and the reddish-pink layer (20 cm thickness) is nearly identical with the normal unconsolidated clayeluvial horizon (E.bi, with strong biological activity), except for its colour (4 YR 3/5, vs 10 YR 5/5 for unaltered E.bi horizon material). No gradual downward decrease in intensity of the colour is observed. The lower boundary of the B.pi horizon is very sharp, but an underlying transitional zone with veins or fragments of reddish-pink material is often present. Mole galleries and casts of other burrows filled with reddish-pink material are very common. Along the top of the reddishpink layer, an iron-eluvial surface gley horizon is commonly present (B.pi.g). The reddish-pink horizon also always contains some sandstone fragments. These are dark and partly red to purple, in contrast to the yellow-brown unaltered sandstone, which is also more porous. Beneath the studied charcoal mound, the soil is only slightly more reddish than the natural soil (9 YR 4/6). In thin sections, the micromass of the reddish-pink surface horizon has a dark colour in plane-polarised light, and a reddish haze is observed in cross-polarised light. These characteristics are less strongly
5.2.2. Soils with yellowish sand cover The yellowish sand layer that covers the soil at one location is heterogeneous, with a field texture ranging from sandy loam (9 YR 5/6) to silt (10 YR 5/4) and with local occurrences of clay-rich bodies. Sandstone fragments are present, as well as some rounded chert fragments and pieces of scoria. The sand layer covers a dark surface horizon, which is partly black with a high organic matter content, and partly grey, without a recognisable admixture of organic matter (Fig. 3b). This buried surface horizon has a thin layer (0.5–2 cm) with surface-gley characteristics along part of its base. The underlying 4
Journal of Archaeological Science: Reports 29 (2020) 102113
F. Mees and R. Langohr
Table 1 Physico-chemical properties of a profile with a reddish-pink surface horizon (P2), a profile with a yellowish sand cover (P5), and a reference profile (P4). Profile
Depth cm
Horizon symbol
Clay %
Silt %
Sand %
C %
N %
C/N
pH H2O
pH KCl
FeDCB %
AlDCB %
CEC cmol(+)/kg
Caexch cmol(+)/kg
Mgexch cmol(+)/kg
Kexch cmol(+)/kg
Naexch cmol(+)/kg
V%
P2
0–5 5–15 13–22 23–39 32–39 5–13 14–26 20–25 20–30 30–34 32–39 40–47 10–20
A B.pi B.pi E.(mn).c B.t.mn B.ys B.ys b.A (black) B.ys b.A (grey) B.ys-E E.c.bl E.bi
8.8 8.4 9.0 9.4 20.9 15.0 15.6 13.1 23.8 17.3 13.9 9.5 10.4
81.0 81.7 80.7 83.4 71.3 32.0 31.9 36.0 61.5 71.7 71.1 79.5 79.8
10.2 9.9 10.3 7.2 7.8 53.0 52.5 50.9 14.7 11.0 15.0 11.0 9.8
3.1 1.4 0.6 0.9 0.4 – 0.8 1.9 – 0.6 – 0.1 1.1
0.16 0.09 0.03 – – – 0.08 0.13 – 0.04 – 0.03 0.08
19.4 15.3 18.1 – – – 9.4 14.9 – 15.5 – 4.0 12.7
3.8 3.9 4.2 4.4 4.4 3.9 4.1 4.0 4.2 4.2 4.3 4.3 4.3
3.0 3.2 3.6 3.7 3.5 – – – – – – – 3.6
1.9 1.7 2.1 0.9 1.5 2.3 2.4 2.1 1.4 1.5 1.4 0.7 0.9
0.1 0.2 0.2 0.2 0.4 0.3 0.3 0.2 0.2 0.3 0.3 0.2 0.2
14.58 9.20 6.90 5.26 10.76 11.16 11.24 16.38 – 11.66 – 3.78 6.56
0.60 0.21 0.06 0.04 0.21 0.35 0.30 0.25 – 0.14 – 0.04 0.07
0.17 0.07 0.03 0.03 0.09 0.10 0.08 0.09 – 0.05 – 0.01 0.04
0.22 0.14 0.14 0.10 0.27 0.17 0.17 0.17 – 0.16 – 0.13 0.14
0.07 0.02 0.01 0.02 0.02 0.04 0.04 0.05 – 0.03 – 0.05 0.05
7.27 4.78 3.48 3.61 5.48 5.91 5.25 3.42 – 3.26 – 6.08 4.57
P5
P4
Horizon symbols – b buried horizon, bi biological activity, bl bleaching, c compaction, mn manganese enrichment, pi reddish-pink colour, t clay illuviation, ys yellowish colour.
6.2. Soil profiles
slightly yellowish transitional horizon is the unconsolidated clay-eluvial horizon of the buried soil. The top of the Bt horizon is 20 cm below the buried surface layer, which is too shallow for an undisturbed soil, in which this level is typically at a depth of 30 to 40 cm. Thin section observations confirm the high fine sand content of the surface layer, with a sand fraction composed of quartz, some glauconite and chert. Considerable amounts of clay are also present. Black humiferous parts of the buried surface horizon show the same characteristics as A horizons of normal soils on loess. Its greyish counterpart is characterised by a high charcoal content. Sandstone fragments in this profile consist of angular to subangular fine sand grains (quartz, glauconite, chert), embedded in a brown iron-rich matrix (up to 60%). Grain size analyses corroborate the relatively high sand and clay contents of the upper horizons (see Table 1, Profile 5). These horizons also yield significantly higher free iron and CEC values than normal soils on loess.
