Late-glacial and holocene malacofaunas from archaeological sites in the Somme valley (North France)

Abstract

The first malacological successions covering the Late-glacial period and the early Holocene are reported from archaeological sites in northern France. Five malacological zones (S1–5) are recognized which are related to the regional lithostratigraphy, Upper Palaeolithic/Mesolithic archaeology and to a radiocarbon chronology. The zone S1, which contains an assemblage of low species diversity but with abundant Vallonia , allows attribution of the basal loess (reworked as a rainwashed silt in the valley bottom) to the early Late-glacial, rather than the Pleniglacial. Overlying the loessic sediments is an organic silt, known as the ‘Belloy soil’w at Belloy, which has yielded birch pollen (but no pine) allowing attribution to the early part of “Allerod” phase of the Late-glacial. The molluscan faunas from this level (S2) are richer and more diverse. Helicopsis striata (Muller) appears to be typical of this period. Above the Allerod deposit is a minerogenic silt containing rhizomes, but this is only represented in the valley floors. The malacofaunas from this unit (S3) are similar to those of the Allerod silt but contain a higher proportion of hygrophilous taxa. These faunas are assigned to the Younger Dryas, a conclusion supported by radiocarbon dating of bones associated with the terminal Upper Palaeolithic “Industrie a pieces mâchurees” which occur at the top of the unit. The Late-glacial succession is overlain by organic silts of Holocene age, which at several sites have yielded Mesolithic artefacts. The molluscan faunas are much more diverse than the earlier Late-glacial assemblages and contain abundant shade-demanding taxa (S4), then a fall in species diversity and a reversion to faunas of open-ground occur (S5). Comparison of these successions with similar sites in Burgundy, the Paris basin and southeastern England demonstrate a closer affinity to the molluscan sequences of Kent rather than to the French sites, although Late-glacial molluscan faunas are still poorly known from France.