In the Beginning

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In the beginning, about 600 million years ago, what is now France was covered by the ocean. Then, during the Devonian Period (about 408 – 360 million years ago) and the subsequent Carboniferous Period (which ended 286 million years ago), a series of mountain ranges developed as a result of a collision between Africa and a North American/North European continent. This is the Hercynian orogonic belt. It extends for more than 3000km from Portugal, Ireland and England in the west, through Spain and France, Germany and Czechoslovakia. Within France the eastern branch comprises the eastern Massif Central, Ardenne, Vosges, and Corsica. The western branch comprises the western part of the Massif Central and the Massif Armoricain, which encompasses the départements of Finistère, Côtes-d’Armor, Morbihan and Ille-et-Vilaine with parts of Manche, Orne, Mayenne, Maine-et-Loire, Loire-Atlantique and Vendée. In mainland France it has the shape of a gigantic V, with the Auvergne at its base.

The folding of the earth’s crust threw up a high, granite, gneiss and schist plateau, with the sea covering the rest of France. At this time the hot, humid climate gave rise to prodigious vegetation. The forests were subjected to tropical rainstorms which carried vegetable debris to a depression bordering the Auvergne at Aurillac and St Flour. Buried under the alluvial mass, and without air, this debris was converted into coal. Monstrous insects, amphibians, reptiles and giant fish existed at this time, but birds had not yet appeared.

Beginning 200 million years ago the Auvergne, subjected to erosion from rain, frost and water currents, had gained some undulations. The climate had developed a seasonal rhythm, and the vegetation lost its exuberance. Conifers, deciduous trees, palm and coconut trees occupied the land. The birds appeared and gigantic tortoises and colossal swimming reptiles inhabited the water. At the end of this period the Massif Central was again almost entirely covered by the sea. Sedimentary soils of clay and chalk settled on the granite ground.

60 million years ago the forces acting on the earth’s crust started to cause a slow process of rising and immersion of the Auvergne. Erosion removed a large part of the secondary deposits.

As result of the tectonic collision giving rise to the Alps and Pyrenees, the southern and eastern areas of the Massif Central were extensively deformed. The centre cracked and dislocated like a granite pavement. Between these fractures, or ‘flaws’, vast parts of the ground were raised, mainly in the east, in particular the Cévennes and Forez. Also, ancient maritime areas were raised by these movements creating the plateaux of the causses. Other areas subsided into the plains and basins, where the sediments accumulated in Limagne, Plaine d’Ambert, Aurillac Basin and Brioude. There is geophysical evidence of a collapsed polygonal area. The contours of this area are roughly under Dienne, Laveissière, Paulhac, Brezons, Narnhac, Pailherols, and in the east of Thièzac. Its base was dislodged in blocks, the deepest of which were a little below sea level. Filled with clay and perhaps gypsum, it perhaps explains the particular instability of the stratovolcanic episodes. The fissures in the rocks allowed the internal magma to erupt, and led to the next phase of building the Cantal.

Classic Geology in Auvergne by Peter Cattermole ISBN 1-903544-05-x available from Blackwells