6.2.1. Reddish-pink surface horizon The presence of reddish material in soils where such colours are not natural is commonly associated with actions that involve heating. Through heating, certain mineral changes take place, which is accompanied by liberation of iron and recrystallisation of iron compounds. The presence of areas with a reddish-pink surface horizon in the vicinity of scoria mounds must clearly be related to some process that requires heat. Because the presence of areas with a reddish-pink surface horizon is not reflected in the topography, and because the soil profile has underwent only minor changes, except for a striking change in colour, it appears that soils with a reddish-pink surface horizon are normal loess soils which were heated, in situ, from the surface. Heating of the soil is most likely related to roasting of the iron sandstone ore. This is suggested by the slight admixture of sand that has occurred and by the changes that affected the sandstone fragments that the heated surface horizon contains. In the course of the years that followed heating of the soil (possibly more than a thousand years), the partially heated unconsolidated clayeluvial horizon was homogenised, from the surface down to the top of the underlying Bt or compacted E horizons, which wiped out any gradient in colour intensity that may have existed. Besides root activity, faunal activity might be partly responsible for this uniformisation. Faunal activity may in fact have been promoted by the improved nutrient status and increased pH after burning. Mole galleries and casts of other burrows, filled with reddish-pink material, are evidences of faunal activity, which is very low in natural soils of the Sonian Forest at present (Langohr and Cuyckens, 1986).
6. Discussion 6.1. Distribution patterns of archaeological structures The distribution patterns of archaeological structures in the study area are characterised by a clear association between scoria mound pairs and zones with a reddish-pink surface horizon. They each represent one site of low furnace activity. Charcoal mounds show a random distribution, which is not unexpected because charcoal production went on for several centuries after the period of metallurgic activity had ended, as demonstrated by historical records such as the Cuerboeck van Zoniën and by absolute age determination (Ghyselbrecht, 2018). Mounds with mixed material are probably merely waste heaps, although a relationship with charcoal production can be assumed for some mounds, based on morphological similarities with charcoal mounds. Other aspects of the distribution patterns include the occurrence of shallow depressions near some scoria mounds. These depressions might correspond to places where a pit was dug to get to the more clayey material of the Bt horizon, which was probably used to build part of the low furnace. Usually such pits were later filled with scoria, and those structures therefore occur beneath the scoria mounds at present (Fig. 3c) (Sanders et al., 1985). Small isolated patches with reddish material that are recognised throughout the study area probably represent superficial disturbance of the soil. This is suggested by the common strong perturbation of the upper part of the soil, marked by a buried surface horizon or a mixed horizon with A-horizon fragments, which is observed in about one third of all augerings.
6.2.2. Yellowish sand cover The yellow sand that occurs as a surficial accumulation at one location is derived from the Paleogene substrate (Lede Formation), as indicated by literature data, laboratory analyses and field observations around pits where iron sandstone was extracted. The zone with a yellowish sand cover in the study area is the site where the iron sandstone was cleaned or where stones were picked out from the load that was brought in from the extraction area. The presence of rounded chert fragments, derived from the stone layer at the contact between the loess and the Paleogene substrate, suggests that gathering of the sandstone was done without much care. The A horizon of the original soil is not preserved and parts of the lower soil horizons were also removed. Humus-rich parts of the buried surface horizon are fragments of the A horizon of a loess soil. The grey charcoal-rich parts developed along the surface of the stripped soil, before being covered by the sand and sandstone debris. 5
Journal of Archaeological Science: Reports 29 (2020) 102113
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7. Conclusions
D'Hondt, D., 1983. Historische schets van het Zoniënbos. In: Sporen van de Mens in Zoniën, Volume 2. Dry Borren Raad, Brussel, p. 1-7. Ghyselbrecht, E., 2018. OSL Dating of Natural and Cultural Heritage. Case Studies on Sonian Forest (B) and Stonehenge (UK). MSc Dissertation. Universiteit Gent 69, p. Haesaerts, P., 1984. Aspects de l'évolution du paysage et de l'environnement en Belgique au Quaternaire. In: Cahen, D., Haesaerts, P. (Eds.), Peuples Chasseurs de la Belgique Préhistorique dans leur Cadre Naturel. Patrimoine de l'Institut Royal des Sciences Naturelles de Belgique, Bruxelles, pp. 27–39. IUSS Working Group WRB, 2015. World Reference Base for Soil Resources 2014, Update, 2015. World Soil Resources Reports n° 106. FAO, Rome, pp. 192. Laban, C., Kars, H., Heidinga, A., 1988. IJzer uit eigen bodem. Grondboor en Hamer 