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- Essai sur le creusement de la vallée de la Somme au Quaternaire
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Essai sur le creusement de la vallée de la Somme au Quaternaire
Abstract
The valleys of the Somme and of the small neighbouring streams which drain towards the English Channel, had their courses determined by tectonic movements, probably at the end of the Tertiary. The Chalk, in which the Somme basin is excavated, regularizes the river almost perfectly. The river was only able to transport coarse gravels during periglacial conditions. V. Commont has shown that these sheets of coarse gravel had a slope much greater than that of the Recent river loams and are related to period of low sea-level. Attempts to apply the eustatic concepts of Lamothe and Depéret fail because two observed facts do not fit. These facts, however, support the idea of climato-sedimentary cycles.
The highest alluvial sheets, entirely decalcified, are still poorly known. The oldest of those dated by faunal means is that of Montières-Grâce which lies near the limit between the old Quaternary (Villafranchian) and the recent Quaternary. At Amiens it stands some 50 meters above the floor of the present river valley. From at least the time of this high alluvial sheet, the progressive cutting of the valley seems to have been determined by glacio-eustatic low sea-levels, although it was perhaps also favoured by a slight uplift of the basin. Interruption of the erosion seems characteristic of the interglacial phases. Since Neolithic times, erosion has begun again due to deforestation resulting from agriculture.