Stratigraphy and correlations with the Carpentier reference sequence

During the revision of the Quaternary Somme terrace sequences at Abbeville25, new fieldwork was first conducted in the Carpentier and Léon 1 quarries (Fig. 2, Supplementary Fig. S3), well known for their rich paleontological assemblages26,27,28,29. These two neighbouring sites exhibit fluvial sequences overlying the same bedrock step (+27 m a.s.l.), located at a relative height of +40 m in relation to the maximal incision of the present-day valley (Alluvial Formation VII, Fig. 3a, Supplementary Fig. S2). Both the age and the interglacial characteristics of the calcareous sandy-silts of the Marne blanche or White marl capping the fluvial sequence of the Carpentier Formation have been firmly established25 using a reliable combination of: (i) ESR (quartz) and ESR/U-series (large mammal tooth) dating, (ii) large and micro mammal and mollusc assemblages, (iii) the occurrence of oncolith layers (algal balls) resulting from cyano-bacterial activity under full temperate conditions, and (iv) finally, the insertion of the Carpentier Formation within the Somme valley Quaternary terrace system (Formation VII in a system where Formation X is dated to about 0.9–1 Ma and formation I is dated to the Late Saalian-Eemian glacial-interglacial cycle). The rich large mammal faunal assemblage recovered from the White marl at Carpentier quarry clearly characterizes a temperate interglacial period and can be biostratigraphically correlated to a Late Cromerian interglacial25. Combined with the geochronological data obtained by both ESR-quartz (n = 6) from the sandy lenses included in the fluvial deposit of the White marl and ESR/U-series on cervid teeth (n = 2) (mean age of the whole set of data: 584 ± 48 ka), this interglacial stage can be allocated to the Cromerian III Interglacial of the formal north-western European stratigraphy and correlated with MIS 1530. The White marl from Carpentier is thus one of the best-dated series from the Cromerian III interglacial in Western European river systems and represents a fundamental cornerstone for the interpretation and dating of the Moulin Quignon sequence and more generally for the Quaternary terrace system of the Somme River.

Figure 3 Cross-section of the terrace system of the right bank of the Somme River at Abbeville. (a) General profile. (b) Detailed stratigraphic relations between Moulin Quignon and Carpentier sites on the +40 m alluvial formation and ESR-quartz dating results (see: Fig. 2 and Supplementary Fig. S5 for location of the various observation points). The geometry of the Quaternary deposits and of the chalk bedrock relies on data originating from the present work for Moulin Quignon, on22, for the Carpentier and Léon quarries, completed by the diagrams and data from (26 to 29). This figure clearly illustrates the stratigraphic relationships between the sequences of Carrière Carpentier and Moulin Quignon. (1) Periglacial fluvial gravels, sands and sandy silts (main unit of the various terrace bodies). (2) Carpentier Marne blanche/White marl (WM) complex: interglacial calcareous silts and sandy silts with oncolith sand layers and large mammal remains (Cromerian III/MIS 15). (3) Undifferentiated slope deposits: reworked gravels and palaeosols. (4) Fluvial organic silts, peat and estuarine sands (Holocene infilling). WM: White marl. Cug: Carpentier Upper gravels: well-sorted medium flint gravels with calcareous sandy matrix, interstratified calcareous silt lenses and large mammal remains. Clg: Carpentier Lower Gravels: poorly sorted and heterometric flint gravels including numerous unrolled flint nodules and chalk blocks packed in a calcareous sandy matrix. Mqg: Non-calcareous, coarse, poorly stratified sandy gravels with iron and manganese oxide coatings at the base. Irb: large rounded blocks of reworked Tertiary sandstone (ice-rafted). Fs: Lens of laminated non-calcareous fluvial sands. Average ESR dating of the WM according to25. Full size image

In this context, thanks to the renovation of the suburb of Abbeville, new investigations were undertaken in 2016–2017 at Moulin Quignon (50°06′18″N/1°50′89″E, Figs 1 and 2), leading, 170 years later, to the rediscovery of this emblematic Palaeolithic site (Supplementary Fig. S4). In 2016, seventeen 4-to-5-m-deep test-pits were excavated, resulting in the discovery of undisturbed, fluvial gravels and sands, well-preserved below thick layers of reworked sandy gravels (from the former quarry) and modern dump deposits. The average bedrock altitude measured at Moulin Quignon (26.5 m a.s.l.) in both test pits and in the archaeological excavation is very close to the altitudes obtained for the Léon 1 and Carpentier quarries, located 200 and 400 m north of the site, respectively (Figs 2 and 3). This approach demonstrates that the three alluvial sequences are located on the same bedrock step (Fig. 2, Supplementary Fig. S5). Consequently, compared to the maximal incision of the valley at Abbeville below the alluvial plain deposits (≈−12 m below sea level), the relative elevation of the Moulin Quignon alluvial formation (+40 m) allows for its attribution to Alluvial Formation VII of the Somme system (Supplementary Figs S1, S2).