42, 1–11. Langohr, R., 2009a. La forêt de Soignes, site unique pour les sciences de la terre et l’archéologie. In: La Forêt de Soignes. Connaissances Nouvelles pour un Patrimoine d’Avenir. Editions Mardaga, Wavre, pp. 181–196. Langohr, R., 2009b. The Zonian Forest – A Unique Site for Earth Science and Archaeology. Understanding Soilscape Evolution of the Belgium Loess Belt – A Review of 30 Years Research. Excursion Guidebook for the German Soil Science Society and Soil Science Society of Belgium 34. Langohr, R., Cuyckens, G., 1986. Une forêt aux pieds de 'limon'. Sol et reliëf en forêt de Soignes: témoins uniques. Réserves Naturelles 3, 51–58. Langohr, R., Pajares, G., 1983. The chronosequence of pedogenic processes in Fraglossudalfs of the Belgian loëss belt. In: Bullock, P., Murphy, C.P. (Eds.), Soil Micromorphology. A B Academic Publishers, London, pp. 503–510. Langohr, R., Pieters, M., 1996. De ijzerindustrie in het Zoniënbos. In: Gullentops, F., Wouters, L. (Eds.), Delfstoffen in Vlaanderen. Ministerie van de Vlaamse Gemeenschap, Brussel, pp. 158–159. Langohr, R., Sanders, J., 1985. The Belgian loëss belt in the last 20,000 years: evolution of soils and relief in the Zoniën Forest. In: Boardman, J. (ed.), Soils and Quaternary Landscape Evolution. John Wiley and Sons Ltd, p. 137-148. Langohr, R., Vermeire, R., 1982. Well drained soils with ‘degraded’ Bt horizons in loess deposits of Belgium: relationship with paleoperiglacial processes. Biuletyn Peryglacjalny 29, 203–212. Mees, F., 1989. Base Maps and Soil Survey of Undisturbed Iron Industry Sites in the Zoniën Forest (Loess Belt, Belgium). MSc Dissertation. Universiteit Gent, pp. 117. Metalidis, I., Deckers, P., Vanmontfort, B., Langohr, R., 2008. Archeologisch onderzoek op GEN Lijn 161, TR 101311 (Groenendaal, Hoeilaart). EPA Report n° 3. Katholieke Universiteit Leuven 55. Sanders, J., Langohr, R., Cuyckens, G., 1985. Bodems en reliëf in het Zoniënbos. Inleiding tot een excursie. De Aardrijkskunde 2, 87–133. Soil Survey Staff, 1999. Soil Taxonomy. A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Second Edition. U.S. Department of Agriculture Handbook n°436, 871 p. Soil Survey Staff, 2014. Keys to Soil Taxonomy. Twelfth Edition. U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, 360 p. Thoen, H., 1983. Overblijfselen van antieke siderurgie in het Zoniënbos. In: Sporen van de Mens in Zoniën Volume 2. Dry Borren Raad, Brussel, pp. 17–20. Verboom, M., Langohr, R., 1982. Cartographie des traces de l’homme en forêt de Soignes. In: La Cartographie de Soignes. Conseil Trois Fontaines, Bruxelles, pp. 29–35. Vincent, A., Vincent, G., 1910. Les anciens ateliers sidérurgiques de la Forêt de Soignes. Annales de la Société d'Archéologie de Bruxelles 24, 79–83.
The ore that was used for metallurgic activity in the Sonian Forest is the iron-rich sandstone of the Eocene Lede Formation that locally occurs at shallow depth. The extraction area, still marked by numerous small round depressions, was situated more to the north, some 100 m away from the area of major metallurgic activity (Verboom and Langohr, 1982). When the sandstone was brought in, it was first cleaned by removing the sand. Traces of this are only observed in one place, which could mean that this activity was centralised, or else that it was usually already done before transportation. Next, the iron ore was apparently roasted, which accounts for the presence of extensive areas with a reddish-pink surface horizon. The survey indicates that roasting was done in separate areas for each low furnace, near the furnace, and not in a central zone for the whole area. The soil was clearly strongly heated, over a large area. Besides the ore, large amounts of charcoal must be available. Some of the charcoal mounds might date from that period, but there is no evidence to support this. Mounds with mixed material might be remnants of charcoal production of the period with metallurgic activity. Material of the Bt horizon was apparently used to build part of the low furnace. The preservation of the various associated anthropogenic structures that are reported in this study, including heat-affected surface horizons and low scoria and charcoal mounds, again confim the pristine Sonian Forest as a unique site, hosting an archive of features that will not be conserved in areas under agriculture. In terms of methodology, the study shows the potential of high-resolution gauge-auger surveys as a largely non-intrusive approach to investigate the presence and spatial distribution of surface-related archaeological structures. References Bonenfant, P.P., Defosse, P., 1994. Les recherches paléosidérurgiques en Belgique: l'exemple de la forêt de Soignes au Sud-Est de Bruxelles. In: Mangin, M. (ed.), La Sidérurgie Ancienne de l!Est de la France dans son Contexte Européen. Archéologie et Archéométrie. Les Belles Lettres, Paris, p. 269-273. Buffel, P., Matthijs, J., 2002. Geologische Kaart van België, Kaartblad 31-39, BrusselNijvel, Schaal 1:50.000. Koninklijk Belgisch Instituut voor Natuurwetenschappen and Afdeling Natuurlijke Rijkdommen en Energie, Brussel. Buffel, P., Matthijs, J., 2009. Toelichtingen bij de Geologische Kaart van België, Kaartblad 31-39, Brussel-Nijvel, Schaal 1:50.000. Koninklijk Belgisch Instituut voor Natuurwetenschappen and Departement Leefmilieu, Natuur en Energie, Brussel, 51 p.
