The Somme valley quaternary (river terraces, loess and palaeosols): a contribution to the national inventory of geological heritage
Le quaternaire de la vallée de la Somme (terrasses fluviatiles, loess et paléosols) : une contribution à l’inventaire national du patrimoine géologique
Résumé
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.
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