THE SOMME VALLEY QUATERNARY (RIVER TERRACES, LOESS AND PALAEOSOLS): A CONTRIBUTION TO THE NATIONAL INVENTORY OF GEOLOGICAL HERITAGE

As part of the National Inventory of Geological Heritage (INPG), initiated in 2007 by the Ministry of the Environment, the Quaternary of the Somme Valley is currently represented by a set of 19 site cards. A preservation approach is also in progress for the most threatened sites. The aim of this contribution is to insert back these reference sites from the Somme area into a synthesis of the stratigraphical, geochronological and palaeoenvironmental data acquired for the last million year. The Somme basin is characterized by the coexistence of two types of complementary sedimentary and morphological contexts: fluvial and slope sequences. These contexts are closely associated in a stepped terrace system including ten stepped alluvial formations covered by slope sequences (loess-palaeosols). In this system. the glacial-interglacial climatic cycles (cycles of about 100 000 years) is recorded both in the various alluvial formations and in the loess-palaeosols sequences, the complexity of which is related to the age of the underlying fluvial sediments. The Somme valley terrace system has played a historical role in the emergence of prehistory and Quaternary geology since the second half of the 19th century. Unfortunately, the scientific and patrimonial importance of these sequences, often located in old quarries in sub-urban areas under strong urbanization pressure, has not always been clearly identified. Finally, only six of them have been preserved owing to their importance for archaeological heritage. So we have lost a lot of these reference sequences and it is urgent to preserve, in this valley, a set of localities offering both good visibility for the general public and perspectives for the development of new research on Quaternary sequences. The protection of a series of Quaternary sites is therefore a fundamental step for the preservation of this unique heritage so that it can retain its prominent place for future scientific issues associated with Quaternary geology, paleoclimatology and prehistory.