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Contents lists available at ScienceDirect
Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep
Undisturbed iron industry sites in the Sonian Forest, Belgium Florias Mees a b
a,b,⁎
, Roger Langohr
T
b
Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium Department of Geology, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: Metallurgy Iron production Loess Sonian Forest
In the Sonian Forest of Middle Belgium, undisturbed remnants of ancient metallurgic activity, dating from the Early Middle Ages, are found. They occur in an area with iron-rich geological formations at shallow depth below a loess cover. Iron-industry traces include scoria mounds, which always occur in pairs. Associated with each pair is a single large zone with a reddish-pink colour of the upper part of the soil. These zones were formed by heating of the soil from the surface, most likely during roasting of the iron sandstone ore as pretreatment. The surface horizon was later homogenised by biological activity, down to the top of the Bt horizon or the compacted part of the E horizon. Charcoal mounds, whose distribution is not clearly related to that of other archaeological structures, date at least partly from more recent periods. Another associated feature is an area with a surface cover of yellowish sands, representing a site where the sandstone was unloaded or cleaned before further treatment. The preservation of these various features related to ancient metallurgic activity is due to the unique land-use history of the Sonian Forest, where a centuries-long continuous prohibition of agricultural practices has prevented the destruction of shallow surface structures.
1. Introduction In comparison with land under agriculture, forest stands have a good potential for conservation of archaeological sites, because they can be largely unaffected by processes such as ploughing, land-levelling and man-induced erosion. In the loess belt of Middle Belgium, a prime example is the Sonian Forest; near Brussels, where no agricultural activities have taken place for several centuries and where acid soil conditions preclude strong bioturbation (Langohr, 2009a, 2009b). Archaeological structures that are preserved in part of the forest include traces of ancient metallurgic activity. The latter were first reported by Vincent and Vincent (1910), in the form of small mounds composed of scoria. Since their discovery, these and other iron-industry remnants in the Sonian Forest have received only little attention. They were considered in a forest-wide survey of anthropogenic features by Verboom and Langohr (1982), followed by some profile observations (Sanders et al., 1985) and by a more extensive study of one representative locality (Mees, 1989). Partial results of the survey by Mees (1989) have been made available (e.g. Langohr and Pieters, 1996; Langohr, 2009b, p. 18-19), but a full report has never been published. Following some occasional later studies (e.g. Bonenfant and Defosse, 1994; Metalidis et al., 2008), new research initiatives have recently been launched, starting with an age-determination feasibility study (Ghyselbrecht,
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2018). In this paper, we present the results of the 1988–1989 survey by Mees (1989), as a baseline study providing information about spatial distribution patterns and soil properties that have not been duplicated by later investigations. It provides further evidence of the uniqueness of the Sonian Forest as an area with exceptional conditions of soil preservation. 2. Setting The Sonian Forest is located south-east of Brussels, in the central part of the Belgian loess belt. The area selected for a detailed survey is situated in the southern part of the forest, near Groenendaal (Fig. 1). It is delimited by a shallow man-made ditch in the east, a 6 m deep flatbottomed valley in the west, and a 4 to 6 m deep V-shaped gully in the south. The area was selected based on its plateau position and on the near-absence of anthropogenic features unrelated to iron-industry activity. Throughout the forest, Pleistocene loess deposits cover a substrate of marine Paleogene sediments. The loess cover, dating from the Pleniglacial B of the Weichsel glaciation (25,000 to 20,000 years BP; Haesaerts, 1984), has a total thickness of several metres. In the study area, the loess cover overlies fine iron-rich sands with sandstone layers (Lede Formation, Middle Eocene). Below these deposits (10 m
Corresponding author at: Department of Geology, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium. E-mail address: fl[email protected] (F. Mees).
https://doi.org/10.1016/j.jasrep.2019.102113 Received 14 February 2019; Received in revised form 7 November 2019; Accepted 18 November 2019 2352-409X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Archaeological Science: Reports 29 (2020) 102113
F. Mees and R. Langohr
Fig. 1. Location of the Sonian Forest, SE of Brussels, and location of areas with iron industry remnants in southern half of the forest, near Groenendaal, investigated by this study and others. Survey area as inset, with indication of valley margins (dashed lines), man-made ditch (full line), and road (double line).
raised to the melting point of iron, but most impurities will be melted. The melt flows into a pit next to the furnace, forming scoria upon cooling. These are still quite rich in iron, due to the low efficiency of the process. Numerous scoria mounds in a region around Groenendaal bear witness of ancient metallurgic activity in the Sonian Forest. The production of crude iron is not mentioned in the ‘Cuerboeck van Zoniën’ of 1371 or in later editions of the same document, which describe and regulate all economic activity in the forest. Metallurgic activity therefore certainly predates the period covered by these reports. In the past, it has been attributed to Roman times (Thoen, 1983) and to the Iron Age (D'Hondt, 1983), but radiocarbon dating of charcoal found in association with iron-processing features has subsequently established that it dates from the Early Middle Ages (Metalidis et al., 2008; Ghyselbrecht, 2018), corroborated by Optically Stimulated Luminescence dating results for associated heated soil material (Ghyselbrecht, 2018). Iron-rich sandstone from the Lede and Brussels Formations, which occur at shallow depths around Groenendaal, was used as ore. Zones where the ore was extracted have a characteristic microrelief, with many small depressions. Scoria mounds are usually present in the vicinity of these zones, but they also occur at greater distances. The Sonian Forest has always been important for the supply of charcoal, from the Roman period till the end of the 18th century. Charcoal was produced by placing wood in a pile, lighting the fire and then covering it with soil taken from around the pile. Remnants of this activity within the forest are the many small mounds with black earth, containing small charcoal fragments. A shallow, sickle-shaped depression often lines part of the mound, marking the place where soil was removed to cover the wood pile. Unlike scoria mounds, charcoal mounds occur throughout the forest.
