Netherlands Journal of Geosciences — Geologie en Mijnbouw |94 – 4 | 387–397 | 2015 doi:10.1017/njg.2014.45 Provenance of the Roman basalt stone at Kotterbos (Lelystad, the Netherlands): A geoarchaeological study based on petrographic and geochemical analysis, and comparison with a synthesis of basalt stones along the northern Limes K. Linthout Petrology, Faculty of Earth and Life Sciences, VU University Amsterdam, the Netherlands. Email: [email protected] Manuscript received: 8 January 2014, accepted: 7 December 2014 Abstract A solitary unprocessed, 26-cm basalt stone recently found at the Roman (69 AD) site at Kotterbos (Lelystad, the Netherlands), situated 40 km north of the Limes, has been examined petrographically and geochemically. The rock is similar to previously investigated Roman basalt stones and blocks from the Limes in the province of Utrecht. All are alkali olivine basalts and basanites, which are also the dominant rock types in the nearby volcanic hinterland in Germany. On the basis of 23 criteria, including all major and minor elements plus a selection of trace elements, the Kotterbos stone correlates with a distinctive subgroup of Limes basalts. This subgroup is characterised by relatively high Mg# and low TiO2 wt% values, as opposed to the other subgroup with lower Mg# and higher TiO2 wt% values. It is argued that the high Mg#–low TiO2 subgroup and the Kotterbos basalt have common geological provenance(s). Five basalt bodies in the hinterland match the criteria of this subgroup, and might be considered as locations of provenance. Of these five, only Rolandsbogen, located next to an ideal anchorage on the west bank of the Rhine, about 12 km upstream from Bonn, can also conform to sensible Roman demands concerning effective logistics and military safety. Consequently, Rolandsbogen is the most likely site of provenance for the high Mg#–low TiO2 subgroup, which includes the Kotterbos basalt. It is argued that the Kotterbos stone is not a rock that somehow got separated from a shipment of building stones to the northern Limes; rather, it is speculated that the stone was picked up as a stray cobble near the basalt body and taken onboard a Roman vessel moored at the Rolandsbogen anchorage. Keywords: archaeometry, Roman, basalt, northern Limes, provenance, Rolandsbogen Introduction cover of the Netherlands, the Romans must have imported the stones from outside the region. Indeed, the discovery of dozens To date in the Netherlands, basalt stones of Roman derivation of basalt blocks aboard the Roman freight vessel De Meern 4 on have only been reported from inside the limits of the Roman the bank of a palaeochannel of the Old Rhine (de Groot & Morel, Empire, in particular from the boundary zone itself, which is 2007) strongly suggests that the Romans imported these stones commonly referred to as the Limes (Fig. 1). For a general and in bulk shipments via the Rhine from the Central European Vol- informal treatise on the Limes of Germania Inferior, the Ro- canic Province (CEVP; Fig. 1). Petrographically, these stones man province along the Lower Rhine, readers should refer to found in the Limes can be classified as alkali olivine basalt and Lendering (2011). Scores of basalt stones have been found in basanite (Linthout, 2007; Linthout et al., 2009), which are pre- revetments, bridges, roads etc., forming part of the infrastruc- cisely the dominant rock types of the CEVP (Wedepohl et al., ture of the Limes in the western Netherlands, and also as cargo 1994). aboard Roman vessels (de Groot & Morel, 2007; Luksen-IJtsma, On the basis of chemical analysis, 60 basalt stones from the 2010; Langeveld et al., 2010; Dielemans & van der Kamp, 2012). Limes in Vleuten-De Meern and Utrecht have been divided into Since basalt does not occur naturally in the bedrock or drift two clearly defined groups, for each of which a matching basalt C Netherlands Journal of Geosciences Foundation 2015 387 Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.234, on 01 Oct 2021 at 03:21:25, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/njg.2014.45 Netherlands Journal of Geosciences — Geologie en Mijnbouw Fig. 1. The Limes (red line), marking the boundary of the Roman Empire, and volcanic fields of the Central European Volcanic Province, after Wedepohl et al. (1994). S, Siebengebirge; WE, HE, EE, West-, Hoch- and East Eifel; W, Westerwald; V, Vogelsberg. The Roman site of Kotterbos is set in the geographical outlines of about 100 AD (after Vos & de Vries, 2013). At that time, the Limes coincides with the Rhine from where the river enters the Siebengebirge volcanic field. Roman basalts from the Limes with established provenances are from Utrecht and Vleuten-De Meern (VM). source could be traced in Roman-controlled