France Geological Evolution 2

In the Cretaceous period, the difference between the Hercinian area and the Alpine area became clearer, on which the folds that prepare those of the Tertiary already appear. The Wealdian, with its clays and its continental or coastal sands, indicates an emergence in the Paris Basin, while in the Jura, during the Lower Cretaceous, zoogenic limestones are still formed and in the Alps muddy limestones of deep sea are deposited (Neocomian, Valanginian, Hauterivian). Subsequently, the corrugations that are delineated in the Alpine area locally decrease the depths and make the coral facies appear with the mighty layers of the Urgonian limestones, which are so important in the relief of the French Pre-Alps. The Hercinian area, on the other hand, is invaded by successive transgressions, which first give rise to sands, then to clay (craie), partly zoogenic muddy limestone. These powerful layers of clay, now white and quite pure, now marly or glauconic, often abundant with flint nodules, aligned in strips, still emerge on large tracts of the Paris Basin and in the north of the Aquitaine Basin where they represent a part of prim order in the geographical landscape, and also form the basis of the tertiary deposits, while the undulations they undergo regulate their distribution.

According to Internetsailors, the physiognomy of the French soil was definitively fixed during the Tertiary. In the Hercian area, the identification of the high massifs is clear, rugged by domed reliefs, fractures and sinking, while the basins are gulfs or even lakes. The Parisian Basin is the scene of numerous transgressions and regressions, whose history is written in the continuous variations of the facies, which have been minutely studied by geologists. One of the most interesting phases is that during which the center of the basin, towards Paris, was occupied by lagoons in which the gypsum was deposited, forming lenses coated with marl, which provided Cuvier with large skeletons of mammals. The limestone bank of the Lutetian corresponds to a larger gulf; that of the Stampiano sands, known as the arenas of Fontainebleau, indicates a maritime transgression that took place very far, as traces of it can be observed as far as Champagne. It was followed by a lacustrine phase, which gave the limestones of Aquitaine, called limestones of the Beauce. After that time, the Paris Basin definitively emerged and was subjected to erosion.

During this period, the Rhine massif broke up. Starting from the Upper Paleogenic, the Vosges and the Black Forest were separated by a rift valley, where an arm of the sea from S. advanced and where deposits of over 100 m. Power accumulated, favored by a gradual lowering. The Armorican Massif was partly invaded by the Lutetian sea, which left traces in its eastern part. The Massif Central was bumped by domed reliefs and by local sinking with faults; and volcanic eruptions began at the beginning of the Neogenic or perhaps even earlier, to continue until the Quaternary. The most ancient flows often preserved the not very consistent tertiary deposits (clays and sands) of the upper Paleogenic and the lower Neogenic, which indicate the extension of lakes on a slightly deformed old penepiano, as in many other Hercinian massifs in central Europe. In the south of the Massif (Montagne Noire), the middle Paleogenic sea had advanced on an even older penepiano, which erosion is beginning to free.

Everything therefore indicates that in the Hercian area, both the ancient massifs and the secondary sedimentary basins in the first part of the Tertiary formed a set of low-relief lands, leveled by erosion and partly occupied by lakes. But at the end of the Tertiary, as a result of movements of the soil and perhaps a lowering of the level of the oceans, erosion was revived in every part, which began to deeply affect the valleys, removing in large quantities the less resistant soils, putting a the hard rocks are bare and often the escarpments of ancient faults reappear. The limestone platforms constituting the Brie, the Beauce etc. appeared in the central Parisian Basin, and the basins, such as the Pays de Bray and the Boulonnais, engraved in the most accentuated domed reliefs. In the east and south-east (Lorraine, Burgundy),côtes arranged in an arch, turning their steep front towards E., SE. and S. The massif of the Ardennes was freed of its secondary and tertiary lining, and the Meuse threw itself into it through a wonderful gorge. In the Massif Central, the excavation of the rift pits of Limagne and Forez took place, where the less resistant tertiary deposits had been preserved, and the lifting of the granite platforms took place; there the erosion was in contrast with the volcanic accumulation, which led to the relief, full of contrasts, of the Auvergne.

To this gradual differentiation of the Hercynian area corresponds the rise of the Mediterranean Alpine area, which ended with the formation of the powerful uplift of the Pyrenees and the Alps, with the branching of the Jura, flanked by depressed corridors, more or less wide, which separated them from the Hercinian massifs, the last remnants of those deep geosynclinals, where the strata violently raised by the tertiary folds accumulated.

France Geological Evolution 2