In addition, we compiled all the available stratigraphic information from Alluvial Formation VII, from the area located between Carpentier and Moulin Quignon, taking in account recent observations at Carpentier, Léon 1 and Moulin Quignon (Fig. 2) made between 2012 and 2017, and all the former data available from D’Ault-du-Mesnil26, Commont27 and Breuil28,29.

This approach was completed by a new test-pit campaign during spring 2019 over an abandoned area further to the north (140 to 180 m) of the Moulin Quignon 2017 excavation, closer to the Carpentier Quarry (Fig. 2). This extensive survey (17 test pits on 2 600 m2) allowed the discovery of in situ remnants of fluvial gravels and sands (thickness about 1 m) overlaying the same chalk bedrock step than in Carpentier Quarry and Moulin Quignon 2017 excavation. Moreover, in one of the test-pits (S4-2019, Fig. 2), ten Acheulean artefacts were discovered within the sandy gravels demonstrating the extension of the Moulin Quignon Acheulean site over more than 150 m to the North. These results strongly support the direct stratigraphic correlation between the fluvial sequences from Moulin Quignon and Carrière Carpentier.

The synthesis illustrated by Supplementary Figs S5 and S6 shows that the Carpentier interglacial deposit, the White marl, extends below the road towards the area corresponding to the Chemin de Fer and Léon 2 quarries located at less than 100 m from the area explored at Moulin Quignon in 2016–2017 (Fig. 2, Supplementary. Fig. S5). It is clear from this that the interglacial calcareous facies of the White marl is only preserved in sequences located in the external part of the alluvial formation. This configuration results from the occurrence of a lateral channel close to the north-east bank of the Palaeo-Somme valley, whereas interglacial deposits are not preserved towards the central part of the former river valley, showing a markedly thicker lower gravel unit (≈3 to 3.5 m), like in the Moulin Quignon area (Fig. 3). Thus, combining the new observations and former data, we can demonstrate: i) that all the fluvial sequences from the area reported from more than 150 years overlie the same bedrock step incised by the Somme River between 26 and 27 m a.s.l. (relative height of 39 to 40 m), and ii) that a clear distinction can be made in all the sections reported by former authors26,27,28,29,31, between the fluvial gravels, systematically located below the interglacial deposit of the White marl, and the slope deposit sequence made up of heterogeneous layers of reddish clayey gravels, clayey sands, sandy loess and palaeosols, which always appear above the White marl (Fig. 3b, Supplementary Fig. S5). Given the synthesis of the stratigraphic information exposed above, the geochronological data from the deposits of the Carpentier quarry, and especially the ESR and ESR/U-series dates from the interglacial deposits of the White marl, can be used to reliably infer the age of the Moulin Quignon findings.

Electron spin resonance (ESR) dating

Three samples of sandy sediments were extracted during the 2017 archaeological excavation from the main stratigraphic units and in situ gamma-ray measurements were performed in each sampling hole (Fig. 4). Details of the Electron Spin Resonance (ESR) dating method applied on sedimentary quartz are given in the Method part.

Figure 4 Stratigraphic sequence exposed during the excavation (height: 4 m). From the top below the reworked sandy gravels from the former Moulin Quignon quarry: Sa-br: Brown-red compact clayey sands preserved in deep dissolution sinkholes. Grs-br: brownish to reddish sandy heterometric clayey rounded gravels with small (reworked) tertiary pebbles. Fs: Yellow fine to medium laminated sands with thick (2–5 cm) red to orange oxidized bands. At its base, in places this unit shows a layer of laminated greyish clayey silts with thin oxidized orange (Fe) and black (FeMn) bands. Grs-j: Heterometric rounded sandy flint gravels with abundant yellow sandy matrix and irregular reddish to orange oxidation (Fe) bands and with (reworked) tertiary pebbles (1–4 cm). Gr-n: Strongly heterometric rounded flint gravels, without any matrix, (mainly 2–4 cm but including irregular elongated nodules up to 30 cm) and scattered (reworked) tertiary pebbles (1–4 cm). This facies is characterized by strong weathering indicated by the occurrence of brownish to blackish Fe-Mn coatings on all the flint nodules and pebbles. All the units described above are free of CaCO 3 due to dissolution and weathering processes that affected the whole sequence after its deposition by the river. Chalk: weathered (soft) Upper Cretaceous chalk bedrock including numerous irregular large elongated flint nodules (20–40 cm in length). Full size image