territory, namely in a former lake that existed in Roman times (van Heeringen at Rolandsbogen and Erpeler Ley. Both these source localities et al., 2014). Many of the stems were carved, showing for in- are on the banks of the Rhine, 12 and 20 km, respectively, stance half-lap joints and cross-connections. Using a procedure upstream from Bonn, as shown on Fig. 6 (Linthout, 2007, 2012, for curve fitting (wiggle match analysis) of two triple AMS- 2014; Linthout et al., 2009). radiocarbon dates, the felling date of one carved alder stem Recently, a 3.25-kg basalt stone was reported from a Ro- was refined to 69 AD (van Heeringen et al., 2014). This un- man archaeological site about 40 km north of the Limes in the usual discovery of a basalt stone at a Roman site some 40 km Netherlands (van Heeringen et al., 2014). The site, just outside outside of the Limes prompted R. van Heeringen to ask the the northern limits of Almere in the municipality of Lelystad, author if the Kotterbos basalt would fit in his study of Roman was discovered during a landscape redevelopment project at basalts from the Dutch Limes. More specifically, the question Kotterbos that was carried out under the commission of the was whether provenance studies could determine if the stone Province of Flevoland and accompanied by archaeological in- could be attributed to one of the two established source sites vestigations by Vestigia Archeologie & Cultuurhistorie (Amers- for Limes basalts (i.e. Rolandsbogen or Erpeler Ley) or came foort). from elsewhere. The single rough basalt stone and a Roman stoneware tile To this end, the petrography and chemistry of the Kotterbos were the only inorganic artefacts amongst some 200 ecofacts; basalt have been investigated and compared with data from these latter remains comprised mainly alder stems in the pro- the author’s previous studies on the subject. The results of this lapsed and drowned archaeological site that was buried by a gyt- geoarchaeological investigation address the question of how tja, namely, a layer of debris dominated by peat moor deposited the stone arrived at Kotterbos. 388 Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.234, on 01 Oct 2021 at 03:21:25, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/njg.2014.45 Netherlands Journal of Geosciences — Geologie en Mijnbouw Fig. 2. The Kotterbos basalt stone, 3.25 kg, maximum orthogonal dimensions 26 × 10 × 9 cm. Photographs by courtesy of Vestigia Archeologie & Cultuurhistorie, Amersfoort. A and B. Opposite faces parallel to volcanic layering. C and D. Across layering. A concave layer is exposed at the bottom of A; coarse layers, apparent in D, are not obvious in C due to previous erosion in situ. Pits, marking the places where xenoliths have fallen out, are most prominent in C. Macroscopic petrography of the Kotterbos The morphological properties of the basalt stones and blocks basalt and the basalt stones from the from the Limes correlate with their functions in the respective Limes constructions. Three groups have been distinguished previously by the author: The stone has a thin greenish grey coating due to weather- ing. Fresh parts show a very dark grey aphanitic matrix. As 1. ‘Small’ (average 5 × 7.5 × 10 cm) angular, blocky stones shown in Fig. 2, the finely granular surface is scattered with with worn edges that were used in bulk, most probably as millimetre-sized dimples (pitting) that reflect selective erosion an admixture in the agger or as statumen ( = foundation) of abundant olivine phenocrysts. Magmatic flow has caused a of Roman roads, as in examples from the Utrecht districts somewhat wavy layering (Fig. 2a and d). Two types of xeno- Kanaleneiland (site code AML) and Hoograven (site code liths up to about 1 cm in size are mostly either olivine-rich or DSL) (Linthout, 2012, 2014). It should be noted that these quartz-rich in composition. blocky shapes have a geological origin; they are formed by In these aspects the Kotterbos stone is very much in line jointing during cooling in the entablure and cuspate zones with the basalt stones found along the Limes in the Netherlands of basalt bodies (Winter, 2001). Such fragments usually ac- (Linthout, 2007, 2012, 2014; Linthout et al., 2009). cumulate in screes, where abrasion over long periods of time will eventually wear off the sharp edges to some extent. Morphology of the Kotterbos basalt Stones of this type are by no means the result of splitting in versus morphologies of basalt stones in Roman quarries or building sites, as over that geologically the Limes short period of time the edges would have preserved their original sharpness. In the two studied Roman roads, stones The shape of the Kotterbos stone is highly irregular and pla- of morphologies other than cuspate are rare.