LATE-GLACIAL AND HOLOCENE MALACOFAUNAS FROM ARCHAEOLOGICAL SITES IN THE SOMME-VALLEY (NORTH FRANCE)

The first malacological successions covering the Late-glacial period a nd the early Holocene are reported from archaeological, sites in north ern France. Five malacological zones (S1-5) are recognized which are r elated to the regional lithostratigraphy, Upper Palaeolithic/Mesolithi c archaeology and to a radiocarbon chronology. The zone S1, which cont ains 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 Plen iglacial. Overlying the loessic sediments is an organic silt, known as the 'Belloy soil' at Belloy, which has yielded birch pollen (but no p ine) allowing attribution to the early part of ''Allerod'' phase of th e Late-glacial. The molluscan faunas from this level (S2) are richer a nd more diverse. Helicopsis striata (Muller) appears to be typical of this period. Above the Allerod deposit is a minerogenic silt containin g rhizomes, but this is only represented in the valley floors. The mal acofaunas from this unit (S3) are similar to those of the Allerod silt but contain a higher proportion of hygrophilous taxa. These faunas ar e assigned to the Younger Dryas, a conclusion supported by radiocarbon dating of bones associated with the terminal Upper Palaeolithic ''Ind ustrie a pieces machurees'' which occur at the top of the unit. The La te-glacial succession is overlain by organic silts of Holocene age, wh ich 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 specie s diversity and a reversion to faunas of open-ground occur (S5). Compa rison of these successions with similar sites in Burgundy, the Paris b asin and southeastern England demonstrate a closer affinity to the mol luscan sequences of Kent rather than to the French sites, although Lat e-glacial molluscan faunas are still poorly known from France. (C) 199 5 Academic Press Limited