The results obtained for the three samples from Moulin Quignon, using the aluminium (Al) and lithium titanium (Ti-Li) centres of quartz, are consistent with each other (overlapping of uncertainty domains at 2 σ) (Supplementary Table S1). This consistency makes it possible to combine the results and calculate a weighted mean age for each sample using the Isoplot 3.0 software32. For samples MQ17-01 and MQ17-03, from the fluvial sand layer (Fs) overlying the main gravel body (Grs-j), the results are respectively 686 ± 58 ka and 650 ± 37 ka (Table 1). Similarly, an average age of 672 ± 54 ka was obtained for the whole fluvial sands and gravels formation of Moulin Quignon (Table 1), confirming the great antiquity of the site.

Table 1 Annual doses, equivalent doses and ages of the Moulin Quignon fluvial sediments obtained using Al and Ti-Li centres and mean ages determined for each sample and the fluvial formation. Full size table

Palaeolithic artefacts

Unexpectedly, typical Palaeolithic artefacts including flakes, flake-tools, cores and one biface were discovered at depths ranging between 3.5 to 4 m in two of the test pits, in the sandy gravels of the terrace deposits preserved by former quarry works. The taphonomy of this archaeological assemblage is described later in Discussion section. In 2017, an archaeological excavation located around the two positive test pits (Fig. 4, Supplementary Fig. S4) yielded several additional new artefacts. With a total of 244 flakes, 13 cores and 5 bifaces (Fig. 5) discovered in situ in the lower part of the alluvial deposits, the excavation provided us with a lithic assemblage large enough to carry out a full technological analysis. The taphonomy of this archaeological assemblage is described later in the Discussion section.

Figure 5 Photographs of three of the bifaces discovered during the archaeological excavation at Moulin Quignon. (a) Biface with extended black coatings of Fe-Mn oxides typical of lithic artefacts discovered in the lower part of the Moulin Quignon sequence (GR-n layer: see Fig. 4 and Supplementary Fig. S6). (b) Biface from Grs-j unit. (c,d) facial and lateral view of a biface from Grs-j unit with thick base. Full size image

The lithic series are made from local flint nodules available both in the alluvial gravels and in the chalk bedrock. The flakes belong to a complete reduction process including mainly large flakes (mostly between 40 and 80 mm long). The cortical flakes (first phases) indicate the use of oval or irregular flint nodules. Unipolar or unipolar convergent removals eliminated the cortex by series of thick flakes. The platform was prepared or remains cortical. The majority of the flakes have a cortical back and for some of them both a cortical back and butt. This attests the use of the nodule sides or core edges for eliminating the cortex and starting the flaking process. Flakes without cortex show that the debitage continued by unipolar, centripetal and/or crossed removals. A frequent back suggests the recurrent use of the core edges as a guide for this debitage. Flakes are thick, some truncating a large part of the cores. The flake cross-sections indicate flaking surfaces with angular facets and deep scars. Some elongated flakes (laminar flakes) are mainly due to the use of prominent scar ridges or the core edges. Butts are plain, but also punctiform, dihedral and facetted. Few flakes are hinged, indicating good general management of the debitage angles. The main characteristic of thick and large butts is an open angle (50–60°). Impact points were regularly located far from the core edges. There are six retouched pieces and a further 30 pieces with possible retouch. They are an end-scraper on a large and elongated cortical flake (Supplementary Fig. S7), two convergent tools (point) and some retouches on a long edge.

The cores indicate either crude and opportunistic flaking, or unipolar/centripetal debitage on one, two (orthogonal or bifacial) or multiple surfaces. Cores are exhausted or still in progress. Scar size indicates a large variety of end-product size and shape. Some cores are crudely flaked, residues of extremities of nodules with one flaking surface, few scars and an occasional prepared platform. When the cortical area is suitable, the platform is not prepared. Others are unifacial cores with unipolar, centripetal and crossed removals more or less covering the surfaces, with a partial prepared platform. Some hinged removals attest poor flaking management for some cores. The orthogonal cores and the bifacial cores sometimes indicate a degree of independence from the raw material by series of removals with no link with nodule shape and thus limited blank constraints. We can mention in particular two cores with peripheral unipolar removals with a prepared platform (semi-rotating-like core). The biggest core (155 mm long) shows several flaking surfaces around part of the periphery and on one large surface by centripetal, invasive and deep removals (Supplementary Fig. S7). The core was turned in the hands according to the suitable angles and re-created some angles.

A few flakes can be related to bifacial technology. They are completed by five bifaces which show wide diversity in shape and shaping mode (Supplementary Fig. S8):