thickness), a 50 m thick layer of sands of the Brussels Formation occurs (Middle Eocene) (Buffel and Matthijs, 2002, 2009). Soils of plateau areas of the Sonian Forest are loamy and well drained, with a clay-illuvial horizon that is penetrated by clay- and iron-depleted vertical planes. Parts of the E and Bt horizons are pedogenetically consolidated. These soils are Dystric Glossic Fragic Retisols (IUSS Working Group WRB, 2015) or Fraglossudalfs (Soil Survey Staff, 1999, 2014), recording physical changes during periglacial stages (Langohr and Vermeire, 1982; Langohr and Pajares, 1983; Langohr and Sanders, 1985). The influence of man on the soil and the landscape is minimal in the Sonian Forest, because the area has never been under agriculture since at least the eleventh century (Langohr, 2009a). In soils of more open areas of the forest, surface gleying often occurs, which is linked to compaction of surface layers, caused by circulation of forestry engines and trampling by horses.
3. Metallurgic activity During the Roman period (75 BC to 450 CE), metallurgic activity was quite extensive in southern Belgium, with Belgian Lorraine and the region between the rivers Sambre and Meuse as the most important centres. Remnants of low furnaces were also discovered in most of the Roman settlements in the sandy area of northern Belgium (Thoen, 1983). According to descriptions of the production process by Thoen (1983) and Laban et al. (1988), the ore was first crushed, washed, and then roasted to make it more reactive. The low furnace, consisting of a shallow pit with a chimney built over it, was filled with alternating layers of charcoal and pretreated iron ore. Large amounts of charcoal are required, because the required ratio between iron ore and charcoal volumes is about 1/10. In a low furnace, the temperature can not be 2
Journal of Archaeological Science: Reports 29 (2020) 102113
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Fig. 2. Distribution of archaeological structures in the study area.
4. Methods
polyester resin.
A map was created by mapping squares of 10 by 10 m at the time (scale 1/250), recording surface features and gauge auger observations (30 cm depth, 2 m spacing). Five types of archaeological structures were recognized and indicated on the field maps: (i) scoria mounds, (ii) charcoal mounds, (iii) mounds with mixed material, (iv) areas with a reddish-pink surface horizon, and (v) surface occurrences of yellowish sand (see Fig. 2). In addition, microrelief characteristics (breaks in the gradient of the slope, isolated depressions), vegetation (trees, ground vegetation), and tracks were mapped. Five profile pits were dug, within a short distance from each other (Fig. 2). Two pits are located in an area with a reddish-pink surface horizon (one in the centre of the area, one at the edge). The three other profiles represent the centre of a charcoal mound, a zone with yellowish more sandy material along the surface, and an area with normal profile development for soils on loess in depressions. Physico-chemical properties were determined by standard methods: grain size by wet sieving and pipette-method analysis, organic carbon and nitrogen contents by Walkley and Black and Kjeldahl procedures, pH using 1:1 soil:solution ratios, cation exchange capacity determination with ammonium acetate at pH 7, and measurement of exchangeable cations and dithionite-extractable Fe and Al with atomic absorption spectroscopy. Thin sections (mostly 24 by 36 mm large) were prepared after impregnation of undisturbed oriented samples with a
5. Results 5.1. Distribution patterns of archaeological structures Figure 2 presents a simplified version of the original base map, at a reduced scale, showing only archaeological structures and tracks. The survey demonstrated that scoria mounds always occur in pairs and that a zone with a reddish-pink surface horizon is associated with each pair, which was not noticed during earlier studies. Seven of these units have been recognised within the study area. One unit, in the north-eastern corner of the study area, is separated from the others, but the other six are grouped close together and appear to be arranged along two lines that are at a right angle. They define a rectangular area, delimited by natural boundaries to the west and to the south. More or less parallel tracks, coming from the main road, seem to lead directly towards some of the scoria mounds. Small isolated patches with reddish material (< 2 m diameter) are scattered all over the area that was mapped. The distribution of charcoal mounds seems to be unrelated to the distribution of scoria mound pairs. The few mounds with mixed material (scoria, charcoal, and fragments of soil horizons) also do not show a clear pattern. The morphology of one of these structures is similar to that of charcoal mounds, with a shallow depression partially 3
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Fig. 3. Soil profiles in areas with (a) a reddish-pink surface horizon, (b) a yellowish sand cover, (c) scoria (Fig. 3c after Sanders et al., 1985). Horizon symbols – pi reddish-pink colour, g surface gley development, c compaction, mn manganese enrichment, t clay illuviation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
expressed in the micromass of the iron-eluvial surface gley horizon that locally developed within the reddish-pink interval. In the transitional horizons and in deep vertical structures, patches with a darker micromass and fragments of clay-eluvial and Bt horizons are observed. Many of the sand grains in the reddish-pink surface horizon are covered with dark reddish-brown to opaque iron oxide coatings, which are often thin and not always continuous. The material of the coatings is similar to that of the matrix of the unaltered iron sandstone, but this sandstone has a markedly smaller grain size than the coated grains. The sandstone fragments in the reddish-pink layers always have an opaque matrix. Physico-chemical analysis confirms that the reddish-pink surface horizon closely resembles the normal unconsolidated clay-eluvial E horizon. The free iron content is about twice the normal value, and the sand content is 2–3% higher, but organic carbon content, nitrogen content, C/N ratio, pH, CEC, base saturation and the relative proportions of exchangeable cations are all the same as for natural soils (Table 1; cf. B.pi of Profile 2 vs E.bi of Profile 4).
