Abstract
The new field research, led on all the Somme valley terraces since 1987 proposes, for this type prehistorical land, a global geological and geomorphological frame integrating ancient data and recent archeological excavation results: longitudinal profiles, cartography and detailed synthetic profiles of alluvial units, stratigraphy and dynamic settlement of river deposits and loess. The reflexion work is based on a study of the geometry of the terraces system, resulting in a nine-stepped alluvial units système valid for 70 km of valley. In this precise frame, it is possible to integrate a study of different alluvial units, allowing us to interpret each alluvial sequence as the morphosedimentary consequence of a glacial-interglacial climatic cycle. This hypothesis is confirmed by confrontation with the pedosedimentary balance of the cover deposits sequences (loess-paleosoils). The work results synthesis proposes an approach of the building of the terraces system and a first chronostratigraphic interpretation hypothesis. The Somme valley terraces system then appears as an original answering model to the quaternary climatic variations since 800 000 BP and a reference frame for paleolithic studies.
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