La limite Pliocène‐Pleistocène en Bretagne

The end of the Pliocene transgression is represented in Brittany, above sands and gravels, by clay or sandy clay containing an estuarian plankton and sometimes shells of molluses. New borings at Pont‐Rouz in Quemperven (Brittany) permit a biostratigraphical study, by means of pollen analysis, of the period corresponding to the Pliocene‐Pleistocene transition. In the lower part of the deposit, rich in Ostrea shells, many floral element characteristics of the Reuver‐clay are present: Sequoia, Taxodium, Sciadopitys , in relatively high percentages, Nyssa, Symplocos, Liquidambar , sporadically. The ‘Tertiary’ elements disappear in the upper part of the clay when other relicts like Tsuga, Pterocarya , and Carya are still present. Thus the Reuverian‐Praetiglian boundary can be traced in the littoral sequence of Pont‐Rouz. As in the Netherlands, the deterioration of the climate is clearly indicated by a sudden increase of Graminaceae pollen. A same evolution of the flora can be noticed at Mernel and Saint‐Jean‐la‐Poterie, also in grey clay, along the river system of the Vilaine. The Upper‐Pliocene transgression has followed the same river system as the present one. The estuarian sediments lie in Brittany between 55 and 30 metres above zero NGF. The maximum of the transgression is situated in the Reuverian at La Salle, near Pont‐Rouz, at 55 m (NGF). The post‐Pliocene regression begins with the Praetiglian and continues in the Lower Pleistocene.