surrounding it. An accumulation of yellow sands was observed in one place, at the southern end of a zone with anthropogenic structures, bordered by a clear break in slope gradient. 5.2. Soil profiles 5.2.1. Soils with reddish-pink surface horizon Soil profiles with a reddish-pink surface horizon (B.pi) are quite similar to undisturbed soils of the area in most respects. The same thin A horizon is present, the top of the Bt horizon is on average at the usual depth (30 to 40 cm) (Fig. 3a), and the reddish-pink layer (20 cm thickness) is nearly identical with the normal unconsolidated clayeluvial horizon (E.bi, with strong biological activity), except for its colour (4 YR 3/5, vs 10 YR 5/5 for unaltered E.bi horizon material). No gradual downward decrease in intensity of the colour is observed. The lower boundary of the B.pi horizon is very sharp, but an underlying transitional zone with veins or fragments of reddish-pink material is often present. Mole galleries and casts of other burrows filled with reddish-pink material are very common. Along the top of the reddishpink layer, an iron-eluvial surface gley horizon is commonly present (B.pi.g). The reddish-pink horizon also always contains some sandstone fragments. These are dark and partly red to purple, in contrast to the yellow-brown unaltered sandstone, which is also more porous. Beneath the studied charcoal mound, the soil is only slightly more reddish than the natural soil (9 YR 4/6). In thin sections, the micromass of the reddish-pink surface horizon has a dark colour in plane-polarised light, and a reddish haze is observed in cross-polarised light. These characteristics are less strongly
5.2.2. Soils with yellowish sand cover The yellowish sand layer that covers the soil at one location is heterogeneous, with a field texture ranging from sandy loam (9 YR 5/6) to silt (10 YR 5/4) and with local occurrences of clay-rich bodies. Sandstone fragments are present, as well as some rounded chert fragments and pieces of scoria. The sand layer covers a dark surface horizon, which is partly black with a high organic matter content, and partly grey, without a recognisable admixture of organic matter (Fig. 3b). This buried surface horizon has a thin layer (0.5–2 cm) with surface-gley characteristics along part of its base. The underlying 4
Journal of Archaeological Science: Reports 29 (2020) 102113
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Table 1 Physico-chemical properties of a profile with a reddish-pink surface horizon (P2), a profile with a yellowish sand cover (P5), and a reference profile (P4). Profile
Depth cm
Horizon symbol
Clay %
Silt %
Sand %
C %
N %
C/N
pH H2O
pH KCl
FeDCB %
AlDCB %
CEC cmol(+)/kg
Caexch cmol(+)/kg
Mgexch cmol(+)/kg
Kexch cmol(+)/kg
Naexch cmol(+)/kg
V%
P2
0–5 5–15 13–22 23–39 32–39 5–13 14–26 20–25 20–30 30–34 32–39 40–47 10–20
A B.pi B.pi E.(mn).c B.t.mn B.ys B.ys b.A (black) B.ys b.A (grey) B.ys-E E.c.bl E.bi
8.8 8.4 9.0 9.4 20.9 15.0 15.6 13.1 23.8 17.3 13.9 9.5 10.4
81.0 81.7 80.7 83.4 71.3 32.0 31.9 36.0 61.5 71.7 71.1 79.5 79.8
10.2 9.9 10.3 7.2 7.8 53.0 52.5 50.9 14.7 11.0 15.0 11.0 9.8
3.1 1.4 0.6 0.9 0.4 – 0.8 1.9 – 0.6 – 0.1 1.1
0.16 0.09 0.03 – – – 0.08 0.13 – 0.04 – 0.03 0.08
19.4 15.3 18.1 – – – 9.4 14.9 – 15.5 – 4.0 12.7
3.8 3.9 4.2 4.4 4.4 3.9 4.1 4.0 4.2 4.2 4.3 4.3 4.3
3.0 3.2 3.6 3.7 3.5 – – – – – – – 3.6
1.9 1.7 2.1 0.9 1.5 2.3 2.4 2.1 1.4 1.5 1.4 0.7 0.9
0.1 0.2 0.2 0.2 0.4 0.3 0.3 0.2 0.2 0.3 0.3 0.2 0.2
14.58 9.20 6.90 5.26 10.76 11.16 11.24 16.38 – 11.66 – 3.78 6.56
0.60 0.21 0.06 0.04 0.21 0.35 0.30 0.25 – 0.14 – 0.04 0.07
0.17 0.07 0.03 0.03 0.09 0.10 0.08 0.09 – 0.05 – 0.01 0.04
0.22 0.14 0.14 0.10 0.27 0.17 0.17 0.17 – 0.16 – 0.13 0.14
0.07 0.02 0.01 0.02 0.02 0.04 0.04 0.05 – 0.03 – 0.05 0.05
7.27 4.78 3.48 3.61 5.48 5.91 5.25 3.42 – 3.26 – 6.08 4.57
P5
P4
Horizon symbols – b buried horizon, bi biological activity, bl bleaching, c compaction, mn manganese enrichment, pi reddish-pink colour, t clay illuviation, ys yellowish colour.
