Auvergne: between volcanoes and valleys

Until the XVIII century volcanoes were considered local phenomena, without a large impact on regional geology or the morphology of the landscape. In the first half of the 18th Century the French self educated naturalist Jean-Ètienne Guettard (as son of a pharmacist at these times he spent a lot of time searching plants and herbs for medical purpose and become interested in natural sciences) discovered that plants are restricted to areas with specific petrological characteristics. In the following years he travelled a lot, and finally with the acquired experience he compiled one of the first geological maps of France.

Fig.1. In 1771 the amateur geologist Nicholas Desmarest studied and published a map depicting the volcanoes of the "Le Puy".

Fig.2. Quarry in the extinct volcano "Puy Narse" with tephra layers.

In 1751 he travelled to the region of Vichy, in Southeast France. With his friend, the lawyer Malesherbes, he visited the "Puy de Dome (1.465m)", a large mountain west of Clermont-Ferrand. Travelling the city of Moulins, he noted a black rock, used as milestone, near the street, and saw that the same lithology was used extensively as building rock. Tracking down the quarry, near the city of Vichy, he saw his presumptions were right, the quarry was situated in an ancient lava flow, and nearby was located an ancient volcano.

Fig.3. Basalt columns used as bricks in a stone wall in the village Murat.

Fig.4. Quarry with columnar basalt near the village of Le Pont de Alleray.

They returned to Clermont-Ferrand, and with a local naturalist they climbed the Puy de Dome and recognized a chain of volcanic craters, known as Le Puys, distributed from south to north.
On 10 May 1752 Guettard handled to the Académie des Sciences a short communication entitled "Observations on some French mountains that in the past were volcanoes".
These observations will begin a scientific controversy on the origin of rocks, were rocks the sediments of an ancient ocean, or were all lithologies erupted by ancient and recent volcanoes?

The region was also important in debates about geologic time and the ability of current river processes to change the landscape in any significant way.
In the1820s the British geologist George Poulett Scrope (1797-1876) established a chronology of lava flows within central France, showing that there had been a continuous history of eruption and river erosion.
In 1822, William Daniel Conybeare (1787-1857) and William Phillips (1775-1828) argued that "to believe them [valleys in general] to have been formed by their actual rivers, however long their action may have endured, involves the most direct physical impossibilities. It is indeed the more extraordinary that a cause so manifestly inadequate, should ever have been embraced, since the fundamental fact of geology, namely, that the continents, now dry land, were once covered with the ocean . . . [and] however that ocean may have been brought to its present level, it could never (on any view of the matter) have drained off the surface of the lands it has deserted, without experiencing violent currents in its retreat . . ."

In 1823 William H. Fitton (1780-1861) wrote that "The effects of water upon the solid strata of the globe have been the subject of much geological debate; but it is now almost universally admitted, that valleys have been excavated by causes no longer in action."

Later, in a 1829 study of the upper Thames, Conybeare characterized "the opposite theories of the fluvialist and diluvialist, the former ascribing such denudations exclusively to the operation of the streams actually existing, or rather to the drainage of the atmospherical waters falling on the districts, which it is supposed have become thus deeply furrowed by the gradual erosion of these waters, continued through a long and indefinite series of ages; the latter contending that such a cause is totally inadequate to the solution of the phenomena, and maintaining that they afford evidence of having been produced by violent diluvial currents."

Fig.5. Glacier-valley on the "Puy Mary", a large volcanic mountain massiv.

References:

DESMAREST, N. (1771): Mémoire sur l'origine et la nature du basalte à grandes colonnes polygones, determinées par l'histoire naturelle de cette pierre, observée en Avergne In: Mémoires de l'Académie Royale des Sciences à Paris pour 1771.
LEWIS, T.A.(ed) (1985): Volcano (Planet Earth). Time-Life Books: 176
MIALLIER, D. ; MICHON, L. ; EVIN, J. ; PILLEYRE, T. ; SANZELLE, S. & VERNET, G. (2004) : Volcans de la chaine des Puys (Massif central, France): point sur la chronologie Vasset-Kilian-Pariou-Chopine. C.R. Geoscience 336 : 1345-1353
MONTGOMERY, K.: The development of the glacial theory, 1800-1870. 2. Tour Europe in the early19th century - Orientation to the issues, scientists, and landscapes of Europe. Acessed 30.05.2010