Lateral river erosion impacts the preservation of Neolithic enclosures in alluvial plains

Abstract

Situating prehistoric sites in their past environment helps us to understand their functionality and the organization of early sedentary human societies. However, this is a challenge as the natural environment constantly evolves through time and erases these constructions, especially along riverbanks, thus biasing the archaeological record. This study introduces a reassessment of the paleo-landscape evolution around the Neolithic enclosures at the Noyen-sur-Seine site based on new field observations as well as the synthesis of (un)published and new radiocarbon dating. Contrary to the initial hypothesis, our results show that the Noyen enclosures were not built along a Neolithic Seine River: the nearby channels were active in the Middle Age and Early Modern periods. Therefore, the results show that the enclosures were originally much larger: only a fraction that survived river erosion (lateral migration rates up to 2-3 m yr^-1 estimated during the nineteenth century) has been preserved. Instead, an abandoned Mesolithic Seine River served as a natural delimitation of the SE part of the Neolithic enclosures. These results indicate that Neolithic enclosures in alluvial settings are often only partly preserved and that societies from that period lived farther away from active rivers than originally thought, where they were protected from floods.

Figure 1

Location of the study area. (a). Geological map showing the valley bottoms together with the main enclosures and necropolises from the Neolithic period. La Bassée, is an alluvial plain of the Seine River upstream of Paris, but downstream of the Aube River and upstream of the Yonne River. It is parallel to the cuesta limiting the Cretaceous chalk and the Tertiary. (b). Details of the geology of the valley bottom showing the Quaternary surface formations.

Figure 2

Examples of non-alluvial (a) and alluvial (b–e) Neolithic enclosures found in northern France^,–. Note that (a) and (b) have a circular shape and that the limit of (c), (d) and (e) correspond to erosional boundaries. (e′) early reconstruction of the Noyen-sur-Seine landscape. For example, it is assumed that the B enclosure was delimited by a Neolithic Seine River, which implied the implantation of early sedentary societies by the river. In this study, this model is challenged by showing that the erosional boundaries occurred in Noyen-sur-Seine much later than the Neolithic.

Figure 3

Geomorphic map of the study area, showing paleochannels (dashed blue lines), crescent bars (dashed white lines) and Neolithic enclosures (orange dashed lines). A chronology with the relative and absolute dating of channels a to h, as well as the two monoxyle pirogues are presented. The white dots correspond to locations where field observations and/or radiocarbon sampling were performed.

Figure 4

Reconstruction of the lateral rates of migration from historical maps (a) Example of the migration distance calculated using the the dynamic time warping (DTW) algorithm for the left bank of the Seine River in 1785 and 1848. The letters b, c and d correspond to the paleochannels located in Fig. 3. (b) Distribution of the estimated migration distance for the left bank (thin black curve) and right bank (thin blue curve) as well as the total (thick black curve). Some corresponding lateral migration rates are indicated in red. The migration rates are calculated by dividing the distance by the time between maps (i.e., 63 yrs).

Figure 5

Sedimentary cross-sections from the abandoned channels with the reported radiocarbon dates: (a) east f channel, (b) west f channel, (c) h channel. See Fig. 6 for locations. Cross-section b is from Mordant et al..

Figure 6

Superimposed paths of the channels based on geoarchaeological mapping and historical maps (see Supplementary Materials). The location of the cross-sections in Fig. 5 is reported. The letters d, f, g and h correspond to the paleochannels shown in Fig. 3.

Figure 7

Reconstructed evolution of the landscape since the Mesolithic showing the progressive erosion of the Neolithic enclosures due to the migration of the Seine River. The reconstruction supports the hypothesis that a system of large, circular enclosures was progressively eroded along the bank of the laterally migrating Seine River. The locus where the two pirogues were abandoned is also reported.