6.2. Soil profiles
slightly yellowish transitional horizon is the unconsolidated clay-eluvial horizon of the buried soil. The top of the Bt horizon is 20 cm below the buried surface layer, which is too shallow for an undisturbed soil, in which this level is typically at a depth of 30 to 40 cm. Thin section observations confirm the high fine sand content of the surface layer, with a sand fraction composed of quartz, some glauconite and chert. Considerable amounts of clay are also present. Black humiferous parts of the buried surface horizon show the same characteristics as A horizons of normal soils on loess. Its greyish counterpart is characterised by a high charcoal content. Sandstone fragments in this profile consist of angular to subangular fine sand grains (quartz, glauconite, chert), embedded in a brown iron-rich matrix (up to 60%). Grain size analyses corroborate the relatively high sand and clay contents of the upper horizons (see Table 1, Profile 5). These horizons also yield significantly higher free iron and CEC values than normal soils on loess.
6.2.1. Reddish-pink surface horizon The presence of reddish material in soils where such colours are not natural is commonly associated with actions that involve heating. Through heating, certain mineral changes take place, which is accompanied by liberation of iron and recrystallisation of iron compounds. The presence of areas with a reddish-pink surface horizon in the vicinity of scoria mounds must clearly be related to some process that requires heat. Because the presence of areas with a reddish-pink surface horizon is not reflected in the topography, and because the soil profile has underwent only minor changes, except for a striking change in colour, it appears that soils with a reddish-pink surface horizon are normal loess soils which were heated, in situ, from the surface. Heating of the soil is most likely related to roasting of the iron sandstone ore. This is suggested by the slight admixture of sand that has occurred and by the changes that affected the sandstone fragments that the heated surface horizon contains. In the course of the years that followed heating of the soil (possibly more than a thousand years), the partially heated unconsolidated clayeluvial horizon was homogenised, from the surface down to the top of the underlying Bt or compacted E horizons, which wiped out any gradient in colour intensity that may have existed. Besides root activity, faunal activity might be partly responsible for this uniformisation. Faunal activity may in fact have been promoted by the improved nutrient status and increased pH after burning. Mole galleries and casts of other burrows, filled with reddish-pink material, are evidences of faunal activity, which is very low in natural soils of the Sonian Forest at present (Langohr and Cuyckens, 1986).
6. Discussion 6.1. Distribution patterns of archaeological structures The distribution patterns of archaeological structures in the study area are characterised by a clear association between scoria mound pairs and zones with a reddish-pink surface horizon. They each represent one site of low furnace activity. Charcoal mounds show a random distribution, which is not unexpected because charcoal production went on for several centuries after the period of metallurgic activity had ended, as demonstrated by historical records such as the Cuerboeck van Zoniën and by absolute age determination (Ghyselbrecht, 2018). Mounds with mixed material are probably merely waste heaps, although a relationship with charcoal production can be assumed for some mounds, based on morphological similarities with charcoal mounds. Other aspects of the distribution patterns include the occurrence of shallow depressions near some scoria mounds. These depressions might correspond to places where a pit was dug to get to the more clayey material of the Bt horizon, which was probably used to build part of the low furnace. Usually such pits were later filled with scoria, and those structures therefore occur beneath the scoria mounds at present (Fig. 3c) (Sanders et al., 1985). Small isolated patches with reddish material that are recognised throughout the study area probably represent superficial disturbance of the soil. This is suggested by the common strong perturbation of the upper part of the soil, marked by a buried surface horizon or a mixed horizon with A-horizon fragments, which is observed in about one third of all augerings.
6.2.2. Yellowish sand cover The yellow sand that occurs as a surficial accumulation at one location is derived from the Paleogene substrate (Lede Formation), as indicated by literature data, laboratory analyses and field observations around pits where iron sandstone was extracted. The zone with a yellowish sand cover in the study area is the site where the iron sandstone was cleaned or where stones were picked out from the load that was brought in from the extraction area. The presence of rounded chert fragments, derived from the stone layer at the contact between the loess and the Paleogene substrate, suggests that gathering of the sandstone was done without much care. The A horizon of the original soil is not preserved and parts of the lower soil horizons were also removed. Humus-rich parts of the buried surface horizon are fragments of the A horizon of a loess soil. The grey charcoal-rich parts developed along the surface of the stripped soil, before being covered by the sand and sandstone debris. 5
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7. Conclusions
D'Hondt, D., 1983. Historische schets van het Zoniënbos. In: Sporen van de Mens in Zoniën, Volume 2. Dry Borren Raad, Brussel, p. 1-7. Ghyselbrecht, E., 2018. OSL Dating of Natural and Cultural Heritage. Case Studies on Sonian Forest (B) and Stonehenge (UK). MSc Dissertation. Universiteit Gent 69, p. Haesaerts, P., 1984. Aspects de l'évolution du paysage et de l'environnement en Belgique au Quaternaire. In: Cahen, D., Haesaerts, P. (Eds.), Peuples Chasseurs de la Belgique Préhistorique dans leur Cadre Naturel. Patrimoine de l'Institut Royal des Sciences Naturelles de Belgique, Bruxelles, pp. 27–39. IUSS Working Group WRB, 2015. World Reference Base for Soil Resources 2014, Update, 2015. World Soil Resources Reports n° 106. FAO, Rome, pp. 192. Laban, C., Kars, H., Heidinga, A., 1988. IJzer uit eigen bodem. Grondboor en Hamer 42, 1–11. Langohr, R., 2009a. La forêt de Soignes, site unique pour les sciences de la terre et l’archéologie. In: La Forêt de Soignes. Connaissances Nouvelles pour un Patrimoine d’Avenir. Editions Mardaga, Wavre, pp. 181–196. Langohr, R., 2009b. The Zonian Forest – A Unique Site for Earth Science and Archaeology. Understanding Soilscape Evolution of the Belgium Loess Belt – A Review of 30 Years Research. Excursion Guidebook for the German Soil Science Society and Soil Science Society of Belgium 34. Langohr, R., Cuyckens, G., 1986. Une forêt aux pieds de 'limon'. Sol et reliëf en forêt de Soignes: témoins uniques. Réserves Naturelles 3, 51–58. Langohr, R., Pajares, G., 1983. The chronosequence of pedogenic processes in Fraglossudalfs of the Belgian loëss belt. In: Bullock, P., Murphy, C.P. (Eds.), Soil Micromorphology. A B Academic Publishers, London, pp. 503–510. Langohr, R., Pieters, M., 1996. De ijzerindustrie in het Zoniënbos. In: Gullentops, F., Wouters, L. (Eds.), Delfstoffen in Vlaanderen. Ministerie van de Vlaamse Gemeenschap, Brussel, pp. 158–159. Langohr, R., Sanders, J., 1985. The Belgian loëss belt in the last 20,000 years: evolution of soils and relief in the Zoniën Forest. In: Boardman, J. (ed.), Soils and Quaternary Landscape Evolution. John Wiley and Sons Ltd, p. 137-148. Langohr, R., Vermeire, R., 1982. Well drained soils with ‘degraded’ Bt horizons in loess deposits of Belgium: relationship with paleoperiglacial processes. Biuletyn Peryglacjalny 29, 203–212. Mees, F., 1989. Base Maps and Soil Survey of Undisturbed Iron Industry Sites in the Zoniën Forest (Loess Belt, Belgium). MSc Dissertation. Universiteit Gent, pp. 117. Metalidis, I., Deckers, P., Vanmontfort, B., Langohr, R., 2008. Archeologisch onderzoek op GEN Lijn 161, TR 101311 (Groenendaal, Hoeilaart). EPA Report n° 3. Katholieke Universiteit Leuven 55. Sanders, J., Langohr, R., Cuyckens, G., 1985. Bodems en reliëf in het Zoniënbos. Inleiding tot een excursie. De Aardrijkskunde 2, 87–133. Soil Survey Staff, 1999. Soil Taxonomy. A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Second Edition. U.S. Department of Agriculture Handbook n°436, 871 p. Soil Survey Staff, 2014. Keys to Soil Taxonomy. Twelfth Edition. U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, 360 p. Thoen, H., 1983. Overblijfselen van antieke siderurgie in het Zoniënbos. In: Sporen van de Mens in Zoniën Volume 2. Dry Borren Raad, Brussel, pp. 17–20. Verboom, M., Langohr, R., 1982. Cartographie des traces de l’homme en forêt de Soignes. In: La Cartographie de Soignes. Conseil Trois Fontaines, Bruxelles, pp. 29–35. Vincent, A., Vincent, G., 1910. Les anciens ateliers sidérurgiques de la Forêt de Soignes. Annales de la Société d'Archéologie de Bruxelles 24, 79–83.
The ore that was used for metallurgic activity in the Sonian Forest is the iron-rich sandstone of the Eocene Lede Formation that locally occurs at shallow depth. The extraction area, still marked by numerous small round depressions, was situated more to the north, some 100 m away from the area of major metallurgic activity (Verboom and Langohr, 1982). When the sandstone was brought in, it was first cleaned by removing the sand. Traces of this are only observed in one place, which could mean that this activity was centralised, or else that it was usually already done before transportation. Next, the iron ore was apparently roasted, which accounts for the presence of extensive areas with a reddish-pink surface horizon. The survey indicates that roasting was done in separate areas for each low furnace, near the furnace, and not in a central zone for the whole area. The soil was clearly strongly heated, over a large area. Besides the ore, large amounts of charcoal must be available. Some of the charcoal mounds might date from that period, but there is no evidence to support this. Mounds with mixed material might be remnants of charcoal production of the period with metallurgic activity. Material of the Bt horizon was apparently used to build part of the low furnace. The preservation of the various associated anthropogenic structures that are reported in this study, including heat-affected surface horizons and low scoria and charcoal mounds, again confim the pristine Sonian Forest as a unique site, hosting an archive of features that will not be conserved in areas under agriculture. In terms of methodology, the study shows the potential of high-resolution gauge-auger surveys as a largely non-intrusive approach to investigate the presence and spatial distribution of surface-related archaeological structures. References Bonenfant, P.P., Defosse, P., 1994. Les recherches paléosidérurgiques en Belgique: l'exemple de la forêt de Soignes au Sud-Est de Bruxelles. In: Mangin, M. (ed.), La Sidérurgie Ancienne de l!Est de la France dans son Contexte Européen. Archéologie et Archéométrie. Les Belles Lettres, Paris, p. 269-273. Buffel, P., Matthijs, J., 2002. Geologische Kaart van België, Kaartblad 31-39, BrusselNijvel, Schaal 1:50.000. Koninklijk Belgisch Instituut voor Natuurwetenschappen and Afdeling Natuurlijke Rijkdommen en Energie, Brussel. Buffel, P., Matthijs, J., 2009. Toelichtingen bij de Geologische Kaart van België, Kaartblad 31-39, Brussel-Nijvel, Schaal 1:50.000. Koninklijk Belgisch Instituut voor Natuurwetenschappen and Departement Leefmilieu, Natuur en Energie, Brussel, 51 p.
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